Prof Giovanni Galfano

Prof Clara Casco

Prof Yves Trotter

Giulio Contemori

Dr Luca Battaglini

Dr Marcello Maniglia

IMPROVING VISUAL FUNCTION THROUGH CONCOMITANT USE OF PERCEPTUAL LEARNING AND BRAIN STIMULATION: THE CASE OF MACULAR DEGENERATION

Macular degeneration (MD) is a common visual disorder in the aging population characterized by a loss of central vision, reduced visual acuity contrast sensitivity, and increased crowding. This impairment strongly affects the quality of life and personal autonomy. There is currently no cure for AMD, available treatment options are only able to slow down the disease, and even palliative treatments are rare. After the emergence of the central scotoma, patients with MD develop one or more eccentric fixation areaspreferred retinal loci (PRLs) -that are used for fixation, reading, tracking, and other visual tasks that require finer ocular abilities. The final goal of the project was to investigate and to improve the residual visual abilities in the PRL. Four studies were conducted in total. Study 1 was conducted in MD patients to investigate whether after the emergence of the scotoma, the PRL acquire enhanced abilities in the processing of the visual information through spontaneous or use-dependent adaptive plasticity. Study 2 aimed to assess the effects of a single administration of transcranial random noise electrical stimulation (tRNS), a subtype of non-invasive transcranial electrical stimulation, on the spatial integration in the healthy visual cortex. Study 3 aimed to assess the between session effect of daily repeated tRNS coupled with perceptual training. The objective of study 4 was to translate the previous findings into a clinically applicable treatment approach by combining tRNS and perceptual training in adult patients with MD. iv Contrary to previous results, we found neither a phenomenon of spontaneous nor use-dependent cortical plasticity undergoing in the PRL before the training. We also found that the tRNS was able to modulate the visuospatial integration in the early visual processing, promoting plastic changes in the stimulated network. Its effects were not limited to the short-term modulation but also produced a boosting of the learning in a crowding task. The final experiment showed that a combination of tRNS and perceptual training could result in greater improvements and larger transfer to untrained visual tasks in adults with MD than training alone. Overall, our results indicate that tRNS of the visual cortex has potential application as an additional therapy to improve vision in adults with bilateral central blindness.

v Résumé La dégénérescence maculaire (DM) est une pathologie visuelle fréquente dans la population vieillissante, qui se caractérise par une perte de la vision centrale, une diminution de la sensibilité au contraste et de l'acuité visuelle. Cette déficience affecte fortement la qualité de vie et l'autonomie . Il n'existe actuellement aucun traitement curatif de la DMLA, les options thérapeutiques disponibles ne permettant que de ralentir l'évolution de la maladie, avec de rares traitements palliatifs. Après l'apparition du scotome central, les patients atteints de DM développent une ou plusieurs zones de fixations excentrées -les lieux rétiniens préférentiels (PRLs) -qui sont utilisées pour la fixation, la lecture, et d'autres tâches visuelles qui nécessitent des capacités oculaires plus fines. L'objectif principal du projet était d'étudier et d'améliorer les capacités visuelles résiduelles dans les PRL. En tout quatre études ont été menées L'étude 1 a été menée chez des patients atteints de DMLA afin de déterminer si, après l'apparition du scotome, le PRL acquiert des capacités accrues dans le traitement de l'information visuelle grâce à une plasticité adaptative spontanée ou dépendante de l'utilisation. L'étude 2 visait à évaluer les effets d'une seule stimulation électrique transcrânienne à bruit aléatoire (tRNS), une variante de stimulation électrique non-invasive, sur l'intégration spatiale dans le cortex visuel sain. L'étude 3 visait à évaluer chez des sujets sains l'effet d'une répétition quotidienne du tRNS associée à un entraînement perceptif entre les séances. vi L'objectif de l'étude 4 était de traduire ces résultats en une approche clinique en combinant la tRNS et l'entraînement perceptif chez des patients adultes atteints de DMLA. Nous n'avons trouvé aucun phénomène de plasticité corticale spontanée ou dépendante de l'utilisation dans la PRL avant l'entraînement contrairement à ce qui avait été montré dans d'autres études. Nous avons cependant constaté que le tRNS était capable de moduler l'intégration visuospatiale dans le traitement visuel précoce, en favorisant les changements plastiques dans le réseau stimulé. Les effets de la tRNS ne se sont pas limités à la modulation à court terme, mais ont également produit un renforcement de l'apprentissage dans une tâche d'encombrement spatial. L'expérience finale a montré chez les adultes atteints de DMLA qu'une combinaison de la tRNS et de l'apprentissage perceptif pouvait induire des améliorations plus importantes et un transfert accentué vers des tâches visuelles non entrainées que le seul apprentissage perceptif. Dans l'ensemble, nos résultats indiquent que la tRNS du cortex visuel peut être utilisée comme thérapie supplémentaire pour améliorer la vision chez les adultes atteints de cécité centrale bilatérale. vii Riassunto La degenerazione maculare (MD) è una patologia visiva che si verifica per lo più nell'età adulta ed è caratterizzata da una perdita della visione centrale, una riduzione della sensibilità al contrasto, una riduzione dell'acuità visiva, e un aumento dell'affollamento visivo. Questo deterioramento compromette in maniera significativa la qualità della vita e l'autonomia dell'individuo. Attualmente non esistono cure per l'MD ed anche le opzioni di trattamento palliativo sono scarse, tuttavia esistono alcuni trattamenti che permettono di rallentare il decorso della malattia. Con l'insorgenza di uno scotoma centrale, i pazienti con degenerazione maculare sviluppano una o più aree eccentriche di fissazioneloci retinici preferenziali (PRLs)-usati per compiti che richiedono fini abilità oculari e stabilità di fissazione quali ad esempio la lettura, la ricerca visiva, ed il riconoscimento di volti o oggetti. Gli obiettivi principali di questo progetto erano due: investigare le abilità visive residue nel PRL e verificare l'efficacia di un nuovo protocollo di riabilitazione visiva nei pazienti affetti da MD. Il progetto si è sviluppato attraverso quattro differenti studi. Il primo è stato condotto in pazienti con MD per analizzare se dopo lo sviluppo dello scotoma, il PRL acquisisca rafforzate abilità nel processamento dell'informazione visiva attraverso processi di plasticità corticale spontanea o uso-dipendente. Il secondo studio era atto a determinare gli effetti sull'integrazione spaziale nella corteccia visiva, di una tecnica di promozione della plasticità neurale basata sulla stimolazione elettrica transcranica non-invasiva a frequenza casuale (tRNS). Con il terzo studio, abbiamo viii testato l'efficacia della tRNS nell'accelerare l'apprendimento percettivo ed il trasferimento dell'apprendimento nella periferia del campo visivo. Infine, l'obiettivo del quarto ed ultimo studio è stato quello di testare la fattibilità e l'efficacia di un protocollo di riabilitazione visiva per i pazienti affetti da MD che prevedesse l'uso combinato della tRNS e del training percettivo. Contrariamente a studi precedenti, nessun fenomeno di plasticità corticale spontaneo o uso-dipendente è stato trovato nei PRL prima del training.

Inoltre, la tRNS si è dimostrata capace di modulare l'integrazione visuo-spaziale nel processamento visivo precoce, promuovendo cambiamenti nella plasticità della popolazione neurale stimolate. Gli effetti della stimolazione non si sono limitati alla modulazione a breve termine, ma hanno prodotto anche un aumento dell'apprendimento in compiti di affollamento visivo. L'ultimo studio ha mostrato come la combinazione tra tRNS e training percettivo nei pazienti affetti da MD produca un incremento dell'apprendimento eparzialmentedel trasferimento verso compiti visivi non allenati.

In generale, i nostri risultati indicano che la tRNS sulla corteccia visiva potrebbe avere Table 1.

Introduction

The continued increase in average age in developed western countries has transformed macular degeneration (MD) into the main cause of visual impairment in the modern age. This condition involves the loss of central vision, including loss of contrast sensitivity and visual acuity, mainly caused by a foveal scotoma. The current treatment options available are limited and mostly aim only to slow progression rather than restore vision. People with MD to cope with the loss of the sharp central vision, begin to retrain their visual habits so that they can fixate with a peripheral preferred retinal locus (PRL) instead of the impaired macula. PRL is usually found in a region near the scotomatous retina [START_REF] Klaver | Age-Specific Prevalence and Causes of Blindness and Visual Impairment in an Older Population[END_REF][START_REF] Klein | Prevalence of Age-related Maculopathy: The Beaver Dam Eye Study[END_REF]. So far, current efforts to monitor and treat the macular disease have focused on the retina, and no therapy has been able to restore patients' vision as it was before the onset of the disease. Pharmacological treatments can slow down or even stabilize the formation of the visual defect; once it has manifested, treatment and rehabilitation must face its irreversibility. In the absence of a restorative treatment, the use of optical aids and compensative strategies might help to reduce the impact of the visual impairment [START_REF] Calabrèse | A Vision Enhancement System to Improve Face Recognition with Central Vision Loss[END_REF][START_REF] Nguyen | Improvement of reading speed after providing of low vision aids in patients with age-related macular degeneration[END_REF][START_REF] Rohrschneider | Low Vision Aids in AMD[END_REF][START_REF] Romayananda | Prismatic Scanning Method for Improving Visual Acuity in Patients with Low Vision[END_REF], on the other hand it is also possible to focus on enhancing the preserved visual functions [START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF]Pijnacker, Campana et al. (2014) showed that when tRNS is repeatedly applied during a contrastdetection training it induced greater transfer (the post-training improvement observed in an untrained task) to visual acuity (VA) with respect to PL alone in both amblyopic and myopic patients [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF].

In general, tRNS appears to boost both the early (within-session (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF] and late (between sessions/days [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF] components of PL. The research presented in this thesis aimed to evaluate the therapeutic potential of tRNS in the treatment of human adults with bilateral central vision loss. The results presented in the thesis also contributed to increase our knowledge of the effects of tRNS on the visual cortex.

Conceptually this thesis has been divided into two parts. The first part is a review on the MD whose purpose is to critically review the evidence of adaptive neural plasticity in central vision loss. Based on the most recent literature, I will propose new interpretations of old results in this field. The second part will focus on my experimental activity.

The first part of the literature review (CHAPTER II) introduces the MD and its clinical features. Particular relevance has been given to the structural and functional alterations that accompany the ocular disease at the level of the central nervous system. I have also discussed how some of the results that were previously interpreted as evidence of adaptive reorganization can be reinterpreted in an alternative more conservative way. I have concluded exposing the importance of developing an effective rehabilitative protocol for MD patients. In CHAPTER III, PL is presented as a tool to improve visual functions. The concepts of learning, transfer, and specificity are discussed. CHAPTER IV provides an overview of the action mechanisms proposed for the tRNS, the main results of previous studies, and some safety considerations. Finally, the possibility of extending the use of tRNS as a therapeutic intervention for other visual disorders is discussed. This thesis presents the data of four experiments, two of these were carried out on patients affected by bilateral maculopathy while the other two involved normal-viewing subjects.

Each experiment was a necessary step to understand the interaction mechanism of tRNS and perceptual learning on the visual cortex of adults. Each subsequent study represented a further advancement from the basic scientific results towards the clinical application of the tRNS in MD. My hypotheses were:

• The spontaneous and use-dependent cortical adaptations in the region around the scotoma produces as perceptual consequences some alterations in the early spatial integration that would be measurable through psychophysical methods (CHAPTER V).

• tRNS modulates early visual-spatial integration by altering the balance between excitatory and inhibitory lateral interactions. (CHAPTER VI)

• tRNS boosts between session perceptual learning and transfer (CHAPTER VII)

• tRNS enhances and accelerate the outcomes of perceptual training in adults with bilateral macular degeneration (CHAPTER VIII).

CHAPTER IX combines datasets from different studies to further test the hypotheses presented in CHAPTER V.

The first study is presented in CHAPTER V. This experiment aimed to evaluate the presence of spontaneous or use-dependent plasticity in the patients' PRL. In the previous literature about plasticity in MD (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF][START_REF] Maniglia | Spontaneous and traininginduced cortical plasticity in MD patients: Hints from lateral masking[END_REF][START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF], there is debate whether the spared periphery undergoes a general process of adaptation triggered by the presence of the scotoma ("use-dependent reorganization" hypothesis) or whether the preferential use of the PRL for active high demanding tasks triggers a more specific use-dependent adaptation (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF]Maniglia et al., 2018;[START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF]. Knowing the type of adaptation process to which the PRL is subject is important to avoid damaging any compensatory mechanisms in place and instead to try to maximize them with the training. In this study, we tried to investigate this issue psychophysically by comparing spatial integration in the PRL, in a symmetrical retinal position (non-PRL) and a region with matched eccentricity in a control group. To do this, we probed the contextual influences by measuring the contrast gain for a vertical Gabor target, flanked by two high-contrast collinear masks compared to the orthogonal baseline condition. In line with previous literature (Maniglia et al., 2018), our prediction was to find evidence for plasticity, in the form of reduced collinear inhibition. Moreover, we expected the reduction of inhibition to be stronger in the PRL.

CHAPTER VI includes the results of a study that aimed at investigating whether the contextual influences are modulated by tRNS applied to the occipital cortex of human observers during task performance. Given that the tRNS main effect is to increase cortical excitability, it could expand the sensitivity of the neurons to weak stimuli and thus lowering the contrast threshold for the target. At the same time, this increased excitability could also modulate how the target is integrated with its context by altering the relative strength of excitatory (E) or inhibitory (I) influences from the flanking elements, depending on which is weaker. In this case, we might expect an effect of tRNS even if tRNS shows no effect at all on target perceived contrast.

The study presented in CHAPTER VII investigated whether tRNS can effectively boost PL on a peripheral visual task over a small number of daily training sessions.

Additionally, we tested whether learning transferred to untrained spatial location and task variation. We expected the tRNS to be able to increase both the learning rate and the transfer of learning.

The final study presented in CHAPTER VIII and CHAPTER IX brought together the findings of the experiments presented in this dissertation, investigating the combined effect of tRNS and perceptual learning in a visual rehabilitation protocol in MD patients.

This thesis explores a new path for visual improvement in MD and opens new possibilities for other clinical population.

Chapter II.

Macular Degeneration: Clinical Presentation

Macular degeneration [MD] can lead to severe visual impairment and blindness.

Throughout the disease, the central retina responsible for our sharp vision undergoes a series of irreversible changes that endangers visual acuity.

The consequences of a bilateral central scotoma are severe visual impairments, especially in reading, face recognition, and visual search. Patients with central scotoma develop one or more eccentric fixation areas -preferred retinal loci (PRLs) -that are used for fixation, reading, tracking, and other visual tasks that require fine ocular abilities. This area, the preferred retinal locus (PRL), is a useful adaptation to central visual loss, but its function is weaker than that of the fovea because of the relatively poor visual resolution and the fixation instability.

There are different types of MD corresponding to different aetiology and progressions. Juvenile macular degeneration (JMD) is a quite rare type and includes several inherited eye diseases like Stargardt's disease, Best disease, and juvenile retinoschisis. In these cases, loss of central vision may begin in childhood or young adulthood. Unfortunately, there is no treatment available for these diseases, which are caused by gene mutations passed down in families. Early age-related macular degeneration (AMD) consists of retinal or subretinal drusen, yellow or white accumulations of extracellular material, and retinal pigment abnormalities. It is not known whether drusen promote AMD or if they are common results of an underlying process. Late AMD is divided into neovascular AMD and geographic atrophy (GA).

Early AMD and GA are called dry AMD as a differentiation from the so-called wet AMD that is a result of neovascularisation within the retina with leakage of fluid in the macula.

There are different outcomes for the three type of AMD: patients with early AMD can retain some visual functionality, on the contrary, GA patients have no residual visual function in the affected area due to permanent loss of photoreceptors. In wet AMD, retinal scarring and deterioration can lead to blindness. No curative treatments are available until now for any of the diseases, but there are therapies able to slow down or freeze the progression. For wet AMD, treatment consists mainly of inhibitors of vascular endothelial growth factor (VEGF) repeatedly injected into the eye. AMD has been identified as the third leading cause of blindness in the world, and the first considering only the developed part [START_REF] Wong | Global prevalence of age-related macular degeneration and disease burden projection for 2020 and 2040: A systematic review and meta-analysis[END_REF]. Current efforts for tracking and treating macular disease have focused on the retina, for instance, quantification of drusen distributions, photodynamic therapy [START_REF] Wormald | Photodynamic therapy for neovascular age-related macular degeneration[END_REF], and even retinal prostheses for degenerations of the entire retina [START_REF] Lohmann | The very large electrode array for retinal stimulation (VLARS) -a concept study[END_REF][START_REF] Roux | Probing the functional impact of sub-retinal prosthesis[END_REF][START_REF] Weiland | Retinal prosthesis[END_REF].

Moreover, the impact of degeneration is not limited within the scotoma, but it may also compromise the peripheral retinal region generally considered as "spared". Some studies have reported a higher frequency of peripheral lesions such as drusen, atrophy, and changes to the retinal pigment epithelium, in the retina outside the scotoma when compared to control eyes. [START_REF] Domalpally | Peripheral Retinal Changes Associated with Age-Related Macular Degeneration in the Age-Related Eye Disease Study 2[END_REF][START_REF] Johansen Forshaw | Peripheral Retinal Lesions in Eyes with Age-Related Macular Degeneration using Ultra-Widefield Imaging: A Systematic Review with Meta-Analyses[END_REF][START_REF] Laíns | Peripheral Changes Associated With Delayed Dark Adaptation in Age-related Macular Degeneration[END_REF][START_REF] Lengyel | A Population-Based Ultra-Widefield Digital Image Grading Study for Age-Related Macular Degeneration-Like Lesions at the Peripheral Retina[END_REF]. This is in line with psychophysical studies reporting reduced visual acuity and contrast sensitivity in MDs in respect to age-matched controls at the same eccentricity [START_REF] Fletcher | Visual function in patients with choroidal neovascularization resulting from age-related macular degeneration: The importance of looking beyond visual acuity[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. The indication that patients might show some impairments even outside the scotoma has also to be considered when discussing results from experimental protocols that match normal viewers and MD patients.

So far, no therapies able to restore patients' vision as it was before the onset of the disease are available. Drug treatments can slow down or even stabilize the formation of the visual defect, but once formed, treatment and rehabilitation must face against its irreversibility. In the case of visual impairment, in the absence of Restorative approaches, it is possible to adopt other types of rehabilitations based on Compensatory and Substitutive techniques [START_REF] Bouwmeester | The effect of visual training for patients with visual field defects due to brain damage: A systematic review[END_REF][START_REF] Lane | Clinical treatment options for patients with homonymous visual field defects[END_REF]. In particular, to ensure a better outcome in the daily life activities, the treatment of MD often require combined approaches that reckon on optical aids (N. X. [START_REF] Nguyen | Improvement of reading speed after providing of low vision aids in patients with age-related macular degeneration[END_REF][START_REF] Rohrschneider | Low Vision Aids in AMD[END_REF] and multiple kinds of training. Some possible training options are reading training [START_REF] Seiple | Reading rehabilitation of individuals with AMD: Relative effectiveness of training approaches[END_REF], scotoma awareness training [START_REF] Verghese | Scotoma Awareness and Eye Movement Training in Age-Related Macular Degeneration[END_REF], oculomotor training [START_REF] Rosengarth | Functional and structural brain modifications induced by oculomotor training in patients with age-related macular degeneration[END_REF] and Microperimetric biofeedback training [START_REF] Vingolo | Microperimetric biofeedback in AMD patients[END_REF][START_REF] Vingolo | MP-1 biofeedback: Luminous pattern stimulus versus acoustic biofeedback in age related macular degeneration (AMD)[END_REF]. What I will focus on in this thesis is a training based on a perceptual learning paradigm whose purpose is to reshape the way in which visual information relating to multiple concomitant elements is integrated to form a single percept (Uri [START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. We will see this technique in more detail in CHAPTER III.

The PRL It is still unclear how the PRL develops and if it acquires over time, particular processing ability with respect to the other quadrants of the functional periphery. Since it may be found in every direction around the scotoma, it is difficult to predict where the PRL might develop. Possibly would be the region with the best-spared sensitivity outside the scotoma [START_REF] Timberlake | Reading with a macular scotoma. II. Retinal locus for scanning text[END_REF] but this idea has been challenged recently since in average the PRL doesn't seem the absolute best retinal spot for visual acuity [START_REF] Bernard | Visual Acuity Is Not the Best at the Preferred Retinal Locus in People with Macular Disease[END_REF][START_REF] Shima | Concept of a functional retinal locus in age-related macular degeneration[END_REF] or contrast sensitivity and crowding [START_REF] Contemori | Contextual influences in the peripheral retina of patients with macular degeneration[END_REF]. Some other factors known to play a role are the form and density of the central scotoma, the distance from the dysfunctional macula, and also the distance from the scotoma border [START_REF] Crossland | Preferred retinal locus development in patients with macular disease[END_REF][START_REF] Erbezci | Preferred Retinal Locus Locations in Age-Related Macular Degeneration[END_REF][START_REF] Markowitz | The relationship between scotoma displacement and preferred retinal loci in low-vision patients with age-related macular degeneration[END_REF]Riss-Jayle, Giorgi, & Barthes, 2008a, 2008b). Sometimes, multiple PRLs may co-exist, and they could be interchanged depending on the viewing distance, the task, or the luminance [START_REF] Crossland | Preferred retinal locus development in patients with macular disease[END_REF][START_REF] Déruaz | Reading with multiple preferred retinal loci: Implications for training a more efficient reading strategy[END_REF][START_REF] González | Mechanisms of Image Stabilization in Central Vision Loss: Smooth Pursuit[END_REF][START_REF] Lei | Using two preferred retinal loci for different lighting conditions in patients with central scotomas[END_REF]. Probably it is worthy of thinking of the PRL as the best region in terms of functional efficiency instead of best sensitivity. Shima and colleagues (2010) have proposed that the oculomotor efficiency is likely the driving force for the functional adaptive changes in MD. They suggest that the spot with the best sensitivity and the one with the best fixational abilities are not necessarily the same but that they are often nearby each other and functionally linked (Samuel N. [START_REF] Markowitz | Letter to the Editor: Visual Acuity Is Not the Best at the Preferred Retinal Locus in People with Macular Disease[END_REF][START_REF] Shima | Concept of a functional retinal locus in age-related macular degeneration[END_REF]. Moreover, [START_REF] Kabanarou | Gaze Changes with Binocular versus Monocular Viewing in Age-Related Macular Degeneration[END_REF] showed that there is a shift in fixation in one or both eyes when comparing monocular versus binocular viewing conditions. Since the two monocular PRLs often fall on noncorresponding areas [START_REF] Kisilevsky | Characteristics of the preferred retinal loci of better and worse seeing eyes of patients with a central scotoma[END_REF] one or both might switch to another position, to facilitate conjugation in binocular tasks [START_REF] Kabanarou | Gaze Changes with Binocular versus Monocular Viewing in Age-Related Macular Degeneration[END_REF][START_REF] Verezen | Eccentric gaze direction in patients with central field loss[END_REF]. In general, visual acuity, eye movement, and fixation stability are driven by the better eye [START_REF] Kabanarou | Gaze Changes with Binocular versus Monocular Viewing in Age-Related Macular Degeneration[END_REF][START_REF] Schneider | Neurological Basis for Eye Movements of the Blind[END_REF][START_REF] Tarita-Nistor | Fixation stability during binocular viewing in patients with age-related macular degeneration[END_REF] but the overall binocular oculomotor efficiency is still disrupted in patients who have a low correspondence between the two PRLs. As we might expect, the lower the difference in quality of vision between the two eyes end the better the oculomotor efficiency. In fact, the best predictors for the oculomotor efficiency are the contrast sensitivity ratio between the two eyes and the amount of stereoacuityif measurablenot the absolute visual acuity [START_REF] Shanidze | Monocular and binocular smooth pursuit in central field loss[END_REF]. The same concept holds also for the reading speed, in this case binocular integration and acuity gain have a stronger impact than absolute visual acuity. Indeed patients with interocular inhibition are reading significantly slower than those with no inhibition or summation [START_REF] Tarita-Nistor | Maximum reading speed and binocular summation in patients with central vision loss[END_REF]. It has been reported that some patients also show characteristics of binocular inhibition at low and medium spatial frequencies [START_REF] Valberg | Binocular contrast inhibition in subjects with age-related macular degeneration[END_REF] but surprisingly in average binocular acuity gain is not different from that of age-matched control participants (Tarita-Nistor, [START_REF] Tarita-Nistor | Binocular interactions in patients with age-related macular degeneration: Acuity summation and rivalry[END_REF]. A better understanding of how the PRLs develop and how people with bilateral MD conjugate the two eyes is fundamental to design accurately any visual rehabilitation protocols that aims at improving the residual visual ability in MD, but this requires ad hoc tools with different characteristics from those generally used in clinical evaluation and rehabilitation [START_REF] Tarita-Nistor | Identifying absolute preferred retinal locations during binocular viewing[END_REF]. As we will see later in CHAPTER III, the rehabilitative protocol discussed in this thesis partially overcomes this limitation. In fact, even if carried out monocularly, PL obtained with the lateral masking paradigm is able to transfer to the untrained eye with consequent benefit in binocular vision [START_REF] Dorais | Contrast masking effects change with practice[END_REF][START_REF] Yu | Perceptual learning in contrast discrimination and the (minimal) role of context[END_REF]).

As we will see in CHAPTER VII and VIII, one of the thesis aims is to improve PL outcome by maximizing the transfer through the concomitant use of the tRNS.

Plasticity in the Adult Visual System

Macular degeneration is the most frequent, but not the only cause of central scotoma. Regardless of aetiology, the onset of the central blindness generally occurs in adulthood. [START_REF] Ferris | Senile macular degeneration: Review of epidemiologic features[END_REF]).

To date it has been established, that once the critical period for the development of the visual system is over, there is still a certain level of residual plasticity that allows for adaptative changes of the system even in advanced age [START_REF] Gilbert | Topographic reorganization in the striate cortex of the adult cat and monkey is cortically mediated[END_REF][START_REF] Kaas | Reorganization of retinotopic cortical maps in adult mammals after lesions of the retina[END_REF][START_REF] Knudsen | Sensitive Periods in the Development of the Brain and Behavior[END_REF]. It is also known, however, that this ability is slowly declining with time and that it is subject to the limits given by the functional equilibrium of the entire system [START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF][START_REF] Knudsen | Sensitive Periods in the Development of the Brain and Behavior[END_REF][START_REF] Morishita | Lynx1, a cholinergic brake, limits plasticity in adult visual cortex[END_REF]. Any change in the network must be compatible with the subsequent, and previous stages of processing, otherwise perceptive distortions due to maladaptive plasticity may arise [START_REF] Rosa | Plasticity in the Human Visual Cortex: An Ophthalmology-Based Perspective[END_REF]. In some cases, visual hallucinations are a necessary step during visual recovery and tend to disappear when the plastic adaptive process matures, but if their presence persists, they could be functionally invalidating [START_REF] Burke | The neural basis of Charles Bonnet hallucinations: A hypothesis[END_REF][START_REF] Tan | Visual hallucinations during visual recovery after central retinal artery occlusion[END_REF]. When visual cortical neurons in adult mammals are deprived of their normal afferent input from retinae, they can acquire new receptive fields by modifying the effectiveness of existing intrinsic connections, a basis for topographic map reorganization. If the retinal lesions are relatively small (< 5 degrees), after long periods, the lesion projection zone (LPZ) shows partial recovery and exhibit normal receptive field properties for high contrast stimuli. However, their maximum response amplitude and the contrast sensitivity is notably reduced (Y. M. [START_REF] Chino | Adult plasticity in the visual system[END_REF][START_REF] Safran | Plasticity in the adult visual cortex: Implications for the diagnosis of visual field defects and visual rehabilitation[END_REF][START_REF] Sur | Mechanisms of plasticity in the developing and adult visual cortex[END_REF]. Some other experiencedependent mechanisms may overlap on top of this spontaneous cortical plasticity, for example by prolonged practice it is possible to trigger a perceptual learning mechanism (PL) that benefits from reinforcement-dependent plasticity (Manfred. [START_REF] Fahle | Perceptual learning[END_REF][START_REF] Karmarkar | Experience-Dependent Plasticity in Adult Visual Cortex[END_REF]. Recent studies have shown how it is possible to take advantage of this residual plasticity to obtain significant improvements during rehabilitation protocols although with great limitations (M. [START_REF] Li | New approaches to visual rehabilitation training for patients with visual field defects[END_REF][START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF]. Wandell & Smirnakis (2009) in their fundamental review state: "...it can be no serious debate as to whether the brain is plastic or not: it is both. A better question is to investigate distinct systems and understand the conditions under which each system is plastic or stable". More recently Haak and colleagues by reviewing contrasting results about cortical remapping in MD suggest that differently from animal studies, evidence for extensive reorganization in the adult human primary visual cortex is just limited. According to their model, this could be due to the costs associated with making changes at the very root of the visual processing hierarchy [START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF]. A retinal lesion during (or before) the sensitive period can modify the architecture of the visual system radically, but after the development is completed, the patterns of connectivity become highly stable. At this point, the higher visual areas for their correct functioning rely on the retinotopic and functional organizations of the lower-level visual areas. Huge alterations of the low-level structures could have negative consequences in the readout of the upper areas. After the end of the sensitive period, the residual plasticity could be able to alter the connectivity patterns exclusively if the change respects the architectural constraints established during development. In case of retinal damage, it would make more sense for the visual system to make slow structural adjustments at later stages of visual processing than a fast and extensive remapping at a lower-level, because the later stages have fewer dependencies that may be adversely affected [START_REF] Ahissar | The reverse hierarchy theory of visual perceptual learning[END_REF][START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF][START_REF] Hochstein | View from the top: Hierarchies and reverse hierarchies in the visual system[END_REF][START_REF] Lillard | Old dogs learning new tricks: Neuroplasticity beyond the juvenile period[END_REF]. It is important to note that this constraint would be less strict in the case of a simpler brain with less processing nodes such as mice or cats that are the main nonprimate models for the study of retinal lesions.

According to the previous literature, a necessary factor for an extensive cortical reorganization might be the presence of a dense bilateral scotoma rather than a monocular scotoma or a bilateral scotoma with some spared islands of vision (Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF][START_REF] Dilks | Reorganization of visual processing in age-related macular degeneration depends on foveal loss[END_REF][START_REF] Schumacher | Reorganization of visual processing is related to eccentric viewing in patients with macular degeneration[END_REF]. Also, the presence of a stable eccentric PRL seems to be relevant (Daniel D. [START_REF] Dilks | Reorganization of visual processing in age-related macular degeneration depends on foveal loss[END_REF][START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF][START_REF] Schumacher | Reorganization of visual processing is related to eccentric viewing in patients with macular degeneration[END_REF].

Undoubtedly if the central vision in one of the two eyes is preserved, there is no need for a re-referencing of the oculomotor activity, much less a cortical remapping, with the result that the mechanism of suppression of the best eye with respect to the defective one could be activated [START_REF] Wiecek | Metamorphopsia and interocular suppression in monocular and binocular maculopathy[END_REF]. This suppressive mechanism could be based on the consolidation of pre-existing inhibitory networks that are also transiently activated in normal viewers, for example during binocular rivalry [START_REF] Holopigian | Clinical suppression and binocular rivalry suppression: the effects of stimulus strength on the depth of suppression[END_REF][START_REF] Nguyen | The depth and selectivity of suppression in binocular rivalry[END_REF]. Possibly it could easily be reversed with perceptual learning or dichoptic training, in the same way as of clinical suppression (Barollo, Contemori, Battaglini, Pavan, & Casco, 2017a;[START_REF] Hess | A new binocular approach to the treatment of Amblyopia in adults well beyond the critical period of visual development[END_REF][START_REF] Hess | A binocular approach to treating amblyopia: Antisuppression therapy[END_REF]. Also, the re-activation at the level of the LPZ that is found in patients with bilateral partially overlapping scotoma could indicate a strengthening of a pre-existing feedback network already active in normal vision rather than an extensive cortical reorganization. This interpretation of the BOLD activity in the LPZ was originally suggested by Masuda, but to date it has not been confirmed [START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF]. We will discuss this possibility extensively in the 'Functional changes' section of this review.

The idea that the reorganization relies more on slow structural adjustments at later stages rather than extensive remapping at lower-levels undoubtedly re-launches the role of visual PLand the active training in generalin the rehabilitation of visual deficits.

Later, we will see how the major evidence of cortical reorganization in MD can be explained as the product of short-term non-specific spontaneous plasticity mechanisms. This form of plasticity contrasts with the one promoted by the perceptual training whose changes are slow but stable over time. [START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF][START_REF] Sabel | Vision restoration after brain and retina damage: The "residual vision activation theory[END_REF][START_REF] Trauzettel-Klosinski | Rehabilitative techniques[END_REF].

Is there a remapping process for the neurons inside the lesion projection zone (LPZ)? Retrograde (presynaptic) degeneration of retinal ganglion cells and retinal nerve fibers following damage to the occipital lobe has been documented extensively [START_REF] Beatty | Direct demonstration of transsynaptic degeneration in the human visual system: A comparison of retrograde and anterograde changes[END_REF][START_REF] Cowey | Projection patterns of surviving neurons in the dorsal lateral geniculate nucleus following discrete lesions of striate cortex: implications for residual vision[END_REF][START_REF] Dinkin | Trans-synaptic Retrograde Degeneration in the Human Visual System: Slow, Silent, and Real[END_REF].

On the other hand, evidence for anterograde (postsynaptic) degeneration are not as prevalent, but the phenomenon is nonetheless accredited [START_REF] You | Anterograde Degeneration along the Visual Pathway after Optic Nerve Injury[END_REF] as well as the fact that retinal degeneration could lead to secondary brain damage to the visual pathways through a trans-synaptic degeneration mechanism [START_REF] Ito | An approach for neuroprotective therapies of secondary brain damage after excitotoxic retinal injury in Mice[END_REF].

On the other hand, we also know that the adult visual system has residual plasticity that can counterbalance this phenomenon through the functional reorganization of neurons whose afferences have been suppressed [START_REF] Gilbert | Adult Visual Cortical Plasticity[END_REF][START_REF] Karmarkar | Experience-Dependent Plasticity in Adult Visual Cortex[END_REF]. In this regard, the long-term visual deprivation might work as a trigger for the replacement (loss and formation) of spines in the deafferented cortex favouring the formation of new networks [START_REF] Keck | Massive restructuring of neuronal circuits during functional reorganization of adult visual cortex[END_REF]. One might wonder what the changes at a structural and functional level after a long period of adaptation to the loss of central vision are and whether these changes lead to compensation for the damage. Over the last two decades, a growing body of research has used magnetic resonance imaging (MRI) to answer these questions [START_REF] Prins | Structural brain MRI studies in eye diseases: Are they clinically relevant? A review of current findings[END_REF]. We will try to summarize the main findings in the following paragraphs.

Structural Changes

In MRI research that studies alterations following the loss of central vision, it is a common practice to compare patients with a corresponding age-matched control group.

The body of evidence that has gradually accumulated over time shows a reduction in the grey matter of the lateral geniculate nuclei, and of the visual cortex as well as in the white matter of the optic radiations [START_REF] Boucard | Changes in cortical grey matter density associated with long-standing retinal visual field defects[END_REF][START_REF] Hernowo | Morphometric analyses of the visual pathways inmacular degeneration[END_REF][START_REF] Plank | Gray matter alterations in visual cortex of patients with loss of central vision due to hereditary retinal dystrophies[END_REF][START_REF] Prins | Structural brain MRI studies in eye diseases: Are they clinically relevant? A review of current findings[END_REF][START_REF] Yoshimine | Age-related macular degeneration affects the optic radiation white matter projecting to locations of retinal damage[END_REF]. Grey matter loss in the visual cortex appears to overlap with the retinotopic region corresponding to the retinal lesion [START_REF] Boucard | Changes in cortical grey matter density associated with long-standing retinal visual field defects[END_REF] and to be correlated with scotoma size [START_REF] Plank | Gray matter alterations in visual cortex of patients with loss of central vision due to hereditary retinal dystrophies[END_REF]. Also, differences between JMD and AMD have been evidenced, with JMD showing more marked signs of grey matter loss [START_REF] Prins | Surface-based analyses of anatomical properties of the visual cortex in macular degeneration[END_REF] and white matter loss in the optic nerves and the chiasm [START_REF] Hernowo | Morphometric analyses of the visual pathways inmacular degeneration[END_REF]. If the long-term visual deprivation would be the mechanism behind those structural changes, we should see alterations in higher-level visual areas as a consequence of specific functional deprivation in the absence of damage in the early visual cortex. This is precisely what has been found in the case of late monocular blind patients where a volumetric decrease in the superior lateral occipital cortices is present in the absence of a decrease in the early visual cortex [START_REF] Prins | Loss of binocular vision in monocularly blind patients causes selective degeneration of the superior lateral occipital cortices[END_REF]. On the contrary, in MD, observed data are consistent with a transsynaptic degeneration propagating from the degenerated retinal axons to the central visual system [START_REF] Malania | Compromised integrity of central visual pathways in patients with macular degeneration[END_REF][START_REF] Ogawa | White matter consequences of retinal receptor and ganglion cell damage[END_REF][START_REF] Yoshimine | Age-related macular degeneration affects the optic radiation white matter projecting to locations of retinal damage[END_REF]. Some very recent studies based on tractography and diffusion-weighted imaging (dMRI) have provided strong support to the existence of transsynaptic anterograde degeneration in case of damage to the peripheral visual nervous system [START_REF] Balk | Bidirectional trans-synaptic axonal degeneration in the visual pathway in multiple sclerosis[END_REF][START_REF] Balk | A dam for retrograde axonal degeneration in multiple sclerosis[END_REF][START_REF] Malania | Compromised integrity of central visual pathways in patients with macular degeneration[END_REF][START_REF] Ogawa | White matter consequences of retinal receptor and ganglion cell damage[END_REF][START_REF] Tur | Longitudinal evidence for anterograde trans-synaptic degeneration after optic neuritis[END_REF][START_REF] Yoshimine | Age-related macular degeneration affects the optic radiation white matter projecting to locations of retinal damage[END_REF].

In 2014 Ogawa and colleagues showed a reduction in fractional anisotropya measure of fiber integrityin the optic tract and the optic radiation of patients with central vision loss. Other two studies have later corroborated their main finding. [START_REF] Malania | Compromised integrity of central visual pathways in patients with macular degeneration[END_REF] found a correlation between photoreceptors loss, atrophy of the ganglion cell axons, and diffusivity in the optic tract fibers. Moreover, they found that retinal nerve fiber layer thinning and the lower fractional anisotropy in the optic radiation was not limited to the foveal afferent fibers but also extended to the peripheral fibers [START_REF] Malania | Compromised integrity of central visual pathways in patients with macular degeneration[END_REF]. This is in agreement with the psychophysical and physiological finding showing that the "spared" periphery outside the scotoma maybe not so spared [START_REF] Domalpally | Peripheral Retinal Changes Associated with Age-Related Macular Degeneration in the Age-Related Eye Disease Study 2[END_REF][START_REF] Fletcher | Visual function in patients with choroidal neovascularization resulting from age-related macular degeneration: The importance of looking beyond visual acuity[END_REF][START_REF] Johansen Forshaw | Peripheral Retinal Lesions in Eyes with Age-Related Macular Degeneration using Ultra-Widefield Imaging: A Systematic Review with Meta-Analyses[END_REF][START_REF] Laíns | Peripheral Changes Associated With Delayed Dark Adaptation in Age-related Macular Degeneration[END_REF][START_REF] Lengyel | A Population-Based Ultra-Widefield Digital Image Grading Study for Age-Related Macular Degeneration-Like Lesions at the Peripheral Retina[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. Despite this, it is clear that the main driver of the degeneration is the loss of central vision. Yoshimine and colleagues (2018) also found a reduction in fractional anisotropy along the optic radiation, but it affected fascicles primarily projecting to the central visual field in respect to the one projecting to mid or far-periphery. Contrary to the previous study, they did not find an alteration in the optic tract, but they found a positive correlation between the integrity of the optic radiation and the visual acuity [START_REF] Yoshimine | Age-related macular degeneration affects the optic radiation white matter projecting to locations of retinal damage[END_REF]. Although the results of these studies have some minor inconsistencies, they provide convergent evidence that cell death within the retina causes significant alterations along the visual pathways to the cortex. This finding challenges the understanding that macular degeneration is confined within the retinal tissue and questions the effectiveness of many restorative treatments (Lemos, Pereira, & Castelo-Branco, 2016a). The structural changes listed above are not, however, the only ones that have been found. Alongside with anterograde degeneration, another more general degenerative mechanism could be related to the etiopathology of AMD. For instance, Hernowo and colleagues (2014) have also found that AMDs but not JMDs show a reduction of white matter volume in frontal areas that are not strictly linked with visual processing. This could suggest a link with other forms of neural degeneration such as mild cognitive impairment and Alzheimer disease (AD). Indeed this hypothesis is corroborated by some studies that evidenced some common histopathologic features found in AD and AMD that might be led back to a common pathogenic mechanism [START_REF] Lynn | The complexities underlying age-related macular degeneration: Could amyloid beta play an important role?[END_REF][START_REF] Ohno-Matsui | Parallel findings in age-related macular degeneration and Alzheimer's disease[END_REF][START_REF] Ong | Ocular amyloid imaging at the crossroad of Alzheimer's disease and agerelated macular degeneration: implications for diagnosis and therapy[END_REF][START_REF] Peiretti | Age-related macular degeneration and cognitive impairment show similarities in changes of neutral lipids in peripheral blood mononuclear cells[END_REF].

Nevertheless, AD and AMD don't have a strong association with each other [START_REF] Keenan | Associations between age-related macular degeneration, alzheimer disease, and dementia: Record linkage study of hospital admissions[END_REF][START_REF] Williams | The prevalence of age-related macular degeneration in Alzheimer's disease[END_REF]. In fact, the hazard ratio for developing probable or possible AD for people with established diagnoses of AMD (more than 5 years) is quite low, approximately 1.50 (C. S. [START_REF] Lee | Associations between recent and established ophthalmic conditions and risk of Alzheimer's disease[END_REF]. A ratio of 1.50 does not surprise if we consider that there might be some common risk factors linked to lifestyle habits [START_REF] Klaver | Is age-related maculopathy associated with Alzheimer's disease? The Rotterdam Study[END_REF]. Not all the structural alterations found in AMD have a negative connotation. Some studies have also reported changes that can be traced to a positive adaptation to the presence of the scotoma. For example, [START_REF] Plank | Gray matter alterations in visual cortex of patients with loss of central vision due to hereditary retinal dystrophies[END_REF] found increased grey matter volume in a region slightly anterior to the frontal eye fields, and this increase was correlated positively with fixation stability. They proposed that this could be a consequence of oculomotor learning. Other evidence for structural changes linked to functional adaptive plasticity comes from the studies of [START_REF] Sabbah | Reorganization of early visual cortex functional connectivity following selective peripheral and central visual loss[END_REF][START_REF] Sanda | Visual brain plasticity induced by central and peripheral visual field loss[END_REF]. In this second study, they provided evidence of structural modification linked to enhanced peripheral visual field processing in associative visual areas that compensate for the central visual field loss. In fact, despite a thinning of the visual cortex, they found an increase in synaptic complexity in some regions of the lateral occipital cortex (hOc4la) and the fusiform gyrus (FG1) that are involved in shape, place, object, and face processing [START_REF] Lorenz | Two New Cytoarchitectonic Areas on the Human Mid-Fusiform Gyrus[END_REF][START_REF] Malikovic | Cytoarchitecture of the human lateral occipital cortex: mapping of two extrastriate areas hOc4la and hOc4lp[END_REF]. This is in agreement with the previous study of [START_REF] Sabbah | Reorganization of early visual cortex functional connectivity following selective peripheral and central visual loss[END_REF] that found increased resting-state functional connectivity between the peripheral early visual cortex and the fusiform gyrus in patients with central visual field loss. If we consider the results presented so far, we can imagine two mechanisms that act oppositely: an anterograde degeneration that spreads from the retinal lesion to the visual cortex and a compensatory mechanism that enhances the connections between the preserved peripheral visual field and the visual associative areas.

In this context, the use of PL as a way to bolster activity-dependent plasticity during visual rehabilitation could have the dual purpose of reducing the effects of anterograde degeneration and guiding the strengthening of connections between the cortical projection of the PRL and the higher-level visual areas [START_REF] Crair | Reconnecting eye to brain[END_REF][START_REF] Ganguly | Activity-dependent neural plasticity from bench to bedside[END_REF]. PL might also work in combination with genetic or pharmacological strategies that reintroduce conditions for reorganization in the adult brain by promoting homeostatic rescaling and synaptic remodelling [START_REF] Crair | Reconnecting eye to brain[END_REF].

Functional Changes

The use of neuroimaging techniques in the study of central vision loss was not limited to structural investigations, and there are functional anomalies that have also been reported (for review on the topic see [START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF]Lemos et al., 2016a;Wandell & Smirnakis, 2009a). Given the retinotopic structure of the visual system, one of the most interesting questions is what happens to the deafferented part of the visual cortex. One hypothesis is that the neurons coding for the diseased part of the retina that is still functional could be recruited in the processing of stimuli outside the scotoma (Daniel D. [START_REF] Dilks | Reorganization of visual processing in age-related macular degeneration depends on foveal loss[END_REF][START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF][START_REF] Morland | Abnormal retinotopic representations in human visual cortex revealed by fMRI[END_REF]. By responding to stimuli originally outside the retinotopic area of their competence, these neurons would, therefore, undergo a functional remapping. This type of extensive reorganization is documented in animal models [START_REF] Gilbert | Axonal sprouting accompanies functional reorganization in adult cat striate cortex[END_REF]U T Eysel & Schweigart, 1999;[START_REF] Eysel | A functional sign of reorganization in the visual system of adult cats: Lateral geniculate neurons with displaced receptive fields after lesions of the nasal retina[END_REF][START_REF] Keck | Massive restructuring of neuronal circuits during functional reorganization of adult visual cortex[END_REF] as well as in other domains such as the somatosensory cortex [START_REF] Flor | Remapping somatosensory cortex after injury[END_REF][START_REF] Gaetz | Massive cortical reorganization is reversible following bilateral transplants of the hands: evidence from the first successful bilateral pediatric hand transplant patient[END_REF][START_REF] Rigato | The neural basis of somatosensory remapping develops in human infancy[END_REF][START_REF] Winship | Remapping the somatosensory cortex after stroke: Insight from imaging the synapse to network[END_REF]. In the human vision, this has been found in early partial blindness [START_REF] Baseler | Reorganization of human cortical maps caused by inherited photoreceptor abnormalities[END_REF][START_REF] Muckli | Bilateral visual field maps in a patient with only one hemisphere[END_REF] and recently also in adults suffering from degeneration of the peripheral retina [START_REF] Ferreira | Primary visual cortical remapping in patients with inherited peripheral retinal degeneration[END_REF]. In case of central vision loss, some authors observed a stimulus-related activation in the lesion projection zone (LPZ) (Chris I. [START_REF] Baker | Reorganization of visual processing in macular degeneration: Replication and clues about the role of foveal loss[END_REF][START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF][START_REF] Schmid | Visually driven activation in macaque areas V2 and V3 without input from the primary visual cortex[END_REF]. There are two competing explanations for the activation in the LPZ: it could indicate structural reorganization of the visual cortex or unmasking of previously silent cortico-cortical connections [START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF]. Although the first hypothesis finds support in animals and juvenile lesions, there are valid reasons to think that this activation does not represent an index of cortical reorganization in adults with MD. First of all the reactivation of the LPZ seems to be present only under some conditions [START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF][START_REF] Masuda | Task-dependent V1 responses in human retinitis pigmentosa[END_REF] and only for some patients (Chris I. [START_REF] Baker | Reorganization of visual processing in macular degeneration: Replication and clues about the role of foveal loss[END_REF] and it has not been reported in all the studies (T. [START_REF] Liu | Incomplete cortical reorganization in macular degeneration[END_REF][START_REF] Sunness | Retinotopic mapping of the visual cortex using functional magnetic resonance imaging in a patient with central scotomas from atrophic macular degeneration[END_REF]. Moreover, such an extensive reorganization could cause dysfunction at higher visual areas that rely on that input. The input could be misinterpreted as producing distortion or illusions (D. D. [START_REF] Dilks | Human Adult Cortical Reorganization and Consequent Visual Distortion[END_REF]. In the case of extensive central vision loss, a huge rearrangement could prevent the long-term stabilization of the initial short-term plasticity [START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF]. Based on the recent literature, here we propose a more conservative version of the second hypothesis based on the concept of reinforcement of already existing (and active) cortico-cortical connection instead of the unmasking of silent connection. Only about 20% of V1 activity comes from excitatory projections from the lateral geniculate nucleus (LGN) [START_REF] Carandini | Do We Know What the Early Visual System Does[END_REF], while most activity in V1 receives major contributions from top-down, feedback and lateral input [START_REF] Budd | Extrastriate feedback to primary visual cortex in primates: A quantitative analysis of connectivity[END_REF]. In particular, if it is true that collicular input drives the earliest spiking activity that is linked with local processing it is also true that there is a prolonged later response that is more influenced by contextual feature of the stimuli (T. S. [START_REF] Lee | The role of the primary visual cortex in higher level vision[END_REF]. Recent research shows that feedback from higher-level areas plays more than a modulatory function in V1 but can drive a task-related activity that can spread to nonstimulated regions (Lars [START_REF] Muckli | Network interactions: Non-geniculate input to V1[END_REF]. For example activation in V1 carries information of task-dependent and position-independent representations of imagined [START_REF] Thirion | Inverse retinotopy: Inferring the visual content of images from brain activation patterns[END_REF], remembered [START_REF] Serences | Neural mechanisms of information storage in visual short-term memory[END_REF], or compared stimuli (Mark A. [START_REF] Williams | Feedback of visual object information to foveal retinotopic cortex[END_REF]. Baseler and colleagues (2011) in analysing the population receptive fields centre location in MD and controls with artificial scotoma, found the presence of ectopic receptive fields that were not restricted to the LPZ and were equally present in both the patient and control group. They also hypothesized that feedback or lateral connections could be the source of this ectopic signal and that it could be more active in the absence of feedforward signals as in the patient group. The fact that the visual processing in V1 might not be strictly retinotopic plays against the interpretation of this "re-activation" of the LPZ as a sign of extensive cortical reorganization. Moreover, reactivation in the LPZ has been proven to be absent during passive viewing but present during an active task like a discrimination task [START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF][START_REF] Masuda | Task-dependent V1 responses in human retinitis pigmentosa[END_REF]. This type of task is indeed difficult for the patient given the central scotoma, the instability of fixation and the narrowness visual field stimulated in the scanner. However, why the reactivation of the LPZ can be measured only with difficult active task and not during the classic retinotopic mapping? A possible explanation comes from the paper of [START_REF] Williams | Feedback of visual object information to foveal retinotopic cortex[END_REF], in fact they have found a foveal activation during a peripheral matching task in normal viewers. This activation was related to task difficulty and performance and was stronger after practice. A series of psychophysical [START_REF] Fan | Psychophysically disrupting the delayed feedback signal to foveal retinotopic cortex selectively impairs extrafoveal object perception[END_REF][START_REF] Weldon | Disruption of foveal space impairs discrimination of peripheral objects[END_REF][START_REF] Yu | Modulating foveal representation can influence visual discrimination in the periphery[END_REF] studies have later demonstrated that delayed foveal feedback is critical for peripheral perception in fine details processing. Moreover, this feedback has a precise time course, altering the processing of the feedback utilizing a foveal mask or a TMS pulse at around 350ms that produces a decrease in the peripheral task performance [START_REF] Chambers | Is delayed foveal feedback critical for extra-foveal perception?[END_REF]. This feedback seems to be activated "on-demand" only for active peripheral task that requires processing of the details of the image. It has been suggested that since the foveal region has higher spatial resolution and small receptive fields, it might act as a sort of "enhancing software" that reconstruct and solve the details of the blurry peripheral image [START_REF] Shim | Redundancy gains in retinotopic cortex[END_REF][START_REF] Williams | Feedback of visual object information to foveal retinotopic cortex[END_REF]. Is seems very plausible that the AMD take advantage of this foveal feedback even more than normal viewers. We might also expect a strengthening of this feedback connection, but this is far away from the concept of cortical remapping as found in the sensorimotor system or case of early visual impairments. However, where this feedback comes from? There is consistent literature about a "frontoparietal control system" active during a wide range of cognitive tasks including perceptual discrimination [START_REF] Cole | The cognitive control network: Integrated cortical regions with dissociable functions[END_REF][START_REF] Duncan | Common regions of the human frontal lobe recruited by diverse cognitive demands[END_REF] that code for a range of different types of taskrelevant information and that is sensitive to changing task demands [START_REF] Duncan | The multiple-demand (MD) system of the primate brain: mental programs for intelligent behaviour[END_REF]. [START_REF] Woolgar | Attention enhances multi-voxel representation of novel objects in frontal, parietal and visual cortices[END_REF] showed that during a peripheral task, representation of visual stimuli in the frontoparietal cortex was stronger when stimuli were hard to perceive, and coding in early visual cortex was weak. On the contrary, coding in higher visual areas was sensitive to the allocation of attention but robust to changes in perceptual difficulty. This result is consistent with a feedback process that reinforces the visual percept at an early stage of visual processing. For the patients with central vision loss the possibility of reinforcing such feedback loop seems more probable than the possibility of an extensive rearrangement occurring at lower-order visual areas [START_REF] Haak | Plasticity, and its limits, in adult human primary visual cortex[END_REF].

Increased prefrontal and parietal activation in AMD patients compared to controls might be a sign of increased top-down involvement in basic visual processing to compensate for the sensory loss [START_REF] Szlyk | An fMRI study of word-level recognition and processing in patients with age-related macular degeneration[END_REF]. However, some visual and non-visual high-level areas could be affected by the trans-synaptic anterograde degeneration which in turn could weaken the feedback to the lower-level areas. Ramanoël et al., (2018) testing AMD patients with low-pass, high-pass and nonfiltered scenes, found that in respect of controls patients had a much worse performance with high-pass scenes than low-pass. In patients, this was associated with reduced BOLD activity in the cortex corresponding to both the central and the peripheral visual fields. Moreover, they found reduced activation in the parahippocampal place area, a cerebral region specialized for scene perception.

Increasing the contrast of the scene produced a benefit in the processing of high-pass images but surprisingly only the activation in the occipital cortex was spurred by the increase in contrast, while the BOLD signal in the parahippocampal place area was not significatively increased (Ramanoël et al., 2018). This could mean that the prolonged decreased sensitivity for high spatial frequencies, and therefore for details, has produced a long-term impoverishment in the area specialized in perception which becomes less responsive.

In contrast, motion sensitivity seems to be improved in MD. It is relevant to mention the case of a monkey who developed juvenile MD. Interestingly the animal showed limited reorganization and no remapping in V1 and V2, but area V5/MT showed increased spread of activation compared to controls [START_REF] Shao | Visual cortex organisation in a macaque monkey with macular degeneration[END_REF]. This finding has been further supported by another animal study that found enhanced peripheral vision as a result of the sensitization for motion processing relying on feedback from V5/PMLS and area 7 to area 17 [START_REF] Burnat | Plasticity Beyond V1: Reinforcement of Motion Perception upon Binocular Central Retinal Lesions in Adulthood[END_REF]. The idea of improved motion sensitivity in MD is partially corroborated by a human behavioural study which shows increased effect of large moving visual fields display in the vection of MD patients than controls (Tarita-Nistor, [START_REF] Tarita-Nistor | Increased role of peripheral vision in self-induced motion in patients with age-related macular degeneration[END_REF]. While in the central visual field the geniculate inputs to the striate cortex are predominantly parvocellular the Parvo to Magno ratio progressively decreases as a function of eccentricity [START_REF] Azzopardi | Uneven mapping of magnocellular and parvocellular projections from the lateral geniculate nucleus to the striate cortex in the macaque monkey[END_REF]. Moreover, the receptive field size of magnocellular neuron is, on average, much larger compared to the parvocellular one [START_REF] Croner | Receptive fields of P and M ganglion cells across the primate retina[END_REF]. If we consider these assumptions together, we might expect increased sensitivity to motion stimuli due to a compensatory mechanism that in the absence of the foveal input rely mostly upon the spared peripheral abilities. Future studies should better address the modification in motion sensitivity in patients. Moreover, the study of a possible dissociation in the performance of MD patients when tested with stimuli that are selective for the Magnocellular pathway (low spatial frequency and fast motion) or the Parvocellular pathway (high spatial frequency and slow motion) could have potentials implication for rehabilitation.

Receptive Field Expansion

In animal research it is well established that neurons in the border of and inside the LPZ after a given period of recovery display an enlarged receptive that might also be shifted outside the scotoma (Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF][START_REF] Chino | Recovery of binocular responses by cortical neurons after early monocular lesions[END_REF]U T Eysel et al., 1999;[START_REF] Gilbert | Receptive field dynamics in adult primary visual cortex[END_REF][START_REF] Heinen | Recovery of visual responses in foveal V1 neurons following bilateral foveal lesions in adult monkey[END_REF][START_REF] Kaas | Reorganization of retinotopic cortical maps in adult mammals after lesions of the retina[END_REF]. In adult mammals, this reorganization occurs within hours of the lesion, but only if associated with the absence of input from the fellow eye (Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF]. The receptive field enlargement can be measured non-invasively by means of a functional MRI method that estimate the neuronal population receptive field size and location (pRF) (Dumoulin & Wandell, 2008;[START_REF] Harvey | The Relationship between Cortical Magnification Factor and Population Receptive Field Size in Human Visual Cortex: Constancies in Cortical Architecture[END_REF]. Results of this measurement in normal human subjects have been found to be in agreement with electrophysiological measurements in the corresponding areas in monkeys (Dumoulin & Wandell, 2008). Shao et colleagues (2013) in a monkey affected by MD found an average increase in the pRF size of non-deafferented V1 voxels of about 20%. Similarly to the MD monkey, even MD patients show increased pRF sizes when compared to healthy controls, but unexpectedly the same effect was also present when healthy controls were tested with an artificial central scotoma (Baseler et al., 2011a). Recent studies with the artificial scotoma suggest that the receptive field enlargement is part of a transient mechanism of homeostatic disinhibition that leads to increased cortical response in the cortical zone deprived of the visual stimulation [START_REF] Haak | Population receptive field dynamics in human visual cortex[END_REF][START_REF] Papanikolaou | Nonlinear population receptive field changes in human area V5/MT+ of healthy subjects with simulated visual field scotomas[END_REF][START_REF] Parks | Neural mechanisms of short-term plasticity in the human visual system[END_REF]. The fact that this expansion is very rapid and present also in case of artificial scotoma lets us think of a reversible, spontaneous and automatic mechanism which has nothing to do with the retinal damage in the strict sense, but which instead comes into play whenever there is a significant decrease in the levels of visual activity in a given part of the visual field, and that is partly modulated by feedback signals from extrastriate visual areas [START_REF] Haak | Population receptive field dynamics in human visual cortex[END_REF]. This short-term plasticity comes at the expense of the neural response tuning as it causes a reduction in the inhibitory shaping of selectivity [START_REF] Gannon | Homeostatic plasticity in human 227 extrastriate cortex following a simulated peripheral scotoma[END_REF]. Even if it cannot be considered a reorganization process in itself, the increase in receptive field size together with the transient increase in cortical response might be the starter for a long-term topographical reorganization [START_REF] Gannon | Homeostatic plasticity in human 227 extrastriate cortex following a simulated peripheral scotoma[END_REF].

At the same time, it could produce behavioural effects that in some cases might be detrimental. In AMD patients, some studies have shown a specific impairment for the elaboration of precise details transmitted from high spatial frequencies in the scenes, while the perception of global forms transmitted by low spatial frequencies remains relatively well preserved. [START_REF] Musel | Residual abilities in age-related macular degeneration to process spatial frequencies during natural scene categorization[END_REF][START_REF] Peyrin | Scene perception in age-related macular degeneration: Effect of spatial frequencies and contrast in residual vision[END_REF]. This could be a consequence of the widening of the receptive fields that leads to a greater spatial summation, but also to a loss of neural tuning. Improving the sensitivity specifically to the medium and high spatial frequencies is one of the objectives of the neural-based perceptual learning that will be discussed in CHAPTER VIII of this thesis.

In the next section, I will discuss how, through the use of psychophysical methods it is possible to investigate the mechanisms of adaptation to the presence of the central scotoma.

Psychophysical Evidence of Cortical Plasticity in MD

We have previously seen what are the main visual deficits that emerge following the presence of bilateral central scotomas, we then saw what are the structural and functional alterations that ensue at the level of the central nervous system. Now we will focus on the psychophysical studies that have investigated the presence of cortical reorganization. Patients with retinal scotomas experience perceptual filling-in for scotomas as large as 6 degrees [START_REF] Gerrits | The filling-in process in patients with retinal scotomata[END_REF][START_REF] Zur | Filling-in of retinal scotomas[END_REF].

According to the classification in [START_REF] Weil | A new taxonomy for perceptual filling-in[END_REF], this completion process falls under Stimulus-independent and instantaneous type. The exact mechanism through which this process takes place is still uncertain. There can be three possible mechanisms that can act independently or combined. First, there might be active lateral propagation of the visual information from the border to interior of the scotoma [START_REF] Weerd | Responses of cells in monkey visual cortex during perceptual filling-in of an artificial scotoma[END_REF]; alternatively there might a be a remapping of receptive fields (Y. [START_REF] Chino | Recovery of binocular responses by cortical neurons after early monocular lesions[END_REF]. Lastly this completion might be driven by the activity of some location-independent large receptive fields like the one found in the lateral occipital and inferior temporal cortex that are activated even in case of partial visual stimulation as in the presence of an occluder [START_REF] Hegdé | Preferential responses to occluded objects in the human visual cortex[END_REF][START_REF] Weigelt | Separate cortical stages in amodal completion revealed by functional magnetic resonance adaptation[END_REF]. How much of this process can be attributed to a permanent and slow adaptive reorganization (long-range rewire) and how much is part of transient plastic mechanisms (receptive field enlargement and shift) is not given. [START_REF] Croner | Receptive fields of P and M ganglion cells across the primate retina[END_REF] in a Vernier task found an advantage when stimuli were presented across the physiological blind spot than over equally eccentric temporal retina, but the same advantage was not present over pathologic retinal scotomas than more intact, equally eccentric retina. This result suggests that the filling-in in the pathological scotoma does not have a functional implication and is not the result of a low-level reorganization. With more complex stimuli De Stefani and colleagues (2011) found perfect discrimination of the curvature of the illusory contours over the pathological retinal scotoma that is usually not found over the physiological blind spot. A more recent study found that MD patients had higher sensitivity to mirror symmetry for configuration displayed at the two sides of the scotoma than controls at the same eccentricity. The same was not true for translational symmetry [START_REF] Clara | Hyper-vision of mirror symmetry in patients with macular degenerationreflects parafoveal cortical reorganization[END_REF]. In respect of the Vernier stimuli, detection of illusory contour and mirror symmetry rely on a higher-level cortical representation as they provide stronger cue in Gestalt processes mediating object recognition. Those stimuli are likely coded in the lateral occipital cortex [START_REF] Bona | The causal role of the lateral occipital complex in visual mirror symmetry detection and grouping: An fMRI-guided TMS study[END_REF] one of the visual areas that despite the anterograde degeneration shows an increase in synaptic complexity after central vision loss [START_REF] Lorenz | Two New Cytoarchitectonic Areas on the Human Mid-Fusiform Gyrus[END_REF][START_REF] Malikovic | Cytoarchitecture of the human lateral occipital cortex: mapping of two extrastriate areas hOc4la and hOc4lp[END_REF][START_REF] Sabbah | Reorganization of early visual cortex functional connectivity following selective peripheral and central visual loss[END_REF]. The dissociation in the detection between low-and high-level visual stimuli in MD is further corroborated by other studies. When tested within their PRL, patients do not show any improved ability to recognize ladder contours, which suggests that there is no use-dependent improvement in contour integration or crowding undergoing [START_REF] Haun | Similar sensitivity to ladder contours in macular degeneration patients and controls[END_REF]. But at the same time, some patients display a spontaneous reduction of impairment over time (Avinoam B. [START_REF] Safran | Plasticity in the adult visual cortex: Implications for the diagnosis of visual field defects and visual rehabilitation[END_REF]. [START_REF] Sunness | Improvement of visual acuity over time in patients with bilateral geographic atrophy from age-related macular degeneration[END_REF] found that patients with bilateral scotoma have a slow improvement in their visual acuity after the initial drop. Casco and colleagues also reported a case of a patient who presented normal acuity for crowded letters and better performance than controls in a lexical decision task and visual search [START_REF] Casco | Hyper-vision in a patient with central and paracentral vision loss reflects cortical reorganization[END_REF]. Maniglia et al. (2018) using a lateral masking paradigm (I will discuss in depth this paradigm in CHAPTER III) measured the extent of the contextual effect of the two collinear flankers over the central target at the PRL of five MD patients. In this paradigm, depending on the reciprocal distance between flankers and target, we might have a facilitatory or inhibitory effect of the flankers over the target.

Each patient was matched with a control subject tested at the same eccentricity.

What they found was a reduction of the inhibition that is generally found at the shortest target-to-flanker distance that was present only in the MD group. They interpreted this result as a sign of spontaneous cortical reorganization that might be related to the receptive field enlargement found in electrophysiological (Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF] and MRI studies (Baseler et al., 2011a) In the previous literature, the amount of lateral inhibition has been linked with the amount of crowding [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. This link is still controversial, but when MDs are tested with a crowding task at the PRL they display some unusual characteristic that might be in line with the reduction of inhibition. Firstly, despite having a worse visual acuity than controls at the same eccentricity, Md patients nominal critical spacing (the smaller target-to-flanker distance at which they can recognize an object when surrounded by similar flanking elements) is comparable with that of the normal fovea (Susana T.L. [START_REF] Chung | Size or spacing: Which limits letter recognition in people with age-related macular degeneration?[END_REF]. Secondly, they do not show the radial-tangential anisotropy that is typical when normal viewers are tested in the periphery (S. T. L. [START_REF] Chung | Cortical Reorganization after Long-Term Adaptation to Retinal Lesions in Humans[END_REF]. Interestingly, the similar reduction in the radial-tangential anisotropy has been observed in normal viewers after a small practice with an artificial central scotoma. Moreover, this effect was correlated with a reduction in the crowding-related suppression measured with fMRI (N. [START_REF] Chen | Cortical reorganization of peripheral vision induced by simulated central vision loss[END_REF]. The authors of this study suggest that this might be due to a reorganization in the local connections that alters the way in which the context is integrated (or segmented) with the target. It is tempting to make a link between this result and the previous ones that have shown a widening of the receptive fields and an increase of the cortical activity following the adaptation to a simulated central scotoma (Baseler et al., 2011a). It is not yet clear how these two results relate to each other, what seems clear is that both are the product of a spontaneous mechanism of adaptation to the absence of central input that acts quickly and is easily reversible. At the basis there might be an attempt by the system of maintenance of the homeostasis that tries to balance the absence of input by increasing the local activity [START_REF] Ganguly | Activity-dependent neural plasticity from bench to bedside[END_REF].

Macular Degeneration: Final Consideration

Up to this point, I have discussed in detail many of the clinical features of MD.

Particular emphasis was given to the theme of neural plasticity with reference to more recent work in the field. It is undoubtedly difficult to unify all the results in a coherent picture; in this section, I will try to summarize systematically the salient points:

• There are currently no restorative treatments for macular degeneration.

• The best option is the combined use of training and visual aids.

• MD patients develop one or more PRLs that are used in the tasks usually performed by the fovea.

• In MD, the visual deficits are not limited to central blindness but also partly involve the peripheral vision.

• For rehabilitation, it is important to also consider binocular visual skills and not just the best eye.

• The adult brain is still plastic, and in the event of injury, spontaneous compensatory mechanisms are activated.

• Retinal degeneration could lead to secondary brain damage to the visual pathways through trans-synaptic degeneration.

• There is little evidence of long-range cortical reorganization in MD patients.

• While the early visual areas show signs of degeneration, those of higher-level show use-dependent functional adaptations.

• The reactivation of the LPZ in V1 is largely due to the strengthening of feedback connections from the higher visual areas already present in normal viewers and not to an extended cortical remapping.

• The sensitivity for high spatial frequency in patients is reduced; on the contrary the sensitivity for low spatial frequency moving stimuli is increased.

• Converging evidence supports the presence of a spontaneous and rapid adaptation to the scotoma that can be the trigger for more complex slow changes.

• The homeostatic mechanisms already present in subjects with healthy vision are sufficient to explain the rapid initial adaptations to the scotoma.

• Visual training can be a useful tool to reduce the effects of transsynaptic degeneration and to guide spontaneous plasticity.

In the next CHAPTER, we will look at the features of perceptual learning in detail and how it can be used in visual rehabilitation. In particular, I will discuss the use of a paradigm based on lateral masking as a means to enhance residual vision in the PRL.

Chapter III.

Perceptual Learning as a Tool to Improve Visual Functions Perceptual learning consists of a practice-dependent improvement in a task that may persist for months; it is specific to the stimulus, to the type of task, to the eye, to the retinal location, to the spatial frequency and the orientation used during the training process [START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF][START_REF] Goldstone | Perceptual Learning[END_REF]. This specificity of perceptual learning has been explained because of a long-term practice-dependent structural modification that alters response properties of neurons in the primary visual cortex (M [START_REF] Ahissar | Attentional Control of Early Perceptual Learning[END_REF][START_REF] Ahissar | Learning pop-out detection: Specificities to stimulus characteristics[END_REF], 1997;[START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF][START_REF] Karni | Where practice makes perfect in texture discrimination: evidence for primary visual cortex plasticity[END_REF], 1993;[START_REF] Kuffler | Neurons in the retina; organization, inhibition and excitation problems[END_REF]. PL seems to occur even in the absence of an explicit task for the subject [START_REF] Ghose | Learning in mammalian sensory cortex[END_REF][START_REF] Guzzon | The effect of visual experience on texture segmentation without awareness[END_REF][START_REF] Herzog | Modeling perceptual learning: difficulties and how they can be overcome[END_REF][START_REF] Presentation | Perceptual Learning[END_REF][START_REF] Watanabe | Perceptual learning without perception[END_REF], but the degree of acquisition, generalization, and retention are strongly related with task difficulty (Merav [START_REF] Ahissar | Task difficulty and the specificity of perceptual learning[END_REF][START_REF] Deloss | Optimization of perceptual learning: Effects of task difficulty and external noise in older adults[END_REF][START_REF] Liu | Mixed training at high and low accuracy levels leads to perceptual learning without feedback[END_REF][START_REF] Wang | Transfer in motion perceptual learning depends on the difficulty of the training task[END_REF], involvement of the subject (T. H. [START_REF] Lee | Evidence for arousal-biased competition in perceptual learning[END_REF][START_REF] Lee | Emotional arousal amplifies the effects of biased competition in the brain[END_REF] and attentional resources allocated on the stimulus [START_REF] Donovan | Exogenous attention facilitates location transfer of perceptual learning[END_REF][START_REF] Paffen | Attention-based perceptual learning increases binocular rivalry suppression of irrelevant visual features[END_REF][START_REF] Roelfsema | Perceptual learning rules based on reinforcers and attention[END_REF][START_REF] Tsushima | Roles of attention in perceptual learning from perspectives of psychophysics and animal learning[END_REF]. Recent results suggest that if the sustained attention is involved, only the processing of characteristics that are relevant to the task is object of learning, while reward-related reinforcement signals promote both non-relevant and task-relevant learning [START_REF] Huang | Task attention facilitates learning of task-irrelevant stimuli[END_REF][START_REF] Sasaki | Advances in visual perceptual learning and plasticity[END_REF][START_REF] Wang | Task relevancy and demand modulate double-training enabled transfer of perceptual learning[END_REF]. As [START_REF] Yotsumoto | Defining a link between perceptual learning and attention[END_REF] suggest, different levels of processing are involved during PL depending on trained feature and training procedure. In general, we can distinguish between three primary levels where learning can take place, and for each of them, there is a particular model that tries to explain the mechanisms:

(A) Early-stage, local network model: [START_REF] Adini | Context-enabled learning in the human visual system[END_REF][START_REF] Tsodyks | Associative learning in early vision[END_REF]) learning may occur intracortically at low-level, and therefore we do not have to assume interactions between cortical areas at different levels in the visual processing hierarchy.

(B) Mid-level stage, reweighting: (B. A. [START_REF] Dosher | An integrated reweighting theory of perceptual learning[END_REF][START_REF] Dosher | Perceptual learning reflects external noise filtering and internal noise reduction through channel reweighting[END_REF], learning occurs by changing the strength of neural connections specific for a given task between the early processing stage and a decision unit. This reweight can account for transfer to very similar tasks that share the same processing network (C) Higher-to-lower stages, Reverse hierarchy theory (RHT): (Merav [START_REF] Ahissar | The reverse hierarchy theory of visual perceptual learning[END_REF] top-down modulation influences the signal to noise ratio during the task, the sustained attention repetition after repetition drives learning triggering a structural change in the whole system. The three models should work as guidelines to achieve the best training procedures. Indeed, a clinical procedure needs feasibility, reliability, and should produce a long-lasting gain in the quality of life of the patients. Thus, it is possible to combine a task with increasing difficulty that ensures top-down modulation [START_REF] Ahissar | Task difficulty and the specificity of perceptual learning[END_REF][START_REF] Wang | Task difficulty modulates electrophysiological correlates of perceptual learning[END_REF] with a stimulus tuned according to low-level perceptual features of the system to target fundamental visual abilities, such as contrast sensitivity. Increasing task difficulty is necessary to engage the participant in the task and is known to positively affect the training results (S.

C. Hung & Seitz, 2014;[START_REF] Wang | Task difficulty modulates electrophysiological correlates of perceptual learning[END_REF]. This is usually achieved by the use of an adaptive procedure like a Levitt staircase procedure with nalternative forced choices (nAFC) [START_REF] García-Pérez | Forced-choice staircases with fixed step sizes: Asymptotic and small-sample properties[END_REF][START_REF] Levitt | Transformed Up-Down Methods in Psychoacoustics[END_REF]. A virtue of this procedure is that of being predictable since it follows a staircase pattern that converges towards the threshold. In this way, the implicit statistical learning of the target variation helps the subject in building up an internal representation of expected target making the task more effective [START_REF] Fiser | Perceptual learning and representational learning in humans and animals[END_REF][START_REF] Fiser | Statistically optimal perception and learning: from behavior to neural representations[END_REF][START_REF] Neger | Relationship between perceptual learning in speech and statistical learning in younger and older adults[END_REF].

This also provides a progression in the difficulty of the task that is supposed to increase learning gain [START_REF] Deloss | Optimization of perceptual learning: Effects of task difficulty and external noise in older adults[END_REF][START_REF] Liu | Mixed training at high and low accuracy levels leads to perceptual learning without feedback[END_REF][START_REF] Wang | Transfer in motion perceptual learning depends on the difficulty of the training task[END_REF] and reduce participants' frustration. Through this procedure, it is also possible to take advantage of the positive effect of the feedback that strengthens the stimulus-response association (task rule) through positive by maximizing decision mechanism through reward [START_REF] Kumano | Neuronal mechanisms of visual perceptual learning[END_REF][START_REF] Lu | Modeling mechanisms of perceptual learning with augmented Hebbian re-weighting[END_REF][START_REF] Petrov | The dynamics of perceptual learning: An incremental reweighting model[END_REF]. Using our previous knowledge, it is possible to refine the training procedure in such a way as to exploit the low-level plasticity and to foster transfer at higher-level untrained visual function. As Chung states in "The Glenn A. Fry Award Lecture 2012: Plasticity of the Visual System Following Central Vision Loss (2013): "The presence of this experiencedependent plasticity offers us an exciting opportunity to adopt PL as an alternative rehabilitative strategy for improving visual functions for people with central vision loss" [START_REF] Atchison | The Glenn A. Fry Award Lecture 2011: Peripheral optics of the human eye[END_REF].

Specificity vs. Generalization

One of the characteristics necessary for PL to become a useful tool for visual rehabilitation is undoubtedly a high degree of generalization. Without this, the definite improvement that is achieved hardly affects the everchanging activities of daily life. Very consistent literature shows the specificity of perceptual learning to many low-level visual features and training characteristics which are encoded in early visual areas such as V1 and V2 or MT. Some examples are orientation and spatial frequency [START_REF] Fiorentini | Perceptual learning specific for orientation and spatial frequency[END_REF], texture (Avi [START_REF] Karni | Where practice makes perfect in texture discrimination: evidence for primary visual cortex plasticity[END_REF], retinal position (M. [START_REF] Fahle | Fast perceptual learning in hyperacuity[END_REF], motion direction [START_REF] Ball | A specific and enduring improvement in visual motion discrimination[END_REF], motion speed [START_REF] Saffell | Task-specific perceptual learning on speed and direction discrimination[END_REF], and the trained eye [START_REF] Fahle | Perceptual learning: A case for early selection[END_REF] but under some conditions it is possible to allow transfer to slightly different stimuli or to totally untrained tasks [START_REF] Ahissar | Task difficulty and the specificity of perceptual learning[END_REF][START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF][START_REF] Liu | Mechanisms of generalization in perceptual learning[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. For example, [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF][START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF]Harris &[START_REF] Harris | Effects of spatiotemporal consistencies on visual learning dynamics and transfer[END_REF] found that complete generalization to a new location can be observed by introducing "dummy" trials with task-irrelevant stimuli counteracting sensory adaptation. Following this research line, Mastropasqua, Galliussi, Pascucci, & Turatto in 2015 showed that transfer of learning was complete and mutual between two tasks ant different retinal locations when the alternation between the two tasks was implemented on a trial-by-trial basis. However, when the amount of additional training was significantly reduced (second task only in 20% of the trials) or the exposure to the second stimuli was passive, then PL remained confined to the trained location (no transfer). With double training, an active task is more likely to transfer to another retinal location, but the frequency of the second task has to be higher than the 20% of the total trials. This study shows that the characteristics that can influence the level of generalization are many and peculiar. Many other studies found evidence of generalization, but the general rule is still unclear.

Experiments with many repetitions and with stimuli that are very difficult to discriminate (high similarity between them) are more prone to produce specific learning [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF] while more transfer has been observed with a small number of trials (Avi Karni & Sagi, 1993), with shorter training sessions (Kristoffer C. [START_REF] Aberg | Perceptual learning with Chevrons requires a minimal number of trials, transfers to untrained directions, but does not require sleep[END_REF], with easy tasks [START_REF] Ahissar | Task difficulty and the specificity of perceptual learning[END_REF] with coarse discrimination [START_REF] Jeter | Task precision at transfer determines specificity of perceptual learning[END_REF], or with interleaved double training (J.-Y. [START_REF] Zhang | Rule-Based Learning Explains Visual Perceptual Learning and Its Specificity and Transfer[END_REF]. It has been suggested, for instance, that sensory adaptation due to repeated visual stimulation is the main causes of specificity. When adaptation is removed, complete generalization to a new location can be observed [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF].

According to this view, perceptual learning would involve a low-level visual network that processes simple stimulus features and a higher-level readout network used to learn how to perform the task based on the output of the lower-level network. Sensory adaptation occurring during training could hinder learning at the level of the low-level network so that the readout higher-level network will not be able to successfully apply the experience previously gained to new conditions (transfer) [START_REF] Harris | Effects of spatiotemporal consistencies on visual learning dynamics and transfer[END_REF]. Stimuli and task choice strongly influence the level at which we might expect the transfer, and as a consequence, the degree of generalization. Performing training on a fundamental visual feature with stimuli able to match the selectivity of the early visual cortex is very likely to drive a neural improvement at early stages of processing. A low-level stimulus with a restricted set of alternatives may lead to over-specificity of learning, significantly reducing the transfer to other stimuli, but strongly increasing the signal-to-noise ratio for the input to all the subsequent processing modules (Sagi, 2011a). On the contrary, using complex stimuli or tasks might push the locus of learning at a higher stage of processing, making learning more generalizable but also less efficient (C. S. [START_REF] Green | Differences in perceptual learning transfer as a function of training task[END_REF]. It has been suggested that the level a which learning occurs in the brain may change from time to time to achieve the greatest improvement possible, for the peculiar task, and stimuli at hand [START_REF] Sasaki | Advances in visual perceptual learning and plasticity[END_REF]. Optimizing PL for clinical purposes, require to train the entire chain from the coding of the stimulus to the decision-making process and even beyond, up to the backward feedback propagation to the coding areas. A means to achieve this, is by providing a feedback to the subject. The feedback is a key factor that could influence both, the learning [START_REF] Holloway | Two cases of a requirement of feedback for perceptual learning[END_REF][START_REF] Shibata | Boosting perceptual learning by fake feedback[END_REF] and the generalization [START_REF] Herzog | The role of feedback in learning a vernier discrimination task[END_REF][START_REF] Liu | Modeling trial by trial and block feedback in perceptual learning[END_REF]. Using two different task, one orientation discrimination and one motion discrimination Holloway and colleagues (2006) found learning only in the trial-by-trial feedback conditions in respect to the no-feedback. [START_REF] Aberg | Different types of feedback change decision criterion and sensitivity differently in perceptual learning[END_REF] demonstrated that criterion and sensitivity are affected both by feedback delivered trial-by-trials while feedback delivered across block affects only the sensitivity. In an fMRI based study, Goldhacker, [START_REF] Rosengarth | Functional and structural brain modifications induced by oculomotor training in patients with age-related macular degeneration[END_REF] find that feedback influenced behavioural performance and, even if to a lesser extent brain activation in areas responsible for monitoring PL, suggesting a top-down influence. In an ERP study, [START_REF] Zhang | ERP P1-N1 changes associated with Vernier perceptual learning and its location specificity and transfer[END_REF] investigated electrophysiological correlate linked with location specificity and transfer in a Vernier PL paradigm. Results show that transfer is associated with P1 reduction and N1 enhancement whereas location specificity is only associated with N1 suppression corresponding to the untrained retinal location. These results are consistent with the proposal of top-down modulatory influences at the untrained location.

Moreover, Kahnt and colleagues (2011) acquiring fMRI data during a training in an orientation discrimination task involving feedback, found that activity in the ACC tracked changes in decision variables during learning. These results provide strong evidence for PL-related changes in higher order areas and suggest that perceptual and reward learning are based on a common neurobiological mechanism.

The Lateral Masking Paradigm Among the various PL paradigms that have been proposed one, in particular, seems to have high potential in promoting neural plasticity from the lowest coding freedom to the highest decision-making process. The lateral masking paradigm consists in a contrast detection task for a low contrast Gabor patch flanked by two high contrast Gabor patches (Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Casco | Specificity and generalization of perceptual learning in low myopia[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]Maniglia et al., , 2018;;[START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]U. Polat, Ma-Naim, Belkin, & Sagi, 2004a;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF][START_REF] Tanaka | Long-lasting, long-range detection facilitation[END_REF]. Among others approaches, contrast detection with lateral masking is one of the leading training paradigms adopted to treat a series of visual diseases such as amblyopia (Barollo et al., 2017a;U. Polat et al., 2004a), presbyopia (Uri [START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF], myopia (Camilleri, Pavan, Ghin, & Campana, 2014; D. T.

H. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF] and Age-Related Macular Degeneration (AMD) [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). Although the method and parameters used are not always the same, all these studies have found an improvement in visual functions following training, in particular in visual acuity and contrast sensitivity. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF] trained myopic subjects with Gabor stimuli manipulating many different parameter such as the presence of flankers, the global and local orientation, the Gaborflanker separation, the exposure time, the contrast and spatial frequency. At the end of the training they found an improvement in visual acuity with the Early Treatment Diabetic Retinopathy Study logMAR chart (ETDRS) (2.1 logMar lines) that was greater in subjects with lower visus. They found also an improvement in the contrast sensitivity for all the spatial frequencies tested and transfer to the contralateral eye. Moreover, follow up after 6 and 12 months showed that the learning was largely maintained. Later, [START_REF] Durrie | Computer-based primary visual cortex training for treatment of low myopia and early presbyopia[END_REF] replicated these positive results in a group of myopic and presbyopic subjects. Recently, Polat and colleagues (2012), after a training paradigm based on a "backward masking" with flankers, found a significant improvement in contrast sensitivity and visual acuity.

At this point a doubt arises, since training the neurons in V1 increase their tuning and thus their selectivity [START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF]U Polat & Sagi, 1994), how is it possible to achieve transfer to complex stimuli and to the untrained eye after PL? Li and coworkers (2011) suggest that the transfer of the lateral masking paradigm could be unidirectional from the high spatial frequencies that represent the limit of acuity towards low spatial frequencies. It is also possible that since the task trains a low-level neuronal network all the higher lever module that readout from the trained one will benefit from the training allowing for a cascade of improvements [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. This task also offers the advantage of allowing to study how early spatial integration of the target and its context occurs. Having been intensely studied, the predictions regarding the contextual effects of flankers are well detailed. From the previous literature we know that when collinear flankers are placed at a distance of 3-4 lambdas -the wavelength of the target Gabor's carrier (λ)they are able to enhance the detection of the target if compared to the threshold for the single Gabor or the orthogonal neutral conditions ( Polat & Sagi, 1993, 1994a, 1994b). On the contrary, for very short target-to-flankers distances (i.e., 1-2λ), the contrast threshold is increased. The mechanism behind this dissociation is not entirely understood, but there is a general agreement between neurophysiological and psychophysical data [START_REF] Grinvald | Cortical Point-Spread Function and Long-Range Lateral Interactions Revealed by Real-Time Optical Imaging of Macaque Monkey Primary Visual Cortex[END_REF][START_REF] Polat | Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex[END_REF]. Lower contrast detection thresholds than the orthogonal are generally considered to be a sign of facilitation between receptive fields that are contiguous in space and that code for the same visual feature.

On the other hand the increase in contrast threshold could be explained by a within receptive field inhibition indicating that the flankers are falling in the proximity of the OFF zone of the receptive field (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. The spatial range of the lateral interactions is also known to increase with eccentricity, in agreement with what is known about the cortical magnification and the spatial integration in the periphery [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Maniglia | The effect of spatial frequency on peripheral collinear facilitation[END_REF]. The last important factor to consider is that extensive training can "reshape" the range and the strength of collinear lateral interactions in both, the fovea and the periphery [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF][START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF]. In particular, PL in the periphery reduces suppression that in turn reduces the critical space necessary to discriminate objects or letters in clutter, a phenomenon usually referred as visual crowding that is particularly critical for MD patients [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. The advantages and possibilities linked to the use of this technique as a visual rehabilitation technique for macular degeneration will be discussed in the next section.

Visual Crowding

Compared to central vision, the peripheral vision has limited performances in contrast sensitivity (CS), orientation discrimination, VA and word identification [START_REF] Strasburger | Ideal observer analysis of crowding and the reduction of crowding through learning[END_REF]. Moreover, the identification of a target among neighbouring similar elements, an effect known as visual crowding [START_REF] Whitney | Visual crowding: A fundamental limit on conscious perception and object recognition[END_REF], is much less efficient in the periphery in respect to the foveal vision. Crowding is usually quantified by the critical space, the minimal distance between the target and the surrounding elements (flankers) that leads to correct target identification. While not hindering healthy foveal vision [START_REF] Huurneman | A systematic review on foveal crowding in visually impaired children and perceptual learning as a method to reduce crowding[END_REF], crowding represents a major difficulty in letter recognition and reading for clinical populations suffering from central vision loss [START_REF] Mansfield | Psychophysics of reading. XV: Font effects in normal and low vision[END_REF]. Recent literature on PL showed that training could reduce crowding effect, however it requires a large number of sessions in order to get significant improvements [START_REF] Astle | The effect of normal aging and age-related macular degeneration on perceptual learning[END_REF][START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF][START_REF] Chung | Learning to identify crowded letters: Does the learning depend on the frequency of training[END_REF][START_REF] Hussain | Perceptual learning reduces crowding in amblyopia and in the normal periphery[END_REF]Yashar, Chen, & Carrasco, 2015a). [START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF] after training participants on crowded letter identification at 10° of eccentricity found a reduction in crowding extent of 38%. Yashar and colleagues (2015) showed that a short (600 trials) training could reduce the critical space of about 32%, with partial retinal transfer and a long-lasting effect. It is possible to obtain a reduction of the critical space even through indirect routes, for example by training lower-level abilities as demonstrated by Maniglia and colleagues (2011). After a long training based on a lateral masking paradigm, they found that the improvement in contrast sensitivity transferred also to the visual acuity and to the crowding, the latter had a reduction of 16% after training. This encouraging evidence of improved peripheral visual abilities in normal sighted participants seem to indicate a way in which PL can be effectively used in patients suffering from central vision loss, similar to what reported for amblyopia, presbyopia, and myopia (Campana & Maniglia, 2015a;[START_REF] Levi | Perceptual learning as a potential treatment for amblyopia: A mini-review[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF][START_REF] Woo | Current methods of treating and preventing myopia[END_REF].

From Basic Experiments to Clinics

We have seen how patients with macular degeneration following the central scotoma appearance begin to use a peripheral region of the retina to perform tasks that require high visual skills usually performed by the fovea. This peripheral region, the PRL, is very sensitive to movement and low spatial frequencies but does not have a high visual resolution and is strongly affected by the phenomenon of visual crowding. This, together with the lousy stability of fixation, strongly compromises the reading and the ability to recognize faces and places, which in turn undermines the patient's autonomy.

Recent literature on PL showed that training could improve contrast sensitivity, visual acuity and reduce crowding [START_REF] Casco | Specificity and generalization of perceptual learning in low myopia[END_REF][START_REF] Chung | Learning to identify crowded letters: Does the learning depend on the frequency of training[END_REF]Chung, 2011;Polat, Ma-Naim, Belkin, & Sagi, 2004;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF][START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF]. Few recent studies used PL with AMD patients in order to improve their visual abilities (Susana T L Chung, 2011;[START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018;[START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF]Plank et al., , 2014;;[START_REF] Rosengarth | Functional and structural brain modifications induced by oculomotor training in patients with age-related macular degeneration[END_REF], however results are generally unsatisfactory and no standard rehabilitation protocol exists to this date (see Maniglia, Cottereau, Soler, & Trotter (2017) for a review). Rosengarth and colleagues (2013), trained eight MD patients in an oculomotor task, and found improvements in both reading speed and fixation stability between pre-tests and mid-tests, but not between pre-tests and post-tests. test. Furthermore, they did not find significant changes in the BOLD signals between the preand post-training tests in the striate or extrastriate visual cortex. Also, [START_REF] Plank | Perceptual learning in patients with macular degeneration[END_REF] trained eight AMD patients in a texture-discrimination task. After six training sessions spread over three weeks, patients began to show improvements in Vernier acuity.

Moreover, they showed a weak positive correlation between the increase in performance and fixation stability. These improvements were also accompanied by a modest change in the BOLD response in early visual cortex. The limited benefit of classic PL in patients with central vision loss might depend on the reduced plasticity typical of the elderly population, but also on the different goal that a visual training must accomplish respect to myopia, amblyopia or presbyopia. While in the latter the aim of a rehabilitative training is to restore the foveal vision to its optimal state, in case of permanent retinal damage, the goal of PL would be to train a retinal region different from the retina to achieve tasks for which it is not supposed to, and that would require to unlock extra plasticity. The aim is to ensure that this retinal region assumes processing abilities like those of the normal fovea. The principal limit is given by the size of the receptive fields that are larger in the periphery together with the increased spatial range of the lateral interactions. If a high spatial integration is a desirable feature in the healthy periphery, in the PRL of patients this becomes an essential limit to the visual acuity since adjacent stimuli might be "packed together". In this context, perceptual learning based on lateral masking able to improve contrast sensitivity and selectivity for orientation appears to be the perfect tool for remodelling spatial integration in the cortex that codes for PRL. Maniglia and coworkers in 2016 explored this possibility and showed that with this task in MD subjects, PL effect was pronounced at the ʎs at which the flankers facilitate target detection. Moreover, they found reduced crowding and improved contrast sensitivity to untrained spatial frequencies for patients that had been trained with the two alternatives forced-choice task, but not for the yes/no task using the same stimuli. The ability for the lateral masking paradigm to modulate lateral interaction in the PRL of the MD patients has been later confirmed with a second study from the same first author published in 2018. In this work they found that before training MD patients exhibited significantly reduced collinear inhibition. This was then interpreted as a sign of spontaneous neural plasticity triggered by the presence of the scotoma and consistent with the hypothesis use-dependent cortical reorganization in the PRL. Moreover, they showed that three AMD patients from the same group after training further reduced the inhibition exerted by the flankers at the shortest target-to-flankers distance. The same was not found in controls where the amount of inhibition before and after training was substantially unchanged. This result suggests that PL might be acting on top of the spontaneous adaptive change in the PRL further boosting neural plasticity, opening promising perspectives for the development of rehabilitation protocols for MD patients. However, similarly to other standard PL procedures this training still presents some drawbacks: First, as most visual PL studies pointing towards structural modification in the early visual cortex the degree of generalization might not be optimal (M Ahissar [START_REF] Ahissar | Attentional Control of Early Perceptual Learning[END_REF][START_REF] Ahissar | Learning pop-out detection: Specificities to stimulus characteristics[END_REF], 1997;[START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF]A Karni & Sagi, 1993;[START_REF] Karni | Where practice makes perfect in texture discrimination: evidence for primary visual cortex plasticity[END_REF][START_REF] Kuffler | Neurons in the retina; organization, inhibition and excitation problems[END_REF]. While in this and some similar cases training basic features have been proven to transfer to higher-level visual functions [START_REF] Deveau | The therapeutic benefits of perceptual learning[END_REF][START_REF] Chung | Size or spacing: Which limits letter recognition in people with age-related macular degeneration?[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]U. Polat et al., 2004a;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF], specificity of learning seems generally more common [START_REF] Casco | Specificity and generalization of perceptual learning in low myopia[END_REF][START_REF] Dosher | Mechanisms of perceptual learning[END_REF][START_REF] Fahle | Perceptual learning: Specificity versus generalization[END_REF][START_REF] Liu | Mechanisms of generalization in perceptual learning[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. Alongside specificity, another back draw is the need for long periods of training to observe significant improvement (Susana T.L. [START_REF] Chung | Learning to identify crowded letters: Does the learning depend on the frequency of training[END_REF]; R. W. [START_REF] Li | Extended perceptual learning results in substantial recovery of positional acuity and visual acuity in juvenile amblyopia[END_REF]. For clinical populations, a lengthy training can be a reason enough to prevent them from participation, especially in cases of serious visual diseases, such as AMD, that render patients not independent and force them to rely on others for transportation. Consequently, it is paramount for a clinical application of PL to reduce the length of the training and to define a better ratio between specificity and generalization. This is where electrical stimulation comes into play. Previously, tRNS has been proven useful to boost PL both between blocks and between sessions and also to increase its generalizability [START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF][START_REF] Conto | tRNS facilitates perceptual learning on cross-task training[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Moret | Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. By modulating the firing rate of the target neurons tRNS is believed to be able to reduce the sensory adaptation during a perceptual task [START_REF] Campana | Opposite effects of high-and low-frequency transcranial random noise stimulation probed with visual motion adaptation[END_REF]. This thesis aims to push the accelerator of perceptive learning by using a neuromodulatory technique such as transcranial random noise electric stimulation to obtain a significant increase in learning rate. Secondly, we set ourselves the objective of verifying whether this increase in the learning rate turns into an increase in the transfer to tasks and stimuli different from those trained. Reducing the number of sessions required to achieve significant improvement would allow more straightforward application of mass perceptual training in clinical and hospital settings. It should also be taken into consideration that given the recent developments in retinal implantation techniques; perceptual learning becomes a fundamental tool for reducing the effects of trans-synaptic anterograde degeneration that could compromise the effectiveness of the implant in patients with long-standing retinal degeneration. Furthermore, even after the implantation, the use of a paradigm like the one proposed in this thesis could facilitate the adaptation of the central nervous system to artificial input, accelerating it and making it more effective.

Chapter IV.

Non-invasive Brain Stimulation Techniques: The tRNS Among the non-invasive brain stimulation techniques [NIBS] the transcranial magnetic stimulation [TMS] and the transcranial direct current stimulation [tDCS] are the most exploited as a therapeutic intervention for cortically based disorders [START_REF] Coslett | Non-invasive brain current stimulation in neurorehabilitation[END_REF]. Both have been proven useful in the rehabilitation for motor, visual, somatosensory, attentional, cognitive and emotional function [START_REF] Clayton | Brain stimulation: Neuromodulation as a potential treatment for motor recovery following traumatic brain injury[END_REF][START_REF] Dionísio | The Use of Repetitive Transcranial Magnetic Stimulation for Stroke Rehabilitation: A Systematic Review[END_REF][START_REF] Martin | Repetitive transcranial magnetic stimulation for the treatment of depression: Systematic review and meta-analysis[END_REF][START_REF] Solomons | A review of transcranial electrical stimulation methods in stroke rehabilitation[END_REF][START_REF] Utz | Electrified minds: Transcranial direct current stimulation (tDCS) and Galvanic Vestibular Stimulation (GVS) as methods of non-invasive brain stimulation in neuropsychology-A review of current data and future implications[END_REF][START_REF] Wessel | Non-Invasive Brain Stimulation: An Interventional Tool for Enhancing Behavioral Training after Stroke[END_REF]. During transcranial direct current stimulation (tDCS), a homogenous electric field is induced in between the electrodes to elicit polarity-specific modulation of cortical excitability, in particular an increase of cortical excitability by the means of sub-threshold depolarization for anodal stimulation and a decrease of cortical excitability for cathodal stimulation as a consequence of hyperpolarization, with effects lasting beyond the stimulation period (M. A. [START_REF] Nitsche | Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation[END_REF][START_REF] Paulus | Transcranial electrical stimulation (tES -tDCS; tRNS, tACS) methods[END_REF][START_REF] Radman | Role of cortical cell type and morphology in subthreshold and suprathreshold uniform electric field stimulation in vitro[END_REF][START_REF] Reato | Low-Intensity Electrical Stimulation Affects Network Dynamics by Modulating Population Rate and Spike Timing[END_REF]. Through the tDCS, a homogeneous electric field is induced between the electrodes. This electric field is able to modulate the cortical excitability of the stimulated zone by increasing or decreasing it depending on the direction of the field. The anodic stimulation produces an increase in cortical excitability through sub-threshold depolarization while the cathodic stimulation produces hyperpolarization. The effect of the stimulation lasts for a period of about an hour even after its interruption. The duration of this after-effect varies depending on the stimulation parameters and the stimulated area (M. A. [START_REF] Nitsche | Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation[END_REF][START_REF] Radman | Role of cortical cell type and morphology in subthreshold and suprathreshold uniform electric field stimulation in vitro[END_REF][START_REF] Rahman | Cellular effects of acute direct current stimulation: Somatic and synaptic terminal effects[END_REF]. The current is delivered through electrodes positioned on the scalp.

In order to improve the flow of the current and reduce the impedance, conductive gel or sponges soaked in saline solution are used. This causes the focus of the stimulation to be limited, and therefore it is advisable to evaluate the actual intensity and extension of the electric field generated for each assembly through simulations on anatomically and physiologically plausible models. The direct current is caused by a battery-driven stimulator. The stimulator is a medical device, and thus in Europe it has to be certificated for the use. An electrode is placed over the target cortical region and the second electrode, usually larger to reduce current density, is placed on a non-target area. The two positions can be identified through neuronavigational tools or more simply by using the international EEG system 10-20 [START_REF] Chatrian | Ten percent electrode system for topographic studies of spontaneous and evoked EEG activities[END_REF]. This approach, however, is susceptible to individual variations in the anatomy of the skull and the grooves that reduce the consistency of the technique (Jurcak, Tsuzuki and Dan, 2007).

Not infrequently in fact unexpected effects of the stimulation are observed that can be attributable to the differences in response to stimulation between the subjects [START_REF] Benwell | Nonlinear effects of transcranial direct current stimulation as a function of individual baseline performance: Evidence from biparietal tDCS influence on lateralized attention bias[END_REF]. One way to explain the apparent inconsistencies of the polarity effect among previous studies is considering that the tES effect might be different in (sub)clinical population compared to healthy participant [START_REF] Hill | Effects of Anodal Transcranial Direct Current Stimulation on Working Memory: A Systematic Review and Meta-Analysis of Findings from Healthy and Neuropsychiatric Populations[END_REF], but also that the task-induced activity is more critical than the polarization to predict the stimulation effect on a participant's behaviour [START_REF] Bortoletto | The interaction with task-induced activity is more important than polarization: A tDCS study[END_REF]. Furthermore it must be considered that the same stimulation acts differently on different neural subpopulations producing a globally nonlinear effect in some cases [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF]. Recently, non-invasive brain stimulation has been used, alone or coupled with PL, to enhance visual abilities [START_REF] Camilleri | The application of online transcranial random noise stimulation and perceptual learning in the improvement of visual functions in mild myopia[END_REF][START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Pirulli | The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation[END_REF][START_REF] Thompson | Brain Plasticity in the Adult: Modulation of Function in Amblyopia with rTMS[END_REF]. In particular, transcranial random noise stimulation (tRNS), in which a weak current is delivered through the scalp on a cortical region at random frequencies, has shown promising results in boosting PL and reducing the number of sessions needed to observe significant improvements [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. Specifically, Camilleri et al. (2014) and [START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF] showed that tRNS during contrast-detection training-induced higher transfer (the post-training improvement observed in a new task) to visual acuity (VA) with respect to PL alone in both amblyopic and myopic patients. In general, tRNS appears to boost both the early (within-session - [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF] and late (between sessions/days - Camilleri et al., 2014) components of PL. So far, PL studies used tRNS coupled with lower-level perceptual tasks, such as contrast detection or orientation discrimination, rather than training directly higher-level visual abilities, such as VA or crowding. As we have seen, sensory adaptation occurring during training could hinder learning at the level of the low-level network, so that the readout higher-level network will not be able to successfully apply the experience previously gained to new conditions (transfer) [START_REF] Harris | Effects of spatiotemporal consistencies on visual learning dynamics and transfer[END_REF]. hf-tRNS has been proven useful to boost the transfer of perceptual learning to untrained visual tasks [START_REF] Moret | Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia[END_REF], a highly desirable outcome when perceptual learning is used for clinical purposes. A possible explanation of this result could rely on the effect of hf-tRNS in limiting sensory adaptation. This hypothesis is corroborated by a study from [START_REF] Campana | Opposite effects of high-and low-frequency transcranial random noise stimulation probed with visual motion adaptation[END_REF] showing that the application of online hf-tRNS over the medialtemporal area (V5/MT) diminished the perceived duration of the motion after-effect.

From a physiological point of view, tRNS might induce by temporal summation of small depolarizing currents, an increase in the activity of the engaged neurons [START_REF] Cash | Input summation by cultured pyramidal neurons is linear and position-independent[END_REF]. In fact, stimulation between 100 and 1000 Hz may be optimal for affecting neuronal communication as it could interact with the activity of the cell body and dendrites whose the time constant is between 1 and 10 ms, s. The repetitive activation of the sodium channels and the consequent influx of Na+ ions inside the membrane (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Ho | Transcranial random noise stimulation: A new approach to stimulating the brain[END_REF] may interfere with the depressed state of the adapted neurons repolarizing the membrane near to its resting state [START_REF] Campana | Opposite effects of high-and low-frequency transcranial random noise stimulation probed with visual motion adaptation[END_REF]. The mechanisms underlying tRNS are still not completely understood: cooccurrence of stimuli in close succession and the temporal summation of small depolarizing currents induced by the random sub-threshold stimulation (D. Terney, Chaieb, Moliadze, Antal, & Paulus, 2008) might therefore facilitate the depolarization of cortical neurons, producing Hebbian LTP-like changes in the network that processes the task [START_REF] Cappelletti | Transfer of Cognitive Training across Magnitude Dimensions Achieved with Concurrent Brain Stimulation of the Parietal Lobe[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF][START_REF] Snowball | Long-term enhancement of brain function and cognition using cognitive training and brain stimulation[END_REF], improving in turn performance over time (D. Terney et al., 2008).

An alternative hypothesis is that the high fluctuating frequency of stimulation (100-640 Hz) prevents the homeostasis of stimulated neurons (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. Actually, tRNS seems to induce greater improvements than anodal tDCS, where the current flows constantly along the same direction (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Moliadze | Electrode-distance dependent after-effects of transcranial direct and random noise stimulation with extracephalic reference electrodes[END_REF][START_REF] Pirulli | The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation[END_REF]. Finally, the introduction of external noise from the electric stimulation might have altered the overall level of neural excitability and the probability of discharge of every single unit, modifying, in turn, the signal-to-noise ratio during stimulus processing (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. Miniussi and collaborators (2013) proposed a model of 'stochastic resonance' to explain the non-linear effects found in brain stimulation studies. Their model takes into account the interaction between internal activity, externally induced noise and stimulus-driven activity, predicting that in case of low target signal an adequate amount of external noise can enhance the signal (alone) above the threshold. At a neurophysiological level, the tRNS has been shown to modulate the activation and inactivation of the Na+ channels exerting the maximum excitability increase at an intermediate level of noise [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF]. This shortterm molecular activity due to the tRNS that increases the neuronal activation might be the neural bases for the behavioural effect observed. Furthermore, repetitive activation and inactivation of the NA + channels could trigger the biological cascade responsible for a long-term plastic mechanism that could explain the offline after-effect and also the long-lasting effect that was found in perceptual learning studies (Chaieb, Antal, & Paulus, 2015). Given the uncertainty surrounding the neurophysiological mechanisms of tDCS, and the observed differences in terms of behavioural effects and time courses between anodal tDCS and tRNS [START_REF] Antal | Transcranial Alternating Current and Random Noise Stimulation: Possible Mechanisms[END_REF][START_REF] Pirulli | The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation[END_REF], the effect of the latter might be different from those of the former and could be better suited to be coupled with perceptual learning.

The advantages offered by tRNS can be summarized as follows:

• it is easier to make predictions about its effect and to interpret the results. This reduces the probability of any unwanted effects.

• coupled with perceptual learning, it proved useful in increasing the learning rate.

• preventing sensory adaptation during the task may be helpful in increasing the transfer One of the objectives of this thesis is to investigate the effect of single-session and repeated sessions of tRNS over the spatial integration in the occipital cortex.

In the next section, I will discuss some considerations on the safety of use of this technique.

Safety considerations

Among the tES techniques, the most commonly used is the tDCS. Over time, guidelines have accumulated in the literature that establish the limits of the safe use of this technique, as well as reviews on the possible risks encountered during experimental and clinical practice [START_REF] Almousa | Safety of transcranial direct current stimulation of frontal, parietal, and cerebellar regions in fasting healthy adults[END_REF]Bikson, Datta, & Elwassif, 2009;[START_REF] Bikson | Safety of Transcranial Direct Current Stimulation: Evidence Based Update 2016[END_REF][START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF][START_REF] Liebetanz | Safety limits of cathodal transcranial direct current stimulation in rats[END_REF][START_REF] Nitsche | Safety criteria for transcranial direct current stimulation (tDCS) in humans [1] (multiple letters)[END_REF][START_REF] Poreisz | Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients[END_REF][START_REF] Thair | Transcranial direct current stimulation (tDCS): A Beginner's guide for design and implementation[END_REF][START_REF] Woods | A technical guide to tDCS, and related non-invasive brain stimulation tools[END_REF]. Given the similarity between the three techniques, the same safety criteria apply also for the tACS and the tRNS (Kerrie-Anne [START_REF] Ho | Comparison of the Effects of Transcranial Random Noise Stimulation and Transcranial Direct Current Stimulation on Motor Cortical Excitability[END_REF][START_REF] Inukai | Comparison of Three Non-Invasive Transcranial Electrical Stimulation Methods for Increasing Cortical Excitability[END_REF]. The first safety criteria for the use of tDCS in humans were based on studies conducted by Nitsche and colleagues and where published in 2003 (Michael A. [START_REF] Nitsche | Safety criteria for transcranial direct current stimulation (tDCS) in humans [1] (multiple letters)[END_REF]. Despite some updates and clarifications, the basic rules for using this technique have not changed.

In a rat model, [START_REF] Liebetanz | Safety limits of cathodal transcranial direct current stimulation in rats[END_REF] found that brain lesions occurred at a current density of at least 14.3 mA/cm2 for durations greater than 10 min.

Even if a direct comparison cannot be made, the most intensive stimulation regimes used in recent human studies are two orders of magnitude lower than the limit density causing lesions in animals (K. A. [START_REF] Ho | The Effect of Transcranial Direct Current Stimulation (tDCS) Electrode Size and Current Intensity on Motor Cortical Excitability: Evidence from Single and Repeated Sessions[END_REF].

The following items are the most important safety considerations when designing a transcranial electrical stimulation study.

• Desired current density, defined as stimulation intensity in mA divided by the electrode size in cm2, in human studies should be 0.06 mA/cm2 or less. This corresponds to an intensity of 2 mA for a 35 cm2 electrode.

• The amount of total charge, calculated as stimulation intensity in mA multiplied by the total stimulation duration and then divided by the electrode size in cm2, should be 500 C/m2 or less. [START_REF] Liebetanz | Safety limits of cathodal transcranial direct current stimulation in rats[END_REF] found that the total charge that guaranteed zero lesions in the animal model was 52400 C/m2, again two order of magnitude higher than the one commonly used in human studies [START_REF] Bikson | Establishing safety limits for transcranial direct current stimulation[END_REF][START_REF] Bikson | Safety of Transcranial Direct Current Stimulation: Evidence Based Update 2016[END_REF][START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF][START_REF] Liebetanz | Safety limits of cathodal transcranial direct current stimulation in rats[END_REF][START_REF] Nitsche | Safety criteria for transcranial direct current stimulation (tDCS) in humans [1] (multiple letters)[END_REF][START_REF] Poreisz | Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients[END_REF][START_REF] Woods | A technical guide to tDCS, and related non-invasive brain stimulation tools[END_REF].

• The temperature increase of the stimulated tissue is to be kept under control [START_REF] Bikson | Establishing safety limits for transcranial direct current stimulation[END_REF]. Datta, Elwassif, & Bikson (2009) modelled the increase in temperature produced at different stimulation intensities and concluded that at intensities commonly used in humans, the increase in heat is not significant.

• Avoid metallic ion accumulation below the electrodes. Metallic ion absorption can cause a rash or other lesions to the skin, this risk increases in case of repeated sessions of stimulation, but can be prevented by using conductive gel or salinesoaked sponges as a medium [START_REF] Loo | Avoiding skin burns with transcranial direct current stimulation: Preliminary considerations[END_REF][START_REF] Woods | A technical guide to tDCS, and related non-invasive brain stimulation tools[END_REF].

• Check for the presence of large skull defects or skull plates that might alter the cortical current flow. The presence of defects in the skull could create dangerous local peaks in the density of the current as well as making the locus of the stimulation unreliable [START_REF] Datta | Transcranial direct current stimulation in patients with skull defects and skull plates: High-resolution computational FEM study of factors altering cortical current flow[END_REF]. Some additional considerations must then be made in the case of the clinical application of tenure techniques. For example, it has been reported that tDCS can have an effect on cortical hemodynamic responses by increasing the cerebral blood flow. This might open potential applications for the clinical treatment of ischemic strokes, but at the same time a regional alteration in the hemodynamic response could be dangerous in case of cerebral arteriovenous malformation [START_REF] Hu | Effect of Anodal Direct-Current Stimulation on Cortical Hemodynamic Responses With Laser-Speckle Contrast Imaging[END_REF]. Likewise, cathodal tDCS has been found helpful in the treatment of drug-resistant epilepsy [START_REF] San-Juan | Transcranial direct current stimulation in epilepsy[END_REF][START_REF] San-Juan | Successful treatment of a drug-resistant epilepsy by long-term transcranial direct current stimulation: A case report[END_REF]), but at the same time familiarity with epilepsy has long been considered an exclusion criterion as the stimulation can increase the risk of having seizure [START_REF] Matsumoto | Adverse events of tDCS and tACS: A review[END_REF].

Over time, numerous questionnaires were developed for screening participants, some of which are available online. A good starting point for the screening is the questionnaire published by [START_REF] Thair | Transcranial direct current stimulation (tDCS): A Beginner's guide for design and implementation[END_REF] in their review. This questionnaire can be easily adapted to the specific needs of the individual study. In the case of clinical applications of the techniques, it is good practice that the inclusion / exclusion of the participant is evaluated together with a competent doctor.

Given the reported absence of serious adverse effects, in recent studies, the recommended limits for the amount of total charge and number of repetitions have been progressively increased. The only persistent adverse effects of tDCS are mainly skin problems, but usually side effects are limited to a tingling sensation at the skin under the electrodes [START_REF] Matsumoto | Adverse events of tDCS and tACS: A review[END_REF]. This sensation mainly pertains to the cutaneous receptor activity of the somatosensory system and usually disappears right after the beginning of the stimulation (Michael A. [START_REF] Nitsche | Safety criteria for transcranial direct current stimulation (tDCS) in humans [1] (multiple letters)[END_REF]. The strength of this sensation also depends on the current intensity, the conductivity of the medium, and the type of electrodes [START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF][START_REF] Minhas | Electrodes for high-definition transcutaneous DC stimulation for applications in drug delivery and electrotherapy, including tDCS[END_REF]. This cutaneous perception can be a limitation in the experimental protocols that require repeated sessions of the actual stimulation and the fake control stimulation, aka the "sham" stimulation. The subject could, in fact, recognize when it is actually stimulated, and this could introduce a bias in the results [START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF]. It is good practice to try to minimize this sensation by slowly "ramping up" and "rampingdown" the current intensity at the beginning and the end of stimulation.

An advantage of tRNS over tDCS is that of causing a lower skin sensation. In most cases, the subject cannot distinguish the difference between tRNS and sham stimulation [START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF].

In general, tACS and tRNS produce fewer adverse effects than tDCS and can be considered safer. [START_REF] Matsumoto | Adverse events of tDCS and tACS: A review[END_REF]. In conclusion, the tES in general, and the tRNS, in particular, can be considered as safe both in the basic research and in their clinical application.

Introduction

Macular degeneration (MD) causes loss of input to the region of the primary visual cortex that represents the fovea. The evidence that the adult brain is capable of plasticity [START_REF] Calford | Plasticity in adult cat visual cortex (area 17) following circumscribed monocular lesions of all retinal layers[END_REF][START_REF] Gilbert | Adult Visual Cortical Plasticity[END_REF]Wandell & Smirnakis, 2009b) raises the question of how the cortical deafferentation in MD affects cortical functionality and, in particular, whether cortical rearrangement occurs.

The cortical reorganization hypothesis is compatible with the results of the animal studies reporting the expansion of receptive fields of neurons near the retinal lesion boundary [START_REF] Calford | Plasticity in adult cat visual cortex (area 17) following circumscribed monocular lesions of all retinal layers[END_REF]; Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF][START_REF] Gilbert | Topographic reorganization in the striate cortex of the adult cat and monkey is cortically mediated[END_REF][START_REF] Gilbert | Axonal sprouting accompanies functional reorganization in adult cat striate cortex[END_REF]U T Eysel et al., 1999;[START_REF] Gilbert | Receptive field dynamics in adult primary visual cortex[END_REF][START_REF] Heinen | Recovery of visual responses in foveal V1 neurons following bilateral foveal lesions in adult monkey[END_REF][START_REF] Kaas | Reorganization of retinotopic cortical maps in adult mammals after lesions of the retina[END_REF]. fMRI studies in humans provide instead conflicting evidence in support of the plasticity hypothesis. Some found clear activation of the foveal cortex to stimuli presented outside the central scotoma, (C. I. [START_REF] Baker | Reorganization of Visual Processing in Macular Degeneration[END_REF][START_REF] Baker | Reorganization of visual processing in macular degeneration: Replication and clues about the role of foveal loss[END_REF][START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF][START_REF] Dilks | Reorganization of visual processing in age-related macular degeneration depends on foveal loss[END_REF][START_REF] Schumacher | Reorganization of visual processing is related to eccentric viewing in patients with macular degeneration[END_REF] while others did not show evidence of this functional reorganization (Baseler et al., 2011a;[START_REF] Liu | Incomplete cortical reorganization in macular degeneration[END_REF][START_REF] Masuda | V1 projection zone signals in human macular degeneration depend on task, not stimulus[END_REF][START_REF] Sunness | Retinotopic mapping of the visual cortex using functional magnetic resonance imaging in a patient with central scotomas from atrophic macular degeneration[END_REF]. Some of the behavioural changes in MDs support, but only indirectly, the cortical reorganization hypothesis. For example, Safran and Landis (A B [START_REF] Safran | From cortical plasticity to unawareness of visual field defects[END_REF][START_REF] Safran | Plasticity in the adult visual cortex: Implications for the diagnosis of visual field defects and visual rehabilitation[END_REF] reported that MD patients, similarly to what was found with artificial scotoma [START_REF] Kapadia | A Quantitative Measure for Short-term Cortical Plasticity in Human Vision[END_REF], experienced apparent displacement of images adjacent to the scotoma toward the field defect leading to perceptual completion and shape distortion. De Stefani and colleagues [START_REF] De Stefani | Illusory contours over pathological retinal scotomas[END_REF] demonstrated perfect discrimination of the curvature of illusory contours across the pathological scotoma. They suggested that, following the loss of bottom-up input, the visual cortex enhances connectivity and/or low-spatial frequency response, thus mediating the formation of a neural representation of complex geometrical shapes across the scotoma. In the preferred retinal locus (PRL) of the MD patients, [START_REF] Chung | Cortical Reorganization after Long-Term Adaptation to Retinal Lesions in Humans[END_REF] showed a "foveal-like" distortion of the scaling of critical spacing with the eccentricity. Casco and co-workers (Casco et al., 2015) provided evidence that MD patients are better than normally sighted observers in using information allowing detection of the mirror, but not the translational symmetry of a two-dot configuration at the opposite side of the scotoma. This last result is compatible with cortical rewiring, whereby detection of the co-aligned low spatial filters crossing the scotoma becomes more efficient with MD. However, a more parsimonious explanation is that MD's peripheral vision takes its functional advantage from more efficient use of the high-level representation of the visual input. The numerous functional changes that have been observed in MD vision [START_REF] Casco | Hyper-vision in a patient with central and paracentral vision loss reflects cortical reorganization[END_REF]; S. T. L. [START_REF] Chung | Cortical Reorganization after Long-Term Adaptation to Retinal Lesions in Humans[END_REF][START_REF] Clara | Hyper-vision of mirror symmetry in patients with macular degenerationreflects parafoveal cortical reorganization[END_REF][START_REF] Safran | From cortical plasticity to unawareness of visual field defects[END_REF] are compatible with the suggestion that the visual response around the scotoma relies more on the integration fields output.

The relationship between the contextual modulation and the contour integration has been described by [START_REF] Dakin | Contex influences contour integration[END_REF] and has been postulated to be the basis for the perceptual fill-in to occur (Golden, Field, & Hayes, n.d.) through recurrent feedback to V1 complex cells within the lesion projection zone [START_REF] Mcmanus | A Computational Model of Perceptual Fill-in Following Retinal Degeneration[END_REF].

To summarize, there is an important question that has no answer yet. Do the functional changes observed in the PRL result from modulation of excitatory and inhibitory contextual influences involved in contrast detection? To answer this question in this work we have looked at whether MD's peripheral vision is associated with a change in contextual influences with respect to controls and whether this change is accounted for by the same model that describes the contrast gain in normal viewer 27 .

There are different ways in which the observed pattern of contextual influences in the periphery might be different in MD with respect to controls. In people with normal vision, accumulating psychophysical studies have shown contextual influences on the threshold for contrast detection coming from outside the receptive field of the channel responding to the target (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Gilbert | Adult Visual Cortical Plasticity[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF]. In particular, it is well established that the visibility of low contrast Gabor patch is affected by collinear flanking Gabors of similar orientation and spatial frequencies but high contrast: Short target-to-flanker separation (1-2 times the wavelength of the target Gabor's carrier, λ) leads to suppression, whereas target-toflanker separations of 3-4λ lead to enhancement (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Kapadia | Improvement in visual sensitivity by changes in local context: Parallel studies in human observers and in V1 of alert monkeys[END_REF][START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Shani | Psychometric curves of lateral facilitation[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. Moreover, these contextual effects can be modulated by task repetition in a perceptual learning paradigm, in both normally sighted observers [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Shani | Psychometric curves of lateral facilitation[END_REF] and patients with impaired vision [START_REF] Barollo | Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia[END_REF][START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF]Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Casco | Neural Restoration Training improves visual functions and expands visual field of patients with homonymous visual field defects[END_REF][START_REF] Casco | Specificity and generalization of perceptual learning in low myopia[END_REF][START_REF] Contemori | learning improves vision in patients with macular degeneration<[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]; D. T. H. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF]. In addition, contextual influences depend on eccentricity: in the periphery, inhibition is more prominent and contextual enhancement occurs at a target-to-flanker distance of 6λ, that is larger than in the fovea [START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Maniglia | The effect of spatial frequency on peripheral collinear facilitation[END_REF].

Since lateral connectivity has been shown to increase with practice in normal [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]U Polat & Sagi, 1994;[START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] and pathological vision [START_REF] Lev | Training improves visual processing speed and generalizes to untrained functions[END_REF][START_REF] Polat | Improving vision in adult amblyopia by perceptual learning[END_REF][START_REF] Polat | Restoration of underdeveloped cortical functions: evidence from treatment of adult amblyopia[END_REF], use-dependent cortical reorganization might change the observed contextual influences. Partial support for the cortical reorganization hypothesis comes from recent papers showing that MD patients exhibited a modulation of contextual influences in respect of controls [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). In the present study we further address this issue attempting to specify the underlying mechanism. In addition, by comparing the effect of flankers distance on contrast detection both in the PRL and in a symmetrical retinal position (non-PRL) we also hope to address the still debated issue of whether the vision in the PRL is enhanced by the use of this region for everyday visual task (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF][START_REF] Schumacher | Reorganization of visual processing is related to eccentric viewing in patients with macular degeneration[END_REF][START_REF] Timberlake | Reading with a macular scotoma. I. Retinal location of scotoma and fixation area[END_REF][START_REF] Timberlake | Reading with a macular scotoma. II. Retinal locus for scanning text[END_REF]. A contextual modulation for target contrast detection with specific properties in the PRL would support the "Use-Dependent Reorganization" hypothesis while a strong similarity between the two tested locations would play in favour of a more conservative "Use-Independent Reorganization" hypothesis (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF].

Finally, we asked whether a modulation of contextual influences in MDs affects the efficiency in performing everyday visual tasks.

Participants

Participants were 13 MD patients (mean age of 61 ± 9.16 years, range: 49-83 years) and 7 controls (mean age of 59 ± 4.12 years, range: 54-64 years). Patients were selected based on the clinical history and the Nidek MP1 microperimetry results. The dispersion of fixation was quantified during the microperimetry, and only patients with at least 80% of fixations in the range of 2° of visual angle around the focal point of the PRL were included in the sample. Patients with concomitant visual diseases other than central vision loss were not included, nor were those with a visual acuity on the ETDRS eyechart lower than 1/20 or above 10/20. All the tests were performed monocularly, and in the case of bilateral scotoma, the chosen eye was the one with the best-spared vision based on visual acuity and microperimetry data. The eye chosen to be tested for the control group was the nondominant eye. Because the non-PRL was defined as the symmetrical retinal location of the PRL, the proximity of the optic disc or irregular shape of the scotoma could reduce the visibility of stimuli presented in this second retinal location. To check for visibility of the stimuli, during both the crowding and lateral masking tasks, patients were asked to report the number of visible elements in the non-PRL. Of the 13 MD patients, all could see the full triplet of stimuli (Gabors and letters) in the PRL, while only 8 of them could do so in the non-PRL. Because the presentation of the stimuli was randomized in the two retinal positions to reduce the frequency and the amplitude of eventual eye movements, all the patients were tested in both locations, but only the ones who were able to see the full triplets were further considered for statistical analysis in the non-PRL position.

Details relative to age, gender, scotoma diameter, visual acuity, and PRL position are summarized in Table 1. The study was performed in accordance with the ethical standards laid down by the Declaration of Helsinki [START_REF] Association | Declaration of Helsinki[END_REF]. The study was approved by the Ethics Committee of the Department of General Psychology, University of Padova (Protocol 1449). We obtained written, informed consent from all participants involved in the study.

Locations tested

The eccentricity of the PRL was individually estimated as a proportion of the distance from the macula and the optic disk, in degrees of visual angle. Using the image of the retinal fundus, the position of the fovea was computed based on the averages of the values determined for normally sighted observers: 15.3° temporally and 1.5° below the centre of the optic disc. The distance between the position of the fovea and the PRL position estimated by the microperimetry was then computed to derive the eccentricity of the PRL. The non-PRL position was defined as approximately correspondent to the horizontally specular area of the retina, using the macula as the centre of symmetry. One example of patients' microperimetry and the respective retinal displacement of the target Gabor in either the PRL or the non-PRL is shown in Figure 1. In the case of very small scotoma, the non-PRL position was set by default at 6° of eccentricity with respect to the PRL on the horizontal axis. This was done to allow reliable discrimination between the two gaze positions by the eye tracker. The eccentricity at which stimuli were presented to each control subject matched that of one patient, randomly chosen. On average, the eccentricity was 4°. Figure 1. Illustrative example of stimuli placement.

Stimuli presented at the PRL and at the non-PRL are superimposed over the microperimetry.

Eye movement recording

Participants' fixation was controlled with an eye tracker to determine the retinal position corresponding to the patients' PRL/non-PRL and to be sure that fixation was maintained. Calibration and recording procedures were as follows. Eye movements were recorded using a Mirametrix S2 eye tracker with a sample rate of 60 Hz and an accuracy of 0.5°. The calibration of the tracker and the gaze check were integrated into the main Matlab script using the Mirametrix Matlab Toolbox and API for Windows. Thanks to a custom calibration software, the calibration dot that normally sighted observers follow with the fovea were shifted by a constant so that, although MD patients could follow it with their PRL, the position of the eye relative to the calibration dot corresponded to that of a normally sighted observer. Due to the instability of fixation and the systematic error of the tracker, a tolerance window of ± 1.5° around the PRL fixation point was set. If the gaze of the subject before the stimulus presentation was out of this window, a warning sound was presented to allow the patient to relocate his or her gaze.

Apparatus and stimuli

Participants sat in a dark room 57 cm from the screen. Stimuli were displayed on an ASUS ML228H LCD LED 21.5-inch monitor with a refresh rate of 60 Hz and a spatial resolution of 1920 x 1080 pixels, with a pixel pitch of 0.248 mm. Stimuli were generated with Matlab Psychtoolbox (Brainard, 1997a;[START_REF] Pelli | The VideoToolbox software for visual psychophysics: Transforming numbers into movies[END_REF]. Gamma correction for each color channel was applied through calibration with the Spyder 4 Elite colorimeter (DataColor). The calibration was further verified using a Minolta LS-100 photometer, which indicated that the mean luminance was 50 cd/m2. In that way, luminance was a linear function of the digital representation of the image.

In order to represent 10.7 bits of luminance (1786 gray levels) on an 8-bit display, we adopted a software solution called "Pseudo-Gray," also known as "Bit-

Stealing" [START_REF] Tyler | Colour bit-stealing to enhance the luminance resolution of digital displays on a single pixel basis[END_REF], implemented via a Psychtoolbox built-in function.

Contrast detection stimuli. Stimuli were Gabor patches consisting of a cosinusoidal carrier enveloped by a stationary Gaussian. Each Gabor patch was characterized by its sinusoidal wavelength (λ), phase (φ), and standard deviation of the luminance Gaussian envelope (λ) in the (x,y) space of the image:

( )

2 2 2 2 , cos x y G x y x e       + -         = +    
Eq. 1

with λ = λ and φ = 0 (even symmetric). Gabors' spatial frequency (SF) was 2 and Increasing target-to-flanker separations of2λ, 3λ, 4λ, and 8λ are shown.

The two high-contrast collinear flankers were placed at various distances from the target (i.e., 2λ, 3λ, 4λ, and 8λ). The patients were asked to maintain their gaze on the PRL, and stimuli were randomly presented over either the PRL or non-PRL position within a block. Controls had to fixate the centre of the screen, and stimuli were randomly presented either left or right of fixation.

Visual acuity and crowding stimuli. Visual acuity (eccentric VA) and crowding were measured at the same eccentricity as for the Gabor configuration. Stimuli were generated using Matlab Psychtoolbox (Brainard, 1997a;[START_REF] Pelli | The VideoToolbox software for visual psychophysics: Transforming numbers into movies[END_REF] according to a psychophysical adaptive procedure (Maximum Likelihood Procedure) [START_REF] Grassi | MLP: A MATLAB toolbox for rapid and reliable auditory threshold estimation[END_REF][START_REF] Green | A maximum-likelihood method for estimating thresholds in a yes-no task[END_REF]; David M. [START_REF] Green | Stimulus selection in adaptive psychophysical procedures[END_REF]) that tracked 55% of the participants' psychometric function within a 60-trial block. The starting stroke width was 30 arcmin. Subjects had to verbally report the target letter, and the experimenter registered the answer. The threshold was the values obtained in the last trial.

The crowding stimulus had two different letters vertically flanking the target letter. The stroke width of both the target and flanking letters was set 30% higher than the VA threshold obtained at the same eccentricity and the same exposure duration. When tested in the PRL, the MD patients were able to detect all three letters at the largest spacing used (5°). This procedure is often used [START_REF] Barollo | Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia[END_REF][START_REF] Hussain | Perceptual Learning Reduces Crowding in Amblyopia and in the Normal Periphery[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Pelli | The uncrowded window of object recognition[END_REF] to avoid an influence of VA on the measurement of critical spacing for crowding. Crowding was indexed by the critical spacing, defined as the edge-to-edge inter-letter distance at which observers could discriminate the target (the central letter) with 55% accuracy.

Differently, from the centre-to-centre distance, edge-to-edge distance prevents overlay masking of the target by the flankers but has the disadvantage of co-varying with letter acuity (the bigger the target, the larger the centre-to-centre distance at zero border-toborder distance), thus ultimately underestimating crowding in people with low acuity.

Procedure

Each subject underwent a testing session of 3 hours in which VA, crowding, and the target contrast thresholds for orthogonal and several collinear configurations were measured monocularly in counterbalanced order.

Contextual influences were estimated by computing the threshold elevation (TE) as:

10 _ log _ CT Collinear TE CT Orthogonal   =     Eq. 2
where CT_Collinear is the contrast threshold estimated in the collinear condition, and CT_Orthogonal is the contrast threshold estimated in the orthogonal condition. TE was calculated separately for each target-to-flanker distance (i.e., 2λ, 3λ, 4λ, and 8λ). We used the orthogonal threshold value obtained at 8λ as a baseline to compute TE (instead of thresholds obtained from the isolated target). The advantage is that the orthogonal configuration still maintains the facilitation induced in the task by the reduced spatial and temporal uncertainty that the presence of the flankers causes so that it is not a confounding factor for the estimation of TE [START_REF] Shani | Eccentricity effects on lateral interactions[END_REF]. We chose the 8λ as the orthogonal configuration because, there is little or no contextual enhancement expected at λs > 6 at the tested eccentricities, and therefore it is an ideal condition as a baseline [START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Maniglia | The effect of spatial frequency on peripheral collinear facilitation[END_REF].

Statistical analysis

Within-and between-group comparisons were carried out with ANOVAs on either contrast threshold or TE using, for pairwise comparisons, t-tests with Bonferroni correction. TE was also analysed using one-sample, one-tail t-tests based on the hypothesis of TE as either > 0 or < 0 for suppressive and facilitatory effects, respectively.

Visual acuity and crowding data were also analysed with two-tailed t-tests.

Results

Contextual influence results

Contrast threshold results and TE results are shown in Figure 3. A mixed-design ANOVA conducted on the contrast threshold data on PRL, including as factors the group (patients vs. controls) and λ (2, 3, 4, and 8λ), revealed that thresholds were significantly higher for the MD patients (F(1,18) = 13.7, p =. 002, partial-η2 = 0.422). The effect of λ was significant (F(3,54) = 3.308, p = .027, partial-η2 = 0.155). The interaction group x λ was also significant (F(3,54) = 4.024, p = .012, partial-η2 = 0.183). Pairwise comparisons revealed a group difference for the 3 λ (p = .005), 4 λ (p < .001), and 8 λ (p = .002) and higher thresholds at 2 λ compared to 3 λ (p = .026), 4 λ (p = .039). None of the differences in thresholds across λs were significant in the MD group.

A mixed-design ANOVA conducted on TE data, including as factors the group (patients vs. controls) and the λ (2, 3, 4, and 8λ) indicated that the effect of group was not significant (F(1,18) = 2.752, p = .114, partial-η2 = 0.133), while both the effect of λ (F(3,54) = 10.09, p < .001, partial-η2 = 0.359), and the group x λ interaction (F(3,54) = 10.561, p < .001, partial-η2 = 0.370) were. Post hoc comparisons showed higher TE for the smallest λ compared to others (2 vs. 3λ, p = .002; 2 vs. 4λ, p < .001; 2 vs. 8λ, p = .009). Most importantly, there was a significant group difference at the smallest λ only (p < .001), and a different effect of the smallest λ in the two groups: For controls, TEs were higher at the smallest λ compared to the others (2 vs. 3λ, p = .001; 2 vs. 4λ, p < .001; 2 vs. 8λ, p = .002); for the MD group, there was not any difference in TE between the different λs (p = 0.99). Since TE > reflects inhibitory lateral interaction and TE <0 facilitatory, one-sample, one-tail t-tests were conducted to test the null hypothesis of 0 TE. As expected TE resulted generally negative for λs ≥ 3 for both MD patients (3λ:

(t(12) = -3.2, p = .004; 4λ: t(12) = -1.4, p = .096; 8λ: t(12) = -2.47, p = .014) and controls (3λ: t(6) = -0.85, p = .21; 4λ: t(6) = -3.74, p = .005; 8λ: t(6) = -2.8, p = .015)), confirming that contextual enhancement occurs at larger λs in the periphery than in the fovea [START_REF] Shani | Psychometric curves of lateral facilitation[END_REF]; D. T. H. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. At the shortest λ, TE assumed, as expected, positive values for controls (t(9) = 4.01, p = .003); however, at the same shortest λ the values of TE for MD patients were negative (t(12) = -2.3, p = .02).

Acuity and crowding results

The visual acuity of the 8 patients who had a reliable measure in both the PRL and non-PRL are shown in Figure 5. The one-way ANOVA conducted on these data, with group as a factor (controls, PRL, non-PRL) showed a significant effect of group (F(2,20) = 6.22, p = .007, partial-η2 = .384), indicating higher acuity for controls than MD patients both when tested at the PRL (difference = 6.917, p = .046) and non-PRL positions (difference = 9.167, p = .007). The difference between PRL and non-PRL was not significant (difference = -2.25, p = .67). Even considering the small sample and the high variability in VA data, this lack of difference confirms that development of a PRL is not strictly linked to an advantage in terms of visual acuity over the other retinal quadrants [START_REF] Cheung | Functional and cortical adaptations to central vision loss[END_REF][START_REF] Schuchard | Preferred retinal loci and macular scotoma characteristics in patients with age-related macular degeneration[END_REF]. Moreover, the two acuity values for the 8 patients are highly correlated (R = .78, p = .022). Individual crowding data of the 8 patients who had a reliable measure in both the PRL and non-PRL are shown in Figure 6. The one-way ANOVA conducted on these data revealed a significant effect of group (F(2,20) = 3.516, p = .049, partial-η2 = .26). Post hoc comparison showed a significant difference between controls and MD patients when tested at the non-PRL (difference = 1.693, p = 0.037) but not when tested at the PRL (difference = .962, p = .3). The difference between PRL and non-PRL was not significant (difference = -0.731, p = .47). Visual acuity and crowding measures should be independent by definition. We checked this assumption by calculating the correlation between the two measures. We found that the negative correlation did not reach significance (R = -.52, p = .068) despite the fact that the operative definition of critical spacing as the edge-to-edge inter-letter distance may have inflated this estimate (see Method section).

Figure 6. Critical space PRL vs. Critical space non-PRL.

Scatter plot represents critical spacing for crowding obtained with stimuli presented at the non-PRL regressed on critical spacing for crowding obtained with stimuli presented at the PRL only for the patients for which the two positions were symmetrical with respect to the fovea. Dashed lines represent 95% confidence intervals. Average critical spacing for crowding for this subgroup (n = 8), and for the control group (n = 7) is also shown in the bar chart.

Discussion

Using a two-interval forced-choice task, contrast threshold for a low-contrast target Gabor flanked by two collinear high-contrast Gabors presented at eccentricities varying between 3º and 8º was measured in a group of subjects with MD and in an agematched control group. Target-to-flanker separation, varied in terms of the Gabor's carrier wavelength unit (λ), was 2λ, 3λ, 4λ, and 8λ. The contextual influence of the flankers was indicated by the threshold modulation (TE), indicating the change in contrast threshold obtained at each of the four λs, relative to the baseline condition with no contextual influence (orthogonal flankers, 8λ).

Results showed contextual enhancement at λs higher than in the fovea (4-8λ). At 2λ, all controls had inhibition. Only 2 of the patients had inhibition, 2 had a TE close to zero, and 9 of them had negative TE, indicating facilitatory contextual influences. This change in contextual influences at the shortest λ in MD patients was associated, both at the PRL and non-PRL, with an increase of contrast threshold for the target, as well as with reduced visual acuity and a larger crowding effect. Is the switch between inhibition and facilitation at the shortest λ an index of cortical plasticity or it could be explained by the same model used to interpret psychophysical data from normal viewers? An answer to this question comes from establishing whether TE in MDs is well described by the variation of TE as a function of flanker/target contrast ratio in normal vision [START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Ross | The effects of adaptation and masking on incremental thresholds for contrast[END_REF][START_REF] Snowden | The effects of surround contrast on contrast thresholds, perceived contrast and contrast discrimination[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination--psychophysics and modeling[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] proposed a model for contextual influences in which:

• At a close distance, the sensitivity to a low contrast target is reduced by the presence of high contrast flankers.

• When the contrast of target increases the reduction in sensitivity progressively decreases and then turns into facilitation. The switch happens when the contrast of the target is still lower (around three times) with respect to that of the flankers.

• When the contrast of target approaches that of the flankers, there is a dip in the facilitation.

• However, when the contrast of target surpasses that of the flankers, the facilitatory effect progressively reduces and then disappear.

The model postulates a contrast dependent modulation of the contextual effect of the flankers that shift progressively from inhibition to facilitation depending on flanker/target contrast ratio. We verified this by pulling the data of the two groups together and regressing the TE at 2λ as a function of log 10(Contrastf lankers/Contrast target threshold) in the 8λ orthogonal condition. We performed a locally-weighted polynomial regression, aka lowess [START_REF] Cleveland | Robust locally weighted regression and smoothing scatterplots[END_REF][START_REF] Cleveland | Lowess: A program for smoothing scatterplots by robust locally weighted regression[END_REF], in R (R Core Team, 2012) with a 50% smoothing span that leads to an R2 of 0.52 (correlation between raw and estimated data). Finally, we superimposed raw data and fit line over the model predictions from [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] at 0λ and 2λ. As the Figure 7 shows, the regression line derived from our dataset approximates very well the one predicted by the model, in particular the line that refers to the 0 λ, as expected by the fact that increasing eccentricity would shift the curve leftwards. Thus, the transition from inhibition to facilitation that most MD patients show at high contrast threshold suggests improved 86 efficiency in integrating/grouping elements, possibly mediated by an integration between the flanker and target within the 2nd order integrative field [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] are shown together with the locally weighted scatterplot smoothing (lowess) from our data.

Our results do not support the hypothesis that MD present cortical reorganization leading to a use-dependent increase of long-range connectivity. In this case, with the low contrast target, we would have obtained increased contrast enhancement at the range of target-to-flanker distances at which facilitation occurs in normal vision. In the previous literature, the reduced collinear inhibition found in MD has been interpreted as a sign of neural plasticity, linked with a change in receptive field size [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). We shed new light on this phenomenon and propose a different interpretation. If we consider together all our participants including the controls, our data show that the reduction in collinear inhibition and the switch towards facilitation are clearly linked with the baseline contrast sensitivity of the single subject in the orthogonal configuration. This switch can be well described by the same model previously proposed for the normal vision [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] and thus our results support the hypothesis that inhibitory target-flanker separations (short) become facilitatory in MD as it would do in controls if their contrast threshold were 4-5 fold

higher. Taking all this information into consideration, the reduction of inhibition cannot be ascribed to neural plasticity in PRL but must be considered as a by-product of the same retinal degeneration that may deplete the patient's vision at the boundary of the scotoma.

To conclude, our result is that a reduction of contrast gain at the boundary of the scotoma is associated not only to reduced resolution (Lev & Polat, 2015), which indeed MDs show for stimuli presented at the boundary of the scotoma, but also to a change in the way neighbouring elements are integrated.

Chapter VI. tRNS Modulates Excitatory and Inhibitory Lateral Interactions in Contrast Detection

This chapter has been accetted for publication in Scientific Reports. Battaglini L., Contemori G., Fertonani A., Miniussi C., Coccaro A., Casco C. Delivering a weak electric current through the scalp on a cortical region at random frequencies, has shown promising results in boosting PL between blocks and reducing the number of sessions needed to observe significant improvements [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. A PL based on a lateral masking task can be effective in improving the visual functions in the PRL of MD patients, but a very long training is required and the amount of transfer is limited [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). Patients with MD could particularly benefit from this training since could lead to a use-dependent restructuring of the lateral connections making them potentially more similar to those of the fovea [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]U Polat & Sagi, 1994;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. The possibility to train MD patients with PL + concomitant tRNS is attractive and in other clinical populations it has produced significant benefits [START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF]. However, it is not known whether tRNS has a specific effect on lateral interactions in V1. Studying this effect could help us to better understand the mechanisms of action the tRNS and the possible specific repercussions for MD patients.

Introduction

Visual performance for a stimulus presented in a given retinal location can be modulated by the simultaneous presence of other stimuli having a different retinal position. This technique, known as lateral masking, consists in measuring contrast sensitivity for a periodic Gabor pattern (target) flanked by high-contrast Gabors collinear and iso-oriented to the target. Psychophysical studies on lateral masking showed that in central vision sensitivity reduces (threshold increases) when the distance from the flankers is ≤ 2 target wavelengths, a result suggesting lateral inhibition by the flankers.

For larger target-to-flankers distances, ranging from 3 to 9 target wavelengths, the flankers facilitate target detection, as demonstrated by a threshold decrease from absolute threshold (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF]; C. C. [START_REF] Chen | Lateral sensitivity modulation explains the flanker effect in contrast discrimination[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination-psychophysics and modeling[END_REF]. Indeed, Mizobe and colleagues [START_REF] Mizobe | Facilitation and suppression of single striate-cell activity by spatially discrete pattern stimuli presented beyond the receptive field[END_REF]) not only showed that the neurometric function to target contrast was modulated by the flankers presented outside the classical receptive field, but also that the modulation was dependent on the relative distance between target and flankers.

Moreover, the separations at which facilitation occurs are larger in the periphery than in the fovea [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. Furthermore, psychophysical (Yael [START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF] C. C. [START_REF] Chen | Lateral sensitivity modulation explains the flanker effect in contrast discrimination[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF], electrophysiological (Uri [START_REF] Polat | Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex[END_REF], and brain imaging studies [START_REF] Tajima | Behavioral/Systems/Cognitive Opposing Effects of Contextual Surround in Human Early Visual Cortex Revealed by Functional Magnetic Resonance Imaging with Continuously Modulated Visual Stimuli[END_REF] showed that the polarity of contextual modulation is also contrast dependent: inhibitory effects occur within a contrast range larger than that at which facilitation occurs.

One major question regards the neurophysiological bases of the facilitatory and inhibitory lateral interactions. Psychophysical evidence suggests that detection thresholds depend on the activation of interconnected local neural network with both excitatory (E) and inhibitory (I) neurons whose synaptic connections are activity dependent. E/I ratio depends on the contrast of the target and on the E, and I lateral input that may favour either facilitation or inhibition by the flankers depending on target-flanker separation (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF].

Contextual influences on contrast detection have been investigated in an accumulating mass of studies for two crucial reasons. First, they are considered to contribute to the perception of contours in natural scenes. Facilitation of detection occurs when the target-flanker configuration is collinear rather than orthogonal, that is consistent with a contour structure in which local-global orientation cohere (Bellacosa Marotti, Pavan, & Casco, 2012; [START_REF] Dakin | Contex influences contour integration[END_REF][START_REF] Field | Contour integration by the human visual system: Evidence for a local "association field[END_REF]. Many authors were refrained from drawing a too close parallel between lateral masking and supra-threshold perceptual phenomena (Robert F. [START_REF] Hess | The role of "contrast enhancement" in the detection and appearance of visual contours[END_REF].

Mechanisms involved in suprathreshold perceptual tasks, such as contour integration and crowding, do not use any simple form of local contrast enhancement to perform grouping and segmentation of local elements, respectively. However, even assuming that contrast enhancement may not be the mechanism involved in either perceptual grouping or segmentation, it is quite likely that these high-level tasks and the low-level effects of contrast enhancement could be explained by common cortical circuit [START_REF] Dakin | Contex influences contour integration[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Maniglia | The effect of spatial frequency on peripheral collinear facilitation[END_REF] H. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF].

Neural plasticity induction can be also achieved through weak currents applied transcranially. The most used of these protocols are transcranial direct current (tDCS), alternating (tACS) and random noise (tRNS) stimulation. Whereas tACS has been suggested to be suitable for interacting with endogenous brain oscillations [START_REF] Asamoah | tACS motor system effects can be caused by transcutaneous stimulation of peripheral nerves[END_REF][START_REF] Fröhlich | Endogenous electric fields may guide neocortical network activity[END_REF][START_REF] Helfrich | Entrainment of brain oscillations by transcranial alternating current stimulation[END_REF][START_REF] Schutter | Cutaneous retinal activation and neural entrainment in transcranial alternating current stimulation: A systematic review[END_REF][START_REF] Thut | Guiding transcranial brain stimulation by EEG/MEG to interact with ongoing brain activity and associated functions: A position paper[END_REF], tDCS, and tRNS have become increasingly popular as tools to induce neural modulation in the visual system [START_REF] Battaglini | Anodal and cathodal electrical stimulation over V5 improves motion perception by signal enhancement and noise reduction[END_REF][START_REF] Behrens | Long-lasting enhancement of visual perception with repetitive noninvasive transcranial direct current stimulation[END_REF][START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. With tDCS that is a constant current, in general, the anodal electrode is associated with an increase in excitability, while an inhibitory effect is observed with the cathodal electrode(Michael A. [START_REF] Nitsche | Transcranial direct current stimulation: State of the art 2008[END_REF]. Instead, with tRNS the direction of the current is not relevant to obtain effects [START_REF] Pirulli | On the Functional Equivalence of Electrodes in Transcranial Random Noise Stimulation[END_REF]. In this framework tRNS similarly to anodal tDCS, has been shown to induce an increase of cortical excitability (Daniella Terney, Chaieb, Moliadze, Antal, & Paulus, 2008) but likely with a different dynamic (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF] avoiding inactivation due to adaptation of ion channels when using a constant current. In contrast to anodal tDCS, it has been hypothesized that tRNS prevents homeostasis of the system. Such stimulation consists in the application of a random electrical oscillation spectrum over the cortex; this fast oscillating field modifies the neurons' synaptic efficiency regardless of the current flow orientation [START_REF] Pirulli | On the Functional Equivalence of Electrodes in Transcranial Random Noise Stimulation[END_REF][START_REF] Terney | Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation[END_REF]. Mechanistically, the tRNS-induced neurophysiological effect has been suggested to originate from modulation of voltage-gated sodium channels (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Terney | Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation[END_REF] specifically acting on the dynamics of in/activation of the sodium channels [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF]. Moreover, the behavioural improvement following tRNS has been interpreted by suggesting that the random frequency stimulation produced by tRNS sustains random neural activity in the system, i.e., noise, which serve as a pedestal to expand the sensitivity of the neurons to weak stimuli, providing in same cases inputs similar to those of the target, thereby increasing the signal-to-noise ratio [START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF]. When applied over visual areas, tRNS increases perceived contrast of targets having low contrast (van der Groen & Wenderoth, 2016). In line with this approach, an intriguing possibility that better defines neurophysiological mechanisms is that, with lateral masking configurations, tRNS might induce a synaptic enhancement at the level of the lateral connections between target and flanker neurons, by inducing a temporal summation of weak depolarizing currents. Hence, specific changes in performance are related to a network-dependent stochastic resonance phenomenon (Anna [START_REF] Fertonani | Transcranial electrical stimulation: What we know and do not know about mechanisms[END_REF] i.e., the balance between excitation and inhibition is strictly related to the specific neuronal population state (E/I) and not just to generalized changes in cortical excitability. In this study, because these evidences that tRNS exceeds the beneficial advantages of tDCS, we aimed to explore if the interaction between tRNS and visual system task dependent activity can modulate cortex excitability and therefore behaviour in a specific way. Depending on which of the two circuitries is involved, either the one accounting for increase in perceived contrast for the target or the one responsible for the modulation of target contrast by lateral interactions, a different perceptual outcome is expected. A simple contrast gain effect would be reflected in an increase in sensitivity (d') for a single target of low contrast and modulate the lateral interactions effect consequently. Based on the evidence that tRNS depolarize neurons, we expected tRNS to increase the E thalamic input only when this is weak, that is when the target contrast is low. In this case, given the evidence that E and I lateral modulation occur when the target contrast is low and high respectively (Yael [START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination-psychophysics and modeling[END_REF] we expected the E lateral input to be weakened with tRNS. That is, the reduced strength of lateral input, as reflected into a reduced facilitation by the flankers, would be an epiphenomenon of the change of contrast gain [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. Alternatively, tRNS effect might be dependent on target-to-flankers distance and reflect a direct modulation of the relative strength of excitatory (E) or inhibitory (I) lateral input from the flankers to a target, depending on which is weaker. In this case, we might expect an effect of tRNS even if tRNS has no effect at all on target perceived contrast.

Observers

In

Stimuli

Stimuli were generated using Matlab Psychtoolbox [START_REF] Brainard | The Psychophysics Toolbox[END_REF][START_REF] Pelli | The VideoToolbox software for visual psychophysics: Transforming numbers into movies[END_REF].

They were formed by a vertical Gabor target patch and, when present, by two collinear Gabor flankers (Figure 8). Each Gabor patch consisted of a co-sinusoidal carrier enveloped by a stationary Gaussian (Eq. 3) and was characterized by its sinusoidal wavelength (), phase (φ), and standard deviation of the luminance Gaussian envelope () in the (x, y) space of the image:

( )

2 2 2 2 , cos x y G x y x e       + -         = +     Eq. 3
with  =  and φ = 0 (even symmetric). Gabor patches had a spatial frequency of 1 cycle per degree (c/deg). Target-to-flankers distance was 6 (wavelengths distance) in Experiment 1 and 3 and 2 in Experiment 2 and 4. Since the thresholds for contrast differ between 6 and 2 in order to sample from the floor to the ceiling, we had to adopt two different ranges of contrasts in the two experiments. Each range has been derived from the literature(Uri [START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | The architecture of perceptual spatial interactions[END_REF] and then adjusted with a pilot experiment. They varied according to eight levels: 0, 0.0020, 0.0028, 0.0039, 0.0055, 0.0077, 0.0105, 0.0150 in Experiment 1 and 3 and 0, 0.0030, 0.0058, 0.0110, 0.0250, 96 0.0410, 0.0800, 0.3000 in Experiment 2 and 4 (see Table 2). The contrast of the flankers was fixed at 0.6. To promote a suppressive effect of the flankers placed at short distances from the target (2), we used a relatively higher range of contrasts with respect to the 6 distance. Note, however, that certain contrast levels in the range .003 to .011 were the same in the two  distances. This allowed us to isolate a specific effect of the flankers, independently on target contrast. Left to right: target-flanker configuration at 2, 6 and single Gabor target.

Procedure

Observers sat in a dark room at a distance of 57 cm from the screen. Viewing was binocular. Stimuli appeared randomly for 100ms to the left or to the right of fixation. The distance from the centre of the screen to the centre of the Gabor configuration was 4 deg.

Observers were required to maintain fixation on the central fixation mark, which was always present except during stimulus presentation, to provide a transient cue for advising observers that the stimulus was present even if, at low contrast, they could not detect it. Observers performed a yes-no task in which they were asked to report whether they could perceive the central target by pressing the response key. The next trial started after 0.5s from the response keypress. Each experiment was devoted to one target-toflankers distance (either 2 or 6) and comprised a repetition of two sessions, each one consisting of 112 trials: 8 target contrast levels × 2 stimulus positions × 7 repetitions. In the first session, the target was flanked by collinear Gabor patches; in the second one, the target was presented alone. Participants performed the two sessions twice, once while they received Sham stimulation and once while they received tRNS. The order of the two configurations sessions was the same in the Sham and tRNS session, but it was counterbalanced across participants. The order of stimulation (Sham vs. tRNS) was also counterbalanced across participants in order to avoid a possible tRNS dependent aftereffects.

tRNS

A battery-driven current stimulator (BrainStim, EMS, Bologna, Italy) delivered high-frequency tRNS through a pair of conductive rubber electrodes inserted in a 5 by 7-cm physiological solution-soaked synthetic sponge. The tRNS consisted of a randomly alternating current of 1.5 mA with a 0 mA offset, whose frequency ranged from 100 to 600Hz. In the main Experiments 1 and 2, the electrode of interest was placed over V1/V2

(Oz) and the other electrode over the vertex (Cz), as in previous studies [START_REF] Accornero | Visual evoked potentials modulation during direct current cortical polarization[END_REF][START_REF] Antal | Direct current stimulation over V5 enhances visuomotor coordination by improving motion perception in humans[END_REF][START_REF] Battaglini | Anodal and cathodal electrical stimulation over V5 improves motion perception by signal enhancement and noise reduction[END_REF]. The experiments 3 and 4 served as a control to test the spatial specificity of stimulation in producing its effect on the contrast gain for the target and/or on E, and I lateral interactions. The only difference with the main experiments was that one electrode was placed over the forehead (between Fpz and nasion) while the reference electrode was kept over the vertex (Cz). tRNS was applied for approximately 12 minutes. It was started at the onset of the first session, and it was stopped at the end of the second session, with no pause between the two experimental sessions. The Sham stimulation consisted of 30 seconds delivered only during the first session. The duration of the fade-in/fade-out period was 15 second for both tRNS and Sham stimulation. At the end of each experimental session we asked the participant to complete a sensation questionnaire [START_REF] Fertonani | What do you feel if I apply transcranial electric stimulation? Safety, sensations and secondary induced effects[END_REF]. Very few participants reported mild skin sensation at the onset of the stimulation, but it disappeared after few seconds. The guessing rate of real/placebo stimulation was at chance levels.

Statistical Analysis

To ascertain that the response to the single and flanked targets depended on Figure 12 shows the Criterion results.

Pooled d's correlated positively with contrast at 6 (R2 =0.64, p < .001) and 2 (R2 =0.46, p < .001), indicating higher sensitivity as contrast increases. The ANOVA on the SC revealed a significant effect of the Contrast Levels: SC became more positive (higher d' in the collinear configuration) with increasing contrast at 6 (F(6,108) = 8.9, p < .001, η2p = .33) and more negative (lower d' in the collinear configuration) with increasing contrast at 2 (F(6,108)=25.3, p < .001, η2p = .58). Moreover, at 6 (Experiment 1) the effect of the Stimulation is significant: tRNS reduced SC at 6 (F(1,18) = 5.9, p = .026 , η2p = .25) independently on the Contrast Levels (Stimulation × Contrast Levels: F(6,108) = .74, p = .62, η2p = .039). At 2 on the other hand, an overall effect of tRNS on SC (reduction of inhibition) was not found (Stimulation: F(1,18) = 2.48, p = .13 , η2p = .12) whereas the Stimulation × Contrast Levels interaction was significant (F(6,108) = 2.21, p = .047, η2p = .11); this indicates that tRNS reduced inhibition, non-significantly in the first (from -.07 to .42, p = .085), and significantly in the second (from -.23 to .44, p = .015) and in the highest level of contrast (from -1.86 to -1.21, p = .01).

Pooled C data correlated negatively with contrast at 6 (R2 = 0.36, p < .001) and 2 (R2 = 0.33, p < .001), indicating less positive criterion as contrast increases. The ANOVA on CC (change in Criterion) didn't reveal neither an effect of Stimulation (6:

F(1,18) = 1.57, p = .23, η2p = .08; 2: F(1,18) = .002, p = .96 , η2p < .001) nor of the interaction between Stimulation × Contrast Levels: (6: F(6,108) = .74, p = .62, η2p =.039; 2: F(6,108) = 2.17, p = .051, η2p = .1), indicating that the stimulation did not affect the criterion.

As Figures 9 and 11 show, not only tRNS reduced either facilitation or suppression at 6 and 2 respectively but, for low contrast values, the effect of tRNS resulted into an inversion of SC sign, in both  sessions. That is, positive SC, at 6, turned into negative whereas negative SC, at 2, became positive. This was confirmed by based on the null hypothesis of 0 SC (Table 3). One-tail t-test to compared whether SC in some contrast levels is significantly different from zero value (no modulation effect).

Discussion

In the present study, we investigated the tRNS effect on either the target contrast

Sensitivity changes (SC), referring to the difference between d' obtained in the collinear and single target (d'collineard'single), are plotted as a function of target contrast with flankers at a distance of 2. Positive values represent facilitation by collinear flankers, whereas negative values represent inhibition. Solid bars indicate confidence interval (95%). tRNS inverts the lateral interaction effect

It has been suggested that the facilitation/suppression of the signal by lateral interactions are the result of the balance between excitatory and inhibitory lateral interactions (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF][START_REF] Chen | Lateral sensitivity modulation explains the flanker effect in contrast discrimination[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination-psychophysics and modeling[END_REF]. Our data seem to indicate that tRNS perturbs this balance. With a target-to-flanker distance of 6, not only tRNS reduced flankers' facilitation (Figure 10) but, at low contrast levels, it changed the sign of SC from positive to negative. Note that this effect of the tRNS on SC is mainly due to an increase of d' by tRNS in the single target configuration and also, to a lesser extent, to a decrease of d' by tRNS in the flanker configuration. With a target-to-flanker distance of 2 (Figure 12) the tRNS modulation not only consists of a reduced suppression by the flankers but also, at lower levels of contrast, tRNS turns suppression into facilitation.

These results indicate a dissociation of the tRNS effect within the range of contrasts levels, relatively low, shared by the two target-to-flanker distances. This strongly suggests that the effect consists of modulation of contextual influences, and not simply of the local detection mechanism.

The effect of the criterion

If the tRNS effect reflects a genuine modulation of visual sensitivity, no difference in the Criterion obtained in the sham and tRNS sessions should be highlighted, regardless of whether the flankers were present or not. Figure 13 shows the criterion obtained as a function of contrast in Experiment 1 and 2 where stimulation was delivered to the occipital lobe. Clearly, the Criterion was more conservative (positive) with single target but, as expected, this effect decreased when the target was more detectable at high contrasts. With both flanker separations, at high levels of contrast there is a small change of Criterion polarity, suggesting an increase of false alarms [START_REF] Zomet | Noise and the Perceptual Filling-in effect[END_REF].

Importantly, there was no effect of tRNS, regardless of Criterion polarity, confirming the hypothesis of a selective effect of stimulation on visual coding mechanisms and the way they are modulated by contextual influences.

In each panel, Criterion C is shown for the single (triangle symbols) and collinear target (dot symbols) presented online with tRNS (grey broken lines) or with Sham (black broken lines). Solid bars indicate confidence interval (95%).

tRNS administered to a control region has no effect When tRNS was delivered, as a control, over the forehead, with the other electrode placed over the vertex (Cz) the stimulation had no effect at all on SC and on the CC. This suggests a genuine effect of tRNS on visual coding and contextual influence mechanisms.

tRNS dependent modulation of E/I balance interpretation

To sum up, behavioural data showed an increase of d' by the flankers at 6 and the decrease of d' at 2, as reflected into a positive and negative SC respectively. These results are consistent with the finding that the flankers facilitate target detection at medium  and low contrast whereas they inhibit target detection at short  and relatively high contrast (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF][START_REF] Chen | Lateral sensitivity modulation explains the flanker effect in contrast discrimination[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination-psychophysics and modeling[END_REF].

It has been suggested that the effect of flankers occurs because low contrast targets and large target-to-flankers separations promote activation of excitatory (E) lateral interactions between target and flankers, whereas relatively high contrast targets and short target-to-flankers separation are appropriate for activating lateral interactions or the summation of target and flankers within the target receptive field (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Adini | Recurrent networks in human visual cortex: psychophysical evidence[END_REF][START_REF] Chen | Lateral sensitivity modulation explains the flanker effect in contrast discrimination[END_REF][START_REF] Chen | Lateral modulation of contrast discrimination: Flanker orientation effects[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination-psychophysics and modeling[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. It should be noted that the highest facilitation with the large separation is found for low contrasts (ranging from .0055 to 110

.015 Michelson contrast), whereas for the same contrast values the effect of flankers was negligible with a short distance. This suggests that target-to flanker distance plays the most relevant role in determining the polarity of contrast effects due to flankers in human observers.

The neurophysiological mechanism accounting for the dissociation in the effects of the contextual influence has received great attention. According to it, contrast detection tasks are mediated by the activation of E, and I subpopulations of neurons in a cortical column, with the ratio between E and I activation increasing as a consequence of two inputs: stimulus contrast (thalamic input) and the lateral input biased versus excitation (Y. [START_REF] Adini | Excitatory-inhibitory network in the visual cortex: Psychophysical evidence[END_REF][START_REF] Chen | Contrast response characteristics of longrange lateral interactions in cat striate cortex[END_REF] Chien [START_REF] Chen | Lateral modulation of contrast discrimination: Flanker orientation effects[END_REF][START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Seriès | The "silent" surround of V1 receptive fields: Theory and experiments[END_REF][START_REF] Stemmler | Lateral interactions in primary visual cortex: A model bridging physiology and psychophysics[END_REF].

We suggest that tRNS might perturb E/I balance. The way tRNS produces this effect has been associated to the way stochastic resonance mechanisms operate (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. tRNS consists of random frequency stimulation that induces random activity into the system; this activity acts as a pedestal to boost the activation of weakly stimulated neurons. When the input signal is too weak and produces a subthreshold neural response, tRNS mediates cooperation between signal and optimal visual noise, with the result of input enhancement, selectively for subthreshold but not suprathreshold response (van der

Groen & Wenderoth, 2016).

As suggested by the concept of stochastic resonance [START_REF] Kitajo | Behavioral Stochastic Resonance within the Human Brain[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF] the input at the threshold level can be better processed within an optimum level of noise compared to without noise. In this framework, the tRNS induced noise serves as a pedestal to increase the sensitivity of the neurons to a given range of weak inputs, and the final effects are related to the functional activation induced by the state of the system. Importantly this result is corroborated by in vitro electrophysiological data [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF] endorsing the hypothesis that electrical RNS of neurons induces facilitation of sodium channels current, at an optimum level of noise for short-term application, via an excitability increase of the stimulated neural system.

We suggest that the modulation by tRNS via stochastic resonance mechanism could account for our three main results: i) tRNS affects sensitivity for the single target only at the low contrasts levels ii) the effect of tRNS on the collinear flankers occurs at both separations and consists in an overall reduction of facilitation with 6 and a more selective reduction of inhibition with 2. iii) At both separations the tRNS inverts the polarity of contextual influences at the lowest levels of contrasts used with the two separations: whereas at 2 the tRNS turns inhibitory contextual influences into facilitatory, at 6 tRNS does the opposite.

The neural mechanism accounting for the tRNS-dependent increase of perceived contrast of the single target when it is low (at 6) might rely on the evidence that tRNS generally boosts weak neural input (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Tang | Making up your mind: Enhanced perceptual decision-making induced by stochastic resonance during non-invasive brain stimulation[END_REF][START_REF] Terney | Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation[END_REF][START_REF] Van Doren | Electroencephalographic effects of transcranial random noise stimulation in the auditory cortex[END_REF]. Given that E/I ratio due to thalamic input increases with the contrast of the isolated target, the ratio would be expected to be higher at 2 than 6. As a consequence, the facilitation of the isolated target resulting from the increase of neural excitability produced by tRNS should occur where the E/I ratio is low, i.e., at 6. In these conditions, we would expect, as we found, an increase of contrast gain for the low contrast target.

With a target of high contrast (Experiment 2) the weight of E and I is strongly biased towards E and tRNS would be ineffective in increasing contrast sensitivity for the isolated target. Our results support the hypothesis of an effect of tRNS based on the modulation E/I ratio.

Moreover, to fit the action of tRNS with that of a stochastic resonance mechanism, we have to accept that both the response to the target (by thalamic input) and to the flankers are modulated by a low/appropriate level of noise. That is, whenever the neural response is weak, it is boosted by the tRNS.

This hypothesis is compatible with the weak tRNS effect obtained when the flankers are 6 apart from the target. At these target-to-flanker separations, as our (Figure 9) and previous data show, lateral input is biased towards excitation [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF][START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF]. Given an excess of lateral excitation, the inhibition would be comparatively weak, and therefore, we would expect tRNS to boost inhibitory connection. Indeed, we found that tRNS slightly reduced facilitation by the flankers.

At 2, the expected effect of the tRNS on lateral input is opposite. The stronger activation of inhibitory lateral input when close flankers are present would reduce the E/I ratio in the cortical column activated by the target. Based on this assumption, by boosting the weaker, excitatory lateral input, the tRNS would produce a reduction of inhibition.

In favour of the modulation of lateral E/I input by tRNS is our third result: the E/I balance perturbation result discussed in paragraph "tRNS inverts the lateral interaction effect". It showed that even when the target contrast matches in the two  distances and therefore there is no change in thalamic input, there is still a boosting effect of lateral interactions by tRNS, but in the opposite direction at the two separations.

It should be remarked that the effects of tRNS, at large separations, may be compatible with the way the polarity of lateral interactions depends on the target contrast in normal vision. It has been shown in previous studies that for target contrast ranging from low to very low with respect to that of the flankers, the lateral input switches from facilitation to inhibition [START_REF] Contemori | Contextual influences in the peripheral retina of patients with macular degeneration[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. Such a contextual modulation contrast-dependent could explain both the reduction of facilitation by tRNS and the switch from facilitation to inhibition at very low contrast (Figure 10). This model however does not explain the tRNS-dependent reduction of inhibition by the flankers at 2 for two reasons: first this mechanism only works for a low contrast range (the contrast range used in this study at 2 is higher than 6) and second tRNS should have had increased the perceived contrast of the isolated target not only at 6 but also at 2, at corresponding contrast levels, but it did not. Therefore, at both separations, lateral interaction modulation by tRNS should be called into cause since the two separations produce opposite modulatory contextual effect by tRNS at corresponding contrast level. At 6 only, the modulation of lateral interactions may also depend on perceived contrast for the isolated target.

A final comment should be made on the evidence (Figure 13) that in Experiment 2 but not 1 tRNS sets observers' criterion to a more conservative value (not significantly). However, it is unlikely that this affects the way tRNS affects lateral interactions since sensitivity does not change [START_REF] Zomet | Noise and the Perceptual Filling-in effect[END_REF].

114

To sum up, we have shown a dissociated of tRNS effect: tRNS can either reduce or increase the modulation that collinear flankers exert on contrast sensitivity of a low contrast target. Overall, tRNS increased the efficiency of whatever lateral interactions are weak: excitatory at short target-flanker separations, inhibitory at medium separations.

The dissociation results from a partially complementary effect of tRNS. At large and facilitatory target-to-flanker separations tRNS increases contrast sensitivity for the low contrast target leading, as it occurs in normal vision and produces a modulation of lateral input towards reduced facilitation or to a switch from facilitation to inhibition. When the target-to-flanker separation is short and inhibitory a target contrast is high, tRNS affects directly the inhibitory lateral interactions reducing its strength.

In conclusion, the evidence that tRNS modulates intracortical lateral interactions at the low level of central visual processing in the human brain can have relevant clinical consequences. tRNS might be used to boost the effect of visual training in restoring lateral intracortical connections in V1 when these are made inefficient by visual disorders such as amblyopia and macular degeneration (Barollo et al., 2017a;[START_REF] Casco | Neural Restoration Training improves visual functions and expands visual field of patients with homonymous visual field defects[END_REF][START_REF] Casco | Specificity and generalization of perceptual learning in low myopia[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF][START_REF] Polat | Improving vision in adult amblyopia by perceptual learning[END_REF], 2009, 2012;[START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF].

Chapter VII. tRNS Boosts Perceptual Learning in Peripheral Vision

In CHAPTER VI we have demonstrated that the tRNS allows to modulate the lateral interactions favouring the feedforward signal over the contextual influences.

Under a training regimen that span over several sessions, this transient property of the tRNS could allow a more rapid and complete restructuring of the lateral interactions than the training alone. Moreover, the tRNS is known to interact with the endogenous activity of the stimulated network increasing the processing of week subthreshold stimuli under the right conditions [START_REF] Fertonani | Transcranial electrical stimulation: What we know and do not know about mechanisms[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. The cellular mechanism of this effect has recently been revealed in an article by [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF] showing that tRNS is able to modulate the activation and inactivation of the Na + channels inducing a stochastic resonance phenomenon in the activity of a subpopulations of the stimulated neurons [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF]. Spatial integration in early visual areas strongly influences visual crowding (N. [START_REF] Chen | Cortical reorganization of peripheral vision induced by simulated central vision loss[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. By favouring the feedforward signal over the contextual influences in the early visual areas the tRNS might reduce the detrimental pooling of features among similar neighbouring elements during an identification task.

The study that I will present in this chapter has investigated the possibility of reducing visual crowding in normally sighted subjects through the combined use of PL and tRNS.

Introduction

In peripheral vision, identification of targets among neighbouring elements is much less efficient than in foveal vision, an effect known as visual crowding [START_REF] Whitney | Visual crowding: A fundamental limit on conscious perception and object recognition[END_REF]. Crowding limits peripheral reading and peripheral letter identification (S. T [START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF][START_REF] Mansfield | Psychophysics of reading. XV: Font effects in normal and low vision[END_REF] and, while almost absent in healthy foveal vision [START_REF] Huurneman | A systematic review on foveal crowding in visually impaired children and perceptual learning as a method to reduce crowding[END_REF], it represents a major difficulty for clinical populations suffering from amblyopia [START_REF] Levi | Improvement in Vernier acuity in adults with amblyopia: Practice makes better[END_REF] or central vision loss (macular degeneration (MD), Stargardt syndrome, rods-cone dystrophy, etc. [START_REF] Mansfield | Psychophysics of reading. XV: Font effects in normal and low vision[END_REF]. Perceptual learning (PL), the improvement in a perceptual task as a product of repeated practice (Manfred. [START_REF] Fahle | Perceptual learning[END_REF][START_REF] Garner | Principles of Perceptual Learning and Development[END_REF]Sagi, 2011a), is a promising technique that has found its way into clinical practice due to its non-invasive and inexpensive approach (Campana & Maniglia, 2015a). Several studies tested the efficacy of PL in reducing crowding, both in healthy and clinical populations [START_REF] Astle | The effect of normal aging and age-related macular degeneration on perceptual learning[END_REF][START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF][START_REF] Chung | Learning to identify crowded letters: Does the learning depend on the frequency of training[END_REF]Hussain, Webb, Astle, & McGraw, 2012;[START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Yashar, Chen, & Carrasco, 2015). However, most of these protocols required a large number of sessions, and in some cases the improvement remained specific to the trained task.

Recently, non-invasive brain stimulation has been used, alone or coupled with PL, to enhance visual abilities [START_REF] Camilleri | The application of online transcranial random noise stimulation and perceptual learning in the improvement of visual functions in mild myopia[END_REF][START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Pirulli | The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation[END_REF][START_REF] Thompson | Brain Plasticity in the Adult: Modulation of Function in Amblyopia with rTMS[END_REF]. In particular, transcranial random noise stimulation (tRNS), in which a weak electric current is delivered through the scalp on a cortical region at random frequencies, has shown promising results in boosting PL and reducing the number of sessions needed to observe significant improvements [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. In general, tRNS appears to boost both the early (within-session - [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF] [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF]. The working hypothesis of our study is that stimulation on early visual cortex would promote learning of low-level features and trigger a trickle-down effect downstream of the visual processing, providing higher visual areas computing letter discrimination (e.g., the visual word form area (VWFA) in the left fusiform gyrus, [START_REF] Cohen | Visual Word Recognition in the Left and Right Hemispheres: Anatomical and Functional Correlates of Peripheral Alexias[END_REF][START_REF] Cohen | The visual word form area. Spatial and temporal characterization of an initial stage of reading in normal subjects and posterior splitbrain patients[END_REF] with a better input. Alternatively, tRNS might promote generalization of learning by reducing sensory adaptation, a phenomenon known to limit transfer of learning [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF][START_REF] Harris | Effects of spatiotemporal consistencies on visual learning dynamics and transfer[END_REF]. Consistently, Campana and colleagues (2016) showed that tRNS over V5 (a cortical area involved in the processing of visual motion, diminished the perceived duration of the motion after-effect, while tRNS over early visual areas allowed transfer of learning to an untrained visual task (visual acuity) [START_REF] Moret | Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia[END_REF]. Visual crowding is known to compromise object discrimination in general and letter/word discrimination in particular. In fact, reading under a crowded condition is slower and less accurate. The exact location at which the pooling of features among neighbouring items happens is still debated. Some authors propose that crowding occurs when elements are grouped into wholes, a process reflected in EEG by the N1 component (J. [START_REF] Chen | Attentiondependent early cortical suppression contributes to crowding[END_REF][START_REF] Chicherov | Neural correlates of visual crowding[END_REF][START_REF] Tripathy | Large crowding zones in peripheral vision for briefly presented stimuli[END_REF] while others place the neural locus of crowding at an early cortical site, such as V1 or V2 [START_REF] Freeman | Substitution and pooling in crowding[END_REF][START_REF] Shin | Crowding, visual awareness, and their respective neural loci[END_REF]). [START_REF] Levi | Crowding-An essential bottleneck for object recognition: A minireview[END_REF] review reconciles this in a multi-stage model where crowding occurs at both, the detection of simple features (early lateral interactions) and integration of features downstream from V1. Online tRNS is particularly effective in enhancing performance when the stimulus is sub-threshold and only if the stimulation is applied over the neural population involved in the task (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Tang | Making up your mind: Enhanced perceptual decision-making induced by stochastic resonance during non-invasive brain stimulation[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. In our training, we adopted an adaptive staircase procedure that moved up and down while tracking the threshold. This way, both supra and sub-threshold trials were interleaved and non-independent from each other since the level of each trial was chosen on the basis of all previous responses. We chose an adaptive procedure in order to ensure an adequate level of difficulty throughout the training and also to better exploit the effects of tRNS given its interaction with the task difficulty. In fact, while anodal and cathodal tDCS are mostly used before the task to profit from the after-effect of the stimulation (increased or reduced cortical excitability) [START_REF] Clayton | Brain stimulation: Neuromodulation as a potential treatment for motor recovery following traumatic brain injury[END_REF][START_REF] Fertonani | Transcranial electrical stimulation: What we know and do not know about mechanisms[END_REF] we wanted to profit from both the ongoing modulation in the signal-to-noise ratio during the task and the general increase in the cortical excitability driven by the online tRNS, which is supposedly the best choice to induce cumulative neuroplastic changes over multiple sessions (K.-A. [START_REF] Ho | Transcranial random noise stimulation: A new approach to stimulating the brain[END_REF]. Here, we aimed to test whether tRNS over the occipital cortex boosts PL during crowding reduction training. Moreover, in order to test whether tRNS increases generalization of learning, as observed in other cognitive [START_REF] Cappelletti | Transfer of Cognitive Training across Magnitude Dimensions Achieved with Concurrent Brain Stimulation of the Parietal Lobe[END_REF][START_REF] Looi | Transcranial random noise stimulation and cognitive training to improve learning and cognition of the atypically developing brain: A pilot study[END_REF] and perceptual training studies [START_REF] Camilleri | The application of online transcranial random noise stimulation and perceptual learning in the improvement of visual functions in mild myopia[END_REF][START_REF] Chung | Size or spacing: Which limits letter recognition in people with age-related macular degeneration?[END_REF][START_REF] Moret | Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia[END_REF], we tested five transfer tasks before and after the training. Results indicate that coupling tRNS to the early visual cortex with PL of a peripheral crowding reduction task is effective in boosting between-session learning but does not increase the transfer of learning to untrained visual functions respective to PL alone (Sham condition).

Materials and Methods

Apparatus.

Stimuli were displayed on a 17" Dell M770 CRT monitor with a refresh rate of 60

Hz. All stimuli were produced using the Psychtoolbox toolbox [START_REF] Pelli | The VideoToolbox software for visual psychophysics: Transforming numbers into movies[END_REF] in MATLAB R2012a. The monitor (1024 × 768 pixels) was placed 57 cm in front of the participants and had a spatial resolution of 1.9 arcmins per pixel. Mean luminance was 47.6 cd/m 2 , as measured with a Minolta CS110 (Konica Minolta, Canada). A chin-and-head rest was used to keep the head position fixed, and the viewing was binocular. The experiment was conducted in a dark room.

Participants.

Thirty-two participants (17 females) with normal or corrected-to-normal vision took part in the study (mean age 25, range 20 -32 years). They were randomly assigned to one of two groups (tRNS or sham). All participants gave their written informed consent prior to their inclusion in the experiment and received compensation for their participation. The study was conducted in accordance with the Declaration of Helsinki (1964). The experimental protocol was approved by the ethical committee at Centre National de la Recherche Scientifique with our institutional review board (CPP, Comité de Protection des Personnes, protocole 13018-14/04/2014).

Visual Acuity Procedure.

In order to define the size of the stimuli for the crowding task, we first measure VA individually for each participant. A central cross was displayed in the centre of the screen, and the participants were asked to fixate it and to identify a white single letter presented at 8° of eccentricity onto a black background. In order to avoid eye movements, stimulus position was randomized in a left/right manner and the presentation time was kept short (50 ms). In the absence of an eye-tracker, the use of these precautions greatly reduces (but not completely excludes) the impact of eye movements in the study. The target letter was randomly selected from a subset of 9 uppercase Sloan letters (D, S, R, Z, N, K, H, V, and C [START_REF] Pelli | the Design of a New Letter Chart for Measuring Contrast Sensitivity[END_REF]. The size of the letters varied according to a one-up three-down adaptive staircase procedure [START_REF] Levitt | Transformed Up-Down Methods in Psychoacoustics[END_REF], with 0.1 log unit steps, leading to an 80% of correct letter identification threshold. An experimental run ended after 12 reversals or 100 trials. Each run typically lasted 60 -90 trials. The threshold was obtained by averaging the last 6 reversals. If the number of 12 reversals was not reached after 100 trials, the first six reversals were always discarded, and only the remaining ones were averaged. To reduce temporal uncertainty, a 50 ms sound was played prior to each target onset. At the end of each trial, participants reported verbally the letter to the experimenter who was sitting outside the experimental room in a position from where he was unable to see the monitor. The experimenter reported the answer by pressing the corresponding key on the keyboard.

Crowding Procedure.

The crowding procedure was similar to the visual acuity procedure with the difference that in each trial a trigram, rather than a single letter, was presented (see Figure 14, leftward panel). Participants were asked to identify the central letter of a peripheral trigram appearing at 8° of eccentricity (calculated from the centre of the middle letter) either on the left or right of the central fixation. The trigrams were composed of randomly selected Sloan letters among a group of nine (D, S, R, Z, N, K, H, V, and C) with no repetitions within the same trigram. Participants were then asked to report the central letter of the trigram and to ignore the two flanking letters. Stimulus position was left/right randomized, and the presentation time was 50 ms. To avoid any influence of the letter size on the task, we followed the common practice to increase the size of the letter 30 % more than the acuity threshold of the participants [START_REF] Barollo | Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia[END_REF][START_REF] Hussain | Perceptual Learning Reduces Crowding in Amblyopia and in the Normal Periphery[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]. The spacing among the three letters was varied according to a one-up three-down adaptive staircase procedure [START_REF] Levitt | Transformed Up-Down Methods in Psychoacoustics[END_REF]. The experimental setup and the stopping rule for the staircase were identical to the one adopted for the VA procedure. The measured threshold defined the Critical Spacing (CS) for letter recognition. At the end of each trial, participants verbally reported the letter to the experimenter who registered the answer on the keyboard. Since crowding is particularly relevant for reading, several studies used letters as stimuli. Some of these studies calculated CS as the Letter-to-Letter distance, some others as the centre-to-centre distance between letters (Z. Hussain et al., 2012). An eventual overlap between target and flankers might reduce the validity of the measure since the task would then become a figure-ground segmentation task. On the other hand, measuring letter-to-letter spacing introduces inter-individual variability since the letter size varied according to the threshold of the participant, and the bigger the letter, the larger the centre-to-centre 123 distance. We chose to avoid overlapping by defining CS as the letter-to-letter distance required by the participant for an 80% discrimination accuracy.

Transfer tasks.

VA was measured to calibrate letter size in the crowding task at the same eccentricity to ensure that the size of the letter was large enough not to affect critical space measurement. VA before and after training was also used to determine whether learning transferred to an untrained but related task. In addition to VA, we measured four other transfer tasks (Figure 14): crowding at 12° (retinal position transfer), vertical crowding at 8° (orientation transfer), 12° (retinal position and orientation transfer), and VA at 12° (retinal and task transfer) (see Figure 14, upper panel). The procedure was the same as for crowding and VA. For pre-tests and post-tests, no brain stimulation (real or sham) was applied.

124 Training.

The training was conducted on the horizontal crowding task (see Figure 14 four training sessions, one per day during four consecutive days. Each daily session consisted of six blocks, for a total of 24 blocks. Each session lasted approximately 30 min (~5 min per block). We did not provide direct auditory or visual feedback. However, participants were aware of the one-up three-down procedure so they could infer their performances from the trial-to-trial variation in spacing.

tRNS Stimulation.

Participants in Group 1 (PL+tRNS) were trained with concomitant electrical brain stimulation, while participants in Group 2 (PL alone) performed the training with sham stimulation. High-frequency tRNS was delivered using a battery-driven stimulator (BrainSTIM, EMS) through a pair of saline-soaked sponge electrodes. The tRNS consisted of an alternating current of 1.5 mA intensity with a 0 mA offset and maximal current density of 0.094 mA/cm 2 . This type of stimulation is characterized by an alternating current of random intensity with zero offset and values ranging from -1.5 mA to 1.5 mA, with frequencies of fluctuation distributed across a range of 100-640 Hz with zero-mean (same as in [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. The total duration of stimulation was 30 min to cover the entire training session. The active electrode had an area of 16 cm 2 and was placed over the occipital cortex measured at 3 cm above the inion. The reference electrode had an area of 27 cm 2 and was placed on the vertex. The current density was maintained well below the safety limits (always below 1 A/m 2 ; [START_REF] Poreisz | Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients[END_REF]).

The electrodes were kept in place with non-conductive elastic bandages. For sham stimulation, we applied sponge electrodes in the same manner. At the beginning of the sham stimulation, the current was ramped up over 15 seconds and then tapered off with an equal amount of time. The same procedure was performed at the end of the stimulation. To better understand the diffusion of the current through the cortex and the size of the stimulated area according to this setup, we calculated and visualized the expected current density with the SimNibs software (Saturnino et al., 2015, Figure 15) which confirmed the current density was mostly localized in early visual areas. This estimate shows that the highest current density corresponded to the early visual cortices.

Data analysis.

Visual inspection of the data suggested a possible inhomogeneity in the variance of the training data between the two groups. We confirmed this by means of a Bartlett test (Bartlett's K-squared = 43.57, df = 11, p-value = 0.001), therefore we adopted nonparametric (distribution-free) inferential statistical methods. The Aligned Rank

Transform for nonparametric factorial ANOVA [START_REF] Wobbrock | The aligned rank transform for nonparametric factorial analyses using only anova procedures[END_REF]) allows a non-parametric analysis of variance to be conducted on factorial models with fixed and random effects for repeated measures. For the main effects, we performed this analysis using the "art" function of the ARTool package available at CRAN [START_REF] Kay | {ARTool}: Aligned Rank Transform for Nonparametric Factorial ANOVAs[END_REF]. To test for interactions, we used another nonparametric ART test specifically developed to test for interactions of repeated measures design with one 'within' and one 'between' factors as described by [START_REF] Beasley | Aligned Rank Tests for Interactions in Split-Plot Designs: Distributional Assumptions and Stochastic Heterogeneity[END_REF] and [START_REF] Higgins | An aligned rank transform test for interaction[END_REF]. This test was performed with the "npIntFactRep" function within the homonymous R package [START_REF] Feys | Package 'npIntFactRep[END_REF]. However, because this package does not allow for testing interactions with more than one 'within' factor, we only ran interaction tests for the pre-post comparison in the trained and transfer tasks and for the between-session learning but not for the between blocks learning. All the comparisons were pre-planned, and we, therefore, reported their statistical significance without correcting for multiple comparisons. Given the number of performed comparisons ( 21), if all the null hypotheses were true, we should expect only 1.05 (5%)

of the comparisons to have uncorrected P values less than 0.05. This consideration is important to better evaluate the strength of the reported results [START_REF] Rothman | No adjustments are needed for multiple comparisons[END_REF].

Results

The reduction in critical space in the trained task was evaluated between sessions.

We also evaluated the transfer of learning to other untrained tasks between pre-and posttest.

PL and tRNS effect between sessions.

A two way Aligned Rank Transformation ANOVA performed on Group (tRNS vs Sham) and Sessions (pre-test, day1, day2 day3, day4, post-test) showed a main effect of Sessions (F [5,150] = 26,24, p < 0.0001) and an interaction between Group and Sessions (F [5,150] = 2.72, p = 0.022). It suggested that the tRNS group improved more than the Sham. However, since directly comparing levels of factors in a non-parametric model is not advised [START_REF] Benavoli | Should we really use post-hoc tests based on mean-ranks? Retrieved from[END_REF][START_REF] Kay | {ARTool}: Aligned Rank Transform for Nonparametric Factorial ANOVAs[END_REF], we did not run post-hoc analysis on this data. Trained and transfer tasks.

We performed an Aligned Rank Transformation ANOVA for the trained and each of the transfer tasks. We tested Group (tRNS vs. Sham) and Training (pre-test vs. posttest) as factors plus their interaction. Results are reported below:

Discussion

In the present study, we compared two training protocols for reducing visual crowding, one in which participants received online electric brain stimulation (i.e., tRNS) and the other in which no electric current was delivered during the task (i.e., sham). After four days of training, both groups improved in the trained task, but the brain stimulation group reduced crowding significantly more than the sham group. Moreover, both groups showed transfer of learning to another retinal position and to visual acuity (VA). This is the first evidence of the efficacy of tRNS in boosting PL to improve performance during a peripheral vision task, a result consistent with previous studies in foveal vision [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF].

Effect of tRNS on learning

Coupling tRNS and PL resulted in greater learning than PL alone, as observed in previous studies [START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. The mechanisms underlying tRNS are still not completely understood: co-occurrence of stimuli in close succession and the temporal summation of small depolarizing currents induced by the random sub-threshold stimulation (D. Terney et al., 2008) might facilitate the depolarization of cortical neurons, producing Hebbian LTP-like changes in the network that processes the task [START_REF] Cappelletti | Transfer of Cognitive Training across Magnitude Dimensions Achieved with Concurrent Brain Stimulation of the Parietal Lobe[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF][START_REF] Snowball | Long-term enhancement of brain function and cognition using cognitive training and brain stimulation[END_REF], improving in-turn performance over time (D. Terney et al., 2008). An alternative hypothesis is that the high stimulation frequency (100-640 Hz)

prevents the homeostasis of stimulated neurons (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. Indeed, tRNS seems to induce greater improvements in performance than anodal tDCS, where the current flows constantly along the same direction, despite the fact that both stimulations produce an increase in cortical excitability [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Pirulli | The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation[END_REF].

Finally, the introduction of external noise from the electric stimulation might alter the overall level of cortical excitability and the probability of discharge of every single unit, modifying in-turn the signal-to-noise ratio during stimulus processing (A. [START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. A model of stochastic resonance was previously proposed to explain the nonlinear effects found in brain stimulation studies [START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF]. This model takes into account the interaction between internal activity, externally induced noise, and stimulus-driven activity, predicting that in the case of a low target signal, an "adequate amount" of external noise (in our case the tRNS) can enhance the signal (alone) above the threshold. Some very recent studies investigated the relationship between the intensity of the tRNS stimulation and the performance in a visual task [START_REF] Tang | Making up your mind: Enhanced perceptual decision-making induced by stochastic resonance during non-invasive brain stimulation[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. In particular, van der Groen & Wenderoth (2016) showed that the window of maximum efficacy in terms of stochastic resonance for a subthreshold visual stimulus has a peak at current intensities around 1 mA for an electrode of 35 cm 2 placed over the occipital cortex. This intensity is indeed lower than the one we used, but also the eccentricity of the task and the contact medium used to deliver the stimulation were different, and thus the expected peak efficacy of the stimulation is shifted towards higher intensities. Moreover, in other domains like auditory perception, improvement in perception attributable to stochastic resonance was achievable with intensities higher than 1 mA [START_REF] Rufener | Transcranial Random Noise Stimulation (tRNS) Shapes the Processing of Rapidly Changing Auditory Information[END_REF]. Given all these considerations, we suggest that the tRNS group in the present study might have benefited from both a general increase in cortical excitability and synaptic plasticity as well as a better signal-to-noise ratio throughout the course of the training, improving the performance between sessions, as suggested by the significant interaction.

Learning retention over time Although our original design did not include any follow-up recording, we decided at the end of the experiment to collect additional data to further test learning retention in the trained task (horizontal crowding at 8°) over time. Ten participants (five in each group) were tested in follow-ups after three months. Because of this small sample size, the results of this follow-up experiment are tentative and are only succinctly reported here (the interested reader can, however, find all the details in the supplementary material). Interestingly, in this subsample, only the tRNS group improved significantly between pre and post-test, in agreement with the main finding of the study, which shows a larger learning effect for this group. For the sham group, despite a tendency to improve, the low statistical power due to the small sample size may have led to a "false negative" finding. The main observed effect was that, after three months, the tested subjects had substantially lost their learning benefit. This is in contrast with previous studies that showed long-term learning retention after a crowding training (Z. Hussain et al., 2012).

Although this result might have been influenced by extreme values, it opens up an important reflection. It is possible that even if tRNS was able to speed up learning over a short number of trials, a larger number of blocks or a different distribution of the training sessions needed to achieve a durable improvement in the task. Indeed, the number of trials used in our training is much lower than in previous studies. Given that consolidation of learning is a central aspect for future applications of PL in healthy and clinical populations, further research is needed to better determine the positive and negative aspects of tRNS over time.

Effect of tRNS on transfer

The tRNS group did not show a greater transfer of learning to untrained visual tasks with respect to the sham group. An argument in favour of expecting a bigger transfer of learning for the tRNS group comes from the evidence that PL specificity can be overcome by removing the sensory adaptation that emerges after prolonged exposure to the same training configuration [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF]. Similarly, the randomly changing electric field induced by tRNS might prevent stimulated neurons from homeostasis, increasing their activity and thereby inducing a greater generalization. Indeed, Campana et al. ( 2016) showed a similar effect of reduced adaptation to motion for tRNS delivered over V5, while [START_REF] Moret | Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia[END_REF] reported generalization to VA for a contrast detection training coupled with tRNS on the early visual cortex. The reason why we did not observe this effect might be related to the difficulty of the training we adopted.

According to the stochastic resonance model [START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF], the interaction between task difficulty and intensity of the stimulation produces an inverted U shaped curve of performance, and thus we opted for an adaptive task that could guarantee an adequate level of difficulty through the whole training regimen. Prolonged training at threshold ('difficult' training) was found to prevent learning from transferring to other retinal positions or tasks (S.-C. Hung & Seitz, 2014). Easy trials during training, on the other hand, restored transfer of learning, consistent with the reverse hierarchy theory [START_REF] Ahissar | Task difficulty and the specificity of perceptual learning[END_REF], 2000;[START_REF] Hochstein | View from the top: Hierarchies and reverse hierarchies in the visual system[END_REF], according to which the difficulty of the task induces a shrinking of the attentional window and an increase in learning specificity. Since the beneficial effects of the tRNS are specific for difficult training conditions, this might prevent the stimulation from altering, in a positive or negative way, the amount of generalization. However, the relationship between training and generalization of learning appears complex and might involve modifications in cortical areas beyond the ones we targeted with our stimulation [START_REF] Maniglia | Towards a whole brain model of Perceptual Learning[END_REF].

tRNS and Crowding

Similarly to previous experiments combining visual PL and brain stimulation, our tRNS targeted the occipital cortex [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF] and therefore mostly stimulated early visual areas (i.e., V1, V2/V3). The cortical substrate of crowding is a debated topic in vision science, with some studies implicating early cortical loci [START_REF] Tripathy | Large crowding zones in peripheral vision for briefly presented stimuli[END_REF] and other higher-level regions [START_REF] Chicherov | Neural correlates of visual crowding[END_REF][START_REF] Ronconi | The neural origins of visual crowding as revealed by event-related potentials and oscillatory dynamics[END_REF]. It is likely that the reduction of crowding observed in PL studies results from neural changes at different levels of the visual processing hierarchy. However, the evidence that tRNS on occipital cortex induced a greater reduction of crowding is in line with the involvement of early cortical loci. Moreover, unlike previous studies [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF], we stimulated upstream of the visual areas where the readout module for trained task (VWFA) is supposedly located. In this way, by applying the stimulation to some cortical areas involved in the early visual processing, we can potentially trigger a trickle-down effect that affects learning and discrimination of complex stimuli at later stages. However, given the absence of a control position for the stimulation, we cannot conclude that this result is specific for the protocol we adopted.

We hope that future researchers will clarify this point.

Comparison with previous studies on crowding reduction.

Previous studies reported crowding reduction through PL (S. T L [START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF][START_REF] Huckauf | How odgcrnwi becomes crowding: Stimulus-specific learning reduces crowding[END_REF][START_REF] Hussain | Perceptual Learning Reduces Crowding in Amblyopia and in the Normal Periphery[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]Sun, Chung, & Tjan, 2010;[START_REF] Xiong | Perceptual learning eases crowding by reducing recognition errors but not position errors[END_REF]. [START_REF] Chung | Learning to identify crowded letters: Does it improve reading speed?[END_REF] showed a reduction of crowding of 38% (but no transfer to other tasks, i.e., reading speed). Hussain and colleagues (2012) trained adult amblyopic patients (in the fovea) and healthy participants (4° of eccentricity) on a crowding task and reported similar reduction of critical space between the two groups (~20%) but no transfer to VA. [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF] used a paradigm based on lateral masking and reported a transfer of learning to crowd reduction of about 16%. A similar paradigm used in AMD patients did produce improvements in VA, but not in crowding reduction [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. More recently, Yashar and Carrasco (2015) showed that short training (600 trials) could reduce critical space of 32%. Our training, constituted by an average of 1600 trials, showed a similar reduction for the sham group (26%), but twice that amount for the tRNS group (63%). Moreover, it is worth noting that, on average, participants in the sham group reached their plateau at the end of the third day of training, while participants in the tRNS group reduced their critical space until the last session (Figure 16).

Recently, Zhu and colleagues (2016) trained a group of healthy participants in a crowding task and reported a reduction of about 68% after 1700 trials. However, differences in the task (orientation discrimination task in their study vs. letter identification in the present one) and paradigm (fixed flanking distance and staircase-onorientation discrimination accuracy in their study vs staircase-on-flanking distance in the present one) make the comparison between their results and ours less straightforward. In general, crowding can be reduced either by training on critical space reduction or by improving target identification for a fixed flanker distance.

The coupling of transcranial electrical stimulation and PL has shown to increase learning when compared with PL alone, offering a fast and effective method to improve peripheral visual functions. Future studies should verify its efficacy in clinical populations that might get practical advantages from crowding reduction, such as patients with amblyopia or central vision loss [START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF]. At the same time, its effect on brain networks should be further examined to increase our understanding in order to improve transfer and learning retention over time. Nonetheless, the present results support the hypothesis that tRNS is a promising tool to improve visual training outcomes in general. These findings have potential implications for vision enhancement in both healthy individuals' periphery and patients suffering from central vision loss who cannot undergo the long training sessions typically needed in classic PL paradigms [START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF].

Introduction

Macular Degeneration (MD) is a visual disease characterized by central vision loss. In the case of bilateral central blindness, patients with MD start to use a peripheral retinal locus (PRL) as a replacement for the fovea. However, fixation with this locus is unstable and the processing of visual details poor [START_REF] Hogg | Visual function and dysfunction in early and late age-related maculopathy[END_REF][START_REF] Macedo | Investigating unstable fixation in patients with macular disease[END_REF]. The functional organization of the periphery is very different compared to the fovea. For example, the receptive fields are larger (V. [START_REF] Virsu | Visual resolution, contrast sensitivity, and the cortical magnification factor[END_REF], the range of the contextual influences is wider [START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF], the distribution of photoreceptor is different [START_REF] Elsner | Distribution differences of macular cones measured by AOSLO: Variation in slope from fovea to periphery more pronounced than differences in total cones[END_REF], the parvocellular / magnocellular ratio is decreased [START_REF] Azzopardi | Uneven mapping of magnocellular and parvocellular projections from the lateral geniculate nucleus to the striate cortex in the macaque monkey[END_REF], the sensitivity to motion is increased, and the contrast sensitivity at low spatial frequencies is increased while at high spatial frequency is decreased [START_REF] Virsu | Temporal contrast sensitivity and cortical magnification[END_REF][START_REF] Wright | Spatiotemporal contrast sensitivity and visual field locus[END_REF]. Due to these differences, tasks that require a stable fixation and detailed spatial processing are very limited in the periphery, which renders basic everyday activities very hard to perform [START_REF] Battista | Visual function: The problem with eccentricity[END_REF]. The emergence of the scotoma is a slow process that occurs in phases, sometimes with acute episodes, but generally in the course of months or years [START_REF] Crossland | Preferred retinal locus development in patients with macular disease[END_REF][START_REF] Gheorghe | AGE-RELATED MACULAR DEGENERATION[END_REF]Riss-Jayle et al., 2008a). In this lapse of time, spontaneous changes take place in the functional structure of the patients' visual system [START_REF] Cheung | Functional and cortical adaptations to central vision loss[END_REF]. These changes are based on transient short-term plasticity mechanisms that maintain the homeostasis of the system and are already present in normal vision (Baseler et al., 2011a;[START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF]. The exact mechanism of action of this spontaneous adaptation, as well as its psychophysical correlates, are not clear yet and are difficult to isolate from the effects of use-dependent plasticity that takes place over time in the PRL (Contemori et al., 2019;Maniglia et al., 2018). The comparison with the periphery of subjects with normal vision has shown that MD patients have better perception of mirror symmetries outside the scotoma (Casco, De Stefani, Pinello, Sato, & Battaglini, 2015), lower critical space in visual crowding tasks (Susana T.L. [START_REF] Chung | Size or spacing: Which limits letter recognition in people with age-related macular degeneration?[END_REF], and illusory contours [START_REF] De Stefani | Illusory contours over pathological retinal scotomas[END_REF]. On the contrary, scene discrimination [START_REF] Musel | Residual abilities in age-related macular degeneration to process spatial frequencies during natural scene categorization[END_REF]Ramanoël et al., 2018), visual acuity and contrast sensitivity for medium and high spatial frequencies are worse [START_REF] Hogg | Visual function and dysfunction in early and late age-related maculopathy[END_REF]. Generally, it is difficult to say whether the spontaneous mechanisms of adaptation produce more positive or negative effects on patients' vision.

In any case, the spontaneous adaptation is not sufficient to counteract the action of the degeneration (Lemos, Pereira, & Castelo-Branco, 2016b;[START_REF] Ogawa | White matter consequences of retinal receptor and ganglion cell damage[END_REF]. The usedependent cortical reorganization also has a limited impact on the visual function of the patients and is especially evident in the strengthening of connections towards the extrastriate visual areas involved in objects and scenes recognition. [START_REF] Beer | Age-related changes in gray and white matter microstucture of patients with macular dystrophies and healthy controls as revealed by DTI[END_REF][START_REF] Hernowo | Morphometric analyses of the visual pathways inmacular degeneration[END_REF][START_REF] Malania | Compromised integrity of central visual pathways in patients with macular degeneration[END_REF][START_REF] Ogawa | White matter consequences of retinal receptor and ganglion cell damage[END_REF].

Moreover, an extensive remapping of the LPZ in humans seems out of the question (Baseler et al., 2011b). Although recent research into the use of genetic therapies and retinal implants have made great strides, there are currently no therapies available to restore damaged foveal vision [START_REF] Gehrs | Age-related macular degeneration -Emerging pathogenetic and therapeutic concepts[END_REF][START_REF] Makin | Four technologies that could transform the treatment of blindness[END_REF]. Current treatment options focus on the use of visual aids coupled with various types of training that allow the patient to partially compensate for the loss of central vision [START_REF] Maniglia | Rehabilitation approaches in macular degeneration patients[END_REF]. Recently, attention has been given to the use of training based on perceptual learning tasks that improve the residual vision in the PRL [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018;[START_REF] Plank | Perceptual learning in patients with macular degeneration[END_REF]. Among the various tasks proposed, Maniglia and colleagues (2016) tested the effectiveness of training based on a lateral masking task promoting contrast sensitivity in the PRL. This training, in its two temporal alternatives forced-choice variant, allowed not only an improvement in the trained task, but also a transfer of learning to visual acuity, visual crowding, and higher spatial frequencies compared to those trained. It should be emphasized that although the training was carried out in the monocular condition, even the untrained eye showed a significant transfer of learning at the end of the training [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. The particularity of this task is to specifically train the lateral interactions between receptive fields in the primary visual cortex, that are responsible for the early stages of integration / segmentation of the visual scene (U Polat & Sagi, 1994;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. To do this a collinear triplet of Gabors, whose distance is manipulated between blocks, is used to train the patients. The central Gabor target varies in contrast in an adaptive manner based on the performance of the subject. The contrast of the two flanking Gabor is usually very high and is kept fixed throughout the procedure.

The distance between the flanks and the target is calculated in multiples of the wavelength of the trained spatial frequency, the λ. When the separation is very small, the flankers produce an inhibitory effect which translates into an increase in the contrast threshold necessary to correctly detect the target with respect to when the latter is presented in isolation or with orthogonal flankers. With increased separations, the contrast threshold decreases and the flankers' effect becomes facilitatory [START_REF] Cass | The mechanisms of collinear integration[END_REF][START_REF] Polat | The architecture of perceptual spatial interactions[END_REF]. Although both are allowed, the orthogonal configuration is a preferable baseline than the single Gabor since it allows to control for the decrease in spatial and temporal uncertainty given by the presence of the flankers (Uri [START_REF] Polat | Functional architecture of long-range perceptual interactions[END_REF][START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination--psychophysics and modeling[END_REF]. Through the logarithm of the ratio between the collinear and the orthogonal configurations, it is possible to estimate the threshold modulation elicited by the collinear context. The neural substrate of this effect lies in the horizontal connections between columns in the early visual cortex. The repeated practice with this task at various λs allows for a restructuring of the facilitatory and inhibitory horizontal connections involved in the early stages of visuo-spatial integration. After practice, there is an increase in the signal-to-noise ratio for the trained stimuli, which in turn facilitate the readout of the higher visual areas (Z.-L. [START_REF] Lu | Perceptual learning retunes the perceptual template in foveal orientation identification[END_REF][START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]U. Polat et al., 2004a;U Polat & Sagi, 1994;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. Furthermore, the introduction of a feedback after the answer increase the involvement of the frontal areas during the training with a consequent strengthening of the perceptual learning through reward-associated learning [START_REF] Gilbert | Brain States: Top-Down Influences in Sensory Processing[END_REF][START_REF] Herzog | The role of feedback in learning a vernier discrimination task[END_REF][START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. The use of the lateral masking paradigm as a visual training, is indicated in all cases where the pattern of lateral interactions might be altered [START_REF] Barollo | Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia[END_REF]Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Campana | Editorial: Improving visual deficits with perceptual learning[END_REF]; U. Polat et al., 2004a).

In the normal visual periphery, the range of interactions in the PRL is greater than that of the fovea of which it should be the functional substitute [START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Shani | Eccentricity effects on lateral interactions[END_REF]. In fact, in the fovea the typical distance at which inhibition is found is between 1 and 2λ. On the other hand, between 3 and 4λ there is facilitation, while above 6λ there is no interaction.

Figure 18. The range of lateral interactions in the paracentral vision.

Illustration of the range of contextual influences as expected in the paracentral vision.

In the periphery, the range varies according to the eccentricity but generally there is inhibition up to 2-3λ and facilitation up to 8-12λ [START_REF] Giorgi | Facilitation of contrast detection in near-peripheral vision[END_REF][START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Shani | Eccentricity effects on lateral interactions[END_REF]. Training the PRL with this type of task at increasing spatial frequencies as well as improving the sensitivity to contrast for the target could allow a restructuring of the lateral connections making them more performing in visual acuity tasks and potentially more similar to those of the fovea [START_REF] Maniglia | Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning[END_REF]U Polat & Sagi, 1994;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. The study presented in CHAPTER V showed that the patients had an altered pattern of facilitation and inhibition in respect to the controls tested at the same eccentricity (Contemori et al., 2019). Maniglia (2018) had previously observed a similar result, concluding that the altered lateral interaction pattern in MD could be due to a spontaneous neural plasticity triggered by the loss of foveal input. In our study, we challenged this hypothesis and showed that this effect is instead due to a general reduction in contrast sensitivity in patients. Through an additional analysis we have seen that the inhibition was strong for subjects who had a very good baseline contrast threshold and that was progressively reduced as the contrast threshold increased.

When the ratio between target contrast and flankers' contrast was higher than 1/3 the inhibition switched to facilitation. We proposed that, not the enlargement of the receptive field, but the reduced sensitivity to contrast, is the main cause of the different pattern of contextual influences in MDs. When the baseline contrast is high, the difference in contrast between target and flankers is low, thus the three elements are more prone to be integrated together by a 2nd order integrative field [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. Prolonged training with this task at different λs could improve the contrast sensitivity in the PRL, by strengthening the lateral interactions. (Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]U Polat & Sagi, 1994). Maniglia and colleagues in 2016 found that MD patients following training of around 24 sessions have decreased inhibition and increased facilitation. In addition, the improvement in the trained task also transferred to the untrained eye, and to some untrained tasks -visual acuity and crowding [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]. There two main objectives in this final thesis study: to confirm the efficacy of the lateral masking paradigm in improving vision in MD, and to see whether we can achieve an even better result by coupling the PL with the tRNS. To do so, two groups of patients have been trained with PL or with PL combined with occipital tRNS. As demonstrated in chapter VI, the tRNS allows modulating the lateral interactions favouring the feedforward signalthe genicular inputover the contextual influenceshorizontal cortical integration. This transient property of the tRNS, within a prolonged training, could allow for a more rapid and complete restructuring of lateral interactions than PL alone. Moreover, the tRNS is known to produce two more general effects in the stimulated area: a general increase in cortical excitability and a resonance phenomenon between the externally induced noise and the stimulus-related signal (Anna [START_REF] Fertonani | Transcranial electrical stimulation: What we know and do not know about mechanisms[END_REF][START_REF] Miniussi | Modelling non-invasive brain stimulation in cognitive neuroscience[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF] recently showed that at a cellular level the tRNS is able to modulate the activation and inactivation of the Na + channels. This modulation acts differently depending on the intensity of the current and the type of the stimulated neurons, but at the sweet point it could facilitate the processing of subthreshold stimuli [START_REF] Remedios | Effects of Short-Term Random Noise Electrical Stimulation on Dissociated Pyramidal Neurons from the Cerebral Cortex[END_REF][START_REF] Van Der Groen | Transcranial random noise stimulation of visual cortex: Stochastic resonance enhances central mechanisms of perception[END_REF]. In CHAPTER VII we have studied the interaction between tRNS and PL in the periphery of sighted subjects [START_REF] Contemori | tRNS boosts perceptual learning in peripheral vision[END_REF]. Our secondary aim was to test if the tRNS was able to increase transfer to an untrained task. Transfer is thought to be limited by habituation [START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF][START_REF] Xie | Perceptual learning of Vernier discrimination transfers from high to zero noise after double training[END_REF] and some preliminary results suggest that tRNS could reduce it preventing the homeostasis of the stimulated population [START_REF] Campana | Opposite effects of high-and low-frequency transcranial random noise stimulation probed with visual motion adaptation[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF][START_REF] Ho | Transcranial random noise stimulation: A new approach to stimulating the brain[END_REF][START_REF] Terney | Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation[END_REF]. The tRNS group showed higher learning in the trained task when compared to the PL alone group, but the amount of transfer was similar in the two groups. We hypothesized that the lack of increase in transfer depended partly on the choice of the training task. In fact, V1 centred occipital stimulation may have increased plasticity and the signal-to-noise ratio in the stimulated area, consequently improving input to higherlevel visual areas and thus producing a boost in perceptual learning. On the other hand, since VWFA, the area where the triplets of letters are identified [START_REF] Vigneau | Word and nonword reading: What role for the Visual Word Form Area?[END_REF], was not directly stimulated, tRNS may not have been able to boost the transfer by preventing habituation in this region. For this reason and given the importance of increasing the contrast sensitivity in MD patients, in this fourth and final study we decided to go back to the lateral masking task as training task.

To test the efficacy of the PL + tRNS in improving residual vision in MD, we trained two groups of patients for 24 sessions over 8 weeks. One group was trained with the lateral masking task alone, while the other underwent PL + tRNS. All patients were subjected to a battery of tests at the beginning, at the half, and at the end of the training to evaluate their improvement. We compared then the improvement of the two groups in the training task and in the transfer tasks at the mid-test and at the post-test.

one retired after a week of training for personal reasons. One underwent chemotherapy shortly after joining the study and was therefore excluded. Two were excluded right after the pre-test as they reported that they could not perceive all of the three Gabors in the training configuration. Data were collected over three years and participants were assigned randomly to one of the two groups. To avoid the participants to bias each other, the time schedule of the testing was structured in a way to minimize possible interaction among participants.

For each MD patient, we determined the PRL position using the same procedure described by Maniglia et al. (2018). First, the fovea was localized through a highresolution scan of the retinal fundus with a Spectral-Domain Optical Coherence Tomography (Spectralis OCT, Heildelberg Enginering, Heidelberg, Germany). Then three anatomical landmarks were selected on the retinal fundus image and their coordinates were calculated with respect to the fovea. Later, the three landmarks were used to triangulate the position of the retinal locus that corresponded to the fixation cross during the OCT acquisition. To check for the presence of multiple PRL position the procedure was repeated three times. An example image of the procedure is shown in Figure 19. Only patients who had a consistent PRL position across the three independent measurements where included in the study. The procedure was repeated at both the midtest and the post-test to ensure that the PRL position was stable all over the training.

Details of the sample are reported in Table 5.

Apparatus

Stimuli were displayed on a 17″ Dell M770 CRT monitor with a resolution of 1024×768 pixels, a refresh rate of 60Hz and mean luminance of 47.6 cd/m2. Except for the visual acuity task, stimuli were generated with Matlab Psychtoolbox [START_REF] Brainard | The Psychophysics Toolbox[END_REF]. Each pixel subtended 2.14 arcmin. Far all the tasks that involved a contrast sensitivity measure a digital-to-analog converter (Bits#, Cambridge Research Systems, Cambridge UK) was used to increase the dynamic contrast range (13-bit luminance resolution). The monitor was linearized thanks to a 12-bit gamma-corrected lookup table (LUT). Participants set in a dark room at 57 cm from the screen (200 cm for the visual acuity test). A chin rest was used to keep the participant head at the right distance.

Experiments were carried on at the Centre de la Retine, Hôpital Pierre-Paul Riquet, Purpan Hospital, Toulouse (France).

Visual acuity

To test the visual acuity we used a computerized letter recognition task known as the FrACT (Freiburg Visual Acuity and Contrast Test) software [START_REF] Bach | The Freiburg Visual Acuity Test -Automatic Measurement of Visual Acuity[END_REF]. This software is very robust, it has been used in over 100 studies and it has been validated over different clinical populations [START_REF] Bach | The Freiburg Visual Acuity Test-Variability unchanged by post-hoc reanalysis[END_REF][START_REF] Wesemann | Visual acuity measured via the Freiburg visual acuity test (FVT), Bailey Lovie chart and Landolt Ring chart[END_REF]. This software allows to reliably measure the visual acuity threshold for single letters by means of an adaptive staircase procedure, the Best-PEST algorithm. Stimuli are randomly selected among 10 Sloan letters: C, D, H, K, N, O, R, S, V, and Z. The patients reported the letter aloud and the experimenter that was sitting in a position from where I couldn't see the screen typed the appropriate letter on the keyboard. Two different measures where collected monocularly for each eye. Observers viewed the letters for a maximum of 30s at a viewing distance of 200 cm for a total of 30 trials. Letters were black on a white background. Acoustic feedback was automatically presented for both correct and incorrect answers. The average duration for one block was around 5 minutes.

Tachistoscopic visual acuity and crowding

To measure visual acuity and visual crowding with fast stimulus presentation, we used a procedure similar to the one presented in CHAPTER V. In this case we tested only one retinal position and thus the use of the eye tracker was not required. The participant was asked to fixate with their preferred retinal locus. In case of loss of fixation they were instructed to memorize the last answer, search for the fixation dot and then provide the verbal answer to the experimenter only once ready for the next trial. At this point the experimenter reported the answer by pressing the corresponding button on the keyboard and the next trial was initiated. Randomly selected Sloan letters (D, N, S, C, K, R, Z, H, O, V) were used as stimuli and presented in the centre of the screen for a time of 100 ms.

Letters were white on a black background. The starting letter size was set to 3 deg, a size fairly above threshold for all participants. In the successive trials, the size was changed according to a psychophysical adaptive procedure from the MLP toolbox. [START_REF] Grassi | MLP: A MATLAB toolbox for rapid and reliable auditory threshold estimation[END_REF]Green, 1993[START_REF] Green | Stimulus selection in adaptive psychophysical procedures[END_REF]. There were 30 trials in each block, an adequate number to obtain a reliable and fast threshold [START_REF] Leek | Experience with a yes-no single-interval maximum-likelihood procedure[END_REF]. A practice dummy block was performed at the beginning to help the participant in familiarizing with the very fast presentation speed. During the dummy block the experimenter performed the task together with the patients. During the actual measurement the experimenter was sitting at a side of the desk, in a position where he could see the participant but not the monitor displaying the stimuli. The patient was forced to give a response even when could not perceive the letter. The threshold was defined as the letter size that leads to an accuracy level of 75% of correct answers. A fixation dot was displayed in the centre of the screen. The dot disappeared right before the target and then reappeared right after the target. A warning sound preceded the stimulus onset of 100ms and lasted for 50 ms. The procedure was self-paced, in a sense that the next trial was not started until the previous answer was recorded. In order to avoid lapses the participant was told that no time limit was applied to the answer and was also instructed to think carefully before answering. The total duration of the procedure was considerably variable between patients, but it never exceeded three minutes. The testing was done monocularly for both eyes.

Crowding was measured using the very same procedure as for the tachistoscopic visual acuity, except that we measured the critical space instead of the letter size. A triplet of letter was presented and the distance between the central target and the flanking distractors was varied. The threshold was defined as the critical distance that allowed for an accuracy of 75%. The size of the three letters was kept constant and corresponded to the letter size obtained in the tachistoscopic visual acuity task multiplied by 1.30. In this way we can exclude that identification was compromised by a limit in the visual acuity [START_REF] Contemori | Transcranial Random Noise Stimulation (tRNS) boosts Perceptual Learning in reducing critical space of crowding[END_REF]. The testing was done monocularly for both eyes.

In order to allow the patient so see all the three letters, the global orientation of the triples could be horizontal or vertical, depending on the size and the shape of the scotoma. Every single letter was always vertical.

Contrast sensitivity

We measured contrast sensitivity at 4 different spatial frequency using a custom procedure similar to the lateral masking paradigm except that in this case the target was an isolated Gabor patch of 4 deg (full width at half maximum) with vertical orientation.

Participants had to perform a contrast detection task with a temporal-2AFC (twoalternative forced-choice) procedure in which the target was present only in one of the two intervals. The patient had to report the interval containing the target by pressing the number "1" or "2" in the number pad of the keyboard. Each interval lasted 133 ms and the interstimulus interval was 500 ms. Target contrast varied according to a 3down/1up staircase, in which three consecutive correct responses reduced the target contrast of 0.1 log units and each wrong response increased the contrast of the same amount. The staircase terminated after 120 trials or 14 reversals. The procedure returned the Michelson contrast corresponding to 79% of correct detection estimated from the algebraic mean of the last 6 reversals. A warning sound of 50 ms indicated the beginning of each interval. Each spatial frequency was tested twice, once for each eye monocularly.

The spatial frequency tested were 1, 3, 5, 7 cpd. All the eyes could be tested up to 3 cpd.

The starting contrast was set to 0.3 Michelson at 1 cpd and was adapted to the patient performance for all the other spatial frequency. The spatial frequencies were tested in ascending order and if in one of the blocks the threshold had gone beyond 0.9 Michelson the testing was interrupted. The new trial started only after the participant response was recorded correctly. Patients were instructed to think carefully before to answer, to avoid lapses. To help participant in keeping the fixation, a dot of 0. Except for these two modifications and for the different solution adopted for the monitor, all the other characteristics of the stimuli and task introduced in study 1 remained unchanged. We have also used the method already introduced in CHAPTER V for the calculation of the threshold elevation (TE) where the contrast threshold is Given that the low numerosity of the sample and the many differences between the two groups, a direct comparison between the two would lead to misleading results. To overcome this issue and to control for individual and group differences at the pre-test, we represented data from the transfer tasks as percentage of change from the baseline and then we performed statistical analysis to test whether there was a significant difference between the two groups. The transformation was done by dividing the score at the midor post-test by the score at the pre-test and then subtracting 1. In this way the performance at the mid-test and at the post-test was normalized using the baseline of the same patients prior to run the statistical tests. For the training task we calculated the thresholds elevation as the log-ratio between the collinear and the orthogonal condition.

Data were pre-processed and analysed using the statistical computing environment R (R Core Team, 2012). To test for statistical significance data were fitted with a linear mixedeffects model with the subject as a random term using the "lme4" package and then p.values were calculated with a Type III Anova with Satterthwaite's approximation with the "lmerTest" package [START_REF] Bates | lme4: Linear mixed-effects models using Eigen and S4[END_REF][START_REF] Kuznetsova | lmerTest Package: Tests in Linear Mixed Effects Models[END_REF]. This approach is proven to be solid in case of clustered data with repeated measures and even in case of missing or unbalanced data [START_REF] Baayen | Mixed-effects modeling with 204 crossed random effects for subjects and items[END_REF][START_REF] Barr | Random effects structure for confirmatory hypothesis testing: Keep it maximal[END_REF]. We checked the homogeneity of variance and the linearity of the residuals through visual inspection.

When necessary, Bonferroni corrected comparisons were performed using the "emmeans" function from the homonymous R package [START_REF] Lenth | Estimated Marginal Means, aka Least-Squares Means[END_REF]. A mixed-design ANOVA conducted on TE data, including as factors the group (PL vs. PL + tRNS) and the λ (3, 4, 6, and 8λ) indicated that the effect of group (F(1,10) = 0.3549, p = 0.56), and the effect of session were not significant (F(2,110) = 2.889, p = 0.059). We found a significant effect of λ (F(3,110) = 4.079, p = 0.008), and also a significant the group x session interaction (F(2,110) = 3.156, p = 0.046). Pairwise comparisons for the PL + tRNS group revealed that the threshold elevation at the mid-test was not different than at the pre-test (PL + tRNS,MID -PL + tRNS,PRE = 0.103, se = 0.087, t = 1.178, p = 0.964). On the contrary, the TE at the post-test was significantly lower than at the mid-test (PL + tRNS,POST -PL + tRNS,MID = -0.3153, se = 0.087, t = -3.590, p = 0.002). For the PL group there was no significant difference between pre and mid-test (PL,MID -PL,PRE = 0.084, se = 0.104, t = 0.817, p = 1), or between mid and post-test (PL,POST -PL,MID = 0.010, se = 0.1039, t = 0.100, p = 1). The effect plot for the TE is shown in Figure 22.

Results

Training

Contrast sensitivity

The contrast sensitivity was calculated as LOG(1/contrast threshold). Then the ratio between the contrast sensitivity value at the mid-and post-test divided by the pre- Estimated improvement in contrast sensitivity plotted as a function of the group (PL vs. PL + tRNS), the eye (trained vs. untrained), the session (mid vs. post) and the spatial frequency (1,3,5,7 cpd). The higher the value the better the improvement. Vertical bars represent 95% confidence intervals. Tachistoscopic visual acuity thresholds plotted as a function of the group (PL vs. PL + tRNS) and the eye (trained vs. untrained) and the session (mid vs. post). The lower the value the better the improvement. Vertical bars represent 95% confidence percentiles.

For the tachistoscopic crowding the mixed-design ANOVA conducted on ratio between the critical space at the mid-and post-test divided by the baseline at the pre-test data, including as factors the group (PL vs. PL + tRNS) and the eye (trained vs.

untrained) and the session (mid vs. post) indicated that the effect of group was significant

Discussion

Trained task

In the trained task we found a significant decrease in contrast thresholds after 12 training sessions for both groups. Between the 12th and the 24th training session, only the PL + tRNS group continued to improve while the PL group reached a plateau. It is essential to note that at the end of the training the contrast thresholds of the two groups were still one order of magnitude higher then what expected in controls subjects at a comparable eccentricity (see [START_REF] Contemori | Contextual influences in the peripheral retina of patients with macular degeneration[END_REF] for the normative data).

Considering the post-training contrast thresholds, the plateau was not due to a ceiling effect but more likely to some intrinsic limitation in the PL and adult plasticity [START_REF] Bavelier | Removing Brakes on Adult Brain Plasticity: From Molecular to Behavioral Interventions[END_REF][START_REF] Harris | Generalized perceptual learning in the absence of sensory adaptation[END_REF].

In this context, the tRNS was able to produce deeper learning that continued throughout the training. As we expected no effect of lateral interactions was observed in the orthogonal condition despite an overall reduction in contrast threshold for both groups.

For the TE we found a significant effect of the λ and a significant group x session interaction. Post-hoc analysis revealed that only the PL + tRNS group had a significant modulation of the lateral interaction after the training at the post-test. As can be seen in figure 22, the curve showing the TE as a function of the λs, in the pre and mid-test is above the zero line (inhibition) while after training it falls below zero (facilitation).

This result is in agreement with previous studies that found an increase in facilitation and a reduction of inhibition after training [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). In our case, however, the modulation of lateral interactions reaches significance only for the PL + tRNS group, indicating that the stimulation played a fundamental part in this process.

A further consideration concerns the duration of the training. The number of sessions was based on the previous literature that showed an improvement after the 24 sessions (Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). A previous study showed that by combining PL with tRNS it is possible to reduce the number of sessions required to achieve improvement from 24 to 12. [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF] Surprisingly in our case both groups showed an improvement in the trained task already after the 12th session and therefore we could not verify that the stimulation accelerates perceptual learning by reducing the number of required sessions. On the other hand, our data are in agreement with what we have found in study 3 of this thesis and support the hypothesis that the tRNS can postpone the plateau effect during PL.

Repeating the study by increasing the number and frequency of the intermediate tests could help clarify how the tRNS alters the temporal dynamics of the PL.

Transfer tasks

The results for the transfer tasks are summarized in Table 6. In general, the results of previous studies [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018) are partly frequencies, but at the same time, this transfer is more likely to pass from lower frequencies to higher than the opposite (for a detailed review see [START_REF] Sagi | Perceptual learning in Vision Research[END_REF]. In MD the processing of low spatial frequencies is much less affected by the visual deficit than that of the high ones [START_REF] Musel | Residual abilities in age-related macular degeneration to process spatial frequencies during natural scene categorization[END_REF][START_REF] Peyrin | Scene perception in age-related macular degeneration: Effect of spatial frequencies and contrast in residual vision[END_REF]Ramanoël et al., 2018). Training directly with medium-high spatial frequencies is not always possible since the task could be too difficult, and the patient could be frustrated by failure. The possibility of obtaining a transfer to higher spatial frequencies is, therefore, a desirable property of PL with lateral masking. The results of the contrast detection task clearly show that the PL + tRNS group improved more than the PL alone.

The improvement is particularly evident at the two highest spatial frequencies tested (5 cpd, 7 cpd). Given that high spatial frequencies are necessary for the processing of details, the gain obtained could increase visual resolution in general (Uri [START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]Sagi, 2011a). This finding is in line with the result of a recent study from our lab (L.

Battaglini, G. Contemori, S. Penzo, M. Maniglia, accepted in Neuroscience Letters) in which we found that tRNS enhance detection performance of an isolated Gabor only when it has a high spatial frequency (12 cpd). We hypothesized that this effect might be due to the shallow depth of tRNS. The superficial layers of the visual cortex where neurons have higher preferred spatial frequencies are more likely to be stimulated than cells in the further layers. Future research should better address this issue.

Visual acuity and tachistoscopic visual acuity.

The previous literature reports improvement in visual acuity following training with lateral masking [START_REF] Camilleri | Improvement of uncorrected visual acuity (UCVA) and contrast sensitivity (UCCS) with perceptual learning and transcranial random noise stimulation (tRNS) in individuals with mild myopia[END_REF]Camilleri, Pavan, Ghin, & Campana, 2014;[START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]. In this study, the training transferred to TVA but did not produce an improvement in visual acuity with Sloan's letters. This seemingly contrasting result can have multiple explanations. First, the colours used for background and letters in the two tasks were reversed and this could have had an impact during the testing. Patients with MD often suffer from visual glare [START_REF] Hogg | Visual function and dysfunction in early and late age-related maculopathy[END_REF]. In the VA test with the FrACT software, the letters presented were black on a white background, while in the homemade tachistoscopic test coded with the Psychtoolbox, the letters were white on a black background. Some of the patients reported difficulties in the first of the two tests due to the bright background colour. The glare was not constant during the measurements, but rather grew over time. This may have affected the measurement by reducing its reliability. The dispersion of the data in this task is, in fact, higher than that in the tachistoscopic task. A second reason could be related to the different duration of the stimuli. During the FrACT test, some patients reported seeing the letter in the first few moments after the onset of the stimuli, but the attempt to keep a prolonged fixation produced a later distortion of the patient's percept. This might be due to some sort of habituation effect compensated by large ocular drifts [START_REF] Déruciz | Can fixation instability improve text perception during eccentric fixation in patients with central scotomas[END_REF]. The stimulus duration in the tachistoscopic test may have prevented this problem.

The last possibility is that given the temporal structure of the task the training has especially effective over some transient visual channels and less over the sustained ones, producing a greater benefit to the tachistoscopic test [START_REF] Tolhurst | Sustained and transient channels in human vision[END_REF]. This last possibility could be an interesting starting point for further research in an attempt to perfect the training.

Tachistoscopic visual crowding.

In visual crowding we found a reduction in the critical space for the PL group, but not for the PL + tRNS group. This result is surprising if we consider that in the trained task as well as in the contrast sensitivity there is an advantage of the PL + tRNS over the PL group. In addition, it is in apparent contrast with the results of studies 6 and 7

presented in this thesis. Undoubtedly a lot is still to be discovered regarding the mechanisms of transfer of the PL. A valid approach for rehabilitation could be to follow the training with lateral masking with a short training based on visual crowding. Future studies should investigate this possibility.

Conclusion

The combined use of PL with tRNS has proved effective in increasing learning in the trained task by postponing the plateau effect. It has also produced a greater modulation of contextual influences than the PL alone. tRNS also produced an increase in transfer to untrained spatial frequencies. Furthermore, in both groups there was a transfer of learning to tachistoscopic visual acuity, but only the PL group showed also transfer to visual crowding.

Although there are still grey areas, we conclude that the combined use of PL and tRNS has been effective in postponing the plateau and partially enhancing transfer. In conclusion, tRNS could be effectively used as an additional tool in the visual rehabilitation of MD patients, but further studies are needed to fine-tune the stimulation parameters and maximize the efficacy of the combined tRNS + PL treatment.

Chapter IX.

Contextual Influences on the Peripheral Retina of Patients with Macular Degeneration: further considerations.

In study 1 we proposed a different interpretation for the reduction of inhibition at the very short target-to-flanker distance that has been previously reported by Maniglia and colleagues (2018). The locally-weighted polynomial regression based on the pooled data of all participants in study 1 (MD and controls) showed that the reduction in collinear inhibition at 2λ and the switch towards facilitation are clearly linked with the baseline contrast sensitivity of the single subject in the orthogonal configuration. This relationship is well predicted by the model proposed by [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] that described by the variation of TE as a function of the log flanker/target contrast ratio in normal vision [START_REF] Foley | Human luminance pattern-vision mechanisms: masking experiments require a new model[END_REF][START_REF] Ross | The effects of adaptation and masking on incremental thresholds for contrast[END_REF][START_REF] Snowden | The effects of surround contrast on contrast thresholds, perceived contrast and contrast discrimination[END_REF][START_REF] Zenger-Landolt | Flanker effects in peripheral contrast discrimination--psychophysics and modeling[END_REF][START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF]. According to the model, the sensitivity to a low contrast target is reduced by the presence of the high contrast flankersas usually observed in normal viewersbut as the target's contrast increases this inhibition progressively decreases and then turns into facilitation. However, when the target's contrast surpasses that of the flankers, the facilitatory effect progressively reduces and then disappear. Overall, we have a range that goes from a log(flanker/target) contrast ratio of 0 to a ratio of 1. According to Zenger's model, the switch between inhibition and facilitation should occur at a ratio around 0.25 and 0.50 for the 2λ and 3λ respectively (Barbara Zenger-Landolt & Koch, 2001;[START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF].

Small deviations from the model are expected given that eccentricity and lambda are not constant between subjects but nevertheless, we expect a positive linear relationship between the threshold modulation and the flanker/target contrast ratio in the baseline orthogonal condition. We wanted to verify this prediction by pooling the data collected in study 1 and those collected in study 4.

This new dataset was thus composed of the measurements carried out in study 1

and study 4 at the shortest target-to-flanker tested distance (respectively 2λ and 3λ). The data were analysed through a linear regression in which the TE of the individual participants was related to their respective contrast ratio at the baseline orthogonal condition. Also, we wanted to verify the hypothesis that training modulates lateral interactions through practice-dependent cortical reorganization. We hypothesized that training produced an improvement in the contrast sensitivity of all participants, but that 181 only those who show inhibition at the pre-test show modulation of lateral interaction in the post-test. After training, we expected a larger increase in facilitation in patients who started with a high inhibition and thus with a flanker/target contrast ratio in the orthogonal condition close to 1. To test this second hypothesis, we have carried out a repeated measure ANOVA by comparing TE as a function of flanker/target contrast ratio and the session for the subgroup that underwent the training. We increased the sample size and therefore the statistical power by also including 4 MD subjects from Maniglia, et al., (2016) that had been trained with the same procedure used in our study 4. Our expectation was a change of slope in the relationship between TE and flanker/target ratio between the pre-and post-test. The results are reported below.

Results

The linear regression conducted on these data revealed a significant positive relationship The repeated measure ANOVA performed on log(flanker/target) contrast ratio as a continuous variable and Session (pre vs post) as an ordered factor, showed no significant main effect of Session (F [1,27] = 0.166, p < 0.687) or contrast ratio (F [1,27] 183 = 2.053, p = 0.163). As for the effect of our interest, namely the Session x ratio interaction, the data shown in figure 27 show a trend that is in agreement with our prediction but we found that the difference in the slope was not statistically significant (F[1,27] = 3.851, p = 0.060). Threshold Elevation (TE) as a function of the log(flanker/target) contrast ratio and session. Pooled data from Maniglia, et al., (2016) (PRL 2λ) and study 4 (PRL 3λ). The grey area represents the 95% confidence interval for the regression.

Conclusion

The regression performed over the pooled dataset supports and confirms the previous result obtained in study 1. This result weakens the previous hypothesis of cortical reorganization proposed by Maniglia (2018) and supports instead our alternative explanation for the reduced collinear inhibition in patients based on improved efficiency in integrating/grouping elements at the stage of the 2nd order integrative field [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF].

Regarding our second hypothesis of specific modulation of lateral interactions for patients with strong inhibition, the data reported are not fully conclusive. A further study could clarify the validity of our hypothesis.

contrast ratio increases the inhibition decreases and then switches to facilitation.

However, when ratio surpasses 1 the facilitatory effect progressively reduces and then disappears. This relationship is well expressed by a 'dipper' function similar to those previously reported by [START_REF] Zenger | Isolating excitatory and inhibitory nonlinear spatial interactions involved in contrast detection[END_REF] for normal vision. Contrary to previous interpretations, we demonstrated that this modulation reflects neither a phenomenon of spontaneous nor use-dependent cortical plasticity. Based on this result and the previous literature it is concluded that the spontaneous reorganization process has limited effects on the PRL and that a neural based perceptual learning may be needed to improve peripheral vision in the MD.

CHAPTER VI includes the results of a study that aimed at investigating whether the contextual influences are modulated by tRNS applied to the occipital cortex of human observers during task performance. Given that the tRNS main effect is to increase cortical excitability, it could expand the sensitivity of the neurons to weak stimuli and thus lowering the contrast threshold for the target. At the same time, this increased excitability could also modulate how the target is integrated with its context by altering the relative strength of excitatory (E) or inhibitory (I) influences from the flanking elements, depending on which is weaker. In this case, we might expect an effect of tRNS even if tRNS shows no effect at all on target perceived contrast. Results show a tRNS-dependent increased sensitivity for the single Gabor signal of low but not high contrast. This is compatible with a signal-to-noise ratio improvement for stimuli that are close to the threshold. Moreover, both the inhibition at the shortest target to flanker distance (2) and the facilitation at the largest (6) are reduced by the stimulation. This result suggests a general boost by the tRNS to the feedforward signal at the expense of the strength of the contextual influence. Based on this result it is concluded that this modulatory effect over the visual-spatial integration in the early visual cortex might help reshape the visual processing in the PRL in a more foveal fashion, also reducing the crowding effect in the periphery.

The study presented in CHAPTER VII investigated whether tRNS can effectively boost PL on a peripheral visual task over a small number of daily training sessions.

Since the tRNS proved effective in modulating contextual influences, we decided to use a training task based on visual crowding. One of the objectives was to verify the effectiveness of tRNS applied to low-level visual areas in increasing perceptual learning in visual tasks performed by higher-level areas.

Additionally, we tested whether learning transferred to untrained spatial location and task variation. We expected the tRNS to be able to increase both the learning rate and the transfer of learning. Results showed a greater learning rate within and between sessions for the PL+tRNS group respect to the sham (PL alone). Contrary to our expectations, PL+tRNS group does not show a higher amount of transfer to untrained conditions. We conclude that the combined use of PL and tRNS has a higher potential in improving peripheral visual abilities over a small number of sessions than PL alone, but that training based on a more basic visual ability such as contrast sensibility might be better suited to achieve a higher degree of transfer.

The final study presented in CHAPTER VIII brought together the findings of the experiments presented in this dissertation, investigating the combined effect of tRNS, and perceptual learning in a visual rehabilitation protocol in MD patients. Since stimulation with tRNS coupled with training in a high-level task in subjects with normal vision, did not produce the increase in the hypothesized transfer, we decided to use the paradigm of lateral masking. Previously this paradigm proved effective in increasing contrast sensitivity in trained subjects, and more importantly, this improvement transferred to the untrained eye and higher-level tasks such as visual acuity and visual crowding [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). Furthermore, in CHAPTER II we saw how the tRNS modulates the contextual influences, our expectation, in this case, was to be able to use the modulatory activity of the tRNS to obtain a greater improvement and a transfer compared to behavioural training alone. To test this hypothesis, we used a lateral masking paradigm to train the PRL of 12 patients, and we tested them also for the crowding and the visual acuity as a transfer task. The patients were split into two groups, 7 underwent PL + tRNS while the other 5 only the behavioural training. Both groups improved after the training, but we found a modulation of the lateral interaction and transfer to some untrained spatial frequencies only for the PL + tRNS group. Conversely, only PL group improved also in the crowding task.

Although some questions remain, we concluded that the tRNS could be an efficacious way of maximizing the gain due to the perceptual learning in MD patients. Overall, this thesis provides useful insights into the functioning of the tRNS and explores a new path for visual improvement in MD with possible implications on other clinical populations.

Current perspective on Macular Degeneration Treatment

In recent years there has been a huge effort towards finding therapies and treatments for retinal degenerative diseases. Gene therapy and stem cell therapeutic strategies are currently under investigation [START_REF] Fahim | Retinitis pigmentosa: Recent advances and future directions in diagnosis and management[END_REF] as well as prostheses and retinal implants [START_REF] Lewis | Restoration of vision in blind individuals using bionic devices: A review with a focus on cortical visual prostheses[END_REF][START_REF] Lohmann | The very large electrode array for retinal stimulation (VLARS) -a concept study[END_REF][START_REF] Zrenner | Retinal prosthesis and Artificial Vision[END_REF]. Also, the tools necessary to screen for early signs of AMD, such as advanced retinal imaging with optical coherence tomography, have had a remarkable development [START_REF] Jarc-Vidmar | Mapping of central visual function by microperimetry and autofluorescence in patients with Best's vitelliform dystrophy[END_REF][START_REF] Mohaghegh | Wearable diagnostic system for age-related macular degeneration[END_REF]. Despite all this now it is not possible to restore the damaged vision.

Patients with AMD who develop loss of central vision later in their life have a higher prevalence of comorbidity of depression. Low vision rehabilitation can give to the patient a sense of control over the disease mitigating the risk of depression and increasing participation in daily life activities [START_REF] Cimarolli | Anxiety and depression in patients with advanced macular degeneration: Current perspectives[END_REF].

Microperimetry systems like the MP-1 are diagnostic techniques which allow to precisely map the scotoma in real-time and to measure the location and stability of fixation. They have been used also to perform biofeedback training in patients with a combination of a structured light stimulus plus acoustic feedback. This training has been proven to increase the stability of fixation, distant and near visual acuity, reading speed, and reading comprehension [START_REF] Midena | Age-Related Macular Degeneration: Prevention of Blindness and Low-Vision Rehabilitation[END_REF]. This training can be easily integrated with a PL + tRNS based training offering to the patients a more refined and complete training protocol.

Recently, Virtual and Augmented Reality (VR and AR) system to help patients incorrectly visualizing the relevant information in the environment have been developed.

With the increasing wearability of smart devices like AR glasses, those instruments are going to rapidly change the word of visual aids [START_REF] Shahshahani | iSight: computer vision based system to assist low vision[END_REF]. Those smart devices might be used not just to highlight useful information about the visual scene, but also to increase the contrast or to train the patients with a PL paradigm in a real-life situation.

However, the cutting edge of the research on visual aids is represented by implantable devices. Despite their invasiveness, the benefits obtained are considerable.

They do not only allow the patient to cope with visual impairment but allow a partial restoration of vision [START_REF] Gupta | Implantable miniature telescope (IMT) for vision loss due to end-stage age-related macular degeneration[END_REF].

To an even higher degree of invasiveness, we find retinal and cortical implants.

They provide visual inputs by direct electrical stimulation of the retina. Retinal prostheses systems have also yielded promising results. However, each type of prosthesis has its advantages and disadvantages. Cortical implants, which are currently in the preclinical study phase, may provide artificial vision to patients with the complete retina and optic nerve destruction, but the technical difficulties are considerable, and the quality of the visual input is poor [START_REF] Lewis | Restoration of vision in blind individuals using bionic devices: A review with a focus on cortical visual prostheses[END_REF][START_REF] Lohmann | The very large electrode array for retinal stimulation (VLARS) -a concept study[END_REF][START_REF] Roux | Focusing on the functional impact of retinal prothesis[END_REF][START_REF] Roux | Probing the functional impact of sub-retinal prosthesis[END_REF][START_REF] Stett | Retinale und kortikale Aktivierung durch elektrische Stimulation mit Netzhautimplantaten[END_REF]. This is due to the difficulty of providing a visual input that reflects the functional specificity of the retinal or cortical organization. After the implant, patients need long training so that they can successfully distinguish scenes or objects and often the advantage is limited to trained stimuli [START_REF] Han | Word recognition: Rethinking prosthetic vision evaluation[END_REF]. These instruments are currently tested only in patients with very low or no residual vision, which were probably subjected to strong trans-synaptic degeneration in the years before implantation. In this case, the combination of PL and retinal implant could help the patient adapt more quickly to the new visual input, also thanks to the combined use of tRNS.

Finally, indirect help could come from recent developments in the field of neural networks and artificial intelligence. Complex computational systems can be used for increasingly accurate diagnoses (Schmidt-Erfurth, Sadeghipour, Gerendas, Waldstein, & Bogunović, 2018). Moreover, biologically plausible neural networks are able to mimic the functioning of the human visual system, from the retina to V1 and beyond [START_REF] Chauhan | Emergence of Binocular Disparity Selectivity through Hebbian Learning[END_REF][START_REF] Hopkins | Spiking neural networks for computer vision[END_REF][START_REF] Testolin | Letter perception emerges from unsupervised deep learning and recycling of natural image features[END_REF][START_REF] Testolin | Probabilistic models and generative neural networks: Towards an unified framework for modeling normal and impaired neurocognitive functions[END_REF]. In these systems it is possible to create a virtual lesion in the input, silencing a part of the visual field, to study how the network adapts [START_REF] Andrade | Simulation of plasticity in the adult visual cortex[END_REF][START_REF] Mcmanus | A Computational Model of Perceptual Fill-in Following Retinal Degeneration[END_REF]. This could lead to understanding how and where the PRL develops, the phases and timing of spontaneous adaptation, even which is the best rehabilitation strategy.

Can tRNS be used in visual rehabilitation?

The idea of using a lateral masking paradigm in the visual rehabilitation of bilateral central blindness comes from the attempt to remodel the lateral interactions in the PRL to make them similar to those in the fovea. Being trained on contrast sensitivity in the first instance, we expected an improvement in contrast sensitivity for the trained stimulus, but the ultimate goal was to achieve an increase in visual acuity and a reduction in visual crowding so as to bring benefit to the patient in the daily life activities. In Training with lateral masking has been already adopted to treat a series of visual diseases (Uri [START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF]) such as amblyopia (U. [START_REF] Polat | Improving vision in adult amblyopia by perceptual learning[END_REF], presbyopia (Uri [START_REF] Polat | Making perceptual learning practical to improve visual functions[END_REF], myopia (D. T. H. [START_REF] Tan | Efficacy of neural vision therapy to enhance contrast sensitivity function and visual acuity in low myopia[END_REF]. With due consideration, our results can be easily generalized to all those clinical populations that have been shown to benefit from training with the lateral masking task. Moreover, by deepening our basic knowledge of the neuromodulatory action of the tRNS over the visual system, it will be possible to extend the use of this technique in conjunction with different perceptual training. This

Limitation of the thesis

Even a meticulously prepared study is not without limitations, even more so when dealing with a clinical population. The major limits are related to the difficulty in finding participants. Undoubtedly the recruitment of patients was slow and complicated, and it took three years to arrive at the current sample size. Because of this, in the two studies that included patients, the research was conducted using relatively small samples; 12 MD patients and 7 control participants with normal vision in study 1 (CHAPTER V), 12 patients subdivided in one group of 7 and one group of 5 in study 4 (CHAPTER VIII).

Although for study 1 (CHAPTER V) these numbers should be considered as an improvement over the previous literature, replicating study 4 (CHAPTER VIII) over an even larger sample will be necessary in order to strengthen the conclusions. Second, in study 3 (CHAPTER VII) and 4 (CHAPTER VIII) despite having a sham condition, a second control group with a stimulated location different from the occipital pole as we have done in experiment 3 (CHAPTER VI) was not tested. Lastly, different procedures and different accuracy thresholds have been used in study 1 (CHAPTER V), study 3 (CHAPTER VII) and 4 (CHAPTER VIII). In fact in study 3, for the training, we had to adjust the procedure (accuracy threshold was increased), so as to present a greater number of stimuli in each block to increase the training effect and prevent the participant from feeling frustrated due to excessive difficulty. Because of this, the size of the critical space between the three experiments is not directly comparable. The studies presented in this dissertation are, however, the first to investigate the effects of tRNS on the contextual requires a high restructuring of the visual information processing network. In this thesis,

we have seen how it is possible to modulate lateral interactions at early stages of processing, favouring the processing of feedforward and allowing restructuring of visualspatial integration. This property of tRNS will surely be useful in the future to accelerate the adaptation to retinal implants by patients.

Another way is to combine the tRNS + PL with drug administration. Numerous studies are showing that some substances allow for extra plasticity in the visual cortex [START_REF] Baker | Increasing cortical GABA levels through dietary intervention[END_REF][START_REF] Fuchs | Adult neuroplasticity: More than 40 years of research[END_REF][START_REF] Greuel | Pharmacological induction of usedependent receptive field modifications in the visual cortex[END_REF][START_REF] Kang | Cholinergic pairing with visual activation results in long-term enhancement of visual evoked potentials[END_REF][START_REF] Rokem | The benefits of cholinergic enhancement during perceptual learning are long-lasting[END_REF]. Particularly promising is the use of acetylcholine reuptake inhibitors that act selectively on muscarinic or nicotinic receptors present in the primary visual cortex. The effect of increasing the bioavailability of acetylcholine on the visual system seems comparable to the effect we found with tRNS [START_REF] Kang | Boosting visual cortex function and plasticity with acetylcholine to enhance visual perception[END_REF]. Cholinergic projections from nucleus basalis activate excitatory nicotinic receptors in layer IV of V1 and thus potentiate pyramidal cells that carry retinal activation. They also activate inhibitory muscarinic receptors in layers 1-11 reducing cortico-cortical conduction [START_REF] Bhattacharyya | Basal forebrain activation controls contrast sensitivity in primary visual cortex[END_REF][START_REF] Pinto | Fast modulation of visual perception by basal forebrain cholinergic neurons[END_REF][START_REF] Yu | Uncertainty, neuromodulation, and attention[END_REF]. This way, activation of Nucleus Basalis enhances the processing of task-relevant stimuli at the detriment of the overall spatial integration.

Simply put, it modulates the strength of horizontal connections downwards and favours the feedforward in a way similar to what we saw in study 2 of this thesis [START_REF] Kang | Boosting visual cortex function and plasticity with acetylcholine to enhance visual perception[END_REF]. It is suggestive of imagining a PL + tRNS paradigm with concomitant Ach treatment. The combined effect of the drug and the stimulation could be additive with the results of bringing the effectiveness of the PL to an even higher level.

In conclusion, although some unresolved questions remain, the results presented in this thesis support the potential of PL + tRNS in the improvement of the residual visual abilities in adults with bilateral central blindness. Before an extensive clinical application, further studies are needed to maximize the effectiveness of the treatment. Moreover, this thesis has brought new results that contribute to the understanding of the plasticity of the human brain in general, with possible repercussions for other clinical populations.

  possibili applicazioni come terapia aggiuntiva al training neuro-comportamentale nel trattamento della cecità centrale bilaterale. Questi risultati hanno inoltre potenziali ricadute anche per altre popolazioni cliniche che soffrono di bassa visione. ix Introduction ..............................................................................................142 Materials and Methods .............................................................................150 Results ......................................................................................................162 Discussion ................................................................................................172 Conclusion ...............................................................................................178 Chapter IX. Contextual Influences on the Peripheral Retina of Patients with Macular Degeneration: further considerations. ....................................................179 Results ......................................................................................................182 Conclusion ...............................................................................................184 Chapter X. Overview and Future Directions .......................................................185 Current perspective on Macular Degeneration Treatment .......................189 Can tRNS be used in visual rehabilitation? .............................................192 Limitation of the thesis ............................................................................194 Future direction and general conclusion ..................................................196 Supplementary material .......................................................................................200 References. ...........................................................................................................201 xiv List of Tables

  Details of participants. ........................................................................................ Table 2. Contrast range for the target at each target-to-flankers distance () .................

Figure 1 .

 1 Figure 1. Illustrative example of stimuli placement. ........................................................

Figure 2 .

 2 Figure 2. Stimuli used for the lateral masking paradigm. .................................................

Figure 3 .

 3 Figure 3. Contrast threshold and TE values for PRL and controls ...................................

Figure 4 .

 4 Figure 4. Contrast threshold and TE values for PRL and non-PRL .................................

Figure 5 .

 5 Figure 5. Visual acuity non-PRL vs. Visual acuity PRL. ................................................

Figure 6 .

 6 Figure 6. Critical space PRL vs. Critical space non-PRL. ................................................

Figure 7 .

 7 Figure 7. TEs as a function of flankers/orthogonal contrast ratio. ....................................

Figure 8 .

 8 Figure 8. The stimulus configuration used in the experiments. ........................................

Figure 9 .

 9 Figure 9. Sensitivity (d') for the single (left) and collinear target at 6. ........................

Figure 10 .

 10 Figure 10. Sensitivity changes (SC) plotted as a function of target contrast with flankers

Figure 11 .

 11 Figure 11. Sensitivity (d') for the single (left) and collinear target at 2. ......................

Figure 12 .

 12 Figure 12. Sensitivity changes (SC) plotted as a function of target contrast with flankers

Figure 13 .

 13 Figure 13. The figure shows the way the Criterion varies as a function of contrast in the

Figure 14 .

 14 Figure 14. Configurations used in the crowding experiment. .........................................

Figure 15 .

 15 Figure 15. Electrode positioning and modeled electrical field strength (normE). ..........

Figure 16 .

 16 Figure 16. Between days crowding thresholds. ..............................................................

Figure 17 .

 17 Figure 17. Training and transfer tasks results for the sham and tRNS groups. ..............

Figure 19 .

 19 Figure 19. PRL position e scotoma size..........................................................................

Figure 20 .

 20 Figure 20. Training configuration with the stimuli and the control of fixation. .............

Figure 21 .

 21 Figure 21. Contrast threshold in the trained task as a function of group, λ, and session.

Figure 22 .

 22 Figure 22. Threshold elevation (TE) in the trained task as a function of group, λ, and

Figure 23 .

 23 Figure 23. Improvement in contrast sensitivity. .............................................................

Figure 24 .

 24 Figure 24. Results from the Sloan visual acuity test. ......................................................

Figure 25 .

 25 Figure 25. Results from the tachistoscopic visual acuity test. ........................................

Figure 26 .

 26 Figure 26. Results from the tachistoscopic crowding test. .............................................

Figure 27 .

 27 Figure 27. Threshold Elevation (TE) as a function of log(flanker/target) contrast ratio.

Figure 28 .

 28 Figure 28. Threshold Elevation (TE) as a function of log(flanker/target) contrast ratio and

Figure 29 .

 29 Figure 29. Training and follow-up results for the Sham (n = 5) and tRNS (n = 5)

3

  cycles/deg (cpd) for MD patients and 3 cpd for controls. A vertical low-contrast Gabor target (Figure2) was collinearly flanked, above and below, by two iso-oriented highcontrast Gabors (0.7 Michelson contrast). In addition, a condition with the vertical lowcontrast Gabor target flanked by orthogonally oriented Gabors patches was added; with this stimulus configuration, the target detection is not modulated by lateral interactions (Uri[START_REF] Polat | Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex[END_REF][START_REF] Polat | Lateral interactions between spatial channels: Suppression and facilitation revealed by lateral masking experiments[END_REF]. The contrast threshold of the target was estimated according to 1 up/3 down. Participants performed a temporal twoalternative forced-choice (2AFC). The target was presented in one of the two-time intervals, whereas the flankers were always presented in both time intervals. Observers had to report in which time interval the target was presented. Feedback was provided for incorrect trials. Each block was terminated after 120 trials or 16 reversals. Contrast thresholds were estimated by averaging the contrast values corresponding to the last 8 reversals. For the PRL/non-PRL testing, contrast levels from two separated staircases were displayed in a random order over the two different retinal positions.

Figure 2 .

 2 Figure 2. Stimuli used for the lateral masking paradigm.

  and presented at 57 cm. The stimuli were 10 letters (D, N, S, C, K, R, Z, H, O, and V) with Sloan[START_REF] Pelli | the Design of a New Letter Chart for Measuring Contrast Sensitivity[END_REF] character type, randomly presented for 133 ms. The target letter was presented randomly at the two eccentricities in the same block, either the PRL/non-PRL for MD patients or 4° left/right from fixation for controls. The size of the letters for measuring acuity threshold and the edge-to-edge spacing for measuring crowding varied

Figure 3 .

 3 Figure 3. Contrast threshold and TE values for PRL and controls

Figure 4 .

 4 Figure 4. Contrast threshold and TE values for PRL and non-PRL

Figure 5 .

 5 Figure 5. Visual acuity non-PRL vs. Visual acuity PRL.

`Figure 7 .

 7 Figure 7. TEs as a function of flankers/orthogonal contrast ratio.

Figure 8 .

 8 Figure 8. The stimulus configuration used in the experiments.

  contrast we pooled sensitivity (d') of the person to the signal, and on the Criterion, that is the cut-off value determined by the observer trying to detect the target and regressed the 99 pooled data against the contrast. d' and Criterion were calculated according to the Signal Detection Theory. Two-ways repeated measures ANOVAs were then used to analyse the changes in sensitivity (SC = d'collineard'single) and the change in Criterion (CC = Ccollinear -Csingle) due to the contextual modulation by the flankers on the target. The main factors were: Stimulation (Sham vs. tRNS) and Contrast levels (seven levels, see Stimuli section). Post-hoc pairwise comparisons were conducted using two-tailed t-tests with Bonferroni correction. Separated ANOVAs were conducted for the data of Experiment 1-3 (6) and Experiment 2-4 (2) because the contrast levels didn't match for the two s. One-tail t-tests based on the null hypothesis of 0 SC were also conducted to assess the polarity of contextual effects: SC > 0 indicated facilitation by the flankers were SC < 0 indicated inhibition. Results Main experiments results Results of Experiment 1 are illustrated in Figure 9 (d'), and Figure 10 (sensitivity change, SC = d'collineard'single). Results of Experiment 2 are illustrated in Figure 11 (d') and Figure 12 (SC).

Figure 9

 9 Figure 9 and 11 show the effects of tRNS on the sensitivity (d') for the single

  Figure 10. Sensitivity changes (SC) plotted as a function of target contrast with flankers

Figure 12 .

 12 Figure 12. Sensitivity changes (SC) plotted as a function of target contrast with flankers

Figure 13 .

 13 Figure 13. The figure shows the way the Criterion varies as a function of contrast in the

Figure 14 .

 14 Figure 14. Configurations used in the crowding experiment.

  , lower panel). Participants were divided into two groups: Group 1 (PL plus tRNS) and Group 2 (PL alone/sham). Each participant underwent three phases: pre-tests, training, and posttests. During pre-and post-tests, thresholds for VA at 8° and 12°, horizontal crowding at 8° and 12°, and vertical crowding at 8° and 12° were estimated. Both groups underwent

Figure 15 .

 15 Figure 15. Electrode positioning and modeled electrical field strength (normE).

Figure 16 .

 16 Figure 16. Between days crowding thresholds.

Figure 17 .

 17 Figure 17. Training and transfer tasks results for the sham and tRNS groups.

Figure 19 .

 19 Figure 19. PRL position e scotoma size.

  test, mid-test and post-test, the patients underwent the same battery of visual tasks. The battery included a contrast sensitivity measure, a visual acuity measure with Sloan letters, a tachistoscopic visual acuity measure, measure of visual crowding, a contrast threshold measure with an orthogonal and collinear lateral masking paradigm at 4 different λ (3λ, 4λ, 6λ, 8λ). The training consisted of a collinear lateral masking task with a collinear configuration. At each session of training 4 blocks were performed for a total of about 25 minutes. The target-to-flanker distance was manipulated between blocks. The 4 trained distances were 3λ, 4λ, 6λ, 8λ. During the training concomitant tRNS stimulation was applied over the occipital cortex for the duration of the training session. Each patient was trained three sessions a week over eight weeks for a total of 24 training sessions. After 12 training session the mid-test was carried out. Post-test was collected after the end of the training.

  5 deg was present in the centre of the screen for the whole procedure. Participants were asked to re-centre the fixation over the dot after each trial.Training procedure.Patients were trained with the collinear configuration of a lateral masking task at 4 different target-to-flanker separations: 3λ, 4λ, 6λ, and 8λ. Overall the task was very similar to the one used to test the contextual influences in study 1 presented in CHAPTER V. There were two main differences: first, we trained only the PRL while in study 1 we tested also a non-PRL position; second, instead of using an eye-tracker to control for fixation stability in such a long training we adopted a strategy based on visual cues that were suggested initially by[START_REF] Astle | The effect of normal aging and age-related macular degeneration on perceptual learning[END_REF] and then re-proposed byManiglia et al. (2018). During the training participants were asked to keep their fixation over the central dot for the whole duration of the task. At the same time, they were trained to "not to see" a cue positioned inside the blind visual field. In case of eye movements or big drifts the cue becomes partially visible and warn the patient to reorient the fixation towards the central dot. In our training we used three red disks of 1 deg. of diameter along the internal border of the scotoma positioned in an arrow-like configuration pointing toward the PRL. The exact position of each disk varied from patient to patient and it was initially derived from the OCT and later adjusted by trial and error until the three of them completely disappeared inside the scotoma, although remaining as close as possible to its edge. The training configuration is shown in Figure18.

Figure 20 .

 20 Figure 20. Training configuration with the stimuli and the control of fixation.

  estimated in the collinear condition and it is divided by the contrast threshold estimated in the orthogonal condition. The logarithm of the ratio is then used to evaluate the TE. A positive TE means inhibition while a negative TE indicates facilitation.tRNS stimulationWe replicated the same stimulation protocol used in study 3 in CHAPTER VII.Participants in Group 1 (PL + tRNS) were trained with concomitant electrical brain stimulation, while participants in Group 2 (PL alone) performed the training with sham stimulation. To deliver the high-frequency tRNS we used a battery-driven stimulator (BrainSTIM, EMS) with the main electrode over OZ and the return electrode over CZ. As in study 3 the stimulation consisted of an alternating current of 1.5 mA intensity with a 0 mA offset with frequencies of fluctuation distributed across a range of 100-640 Hz with zero-mean (same as inContemori et al., 2019) and maximal current density of 0.094 mA/cm2. The duration of the stimulation covered the whole training session that was approximately 25 minutes. The stimulation was applied only during the training and was not applied during the pre-test, mid-test, and post-test and thus all the data used for the analysis have been collected in the exact same condition for both, the PL and the PL + tRNS groups.Statistical analyses.

A

  Figure 21.

Figure 21 .

 21 Figure 21. Contrast threshold in the trained task as a function of group, λ, and session.

Figure 22 .

 22 Figure 22. Threshold elevation (TE) in the trained task as a function of group, λ, and

Figure 23 .

 23 Figure 23. Improvement in contrast sensitivity.

Figure 24 .

 24 Figure 24. Results from the Sloan visual acuity test.

Figure 25 .

 25 Figure 25. Results from the tachistoscopic visual acuity test.

  Figure 26.

Figure 26 .

 26 Figure 26. Results from the tachistoscopic crowding test.

  It is indeed difficult to accurately test this 'dip function' with the typical lateral masking paradigm in normal viewers since there are only two ways to manipulate the contrast ratio between targets and flankers, to reduce the contrast of the flankers or to use a pedestal below the target. Both solutions are suboptimal since in the first case we would also have a reduction in the effectiveness of the flankers and in the second case, we would alter the sensitivity to the target. Here we can verify the contrast dependent modulation of the contextual effect of the flankers by studying the inter-individual differences between participants. Due to the difference in the extent of the retinal degeneration, MDs have subjective differences in the baseline sensitivity to contrast that influence also their flanker/target contrast ratio in the baseline orthogonal condition, in which no contextual influence is expected. By pooling together the individual data of the patients we can sample the dip of the function predicted by Zenger's model. By including the control group from study 1 we can sample also the opposite extreme of the function.

  between the log-ratio and the TE (estimate = 0.505, t(1,38) = 5.022, p = 0.001, adjusted R-squared: 0.383) with a predicted intersection with zero (TE=0the switch between inhibition and facilitation) at a ratio of 0.32. Results are shown in figure27.

Figure 27 .

 27 Figure 27. Threshold Elevation (TE) as a function of log(flanker/target) contrast ratio.

Figure 28 .

 28 Figure 28. Threshold Elevation (TE) as a function of log(flanker/target) contrast ratio and

  CHAPTER VI we demonstrated how tRNS modulates lateral interactions by promoting feedforward processing. In CHAPTER VII we have shown how this contributes to a greater reduction in visual crowding following perceptual learning. These two results obtained in groups of subjects with normal vision testify to the general effect of tRNS. In CHAPTER VIII we saw how tRNS increases perceptual learning and transfer following training based on lateral masking in a clinical population. In CHAPTER IX, combining different datasets we confirmed and extended the result presented in study 4.

Table 3 .

 3 Results one-tail t-tests based on the null hypothesis of 0 sensitivity change. ..

Table 4 .

 4 Results table form statistical data analysis .......................................................

Table 5 .

 5 Details of the experimental sample. ..................................................................

Table 6 .

 6 Summary

Table 1 .

 1 Details of participants.The MD group consisted of six patients with AMD, one patient with juvenile macular degeneration (JMD), two patients with Stargardt disease, one with Best disease, one with a macular hole, one with cone-rod dystrophy (CRD), and one with central serous chorioretinopathy. The patients underlined are those considered in the non-PRL analyzes.

	Patients	Deficit	Gender	Age	Scotoma diameter	Position of PRL	Tested eye	(VA)
	MD1	CRSC	Male	50	4°	Left-up 2.0°-1.0°	LE	2/10
	MD2	Macular hole	Female	49	3°	Right-up 1.5°-1.0°	RE	7/10 (LAC)
	MD3	Best disease	Male	58	8°	Left-up 4°-2.7°	LE	2/10
	MD4	Stargardt	Male	69	7°	Left-down 2.5° -6	LE	1/10
	MD5	JMD	Female	56	4°	Right-up 2°-1°	RE	2/10
	MD6	AMD	Female	62	3	Right-down 1°-1°	LE	2/10
	MD7	AMD	Female	65	3°	Left 1°.5	RE	2/10
	MD8	AMD	Female	63	6°	Left-down 4.5°-2°	LE	2/10
	MD9	AMD	Male	61	10°	Left-down 8°-1°	LE	3/10
	MD10	AMD	Male	65	6°	Left-up 4.°-2°	RE	1/10
	MD11	AMD	Male	83	9°	Left-down 2°-2°	LE	3/10
	C1	none	Male	58	none	none	Non-dominant	10/10
	C2	none	Female	64	none	none	Non-dominant	10/10

  . The second reason for which contextual influences in contrast detection have caught the attention of several investigations in the last decades is that when they are made inefficient by a visual disorder, they can be partially restored by promoting, through training, neural plasticity at the level of lateral intracortical

	connections in V1. Thus, modulation of intracortical connections may result in a
	powerful rehabilitation tool for low vision patients. Most studies have used perceptual
	learning to induce neural plasticity in normal and in low vision population (Barollo et al.,
	2017a; Casco et al., 2018, 2014; Maniglia, Pavan, et al., 2016; Uri Polat, Ma-Naim,
	Belkin, & Sagi, 2004; Uri Polat, Ma-Naim, & Spierer, 2009; Uri Polat et al., 2012; D. T.

  Cambridge, UK) was used to increase the dynamic contrast range (12-bit luminance resolution). A 12-bit gamma-corrected lookup table (LUT) was applied so that luminance was a linear function of the digital representation of the image. The screen resolution was 1600 × 1200 pixels.

	total, 68 young subjects participated in this study (46 females; mean age 24 ± 3
	years). All participants had normal or corrected-to-normal vision and were naïve as to the
	purpose of the experiments. Thirty-eight participants were involved in the main
	Experiments 1 (N=19; 15 females; mean age 24 ± 4) and 2 (N=19; 12 females; mean age
	24 ± 4) and 30 participants were involved in the Control Experiments 3 (N=15; 10
	Apparatus
	Stimuli were displayed on a 22-inch. Philips 202P4 CRT monitor with a refresh
	rate of 85 Hz. The minimum and maximum luminance of the screen were 0.63 and 112.1
	cd/m2, respectively, and the mean luminance was 56.8 cd/m2. Luminance was measured

females; mean age 24 ± 2) and 4 (N=15; 9 females; mean age 24 ± 3). Participants in Experiment 1 and 3 were tested with flankers distant from the target, whereas participants in Experiment 2 and 4, were tested with the flankers close to the target. All participants took part voluntarily, and informed consent was obtained from all participants before the study initiated. The study conformed to the tenets of the Declaration of Helsinki, and the experimental methods have approval from the Ethical Committee of the University of Padova (protocol 1719). with a CRS Optical photometer (OP200-E; Cambridge Research System Ltd., Rochester, Kent, UK). A digital-to-analog converter (Bits#, Cambridge Research Systems,

Table 2 .

 2 Contrast range for the target at each target-to-flankers distance ()

			Contrast
	Very low		low		medium
	6 0.002 0.0028 0.0039 0.0055 0.0077 0.01005 0.015
	2	0.003	0.0058	0.011	0.025 0.041 0.08 0.3

Table 3 .

 3 Results one-tail t-tests based on the null hypothesis of 0 sensitivity change.

	Configuration	Contrast	Modulation	t	p
	6	.0028	-.39	-2.50	.011
		.0039	-.26	-1.65	.059
	2	.0030	+.42	1.76	.047
		.0058	+.44	2.35	.015

  and late (between sessions/days -Camilleri et al., 2014) components of PL. So far, PL studies used tRNS coupled with lower-level perceptual tasks, such as contrast detection or orientation discrimination, rather than training directly

higher-level visual abilities, such as visual acuity (VA) or crowding. Interestingly, tRNS during contrast-detection training has been shown to induce greater transfer (the posttraining improvement observed in an untrained task) to VA with respect to PL alone in both amblyopic and myopic patients

  Generalization of learning is a highly desirable training outcome since it could inform rehabilitative interventions for clinical populations such as amblyopic patients or agerelated macular degeneration (AMD) patients. Indeed, from a clinical point of view, task improvement is important but it remains of limited value if the effects are specific. We

chose five transfer conditions (retinal location, orientation, task [VA] separately, and retinal location + orientation, retinal location + task [VA]) and we expected different levels of generalization based on the number of manipulated features (more transfer of learning for a single property manipulation and less transfer for combined manipulations).

Table 4 .

 4 Results table form statistical data analysis

Table 5 .

 5 Details of the experimental sample.

	Patients	Deficit	Gender	Age	Scotoma diameter	Position of PRL	Tested eye	(VA)
	MD1	AMD	Female	59	17°	Left -8.0° 0.2°	LE	1/10
	MD2	AMD	Female	82	22°	Left-down -5.5° 1.0°	RE	1/10

Acknowledgments

Firstly, I would like to express my gratitude to my supervisor, Professor Clara Casco, for her priceless continuous guidance.

Chapter V.

Contextual Influences on the Peripheral Retina of Patients with Macular Degeneration

This chapter has been published in Scientific Reports, 9 (1), 9284. [START_REF] Contemori | Contextual influences in the peripheral retina of patients with macular degeneration[END_REF]. https://doi. org/10.1038/s41598-019-45648-4. We have seen in CHAPTER II that adaptation to the absence of central input produces an enlargement of the receptive fields and loss of neural tuning. These adaptations might also influence the organisation of the lateral connections in V1 (Baseler et al., 2011b; Y. M. [START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF]. In the normal visual periphery, the range of interactions in the PRL is greater than that of the fovea [START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Shani | Eccentricity effects on lateral interactions[END_REF]. In fact, in fovea inhibition is found is between 1 and 1.5λ while facilitation is found between 2 and 4λ. In the periphery the range varies according to the eccentricity but generally inhibition is found up to 2-3λ and facilitation up to 8-12λ [START_REF] Giorgi | Facilitation of contrast detection in near-peripheral vision[END_REF][START_REF] Lev | Collinear facilitation and suppression at the periphery[END_REF][START_REF] Shani | Eccentricity effects on lateral interactions[END_REF]. Studying the neural adaptation to the central blindness might produce better visual rehabilitation protocols by taking advantage of undergoing plastic processing. A non-invasive and convenient way of studying these neural adaptive changes is by means of psychophysical testing.

In the previous literature, there are reports of reduced collinear inhibition found in MD and this has been interpreted as a sign of neural plasticity [START_REF] Maniglia | Perceptual learning leads to long lasting visual improvement in patients with central vision loss[END_REF]Maniglia et al., 2018). However, these studies had some important limitations.

First, the sample size was very small (3-5 subjects). Second, only the PRL was tested. In the study I am going to present to you, we have tried to confirm and extend the previous results by overcoming these limitations.

Comparison of contrast thresholds and TE data obtained in the PRL and non-PRL of MD patients are shown in Figure 4. Repeated-measures ANOVAs, including as factors the retinal locus (PRL and non-PRL) and the λ (2, 3, 4, and 8λ), were conducted on both the contrast thresholds and TE data of the subgroup of 8 patients that were tested with stimuli presented in both the PRL and non-PRL positions.

The ANOVA on contrast threshold revealed that neither the main factors (PRL: F(1,19) = .077, p = .785, partial-η2 = .004; λ: F(3,57) = .947, p = .424, partial-η2 = .047) nor the PRL x λ interaction (F(3,57) = .463, p = .709, partial-η2 = .024) were significant.

Similarly, the ANOVA on TE data did not reveal a significant effect of the retinal locus (F(1,19) = .027, p = .872, partial-η2 = .001), of λ (F(3,57) = .546, p = .653, partial-η2 = .028), and of the interaction between PRL and λ (F(3,57) = .389, p = .762, partial-η2 = .02).

Control Experiments results

Pooled d's obtained in the control experiments correlated positively with contrast, both at 6 (R2 = 0.46, p < .001) and 2 (R2 = 0.47, p < .001), indicating higher sensitivity as contrast increases. The ANOVA didn't show a significant effect of Stimulation on SC, neither at 6 (F(1,14) = .67, p = .43, η2p = .046) nor at 2 (F(1,14) = 1.83, p = .20, η2p = .12). The interaction between Stimulation × Contrast Levels was also not significant, either at 6 (F(6,84) = 1.21, p = .69, η2p =.044) or at 2  (F(6,84) = .50, p = .80, η2p = .035).

Pooled C data correlated negatively with contrast at 6 (R2 = 0.25, p < .001) and 2 (R2 = 0.32, p < .001), indicating less positive criterion as contrast increases. The ANOVA on CC reveal neither the effect of Stimulation (6: F(1,14) = .44, p = .51, η2p = .032); 2: F(1,14) = 0.1, p = .92, η2p = .001) nor of the interaction between Stimulation × Contrast: (6: F(6,84) = .65, p = .69, η2p = .044; 2: F(6,84) = .5, p = 0.8, η2p = 0.035), indicating that the stimulation did not affect the criterion. effect of stimulation in the two configurations is opposite may account for the relevant result, discussed later, of a switch of SC from positive to negative at low contrasts.

Chapter VIII. tRNS Boosts Perceptual Learning in MD Patients

In the introduction, we have seen that one of the effects of adaptation in the absence of central input is the enlargement of the receptive fields with consequent loss of neural tuning (Baseler et al., 2011a;[START_REF] Chino | Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina[END_REF]. Moreover, the processing in the PRL of patients is not optimized for fine vision, and the effect of the adaptive plasticity is limited as confirmed by the study presented in CHAPTER V [START_REF] Musel | Residual abilities in age-related macular degeneration to process spatial frequencies during natural scene categorization[END_REF][START_REF] Peyrin | Scene perception in age-related macular degeneration: Effect of spatial frequencies and contrast in residual vision[END_REF]. In CHAPTER IV we have seen that the tRNS stimulation might be useful to improve PL both between and within sessions [START_REF] Campana | Improving visual functions in adult amblyopia with combined perceptual training and transcranial random noise stimulation (tRNS): A pilot study[END_REF][START_REF] Fertonani | Random Noise Stimulation Improves Neuroplasticity in Perceptual Learning[END_REF]. In CHAPTER VI we have demonstrated that the tRNS might be able to help in modulating the strength of the lateral interactions in the early visual cortex. In CHAPTER VII we have seen that this modulatory effect together with the more general ability of tRNS in boosting PL, helps in the reduction of visual crowding in the periphery normal viewers. Here I present a translational study in which we applied a combined PL + tRNS approach improving residual vision in the PRL of AMD patients.

Materials and Methods

Participants.

Study candidates were selected from the list of the ophthalmology patients of the Paul Riquet hospital in Toulouse.

The first contact was made by their ophthalmologist. If they expressed the will to participate in the study, they were invited to the hospital for a free ophthalmological assessment on the basis of which we evaluated their inclusion. All participants gave their written informed consent prior to their inclusion in the study. This study follows the ethical standards of the Declaration of Helsinki (1964) and the experimental protocol was approved by the CNRS ethical committee before the beginning of the study (Comité de Protection des Personnes, protocole 13018). Participants received reimbursement for all the travel expenses related to their participation.

Only patients with an absolute central bilateral scotoma were included in the study. Binocular visual field test was performed by means of an Octopus® 300 perimeter; Köniz, Switzerland. Only patients with visual acuity between below 3/10 and above 1/10 were considered in the study. Since the training was performed monocularly in the best eye, we only included in the study patients with a single and stable PRL in the trained eye. The presence of concomitant ocular diseases or a non-stabilized scotoma were considered grounds for exclusion. The same for the presence of diagnosed cognitive or mood disorders. Of all contacted patients, only 16 were eligible after the initial ophthalmological screening. Of these, only 12 completed the study. Of the remaining 4, test minus 1 ((mid /pre)-1) was calculated as an index of improvement. In this case the higher the index the better the improvement. An index of zero means no improvement at all. For The mixed-design ANOVA conducted on index of improvement, including as factors the group (PL vs. PL + tRNS) and the eye (trained vs. untrained) and the session (mid vs. post) and the spatial frequency (1,3,5,7 cpd) indicated that, with a p.value slightly higher than the cut-off, the effect of group was not significant (F(1,10.1) = 4.60, p = 0.057). The effect of eye (F(3,149.24) = 1.09, p = 0.297), the effect of session (F(1,149.2) = 0.805, p = 0.371), and the group x session interaction (F(1,149.2) = 0.841, p = 0.360) were all not significant. We found a significant effect of the spatial frequency (F(3,151) = 7.123, p = 0.0001) and interestingly we found a significant effect of the group x spatial frequency interaction (F(3,151) = 5.605, p = 0.001). Post hoc comparisons for the group factor with the null hypothesis of zero mean showed that only PL + tRNS group differed from zero (mean PL = 0.985, se = 3.25, t = 0.303, p =1; mean PL + tRNS = 10.238, se = 2.83, t = 3.612, p = 0.008). Moreover, pairwise post hoc comparisons for the group x spatial frequency interaction revealed that the amount of improvement in the two groups was different for the two higher spatial frequencies tested (PL -PL + tRNS at 1 cpd = -0.455, se = 5.32, t = -0.085, p = 1; PL -PL + tRNS at 3 cpd = -2.160, se = 5.32, t = -0.406, p = 1; PL -PL + tRNS at 5 cpd = -14.633, se = 5.44, t = -2.689, p = 0.050; PL -PL + tRNS at 7 cpd = -19.765, se = 5.82, t = -3.397, p = 0.008). Results are shown in Figure 23.

Visual acuity

For the visual acuity the index of change was calculated upon the LogMar output from the FrACT Sloan test and can be interpreted as follow: a negative index means improvement, an index of zero means no change, and a positive index means a worsening in the performance after the training. The mixed-design ANOVA conducted on ratio between the letter size at the mid-and post-test divided by the baseline at the pre-test data, including as factors the group (PL vs. PL + tRNS) and the eye (trained vs. untrained) and the session (mid vs. post) indicated that the effect of group (F(1,40) = 0.124, p = 0.726), the effect of eye (F(1,40) = 0.172, p = 0.726), the effect of session (F(1,40) = 3.152, p = 0.083), the group x eye interaction (F(1,40) = 0.0223, p = 0.882) were not significant. Post hoc comparisons for the group factor with the null hypothesis of zero mean showed that the average both groups was not different from zero (mean PL = -0.049, se = 0.147, t = -0.338, p = 1; mean PL + tRNS = -0.117, se = 0.124, t = -0.946, p = 0.732). Results are shown in Figure 24.

untrained) and the session (mid vs. post) indicated that both, the effect of group (F (1,40) = 7.72, p = 0.008), and the effect of eye (F(1,40) = 11.1, p = 0.002) were significant, while the effect of session (F(1,40) = 1.04, p = 0.314), and the group x eye interaction (F(1,40) = 1.02, p = 0.318) were not. Post hoc comparisons for the group factor with the null hypothesis of zero mean showed that both groups differed from zero (mean PL = -0.349, se = 0.058, t = -5.985, p < 0.001; mean PL + tRNS = -0.137, se = 0.049, t = -2.777, p = 0.0391). Post hoc comparisons for the eye factor with the null hypothesis of zero mean showed that both the trained and untrained eye differed from zero (mean Trained = -0.730, se = 0.054, t = -6.857, p < 0.001; mean Untrained = -0.116, se = 0.054, t = -2.142, p < 0.081). Results are shown in Figure 25. confirmed since the PL has transferred to the untrained eye, to tachistoscopic visual acuity, and visual crowding. Furthermore, only the PL + tRNS group had transferred to higher spatial frequencies than the trained ones. But two unexpected results came out: we did not find transfer to visual acuity with Sloan letters and the PL + tRNS group did not have the expected transfer to visual crowding. Moreover, we did not find significant differences between the mid-tests and the post-tests for the two groups. This leaves us thinking that 12 sessions are enough to trigger the transfer. In the following paragraphs, I will discuss these results in detail. Table 6. Summary of the results relative to the transfer tasks.

The results of the transfer tasks are summarized in the table, the green "V" symbolizes significant results, while the red "X" indicates non-significant ones. From left to right: Contrast sensitivity (CSC), spatial frequencies (SF 1, SF 3, SF 5, SF 7), visual acuity with Sloan letters (VAS), tachistoscopic visual acuity (TVA), tachistoscopic visual crowding (TVC).

Contrast sensitivity

The measurement of the contrast sensitivity curve among the various transfer tasks is certainly the closest one to the trained task. The fundamental difference is the absence of the two flankers and therefore the absence of contextual influences. In the previous literature, it has been shown that the improvement in contrast sensitivity achieved by training with lateral masking could also transfer to untrained spatial Chapter X.

Overview and Future Directions

The first study was presented in CHAPTER V. This experiment aimed to evaluate the presence of spontaneous or use-dependent plasticity in the patients' PRL. In the previous literature (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF]Maniglia et al., 2018;[START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF] there is debate whether the spared periphery undergoes a general process of adaptation triggered by the presence of the scotoma ("use-dependent reorganization" hypothesis) or whether the preferential use of the PRL for active high demanding tasks triggers a more specific use-dependent adaptation (D. D. [START_REF] Dilks | Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the "Preferred Retinal Locus[END_REF]Maniglia et al., 2018;[START_REF] Plank | FMRI with central vision loss: Effects of fixation locus and stimulus type[END_REF]. Knowing the type of adaptation process to which the PRL is subject is important to avoid damaging any compensatory mechanisms in place and instead to try to maximize them with the training. In this study, we tried to investigate this issue psychophysically by comparing spatial integration in the PRL, in a symmetrical retinal position (non-PRL) and a region with matched eccentricity in a control group. To do this, we probed the contextual influences by measuring the contrast gain for a vertical Gabor target, flanked by two high-contrast collinear masks compared to the orthogonal baseline condition. In line with previous literature (Maniglia et al., 2018), our prediction was to find evidence for plasticity, in the form of reduced collinear inhibition. Moreover, we expected the reduction of inhibition to be stronger in the PRL. Surprisingly, the betweengroups analysis revealed that in both PRL and non-PRL, at the shortest target-to-flankers distance (2λ), the contextual influence was facilitatory, rather than inhibitory as in controls. Further analysis with data collapsed across groups showed that this effect depends on the individual contrast sensitivity at the baseline. When the target-to-flankers will open the doors for the application of PL + tRNS protocols for many more clinical populations.

influences in the visual cortex of MD patients and healthy controls. We used well established and replicable measures that lead to novel results.

Future direction and general conclusion

The studies presented in this thesis are a necessary brick in the development of an