Ómar I Jóhannesson

Jérôme Tagu
email: jerome.tagu@parisdescartes.fr

Árni Kristjánsson

Asymmetries of the visual system and their influence on visual performance and oculomotor dynamics

Keywords: Nasal-temporal asymmetries, saccadic eye movements, peak velocities, visual attention, visual neuroscience

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

the visual system manifest in various measures, in basic psychophysical tests of visual performance, attentional processing, choice behaviour, saccadic peak velocity and latencies. Nasal-temporal asymmetries on saccadic latency seem primarily to occur for express saccades. Neural asymmetries between the upper and lower hemifields are strong and cause corresponding differences in performance between the hemifields.

There are interesting individual differences in asymmetric processing which seem to be related to the strength of eye dominance. These neurophysiological asymmetries and the corresponding asymmetries in visual performance and oculomotor behaviour can strongly influence experimental results in vision and must be considered during experimental design and the interpretation of results.

Introduction

When sudden events in the visual field grab our attention, such attentional capture is the result of a process that rapidly converts light into neural signals. This process must be fast so that we can quickly react to danger, such as a predator, or an approaching car.

There is, indeed, evidence that recognition of visual scenes is fast, and can occur as quickly as in 150 ms [START_REF] Thorpe | Speed of processing in the human visual system[END_REF]. If our attention is drawn to visual events that occur in the periphery, we typically quickly move the centre of our gaze there by making a saccade (Kristjánsson, 2011). Suddenly appearing stimuli in the periphery might be important since they may signal danger. Therefore, prioritizing processing of stimuli in the periphery might be useful from an evolutionary point of view. Neural projections from the retina to eye movement control centres in the mesencephalon are accordingly very fast, since the fastest human eye movement responses to visual events can occur in around 100 ms [START_REF] Edelman | The influence of objectrelative visuomotor set on express saccades[END_REF][START_REF] Edelman | Activity of visuomotor burst neurons in the superior colliculus accompanying express saccades[END_REF][START_REF] Fischer | Saccadic eye movements after extremely short reaction times in the monkey[END_REF].

In this review, our focus will be on attention shifts and eye movements to peripheral visual stimuli. Our main goal is to assess to what degree it matters where in the visual field the stimuli appear, for example on which part of the retina the stimuli are projected to. Does it matter where sudden events appear for how strongly and how quickly we react to them and shift our gaze there? While this is a simple question, it nevertheless requires a rather complicated answer, involving different levels of explanation. One part of the answer involves discussion of neurophysiology, or the basic characteristics of the neural mechanisms responsible for these eye movements.

Secondly, any attempt to answer this question requires discussion of the particularities of the tests used to assess any asymmetries. This latter way of answering the question involves a more functional viewpoint.

To cut to the chase, there is indeed good evidence for asymmetrical processing within our visual system, depending on where in the visual field stimuli appear. The visual system preferentially processes stimuli within certain parts of the visual field, for example by devoting more neural machinery to those stimuli [START_REF] Anderson | Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors[END_REF][START_REF] Curcio | Distribution of cones in human and monkey retina: individual variability and radial asymmetry[END_REF][START_REF] Osterberg | Topography of the layer of rods and cones in the human retina[END_REF][START_REF] Perry | Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey[END_REF][START_REF] Silva | Radial asymmetries in population receptive field size and cortical magnification factor in early visual cortex[END_REF]; see further discussion below). Asymmetries in the visual system are interesting from a number of perspectives. They are important for understanding vision not only from a 4 neurophysiological viewpoint, but also for understanding both attentional orienting within the visual field and eye movements, since attention and eye movements are tightly coupled [START_REF] Deubel | Saccade target selection and object recognition: Evidence for a common attentional mechanism[END_REF][START_REF] Kowler | The role of attention in the programming of saccades[END_REF]; see e.g. Kristjánsson, 2011 for review). We discuss whether there may be any benefits to a particular organization that causes such asymmetries. Such asymmetries may in some cases serve a function, while in others the asymmetries may have developed as by-products of other functions.

Definitions

Before proceeding further, some basic definitions should be made. Figure 1 shows the neural projections from the retina to the main brain regions involved in vision and eye movement generation and how the visual fields are projected onto the retina. Figure 2 then shows how different saccade types are defined with respect to the stimulation sites and saccade projections. Abducting/adducting and centripetal/centrifugal saccades, all have fixed reference points with respect to the body. Centripetal saccades are made from the periphery of the orbit to its centre (the primary position of the eyeball) and centrifugal saccades are made from the centre into the periphery. Note that S1 and S2 of Figure 2 are so-called symmetric saccades [START_REF] Becker | The neurobiology of saccadic eye movements[END_REF] as they include both a centripetal and a centrifugal component. Adducting saccades are directed from the periphery of body towards its centre line. For abducting saccades, this is reversed.

Because the nasal and temporal sides are defined with respect to the visual fields (see Figures 1 and2) the position of the eyeball within the orbit does not affect what is nasal and what is temporal. The left part of the visual field of the right eye is its nasal hemifield and projects to the temporal hemiretina and the right part of the right eye's visual field is its temporal hemifield and projects to the nasal hemiretina. The only difference between adducting/abducting and nasal/temporal saccades is therefore in how they are defined; the former are defined with respect to the anterior-posterior centre line of the body, while the latter are defined with respect to the visual hemifields.

All adducting saccades are nasal saccades and all abducting saccades are temporal saccades.

There are three commonly used methods used for eliciting saccades: the gap (the fixation point disappears prior to the onset of the target), the step (the offset of the fixation point and the onset of the target happen simultaneously) and the overlap paradigms (the offset of the fixation point happens after the onset of the target).

5

Saccadic latency has been found to be shorter in the gap paradigm (about 150 ms or less) than in the step and overlap paradigms. In the step paradigm, the latency is typically around 200 ms and in the overlap paradigm the latency in commonly observed to be about 250 ms or longer (e.g. [START_REF] Saslow | Effects of components of displacement-step stimuli upon latency for saccadic eye movement[END_REF].

Figure 1. The visual pathways. The figure shows the projections from the nasal (red lines) and the temporal (green lines) hemifields through the nasal and temporal hemiretinae, to the SC (the retinotectal pathway), and through the LGN to V1 (thalamocortical pathway). The neurophysiological nasal-temporal asymmetry (NTA) is reflected by wider red than green lines. (Abbreviations: SC: Superior colliculi; LGN: Lateral geniculate nucleus; V1: primary visual cortex).

Saccadic eye movements can involve different cortical and subcortical networks

according to the situation triggering the saccade. Theoretically, saccades can be defined 6 as reflexive and voluntary saccades (e.g. Walker et al. 2000a). Reflexive saccades are triggered by sudden events appearing in the environment and are fast and automatic (these are for example the saccades you make when danger approaches your car while you are driving on the highway). Under laboratory settings, the target usually appears at random locations after a random time interval. But in contrast to the unexpected appearance of a "target" in natural settings the appearance of a target in an experiment is not completely unexpected; the participant knows that a target will appear but does not know exactly where and when. This means that reflexive saccades under laboratory settings might, at least to some extent, be influenced by top-down factors. Express saccadesa subclass of reflexive saccadeshave very short latencies (< 120-130 ms; [START_REF] Delinte | Amplitude transition function of human express saccades[END_REF][START_REF] Fischer | Human express saccades: extremely short reaction times of goal directed eye movements[END_REF][START_REF] Heeman | The influence of distractors on express saccades[END_REF] and have been thought to be the fastest possible saccade type and to reflect a visuomotor "grasp" reflex (e.g. [START_REF] Fischer | Human express saccades: extremely short reaction times of goal directed eye movements[END_REF], partly driven by the subcortical retinotectal pathway [START_REF] Dorris | Neuronal activity in monkey superior colliculus related to the initiation of saccadic eye movements[END_REF][START_REF] Hall | Interlaminar connections of the superior colliculus in the tree shrew. III: the optic layer[END_REF]; see also [START_REF] Koller | Saccade latency bias toward temporal hemifield: evidence for role of retinotectal tract in mediating reflexive saccades[END_REF]. Express saccades occur most often in the gap paradigm. The other "regular" reflexive saccades are thought to be driven by the thalamocortical pathway, which projects to the parietal eye field and then to the superior colliculus (SC) [START_REF] Pierrot-Deseilligny | Cortical control of saccades[END_REF][START_REF] White | The superior colliculus[END_REF]. Note that whether express saccades can be considered a distinct class of saccades has been hotly debated [START_REF] Fischer | Express saccades and visual attention[END_REF][START_REF] Heeman | The influence of distractors on express saccades[END_REF]Jüttner and Wolf 1992;[START_REF] Kingstone | Visual offsets facilitate saccadic latency: Does predisengagement of visuospatial attention mediate this gap effect?[END_REF][START_REF] Wenban-Smith | Express saccades: is there a separate population in humans?[END_REF].

Voluntary saccades, on the other hand, are strongly mediated by top-down influences.

These saccades are those we make during reading or while looking for a friend in a crowded square. They are driven by the thalamocortical pathway which projects to the parietal eye field and frontal eye field, and then to the SC [START_REF] Pierrot-Deseilligny | Cortical control of saccades[END_REF][START_REF] White | The superior colliculus[END_REF]. The signature of these saccades is a longer and more variable latency. The presence of nasal-temporal asymmetries (NTAs) in these different saccade types has been examined with the gap (e.g. [START_REF] Bibi | The influence of motor training on human express saccade production[END_REF][START_REF] Jóhannesson | Effects of saccade training on express saccade proportions, saccade latencies, and peak velocities: an investigation of nasal/temporal differences[END_REF]), step (e.g. Jóhannesson et al. 2012;Tagu et al. 2018a) and overlap paradigms (e.g. [START_REF] Knox | Temporal stability and the effects of training on saccade latency in "Express Saccade Makers[END_REF][START_REF] Knox | Express saccades in distinct populations: east, west, and in-between[END_REF]Tagu et al., 2018a). 7

Overview

This review is divided into two main sections: In Section 2 we discuss NTAs in the visual systems, followed by discussion of other asymmetries in the visual system (Section 3). In Section 2.1 we start by reviewing evidence showing biological (or neurophysiological) NTAs in the retina and the optic nerve. NTAs in the development of the visual system are discussed in Section 2.2. In Section 2.3 we discuss NTAs in visual sensitivity, acuity and resolution. We then discuss NTAs in visual attention in section 2.4 and in Section 2.5 we review evidence concerning NTAs in saccades. While the previous sections review NTAs in studies involving monocular viewing conditions, the Section 3.1 is devoted to studies of NTAs during binocular viewing. We then discuss evidence regarding other visual asymmetries, such as vertical asymmetries in eye movements and attention (Section 3.2) and asymmetries between centrifugal and centripetal saccades (Section 3.3). Finally, we discuss individual variability in NTAs in Section 4. Adducting (S1, S4, S5 and S7) and abducting (S2, S3, S6 and S8) saccades are defined with respect to the anterior-posterior centre line of the body. Centripetal (S3 and S4) and centrifugal (S5, S6, S7 and S8) saccades are defined with respect to the anteriorposterior centre line of the orbit. Nasal (S1, S4, S5 and S7) and temporal (S2, S3, S6 and S8) saccades are defined with respect to the centre of gaze. Dotted lines represent centripetal saccades and whole lines represent centrifugal saccades. Red lines represent saccades (abducting) into the temporal hemifield and green lines represent saccades (adducting) into the nasal hemifield.

Nasal-temporal asymmetries

When studying NTAs -whether in oculomotor behaviour or visual performance -the view of one eye is typically blocked, since what is nasal for one eye is temporal for the other (see Figure 1). The SC, a key area in the mesencephalon for eye movement control, contains a retinotopic map [START_REF] White | The superior colliculus[END_REF], where a substantially bigger area is devoted to the foveal retina than the peripheral retina [START_REF] White | The superior colliculus[END_REF]. Under monocular viewing the right visual field of the right eye (the temporal hemifield) is projected to the nasal hemiretina and its left visual field (nasal hemifield) is projected to its temporal hemiretina (Figure 1). During binocular viewing, the signals from the temporal hemiretina of one eye and signals from the nasal hemiretina of the other are combined in the retinotopic map in SC and -at least to some extent -in the optic nerve, counteracting effects of biological NTAs.

Neurophysiological asymmetries

Photoreceptors in the primate retina turn light into neural messages that are then conveyed to visual processing centres in the cerebral cortices and the midbrain. The distribution of these receptors is not uniform. Firstly, there is good evidence that visual receptor density decreases the further away from the fovea we go [START_REF] Anderson | Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors[END_REF][START_REF] Osterberg | Topography of the layer of rods and cones in the human retina[END_REF]. This is also true for ganglion cell density [START_REF] Perry | Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey[END_REF]. This causes corresponding decreases in visual resolution in the periphery (e.g. [START_REF] Anderson | Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors[END_REF][START_REF] Anstis | A chart demonstrating variations in acuity with retinal position[END_REF][START_REF] Thibos | Retinal limits to the detection and resolution of gratings[END_REF]. But more importantly for our purposes, there are asymmetries in the distribution of cells in the nasal and temporal parts of the retina: [START_REF] Perry | The ganglion cell and cone distributions in the monkey's retina: Implications for central magnification factors[END_REF] found that at peripheral locations, cone density was higher in the nasal-than in the temporal hemiretina of each eye. [START_REF] Curcio | Distribution of cones in human and monkey retina: individual variability and radial asymmetry[END_REF] then confirmed this finding for monkeys and humans.

Importantly, this asymmetry in cone density is associated with an asymmetry in ganglion cell density in primates, with higher density in the nasal than the temporal part of the retina [START_REF] Curcio | Topography of ganglion cells in human retina[END_REF][START_REF] Rolls | Topography of the retina and striate cortex and its relationship to visual acuity in rhesus monkeys and squirrel monkeys[END_REF][START_REF] Whitteridge | Geometrical relations between the retina and the visual cortex[END_REF]. This NTA is particularly prominent at higher eccentricities, which is important for the current question of attentional and oculomotor responses to peripheral events. [START_REF] Perry | The ganglion cell and cone distributions in the monkey's retina: Implications for central magnification factors[END_REF] found that the number of ganglion cells in the retinae of their monkeys varied from 1.4 × 10 6 to 1.8 × 10 6 . At about 0.5 mm from the fovea the 9 ganglion cell density peaked with higher density in the nasal than in the temporal part of the retina. This asymmetry was considerably higher in the periphery where the density in the temporal hemiretina dropped to half the nasal hemiretina density.

Additionally, the ratio of cones versus ganglion cells is not uniform across the retina, since in the eyes of Macaca fascicularis, the ganglion cell to receptor ratios change from 4 to 1 at centre to 1:1 about 15° away from the fovea [START_REF] Wässle | Cortical magnification factor and the ganglion cell density of the primate retina[END_REF][START_REF] Wässle | Retinal ganglion cell density and cortical magnification factor in the primate[END_REF]. Within 3° temporally and 4° nasally there are more than 2 ganglion cells per cone but at 14° temporally and 20° nasally the ratio is about 1:1. Further out in the periphery, cones are more numerous than ganglion cells. Furthermore, [START_REF] Wässle | Retinal ganglion cell density and cortical magnification factor in the primate[END_REF] found that the density of ganglion cells decreased faster with eccentricity for the temporal than the nasal hemiretina. If there are fewer ganglion cells per cone in the temporal than nasal hemiretina this may lead to correspondingly lower spatial acuity.

Overall, the density of both cones and ganglion cells is higher in the nasal than temporal hemiretina at the same eccentricity (e.g. [START_REF] Curcio | Topography of ganglion cells in human retina[END_REF].

The axons of the ganglion cells from each eye are then grouped to form the optic nerves and the NTAs in ganglion cell density produce NTAs in the optic nerves. Indeed, 60% of the axons in each optic nerve project from nasal ganglion cells while the remaining 40% project from temporal ones [START_REF] Rachel | Optic nerve, optic chiasm, and optic tracts[END_REF][START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF]. These NTAs in the optic nerves have consequences at all stages of the visual pathways, as we will discuss.

While the main visual pathway in vertebrates, other than mammals, runs from the retina to the midbrain (the retinotectal pathway), the main visual pathway for mammals runs from the retina to the cortex -the thalamocortical pathway. In mammals, only about 10% of the optic nerve projects from the eye to the SC to form the retinotectal pathway, while about 90% project to the lateral geniculate nucleus (LGN) and then to the visual cortex to form the thalamocortical pathway [START_REF] Perry | Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey[END_REF].

Note that in the thalamocortical pathway, the visual cortex relays information to the SC (both directly and indirectly; see White & Munoz, 2011 for review). This is important as the SC play a crucial role in generating eye movements. There is evidence that the SC receive more direct input from the nasal than the temporal hemiretina, which suggests that NTAs occur in the retinotectal pathway. Neurophysiological studies on cats [START_REF] Hubel | Mode of termination of retinotectal fibers in macaque monkey: an autoradiographic study[END_REF][START_REF] Sterling | Quantitative mapping with the electron microscope: Retinal terminals in the superior colliculus[END_REF]) have revealed NTAs in projections from the hemiretinae to the SC, findings that have then been replicated in both old 10 world [START_REF] Itaya | Retinal projections to the inferior and medial pulvinar nuclei in the old-world monkey[END_REF] and new world monkeys [START_REF] Tigges | Distribution of retionfugal and corticofugal axon terminals in the superior colliculus of squirrel monkey[END_REF]. Furthermore, [START_REF] Sylvester | Visual fMRI responses in human superior colliculus show a temporal-nasal asymmetry that is absent in lateral geniculate and visual cortex[END_REF] found that fMRI responses to contrast reversing checkerboards were stronger for temporal hemifield stimuli (that project to the nasal hemiretina) than nasal hemifield stimuli (that project to the temporal hemiretina).

Notably, this NTA in the blood-oxygen-level dependent (BOLD) signal was only observed in the SC, not the LGN nor the visual cortex. This argues for a unique asymmetry in the retinotectal projections that does not apply to the other pathways. [START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF] found that the ratio between nasal and temporal projection to the SC was on average 1.57 favouring the nasal hemiretina in rhesus monkeys (Macaca mulatta). Importantly, [START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF] found similar asymmetries in projections to the LGN where the same ratio was 1.54 favouring the nasal hemiretina. In contrast to [START_REF] Sylvester | Visual fMRI responses in human superior colliculus show a temporal-nasal asymmetry that is absent in lateral geniculate and visual cortex[END_REF], [START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF] concluded that there were NTAs in both retinotectal and thalamocortical pathways. Note that NTAs in the LGN of macaque monkeys had already been found by Connoly and Van Essen (1984). The idea of NTAs in the thalamocortical pathway has been reinforced by findings in both monkeys [START_REF] Levay | The complete pattern of ocular dominance stripes in the striate cortex and visual field of the macaque monkey[END_REF][START_REF] Tychsen | Nasotemporal asymmetries in V1: Ocular dominance columns of infant, adult, and strabismic macaque monkeys[END_REF] and humans [START_REF] Toosy | Asymmetrical activation of human visual cortex demonstrated by functional MRI with monocular stimulation[END_REF] arguing for NTAs in the primary visual cortex (V1). Indeed, [START_REF] Tychsen | Nasotemporal asymmetries in V1: Ocular dominance columns of infant, adult, and strabismic macaque monkeys[END_REF] found that ocular dominance columns in V1 formed by the input of the nasal hemiretina were larger than those formed by information from the temporal hemiretina.

Altogether, there is strong evidence for neurophysiological NTAs, both in the retinotectal-and thalamocortical pathways. These neurophysiological NTAs are likely to strongly influence the processing of visual information by both subcortical and cortical structures. 11 [START_REF] Lewis | The development of the temporal and nasal visual fields during infancy[END_REF] used visual perimetry measurements to assess the development of the functional visual field of children from birth to 6 month old, comparing their performance with adult performance. One half of each group had their left eye patched and the other their right eye. On half the trials, two light spots of different sizes (3° and 6°) were flashed in 15° steps from the centre to 120° in the nasal and temporal visual hemifields. In the other half, no lights were flashed and those trials were used as baseline. Lewis and Maurer found that with a stimulus of 3°, four week old infants did not orient more often to the stimulus than to a blank area in the periphery.

Nasal-temporal asymmetries during development

But with increased age, the infants looked further out into the periphery and significantly longer into the temporal visual hemifield (processed by the nasal hemiretina) than into the nasal hemifield (processed by the temporal hemiretina). For example, 4 month old infants continued to orient towards a 3° stimulus until eccentricities as high as 75° were reached in the temporal visual hemifield while in the nasal hemifield, orientation towards the 3° stimulus was only found until eccentricities of 30° were reached. With a larger stimulus (6°) they oriented towards stimuli located at eccentricities reaching 105° and 45° in the temporal and nasal visual hemifields, respectively. Interestingly, the development of the temporal hemiretina seemed to lag behind the nasal hemiretina since this asymmetry was affected by age.

This asymmetry in infants also shows up in how they orient to facelike stimuli. [START_REF] Simion | Preferential orienting to faces in newborns: A temporal-nasal asymmetry[END_REF] tested how newborns, with one eye closed with a patch, oriented to three dots arranged in a facelike arrangement that conveyed eyes and mouth, compared to non-facelike stimuli (the same dots presented upside down). They found that newborns oriented preferentially to the facelike arrangement of the dots, but interestingly, this asymmetry only occurred for the temporal hemifield. Infants do indeed seem to prefer looking into their temporal visual hemifields. [START_REF] Johnson | Preferential orienting to faces in 4-month-olds: analysis of temporal-nasal visual field differences[END_REF] presented upright and inverted facelike stimuli in the nasal and temporal hemifields. The infants looked more frequently (although not significantly) at facelike stimuli in the temporal than the nasal hemifield. But when inverted faces and stripes that looked similar to the inverted faces were presented, the main effect of visual hemifield was significant and the infants looked more often at stimuli in the temporal than the nasal hemifield.

Previously, [START_REF] Simion | Inhibition of return in newborns is temporo-nasal asymmetrical[END_REF] had shown that inhibition of return was stronger for the temporal than nasal hemifields in newborns. From such findings, [START_REF] Johnson | Subcortical face processing[END_REF] argued that these looking preferences and processing asymmetries of newborns reflect 12 the operation of a fast, low-spatial-frequency, subcortical face-detection pathway (Vuilleumier & Scwartz, 2001). Note that [START_REF] Tomalski | Temporal-nasal asymmetry of rapid orienting to face-like stimuli[END_REF] showed that this asymmetry in orienting to faces persists into adulthood, but the effect was not as large for adults as for the children. [START_REF] Lewis | The development of young infants' ability to detect stimuli in the nasal visual field[END_REF] observed converging results for the ability of 1 to 2 month old infants to detect lines of varying width (by shifting gaze towards them). Their findings showed that one month old infants were able to detect 1.5° wide lines at 30° in the temporal visual field but not 12.8° wide lines at 20° in the nasal visual field. This suggests that the infants were over eight times less sensitive to stimuli at 20° of eccentricity in the temporal part of the retina than at 30° in the nasal part.

What holds for line detection might not hold for detecting intensity differences.

In [START_REF] Bowering | Sensitivity in the nasal and temporal hemifields in children treated for cataract[END_REF] the task was to detect a light of varying intensity (0.43° in diameter) presented for 100 ms either 20° nasally or 30° temporally. They found that the sensitivity of 7 to 9 year old children was higher at 20° of eccentricity in the nasal hemifield than at 30° in the temporal hemifield. [START_REF] Bowering | Sensitivity in the nasal and temporal hemifields in children treated for cataract[END_REF] found the same pattern for their adult participants, suggesting that this advantage of the nasal visual hemifield (temporal hemiretina) emerges early in life and does not disappear in adulthood. When considering these results, it is important to note that Vernier acuity1 is similar for both hemifields only for the first 5° away from the fovea, but after that there is a clear NTA, at least for adults [START_REF] Fahle | Naso-temporal asymmetry of visual perception and of the visual cortex[END_REF]. For example, at 20° away from the fovea, Vernier acuity is approximately 70% higher in the nasal hemiretina than the temporal hemiretina.

Finally, a study of children with unilateral congenital cataracts [START_REF] Bowering | Sensitivity in the nasal and temporal hemifields in children treated for cataract[END_REF] shows that the nasal visual hemifield is affected more than the temporal visual hemifield by visual deprivation, most likely stemming from interocular competition during development and that this can have large effects upon vision later in life.

All-in-all, this evidence suggests that people are born with a strong asymmetry favouring the temporal hemifield, but also that this asymmetry decreases with age, but probably never fully disappears. 13

Neurophysiological asymmetries are reflected in visual performance

We next turn to the question of whether the physiological asymmetries discussed above are reflected in visual perception and performance -such as visual acuity and visual sensitivity. Consistent with asymmetries in neural density and neural projections, various measures of visual perception show that corresponding asymmetries are observed in visual performance. We first discuss effects on fundamental psychophysical measures of vision and then turn to findings on visual attention.

Measures of visual sensitivity, acuity and resolution

Sutter and Tran (1992) used electroretinogram (ERG) measures to assess the photonic luminance response of retinal cells, finding higher response densities in the temporal visual hemifield (projecting to the nasal hemiretina), particularly when stimulating at locations eccentric to the blind spot. In general, the ERG amplitude as a function of eccentricity decreased by r -2/3 (r = eccentricity). The main conclusion of [START_REF] Sutter | The field topography of ERG components in man-I. The photopic luminance response[END_REF] was that the known anatomical asymmetries induced asymmetries in the photonic luminance response. [START_REF] Anderson | Psychophysical localization of the human visual streak[END_REF] attempted to find a behavioural analogue of NTAs in ganglion cell density [START_REF] Curcio | Topography of ganglion cells in human retina[END_REF]. They used an interferometer which minimizes the defocusing effects of the optical properties of the eye. They found that at 20° and 25° away from the fovea the spatial frequency limit was higher for the nasal than the temporal hemiretina (i.e., for the temporal than the nasal visual hemifield), arguing that peripheral resolution reflects the asymmetrical spacing of ganglion cells. [START_REF] Anderson | Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors[END_REF] examined whether optical, receptoral or higher-order neural properties impose limits on visual acuity or visual spatial resolution, focusing in particular on the peripheral regions of the visual field. They measured spatial contrast sensitivity functions for different parts of the retina, finding an NTA in sensitivity that corresponded roughly to the Nyquist limits 2 imposed by ganglion cell density. They also found a clear NTA in contrast sensitivity, that once again accorded well with ganglion cell density. No NTAs in achromatic contrast sensitivity were found at 8°. However, at higher eccentricities (25°, 40° and 55°) NTAs increased, favouring the 14 temporal hemifield. Chromatic contrast sensitivity was higher for the temporal than the nasal hemiretina until further than 8° from the fovea where it became higher for the nasal than the temporal hemiretina. [START_REF] Rovamo | An estimation and application of the human cortical magnification factor[END_REF] argued that the functional and structural properties of the visual system show a very tight correspondence and that this is true over the whole retina. They argued, for example, that visual acuity has the same relation to the density of sampling, as seen for ganglion cells, over the whole visual field. In other words, they assumed that performance on psychophysical visual measures shows asymmetries that strongly correspond to the physiological findings. In support, [START_REF] Rovamo | An estimation and application of the human cortical magnification factor[END_REF] showed that contrast sensitivity at 15° in the nasal hemifield is similar to the contrast sensitivity at 30° in the temporal hemifield. As discussed, cone and ganglion cell density decreases with increased eccentricity, and to a larger degree in the temporal than nasal hemiretina (e.g. [START_REF] Curcio | Distribution of cones in human and monkey retina: individual variability and radial asymmetry[END_REF]. Note that [START_REF] Rovamo | An estimation and application of the human cortical magnification factor[END_REF] only examined the left visual hemifield (measuring the nasal hemifield of the right eye and the temporal hemifield of the left eye). [START_REF] Silva | Retinal and cortical patterns of spatial anisotropy in contrast sensitivity tasks[END_REF] found higher contrast sensitivity performance for stimuli projected to the nasal than temporal hemiretina for both eyes. [START_REF] Keltgen | Estimation of spatial scale across the visual field using sinusoidal stimuli[END_REF], using Gabor sinusoids of various spatial frequencies, found a performance benefit for the nasal hemiretina and [START_REF] Thibos | Retinal limits to the detection and resolution of gratings[END_REF] found NTAs in grating discrimination (see also [START_REF] Fahle | Naso-temporal asymmetry of binocular inhibition[END_REF][START_REF] Fahle | Naso-temporal asymmetry of visual perception and of the visual cortex[END_REF][START_REF] Paradiso | Orientation discrimination as a function of stimulus eccentricity and size: Nasal/temporal retinal asymmetry[END_REF][START_REF] Paradiso | Orientation discrimination as a function of stimulus eccentricity and size: Nasal/temporal retinal asymmetry[END_REF] for converging findings). [START_REF] Grigsby | Grating and flicker sensitivity in the near and far periphery: Naso-temporal asymmetries and binocular summation[END_REF] investigated spatial frequency sensitivity (ranging from 0.4 to 16 cycles per degree) and flicker discrimination performance (flicker range:

1.0 to 40.0 Hz) as a function of eccentricity during binocular and monocular presentation. During monocular presentation, sensitivity to spatial frequency was higher in the nasal hemiretina (temporal hemifield) than in the temporal hemiretina (nasal hemifield) at 8° away from the fovea and outwards. At 24° and 32° the spatial frequency sensitivity of the nasal hemiretina was similar to the measured binocular sensitivity but the sensitivity of the temporal hemiretina was lower than of the nasal hemiretina and lower than for binocular presentation. For flicker discrimination, no significant differences between the nasal and temporal hemiretinae were found at 4° and 8° but at 24° and 32° the sensitivity of the nasal hemiretina was higher than of the temporal hemiretina. Binocular flicker discrimination performance was better than monocular performance under all conditions.

An interesting perspective comes from the results of [START_REF] Fahle | Naso-temporal asymmetry of binocular inhibition[END_REF] who assessed the amount of binocular inhibition over different parts of the retina using binocular rivalry. While the ratio of dominance period of the right eye divided by the dominance period for the left was roughly 1 in the fovea, it was 0.6 in the left visual field, reflecting the dominance of the left eye's temporal hemifield over the right eye's nasal hemifield.

Moreover, this ratio was around 2 in the right visual field, reflecting the corresponding dominance of the right eye's temporal hemifield over the left eye's nasal hemifield.

In sum, the evidence reviewed above shows that neurophysiological findings on asymmetries in neural mechanisms are well reflected in measures of visual performance such as contrast sensitivity, Vernier acuity, and binocular inhibition. In the next section, we turn to the question of whether NTAs in the retinotectal and thalamocortical pathways induce NTAs in higher order visual processes such as visual attention.

Nasal-temporal asymmetries in visual attention

Above, we discussed NTAs in basic visual psychophysical measures favouring the nasal hemiretina. But many studies show that visual attention is crucial for conscious visual perception. If attention is not applied to the stimuli in the visual field, observers perform more poorly on various basic performance measures (Carrasco et al., 2004a;Kristjánsson & Nakayama, 2003;[START_REF] Posner | Attention and the control of movements[END_REF][START_REF] Kristjánsson | On the benefits of transient attention across the visual field[END_REF]. It is therefore also important to assess whether there are NTAs in visual attention performance. One reason to expect them are the NTAs in retinotectal projections to the SC, since the SC play a key role in attentional orienting (for a review, see e.g. [START_REF] Knudsen | Control from below: the role of a midbrain network in spatial attention[END_REF][START_REF] Krauzlis | Superior colliculus and visual spatial attention[END_REF]). To investigate potential asymmetries of attentional orienting, one research line has involved testing performance with traditional measures of visual attention in healthy populations under monocular viewing to isolate nasal and temporal hemifield processing, while another line involves studies of patients with damage to certain parts of either the visual system or attentional networks.

When the visual cortex in one hemisphere is damaged, humans suffer from hemianopia, where they have no conscious vision in the hemifield projecting to the 16 damaged site. But some studies have shown that people with hemianopia can nevertheless use visual information presented in their 'blind' hemifield for certain actions, a phenomenon called blindsight [START_REF] Weiskrantz | Blindsight revisited[END_REF][START_REF] Goodale | Separate visual pathways for perception and action[END_REF]. [START_REF] Dodds | A temporal/nasal asymmetry for blindsight in a localisation task: evidence for extrageniculate mediation[END_REF] tested hemianopic patients who had limited vision in one visual hemifield due to cortical damage. Their performance was at chance for forced-choice localization in the nasal hemianopic visual hemifield, while for the temporal hemianopic visual hemifield performance was intact. This finding suggested that retinotectal projections (that are stronger from the nasal than temporal hemiretina, as explained above) helped participants perform the task. A related finding is that distractor signals presented to the blind hemifield of people with hemianopia slowed saccades into their intact visual hemifield [START_REF] Rafal | Extrageniculate vision in hemianopic humans: Saccade inhibition by signals in the blind field[END_REF], but importantly this only applied to their temporal hemifield, again arguing that the attentional grab effect is stronger for the nasal hemiretina. The important point for the current purposes is that these particular blindsight phenomena occur in the temporal hemifield that projects to the nasal hemiretina but not for the nasal hemifield, projecting to the temporal part of each retina. A likely explanation for this could be the larger neural machinery available in the nasal hemiretina, and stronger projections towards the SC from the nasal than the temporal hemiretina. It is important to note however, that [START_REF] Walker | The oculomotor distractor effect in normal and hemianopic vision[END_REF] did not observe a similar NTA. They tested the effect of flickering and static distractors on saccadic latency in hemianopic and healthy participants under monocular viewing. The distractors were always presented in the blind hemifield of the hemianopic participants.

For the healthy participants, the inhibitory effect of the distractors was stronger for the temporal visual hemifield than the nasal visual hemifield but for the hemianopic participants no such effect was found. Because the only visual pathway which is impaired in hemianopic patients is the thalamocortical one, the NTAs found for the healthy participants and not the hemianopic patients reinforce the idea that the thalamocortical pathway also involves NTAs.

But these studies also highlight a fundamental property of behavioural NTAs: their individual variability. The patients in [START_REF] Dodds | A temporal/nasal asymmetry for blindsight in a localisation task: evidence for extrageniculate mediation[END_REF] and [START_REF] Rafal | Extrageniculate vision in hemianopic humans: Saccade inhibition by signals in the blind field[END_REF] may have had NTAs in the retinotectal pathway while those in [START_REF] Walker | The oculomotor distractor effect in normal and hemianopic vision[END_REF] did not. This individual variability in NTAs will be discussed in the Section 4 of this review.

Other studies of attentional NTAs have involved testing of non-clinical populations, where the effects of stimulation in either hemifield upon attentional performance have been investigated. An example of such a study was reported by [START_REF] Posner | Attention and the control of movements[END_REF] who asked their observers to choose whether to saccade into the nasal or temporal visual hemifield (a saccadic choice task) with one of their eyes covered. Potential targets were presented simultaneously in both hemifields.

Interestingly, observers showed a clear preference for making saccades into the temporal hemifield (79% and 21% into the temporal and nasal visual fields, respectively), and this was taken as indication that stronger retinotectal projections from the nasal than the temporal hemiretina caused this asymmetrical saccade choice.

However, Bompas and Sumner (2008) reported that there was an NTA in saccadic choice even for so-called s-cone stimuli that are thought to be invisible to the retinotectal pathway and therefore not directly projected to the SC, which argues that the NTA in choice may not be unique to the retinotectal pathway, which is consistent with the physiological results of [START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF]. Levy-Schoen (1969) was the first to notice an increase of 20 to 40 milliseconds in saccade latency when a distractor in the opposite visual field was presented along with the saccade target. [START_REF] Findlay | Global visual processing for saccadic eye movements[END_REF] and [START_REF] Walker | Effect of remote distractors on saccade programming: evidence for an extended fixation zone[END_REF] have also shown that when a stimulus is presented on the opposite side to a target during a saccade task, saccadic response times are longer than otherwise. Some researchers have used such findings to investigate the effects of an irrelevant distractor presented in the nasal or temporal hemifield. [START_REF] Rafal | Extrageniculate contributions to reflex visual orienting in normal humans: A temporal hemifield advantage[END_REF] measured the effects of such cues on attentional orienting of subjects who had one eye closed with an eye patch and found that cueing effects were stronger in the temporal than the nasal hemifield. Additionally, [START_REF] Rafal | Saccade preparation inhibits reorienting to recently attended locations[END_REF] reported NTAs for the inhibition of return effect. Inhibition of return to the previous cue location following a peripheral cue was stronger in the temporal than in the nasal hemifield. [START_REF] Mulckhuyse | Unconscious cueing effects in saccadic eye movements -Facilitation and inhibition in temporal and nasal hemifield[END_REF] found that briefly presented (16 ms) cues influenced attentional orienting, even when they were not consciously perceived. Specifically, they observed inhibition of return for cues presented in the temporal hemifield but this was not the case for cues presented to the nasal hemifield. Again, this argues that temporal hemifield stimuli (that project to the nasal hemiretina) have a stronger influence on attentional orienting than nasal hemifield stimuli. Similarly, 18 [START_REF] Ansorge | Asymmetric influences of temporally vs. nasally presented masked visual information: Evidence for collicular contributions to nonconscious priming effects* 1[END_REF] showed that masked primes had a stronger effect on performance when they were presented in the temporal than the nasal hemifield. [START_REF] Berger | The endogenous modulation of IOR is nasal-temporal asymmetric[END_REF] measured the influence of both endogenous precues (such as an arrow at centre pointing to a peripheral location) and exogenous precues (appearing at the target location) on the inhibition of return (IOR) effect. When they tested this influence monocularly, endogenous cues to the nasal hemifield reduced IOR while no such modulation of IOR was observed for cues to the temporal visual hemifield where the effects of the two cue types were independent. This suggested that attentional capture from an exogenous precue is stronger towards the temporal than the nasal hemifield, again consistent with the proposal that stimuli projecting to the nasal hemiretina induce a stronger effect upon attention.

NTAs in attention have also been found with electroencephalography (EEG).

Huber-Huber et al. ( 2015) found higher amplitudes of the N2pc (second negative deflection, that appears typically around 200 ms after the appearance of the stimulus; pc stands for posterior-contralateral) for targets presented in the nasal than the temporal visual hemifield while an irrelevant distractor was presented in the other hemifield. When the cue was a fearful face and valid, the latency was longer than when the cue was a neutral face. However, this was reversed when the cue was invalid. [START_REF] Framorando | Naso-temporal asymmetries: Suppression of emotional faces in the temporal visual hemifield[END_REF] studied NTAs in the EEG signal depending on whether faces had fearful or neutral expressions. The faces were always presented in pairs to the left and right of fixation, either one neutral and one fearful or two neutral faces. When the faces disappeared, a target ("n" or "m") was presented either at the location of the fearful or 19 neutral face. The task was to judge whether the target was an "m" or "n" by pushing the corresponding button. Behaviourally, there were no differences between conditions. But fearful faces in the temporal hemifield elicited larger N2pc amplitudes than faces in the nasal hemifield. Targets elicited greater P1 responses (140-170 ms after target presentation) in the nasal than temporal hemifield but no difference was found in N1 (210-240 ms after target appearance). [START_REF] Framorando | Naso-temporal asymmetries: Suppression of emotional faces in the temporal visual hemifield[END_REF] suggested that the larger P1 for nasal hemifield than temporal hemifield targets might reflect stronger inhibition of neutral faces in the temporal than nasal hemifield.

Overall, the evidence from attentional tasks shows that the neurophysiological asymmetries between the nasal and temporal hemifields reviewed above do not only cause performance asymmetries in basic visual tasks, but also in the function of visual attention.

Nasal temporal asymmetries in saccades

There is a general consensus in the literature that visual attention and saccadic eye movements are strongly interdependent, while the strength and nature of this relationship is still hotly debated [START_REF] Deubel | Saccade target selection and object recognition: Evidence for a common attentional mechanism[END_REF]Hoffman & Subramaniam 1995;[START_REF] Kowler | The role of attention in the programming of saccades[END_REF]Kristjánsson et al. 2001;[START_REF] Rizzolatti | Reorienting attention across the horizontal and vertical meridians: evidence in favor of a premotor theory of attention[END_REF]; see Kristjánsson, 2011 for review). Findings on attentional asymmetries have therefore led to a number of studies of NTAs in the characteristics of saccadic eye movements.

A question that has received considerable interest is whether anatomical asymmetries correspond to asymmetries in saccadic latency. In spite of many studies on this topic, the answer cannot at present be considered unequivocal in either direction. Kristjánsson et al. (2004, experiment 1) tested observers in a step paradigm on a reflexive prosaccade and voluntary antisaccade task (antisaccades involve eye movements in the opposite direction to a presented target), while observers had one eye closed with an eye patch. They found an interesting interaction where the latencies of prosaccades towards stimuli in the temporal hemifield were lower than for saccades into the nasal hemifield. Antisaccades made in response to stimuli in the temporal hemifield were also slower than antisaccades made in response to nasal hemifield targets. In the antisaccade task, observers had to make an eye movement in the opposite 20 direction to where the target appeared and if the target was in the temporal hemifield it seemed to have a stronger pull effect than nasal hemifield targets. This result is very interesting since it suggests that NTAs in saccade latency are more related to the location of the visual stimulus than to the saccade direction. Indeed, if the NTA would be related to saccade direction, it should have reversed during the antisaccade task with shorter saccade latencies when the saccade (and not the visual stimulus) was directed toward the temporal hemifield. This result of faster prosaccades into the temporal hemifield has, however, not always been replicated (Bompas et al., 2008;[START_REF] Honda | Idiosyncratic left-right asymmetries of saccadic latencies: examination in a gap paradigm[END_REF][START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF]. The training increased the proportion of express saccades (latency between 75 and 130 ms) and reduced saccadic latency, overall. Initially, the participants made more express saccades into the nasal visual hemifield than into the temporal visual hemifield but after the training this was reversed. Although [START_REF] Jóhannesson | Effects of saccade training on express saccade proportions, saccade latencies, and peak velocities: an investigation of nasal/temporal differences[END_REF] did not find significant differences in latency between hemifields overall, the interaction between 21 visual field and training was significant. Before training, latencies were higher towards the temporal than nasal visual hemifields but with training this reversed. [START_REF] Koller | Saccade latency bias toward temporal hemifield: evidence for role of retinotectal tract in mediating reflexive saccades[END_REF] recently assessed NTAs in express saccade latency.

Their study involved the gap paradigm with monocular viewing. They separately analysed saccades with latencies between 70 and 150 ms (considered express saccades), and saccades with latencies between 150 and 350 ms (considered regular saccades).

Their analyses revealed that NTAs in saccade latency occurred only for express saccades; not regular saccades. Together, the findings of [START_REF] Jóhannesson | Effects of saccade training on express saccade proportions, saccade latencies, and peak velocities: an investigation of nasal/temporal differences[END_REF] and of [START_REF] Koller | Saccade latency bias toward temporal hemifield: evidence for role of retinotectal tract in mediating reflexive saccades[END_REF] suggest that there are considerable NTAs in latency for express saccades. This contrasts with the small, or non-existent NTAs for regular saccades [START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF]Tagu et al., 2018a).

If NTAs in saccade latency are restricted to express saccades, this raises the question of why an attentional benefit does not always translate into a benefit for regular saccades. Express saccades differ from regular saccades in that they are assumed to primarily involve the retinotectal pathway [START_REF] Dorris | Neuronal activity in monkey superior colliculus related to the initiation of saccadic eye movements[END_REF][START_REF] Edelman | Activity of visuomotor burst neurons in the superior colliculus accompanying express saccades[END_REF], 1998). It is therefore possible that the NTAs for express saccades are driven by retinotectal NTAs. However, as mentioned above, the thalamocortical pathway also shows anatomical and functional NTAs [START_REF] Connolly | The representation of the visual field in parvicellular and magnocellular layers of the lateral geniculate nucleus in the macaque monkey[END_REF][START_REF] Williams | Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight[END_REF][START_REF] Toosy | Asymmetrical activation of human visual cortex demonstrated by functional MRI with monocular stimulation[END_REF]Tyschen & Burkhatler, 1997). The absence of NTAs in the latencies of regular saccades therefore still remains to be explained. We speculate that for the thalamocortical pathway, the saccade generation network receives a lot of topdown influence from other cortical areas [START_REF] Pierrot-Deseilligny | Cortical control of saccades[END_REF] which are known to induce strong modulations in saccade latency [START_REF] Sommer | Composition and topographic organization of signals sent from the frontal eye field to the superior colliculus[END_REF][START_REF] Leigh | The saccadic system[END_REF]. Cortical input to mesencephalic saccade control centres may therefore dilute any saccadic NTAs. These top-down influences in the generation of regular saccades may compete with the NTAs of the visual system and therefore prevent them from appearing in the latency of regular saccades (see also Bompas et al., 2008 andTagu et al., 2018a for similar arguments). Another possibility is that attentional NTAs only cause speeded attentional selection but not speeded saccade execution. The attentional benefit may simply not translate into a quicker saccade response (as argued by [START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF].

22

In addition to latency, a common measure of saccade performance involves assessing their peak velocity. In contrast to saccade latency, saccadic peak velocity has been shown to involve robust NTAs, with higher peak velocity for temporal than for nasal saccades. This initial finding of Hyde (1959) has been replicated many times with different methods [START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF][START_REF] Cook | Horizontal eye movements studied with the on-line computer[END_REF][START_REF] Fricker | Dynamic measurements of horizontal eye motion I. Acceleration and velocity matrices[END_REF][START_REF] Jóhannesson | Violating the main sequence: asymmetries in saccadic peak velocities for saccades into the temporal versus nasal hemifields[END_REF][START_REF] Robinson | The mechanics of human saccadic eye movement[END_REF]Tagu et al., 2018a). For example, using monocular recordings of eye movements under monocular viewing conditions, [START_REF] Jóhannesson | Violating the main sequence: asymmetries in saccadic peak velocities for saccades into the temporal versus nasal hemifields[END_REF] measured peak velocities as a function of hemifield, by performing a reanalysis of the data from [START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF]. Their results revealed that saccadic peak velocities were higher for saccades towards temporal than nasal hemifield targets. Tagu et al. (2018a) using binocular recordings of eye movements also found strong NTAs in saccadic peak velocity. Interestingly, this NTA was found for both reflexive and voluntary saccades elicited in step and overlap paradigms. [START_REF] Jóhannesson | Effects of saccade training on express saccade proportions, saccade latencies, and peak velocities: an investigation of nasal/temporal differences[END_REF] showed that the NTA in peak velocity was also found for express saccades in the gap paradigm. On the one hand, this result is in line with the neurophysiological asymmetries, that the denser neural circuitry in the nasal hemiretina would lead to higher peak velocity into the temporal visual hemifield. But, on the other hand, this was surprising as it had been assumed for a while that a constant ratio (the main sequence) determined the relationship between velocity and amplitude [START_REF] Bahill | The main sequence, a tool for studying human eye movements[END_REF]. There was no corresponding increase in amplitude in the data that could have explained this.

The intriguing question is why there are strong and robust NTAs in saccadic peak velocity while such NTAs are not consistently found in saccade latency. One explanation could be that, in contrast to saccadic latency, peak velocity is not affected by top-down influences such as decision making or visual attention [START_REF] Galley | Saccadic eye movement velocity as an indicator of (de) activation: A review and some speculations[END_REF]see also Di Stasi et al., 2013;[START_REF] Leigh | The saccadic system[END_REF]. For saccade latency, the NTAs may therefore be present but not observable because of top-down influences, which mask the NTAs. Importantly, the fact that NTAs in saccadic peak velocity are found whatever the saccade type, reinforces the idea that NTAs are present in all the visual pathways; in the retinotectal pathway which contributes to the generation of express saccades, and in the thalamocortical pathway which is involved in reflexive and voluntary saccade generation.

Other asymmetries in the visual system

Although this review has mainly focused on NTAs, the visual system presents many asymmetries. One asymmetry that has raised interest is pseudoneglect [START_REF] Bowers | Pseudoneglect : Effects of hemispace on a tactile line bisection task[END_REF]; for review, see [START_REF] Jewell | Pseudoneglect : a review and meta-analysis of performance factors in line bisection tasks[END_REF]. Pseudoneglect refers to the leftward error typically found in healthy individuals during line bisection tasks, and has been proposed to be related to the specialization of the right cerebral hemisphere for visuo-spatial attention (e.g., [START_REF] Toba | Attention biases the perceived midpoint of horizontal lines[END_REF][START_REF] Zago | Pseudoneglect in line bisection judgement is associated with a modulation of right hemispheric spatial attention dominance in right-handers[END_REF].

As pseudoneglect is a well-documented phenomenon and because several literature reviews have already been devoted to it (e.g., [START_REF] Jewell | Pseudoneglect : a review and meta-analysis of performance factors in line bisection tasks[END_REF][START_REF] Ribolsi | A critical review and meta-analysis of the perceptual pseudoneglect across psychiatric disorders: Is there a continuum?[END_REF], we will not discuss it further in this section. We will however discuss other visual asymmetries, first in Section 3.1 between abducting and adducting eye movements (i.e., studies of NTAs during binocular viewing). Then, vertical asymmetries in visual and attentional functions are discussed in Section 3.2, followed by a discussion of centripetal and centrifugal saccades in Section 3.3.

Asymmetries found in binocular studies

In the studies reviewed above on saccadic eye movements, NTAs were assessed with monocular recording during monocular viewing, by contrasting leftward and rightward saccades from the same eye. However, by using binocular recordings of eye movements in more ecologically valid viewing conditions, it is possible to assess NTAs by contrasting the saccadic parameters of the left and right eyes for the saccades generated by each eye at the same time. This method, that eliminates the effects of biological NTAs by combining the nasal and temporal hemifields, has revealed that the two eyes are not always strictly conjugated, and that saccades of the abducting eye have shorter latency and higher peak velocity than concomitant saccades of the adducting eye [START_REF] Miyoshi | Dissociation of the eyes in saccadic movement[END_REF][START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF]. It is important to note that adducting and abducting eye movements can be from one visual field to the other (see Figure 2) and can therefore be affected by the properties of nasal and temporal eye movements as well as properties of centripetal and centrifugal saccades. [START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF] performed a very detailed investigation of a large number of saccadic parameters during such binocular recordings. They showed, using scleral coil binocular eye tracking, that saccades made by the abducting eye (the one moving away from the anterior-posterior centre line of the body) consistently had shorter duration and higher peak velocity than the concomitant saccade of the adducting eye (the one moving towards the anterior-posterior centre line of the body). Tagu et al. (2018a) systematically investigated NTAs in saccade latency, amplitude, peak velocity and duration with infrared video-based binocular eye tracking. Moreover, their study involved a large variety of horizontal saccades triggered by step and overlap paradigms, and centripetal and centrifugal saccades, made from different starting positions on the horizontal axis. They showed that asymmetries in peak velocity between saccades made simultaneously by the left and right eyes were very robust for all saccade types tested.

For a given saccade, the abducting eye did indeed move faster than the adducting eye.

However, Tagu et al. (2018a) did not find any NTAs or abducting-adducting differences in saccade latency, amplitude nor duration for any saccade type. Again, this shows that saccade asymmetries are more clearly found in saccadic peak velocity than other parameters which are more sensitive to other top-down influences [START_REF] Di Stasi | Saccadic velocity as an arousal index in naturalistic tasks[END_REF][START_REF] Galley | Saccadic eye movement velocity as an indicator of (de) activation: A review and some speculations[END_REF][START_REF] Leigh | The saccadic system[END_REF].

All in all, by either using monocular recordings of eye movements in monocular viewing conditions [START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF][START_REF] Jóhannesson | Violating the main sequence: asymmetries in saccadic peak velocities for saccades into the temporal versus nasal hemifields[END_REF]Kristjánsson et al., 2004) or binocular recordings of eye movements in binocular viewing conditions [START_REF] Miyoshi | Dissociation of the eyes in saccadic movement[END_REF][START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF]Tagu et al., 2018a), research has shown that biological NTAs, found in the retina and the optic nerve, affect saccadic eye movements, leading to clear NTAs in peak velocity. Moreover, these NTAs can also affect the saccade latency of express saccades triggered by the gap paradigm [START_REF] Jóhannesson | Effects of saccade training on express saccade proportions, saccade latencies, and peak velocities: an investigation of nasal/temporal differences[END_REF][START_REF] Koller | Saccade latency bias toward temporal hemifield: evidence for role of retinotectal tract in mediating reflexive saccades[END_REF]; although they are not as easily observable for reflexive and voluntary saccades triggered by the step and overlap paradigms, respectively [START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF]Tagu et al., 2018a). Interestingly express saccades are the eye movement that is probably the least affected by top-down influences (see Section 2.5).

Vertical asymmetries in the visual system

Until now, we have reviewed asymmetries found between the temporal and nasal hemiretinae and abducting and adducting eye movements. However, asymmetries between the upper and lower visual hemifields, or between the processing of visual information presented straight-ahead (see section 3.3) and presented elsewhere in the periphery have also been found.

Curcio and colleagues [START_REF] Curcio | Topography of ganglion cells in human retina[END_REF][START_REF] Curcio | Distribution of cones in human and monkey retina: individual variability and radial asymmetry[END_REF] showed not only that the density of cones and ganglion cells was higher in the nasal than temporal hemiretina as discussed previously, but also that it was higher in the upper than in the lower hemiretina. As is the case for NTAs, this asymmetry between the upper hemiretina (lower visual hemifield) and the lower hemiretina (upper visual hemifield) seems to spread across the thalamocortical pathway. Indeed, visual objects have been shown to be predominantly represented by neural mechanisms corresponding to the lower visual hemifield in the LGN [START_REF] Schein | Mapping of retinal geniculate neurons onto striate cortex of macaque[END_REF] and in V1 [START_REF] Van Essen | The visual field representation in striate cortex of the macaque monkey: asymmetries, anisotropies, and individual variability[END_REF]. Importantly, the asymmetry continues to spread further into extrastriate visual areas [START_REF] Rossit | fMRI reveals a lower visual field preference for hand actions in human superior parietooccipital cortex (SPOC) and precuneus[END_REF][START_REF] Van Essen | The projections from striate cortex (V1) to areas V2 and V3 in the macaque monkey: asymmetries, areal boundaries, and patchy connections[END_REF]) and influence the well-known dorsal stream of vision for action, and ventral stream of vision for perception [START_REF] Goodale | Separate visual pathways for perception and action[END_REF][START_REF] Ungerleider | Two cortical visual systems[END_REF]. The lower visual hemifield may have stronger connections with the dorsal stream of vision for action whereas the upper visual hemifield is more connected to the ventral stream of vision for perception [START_REF] Galletti | Brain location and visual topography of cortical area V6A in the macaque monkey[END_REF][START_REF] Gamberini | Is the medial posterior parietal area V6A a single functional area?[END_REF][START_REF] Previc | Functional specialization in the lower and upper visual fields in humans: Its ecological origins and neurophysiological implications[END_REF][START_REF] Rossit | fMRI reveals a lower visual field preference for hand actions in human superior parietooccipital cortex (SPOC) and precuneus[END_REF]. In a recent study, [START_REF] Zhou | Asymmetric representations of upper and lower visual fields in egocentric and allocentric references[END_REF] proposed that this asymmetry was related to peripersonal and extrapersonal spaces. Accordingly, visually-guided actions initiated by the dorsal stream are mainly executed in peripersonal space; which often occurs in the lower visual hemifield. In contrast, the extrapersonal visual environment is mostly perceived in the upper visual hemifield and would mainly involve object recognition or other perceptual processes. Several psychophysical experiments have indeed shown better performance for visually-guided actions in the lower visual hemifield than in the upper one [START_REF] Amenedo | Vertical asymmetries in preattentive detection of changes in motion direction[END_REF][START_REF] Genzano | Upper/lower visual field asymmetry on a spatial relocation memory task[END_REF][START_REF] Rubin | Enhanced perception of illusory contours in the lower versus upper visual hemifields[END_REF][START_REF] Thomas | Upper and lower visual field differences in perceptual asymmetries[END_REF]. Moreover, discrimination performance in egocentric reference frames has been shown to be enhanced in the lower visual hemifield whereas it was enhanced in the upper visual hemifield when the task was performed within an allocentric reference frame [START_REF] Levine | The relative capabilities of the upper and lower visual hemifields[END_REF][START_REF] Zhou | Asymmetric representations of upper and lower visual fields in egocentric and allocentric references[END_REF].

Vertical asymmetries in saccadic latency has been found. [START_REF] Abegg | A Vertical Asymmetry in Saccades[END_REF] they presented targets at 45°, 135°, 225° and 325° from the horizontal midline at 10° eccentricity. Latencies were significantly shorter for saccades into the upper, than the lower hemifield. Previously, [START_REF] Zhou | Attentional sensitivity and asymmetries of vertical saccade generation in monkey[END_REF] had shown that saccades made by macaque monkeys (Macaca mulatta) had shorter latency and higher peak velocity when they were directed toward the upper hemifield than the lower one. [START_REF] Carrasco | Characterizing visual performance fields: Effects of transient covert attention, spatial frequency, eccentricity, task and set size[END_REF] showed that contrast sensitivity was higher for the lower than the upper half of the vertical meridian -a so-called vertical meridian asymmetry. [START_REF] Talgar | Vertical meridian asymmetry in spatial resolution: Visual and attentional factors[END_REF] then concluded that this was due to visual rather than attentional factors. But asymmetries between the upper and lower visual hemifields have since been found (e.g. [START_REF] Kristjánsson | On the benefits of transient attention across the visual field[END_REF]; see also [START_REF] Rezec | Attentional weighting: A possible account of visual field asymmetries in visual search?[END_REF] where the attentional boost from precues was higher for the upper than the lower hemifield. This was only true for a disk displacement discrimination task but not Gabororientation as tested by Talgar and Carrasco, however. Kristjánsson and Sigurdardóttir speculated that this asymmetrical effect of attention reflected that because of the lower density of receptors devoted to the upper visual hemifield this part requires a larger attentional boost for stimulus detection (see also [START_REF] Breitmeyer | Dynamic random-dot stereograms reveal up-down anisotropy and left-right isotropy between cortical hemifields[END_REF][START_REF] Carrasco | Temporal performance fields: visual and attentional factors[END_REF] for converging results).

Centripetal and centrifugal asymmetries

Another well-known visual asymmetry concerns the perceptual advantage of visual information presented straight-ahead compared to information presented elsewhere [START_REF] Durand | Privileged visual processing of the straight-ahead direction in humans[END_REF][START_REF] Kardamakis | Optimal control of gaze shifts[END_REF][START_REF] Tatler | The central fixation bias in scene viewing: Selecting an optimal viewing position independently of motor biases and image feature distributions[END_REF]. This has been linked to privileged processing in V1 of information presented straight-ahead, both in monkeys [START_REF] Durand | Privileged processing of the straightahead direction in primate area V1[END_REF][START_REF] Przybyszewski | Primate area V1: Largest response gain for receptive fields in the straight-ahead direction[END_REF]) and humans [START_REF] Strappini | Eye position modulates retinotopic responses in early visual areas: A bias for the straight-ahead direction[END_REF]. This advantage has been proposed to be behaviorally driven, allowing quick orientation to stimuli presented straight-ahead which could be obstacles during locomotion [START_REF] Durand | Privileged processing of the straightahead direction in primate area V1[END_REF][START_REF] Camors | Visual straight-ahead preference in saccadic eye movements[END_REF]. Moreover, the asymmetry between the straight-ahead direction and other positions in the visual field influences saccadic parameters, as centripetal saccades (directed toward the straight-ahead direction) have been shown to have shorter latency [START_REF] Krebs | The saccadic recentering bias is associated with activity changes in the human superior colliculus[END_REF][START_REF] Krebs | Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades[END_REF], shorter duration [START_REF] Camors | Visual straight-ahead preference in saccadic eye movements[END_REF][START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF][START_REF] Pelisson | Kinematics of centrifugal and centripetal saccadic eye movements in man[END_REF] and higher peak velocity [START_REF] Abel | Saccades in extremes of lateral gaze[END_REF][START_REF] Camors | Visual straight-ahead preference in saccadic eye movements[END_REF][START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF][START_REF] Frost | Different programming modes of human saccadic eye movements as a function of stimulus eccentricity: Indications of a functional subdivision of the visual field[END_REF][START_REF] Inchingolo | The characteristic peak velocity-Mean velocity of saccadic eye movements in man[END_REF][START_REF] Jürgens | Natural and drug-induced variations of velocity and duration of human saccadic eye movements: Evidence for a control of the neural pulse generator by local feedback[END_REF][START_REF] Pelisson | Kinematics of centrifugal and centripetal saccadic eye movements in man[END_REF]Tagu et al., 2018a) than centrifugal saccades (directed away from the straight-ahead).

Importantly, Tagu et al. (2018a) showed that this centripetal-centrifugal asymmetry (CCA) in saccadic parameters was found for temporal saccades only, not nasal saccades. This finding suggests that NTA and CCA share some links. The CCA has been proposed to be related to asymmetrical mechanical constraints at the muscular level on the lateral-and medial extraocular rectus [START_REF] Koene | Cause of kinematic differences during centrifugal and centripetal saccades[END_REF]. However, other studies have shown that the CCA also had subcortical and cortical correlates. [START_REF] Paré | Expression of a recentering bias in saccade regulation by superior colliculus neurons[END_REF] found that the preparatory activity of the buildup neurons in the SC was higher for centripetal, than for centrifugal saccades (in the monkey, using single cell recording) while [START_REF] Krebs | The saccadic recentering bias is associated with activity changes in the human superior colliculus[END_REF] in humans using fMRI) found the activity in the SC to be lower for centripetal, than for centrifugal saccades. In Paré and Munoz's study the centripetal-centrifugal direction did neither affect the activity of the fixation nor the burst neurons which, along with different methods, might explain the seeming contradiction between those studies. In an EEG study, [START_REF] Krebs | Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades[END_REF] found that parieto-occipital regions contralateral to the saccade direction showed a negative deflection 160 ms before saccade onset. Importantly, this deflection was shorter and weaker prior to centripetal than centrifugal saccades. [START_REF] Krebs | Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades[END_REF] proposed that the weaker deflection suggest a lower need for attentional guidance for centripetal than for centrifugal saccades.

When the eye is in its primary position, the lateral and medial recti are mostly relaxed [START_REF] Sparks | The brainstem control of saccadic eye movements[END_REF] and the mechanical forces of the socket can keep the eye stable in this position. As soon as the eye is to be rotated, one rectus must be contracted while the other is kept in its relaxed condition. However, the elastic force of the relaxing rectus increases, the further the eye is rotated away from its primary position. After the eye has been rotated away from its primary position the force of the pulling rectus is adjusted sufficiently to overcome the elastic force of the relaxed rectus and to keep the eyeball at the new position [START_REF] Sparks | The brainstem control of saccadic eye movements[END_REF]. When the eye rotates back to its primary position the relaxing of the pulling rectus should suffice to bring the eye back to its primary position. But some force of the previously relaxed rectus may be added to the rotation to speed up the rotation process which can increase the peak velocity of centripetal saccades over centrifugal saccades [START_REF] Abel | Saccades in extremes of lateral gaze[END_REF][START_REF] Camors | Visual straight-ahead preference in saccadic eye movements[END_REF][START_REF] Collewijn | Binocular co-ordination of human horizontal saccadic eye movements[END_REF][START_REF] Frost | Different programming modes of human saccadic eye movements as a function of stimulus eccentricity: Indications of a functional subdivision of the visual field[END_REF][START_REF] Inchingolo | The characteristic peak velocity-Mean velocity of saccadic eye movements in man[END_REF][START_REF] Jürgens | Natural and drug-induced variations of velocity and duration of human saccadic eye movements: Evidence for a control of the neural pulse generator by local feedback[END_REF][START_REF] Pelisson | Kinematics of centrifugal and centripetal saccadic eye movements in man[END_REF]Tagu et al., 2018a). This could also contribute to the shorter latency of centripetal than of centrifugal saccades [START_REF] Krebs | The saccadic recentering bias is associated with activity changes in the human superior colliculus[END_REF][START_REF] Krebs | Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades[END_REF]

Individual variability in NTAs

While NTAs in saccadic eye movement parameters have been found in several studies from different laboratories using different eye-tracking techniques, it is important to highlight at this point that a subgroup of participants do not show any NTAs.

Vergilino-Perez et al. ( 2012) investigated potential left-right asymmetries in saccadic latency, amplitude and peak velocity of saccades under binocular viewing (during binocular viewing biological NTAs are eliminated, see Section 2), with both monocular and binocular recording. They did not find any asymmetries in latency and only a weak NTA in saccade amplitude. However, their most interesting result was that during binocular recordings of eye movements, some participants showed robust NTAs in peak velocity but others did not. Instead, these participants showed either higher peak velocities for leftward than rightward saccades, or for rightward than leftward saccades in both eyes; irrespective of whether the saccades were temporal or nasal. Moreover, the participants with higher leftward than rightward peak velocities had a dominant left eye while those showing higher peak velocities rightward than leftward had a dominant right eye. In other words, for these participants peak velocities were consistently higher for saccades into the hemifield ipsilateral to the dominant eye. Vergilino-Perez et al.

(2012) therefore proposed that the peak velocity pattern was linked to eye dominance and that these participants could have stronger eye dominance than those exhibiting standard NTAs where peak velocities were not linked to eye dominance. Interestingly, this shows that studies of NTAs in saccadic parameters could reveal individual characteristics such as the strength of eye dominance. Moreover, other studies contrasting participants with "strong" eye dominance (without any NTA in saccadic peak velocity) against participants with "weak" eye dominance (exhibiting standard NTA in saccadic peak velocity) showed that this measure of eye dominance strength based on oculomotor behavior influenced perceptual [START_REF] Chaumillon | Asymmetry in visual information processing depends on the strength of eye dominance[END_REF] and visuo-motor [START_REF] Tagu | How eye dominance strength modulates the influence of a distractor on saccade accuracy[END_REF] performance.

In [START_REF] Tagu | How eye dominance strength modulates the influence of a distractor on saccade accuracy[END_REF] participants with left-or right eye dominance made saccades toward a target presented along with a proximal distractor. This situation is known to induce a "global effect" [START_REF] Findlay | Global visual processing for saccadic eye movements[END_REF][START_REF] Vitu | About the global effect and the critical role of retinal eccentricity: Implications for eye movements in reading[END_REF][START_REF] Walker | Effect of remote distractors on saccade programming: evidence for an extended fixation zone[END_REF]: the saccade landing position falls in between the saccade target and the distractor. [START_REF] Tagu | How eye dominance strength modulates the influence of a distractor on saccade accuracy[END_REF] measured this global effect separately for the left-and right visual hemifields.

Participants with strong eye dominance had higher saccade accuracy (weaker global effect) in the hemifield contralateral to the dominant eye than in the ipsilateral hemifield. This contralateral advantage was not found in participants with weak eye dominance (exhibiting standard NTAs). These results reinforce the idea that NTAs can be used to quantify eye dominance strength. Two questions remain however: why the hemifield contralateral to the dominant eye has such advantages, and why NTAs are linked to eye dominance strength.

It is well known that similar factors can influence eye dominance and the ocular dominance columns in the visual cortex, such as lack of visual input as shown in deprivation studies [START_REF] Levay | The development of ocular dominance columns in normal and visually deprived monkeys[END_REF][START_REF] Shatz | Ocular dominance in layer IV of the cat's visual cortex and the effects of monocular deprivation[END_REF]. Neuroimaging studies have shown that V1 ipsilateral to the dominant eye is larger [START_REF] Erdogan | Right and left visual cortex areas in healthy subjects with right-and left-eye dominance[END_REF] and more highly activated when the dominant eye was stimulated than when the nondominant eye was stimulated [START_REF] Shima | Ocular dominance affects magnitude of dipole moment: an MEG study[END_REF]. This suggests that there is a particular relationship between the dominant eye and ipsilateral V1. Due to the architecture of the thalamocortical pathway, V1 ipsilateral to the dominant eye processes information presented in the contralateral hemifield. [START_REF] Tagu | How eye dominance strength modulates the influence of a distractor on saccade accuracy[END_REF] proposed that the higher saccade accuracy found in the hemifield contralateral to the dominant eye reflected this. Moreover, this only occurs for participants with strong eye dominance (no NTA in saccadic peak velocity), so the relationship between the dominant eye and the ipsilateral V1 should be present in this population only. Interestingly, using magnetoencephalography, [START_REF] Shima | Ocular dominance affects magnitude of dipole moment: an MEG study[END_REF] found that V1 showed higher activation when the dominant eye was stimulated (by presenting a reversing checkerboard stimulus) than when the non-dominant eye was stimulated; but only when the stimulus was presented in the nasal hemifield. This suggests that the relationship between the dominant eye and ipsilateral V1 is strongly related to input from the temporal hemiretina. We believe that this could be the reason why NTAs and eye dominance strength are linked. Interestingly, V1 ocular dominance columns receiving input from the nasal hemiretina are larger and occupy more area than the ones receiving input from the temporal hemiretina [START_REF] Tychsen | Nasotemporal asymmetries in V1: Ocular dominance columns of infant, adult, and strabismic macaque monkeys[END_REF], while ocular dominance columns in V1 ipsilateral to the dominant eye (input from temporal hemiretina) seem more activated than the ones receiving input from the nasal hemiretina [START_REF] Shima | Ocular dominance affects magnitude of dipole moment: an MEG study[END_REF]. Competition between these two phenomena may therefore attenuate, or even eliminate, behavioral NTAs. This would suggest that the stronger the relationship between the dominant eye and ipsilateral V1 (i.e., the higher the eye dominance strength), the lower the observed NTA.

Interestingly, as for NTAs, other asymmetries found in the visual system also show individual variability (e.g., [START_REF] Camors | Visual straight-ahead preference in saccadic eye movements[END_REF][START_REF] Galley | Saccadic eye movement velocity as an indicator of (de) activation: A review and some speculations[END_REF]Tagu et al., 2018a). We therefore believe that these asymmetries could be linked to individual characteristics such as eye dominance. Indeed, [START_REF] Tagu | Quantifying eye dominance strength -New insights into the neurophysiological bases of saccadic asymmetries[END_REF] showed that simultaneously studying individual variabilities in NTA and CCA provided a graded measure of eye dominance, reinforcing the idea that NTA and CCA share some links. We however acknowledge that biological evidence regarding this question is sparse. Future studies should be carried out to further understand this individual variability found in the asymmetries of the visual system, and how it may relate to the neuroanatomy and physiology of the visual system.

Conclusions and future directions

The evidence that we have reviewed clearly shows that there are various forms of asymmetries in the neurophysiological mechanisms supporting visual perception and oculomotor mechanisms. The evidence also shows that all these asymmetries found in the visual system influence visual performance, attentional function and eye movements. Most surprising, perhaps, is that these asymmetries seem to interact. One recommendation that we can therefore make is that these asymmetries must be taken into account in current and future research in visual attention, visual perception, oculomotor control and eye movements before experimental results are interpreted. Additionally, during eye tracking studies, attention must be paid to which eye is measured -the left or right, dominant or non-dominant -(which is not always reported), since performance can clearly differ by which eye is measured, and the saccade direction (for example for studies involving only one saccade direction). Finally, studies of asymmetries of the visual system can clearly inform our understanding of the visual system, and can in many cases lead to surprising insights into visual perception, oculomotor behavior and the neural mechanisms involved.

Although this review has provided new insights into visual asymmetries, it also has highlighted unresolved issues, that future studies may address. Firstly, the study of NTAs in saccade latency suggest that they are found only for express, not regular saccades. This discrepancy is important as it directly concerns the neurophysiology of saccadic asymmetries, and could for example be driven by different expression of NTAs in the retinotectal and thalamocortical pathways. The differences between NTAs in express and regular saccades may be examined in future studies involving both behavioral and neurophysiological measures. Furthermore, the particular emphasis of the NTAs in express saccades and retinotectal projections in different species could provide an evolutionary perspective of the NTAs, as this pathway is shared by almost all vertebrates. Although we reviewed evidence regarding the development of NTAs, none of these studies included investigations of the development of saccadic NTAs.

Yet, the huge advances in eye-tracking technologies made over the last decades now allow examining the saccadic eye movements of preschool children with high temporal and spatial accuracy (e.g., [START_REF] Alahyane | Development and learning of saccadic eye movements in 7-to 42-month-old children[END_REF][START_REF] Lemoine-Lardennois | Saccadic Adaptation in 10-41 Month-Old Children[END_REF].

Future studies are needed to examine how the NTAs in saccadic parameters evolve during development. This point could also give new insights about the links between saccadic asymmetries and eye dominance. Indeed, although evidence is sparse, it seems that eye dominance is not fully established until after three years of age. Infants often switch between the two eyes to perform monocular tasks [START_REF] Coren | Lateral preference behaviors in preschool children and young adults[END_REF][START_REF] Updegraff | Ocular dominance in young children[END_REF]. As the saccadic NTAs in peak velocity seem mostly observed in case of weak eye dominance, they may be enhanced in preschool infants, and decrease after three years of age when eye dominance is established.

Future studies concerning these questions may furthermore provide new information about the links between the different saccadic asymmetries (e.g., Tagu et al., 2018a) and between saccadic asymmetries and eye dominance (e.g., [START_REF] Tagu | Quantifying eye dominance strength -New insights into the neurophysiological bases of saccadic asymmetries[END_REF].

A final puzzle that we are left with concerns two proposals that have been reviewed regarding asymmetrical processing; firstly, the point that nasal-temporal asymmetries in visual and attentional processing may reflect the need to quickly orient to events in the periphery, and secondly that the centripetal-centrifugal asymmetry reflects a bias towards potentially important items straight ahead in the visual field. We consider it a highly interesting endeavor to address this seeming contradiction. But keep in mind, that there may not be contradiction since the somewhat haphazard nature of evolution leaves open the possibility that two independent mechanisms may have 32 developed in parallel or at different evolutionary stages serving these two different functions.

Figure 2 .

 2 Figure 2. Saccadic movements types of the right eye.Adducting (S1, S4, S5 and S7) and abducting (S2, S3, S6 and S8) saccades are defined with respect to the anterior-posterior centre line of the body. Centripetal (S3 and S4) and centrifugal (S5, S6, S7 and S8) saccades are defined with respect to the anteriorposterior centre line of the orbit. Nasal (S1, S4, S5 and S7) and temporal (S2, S3, S6 and S8) saccades are defined with respect to the centre of gaze. Dotted lines represent centripetal saccades and whole lines represent centrifugal saccades. Red lines represent saccades (abducting) into the temporal hemifield and green lines represent saccades (adducting) into the nasal hemifield.

A

  highly pertinent question in relation to NTAs of the visual system is how such asymmetries develop. Research suggests that newborns have strong NTAs, favouring the temporal hemifield, that become weaker over the course of development. Rothbart et al. (1990) argued for faster development of neural mechanisms in the infant nasal hemiretina than the temporal hemiretina.

  They concluded that this somewhat unexpected NTA in N2pc reflected stronger inihition of distractors in the temporal than the nasal hemifield. Recent evidence[START_REF] Li | Further evidence that N2pc reflects target enhancement rather than distracter suppression[END_REF] suggests, however, that N2pc reflects target enhancement rather than distractor inhibition. The NTA in N2pc found by[START_REF] Huber-Huber | Nasotemporal ERP differences: evidence for increased inhibition of temporal distractors[END_REF] could therefore also reflect stronger target enhancement in the temporal than the nasal hemifield.It has been shown that facial emotion can affect visual search[START_REF] Jóhannesson | Violating the main sequence: asymmetries in saccadic peak velocities for saccades into the temporal versus nasal hemifields[END_REF]) and eye movements[START_REF] Bannerman | Attentional bias to brief threatrelated faces revealed by saccadic eye movements[END_REF].Kristjánsson et al. found that response times following fearful faces were longer than following neutral faces. In the saccade task, Bannerman et al. used fearful or neutral faces as valid or invalid cues.

  In a comprehensive test involving a number of different experiments,[START_REF] Jóhannesson | Saccade performance in the nasal and temporal hemifields[END_REF] studied potential NTAs in latency under various conditions, testing different amplitudes, and different amounts of concurrent attentional load. They found no NTAs in latency in any of their testing conditions, and only a minor effect on landing point accuracy. They speculated that cortical input to eye control centres in the mesencephalon may mitigate any asymmetries in visual pathways. In a recent paper, however, Jóhannesson et al. (2018) investigated training effects during express saccade generation. Their experiments were inspired by the work of Bibi and Edelman (2009) who found that express saccade proportions increased with training when participants made saccades in the gap paradigm. The logic of Jóhannesson et al. (2018) went as follows: Perhaps any saccade latency differences by hemifield are masked by differences in saccade training between individuals, and measuring potential NTAs at different levels of training may actually mask the NTAs. They trained their participants over 12 training sessions of 280 trials in the gap paradigm while participants wore an eye patch over their non-dominant eye. Overall, the proportion of express saccades increased, but the most interesting finding was how asymmetries in both express saccade proportion and latency developed over training.

Eur J Neurosci, doi: 10.1111/ejn.14225

Vernier acuity is the ability to detect misalignment between two lines where one is at the end of the other and both have the same orientation.

The distance between the photoreceptors in the retina limits the minimal visual acuity, i.e. stimuli or changes in a stimulus smaller than the distance between the photoreceptors cannot be detected.

Author contributions: OIJ made the figures and took care of the reference list.

Otherwise the authors contributed equally to all parts of the paper.

Conflict of interest: The authors declare that there is no conflict of interest regarding this paper.