Jonathan Levy

Juan R Vidal

Robert Oostenveld

Ian Fitzpatrick

Jean-Francois Démonet

Pascal Fries

Alpha-band Suppression in the Visual Word Form Area as a Functional Bottleneck to Consciousness

Abstract

The current state of empirical investigations refers to consciousness as an all-or-none phenomenon. However, a recent theoretical account opens up this perspective by proposing a partial level (between nil and full) of conscious perception. In the well-studied case of single-word reading, short-lived exposure can trigger incomplete word-form recognition wherein letters fall short of forming a whole word in one's conscious perception thereby hindering word-meaning access and report. Hence, the processing from incomplete to complete word-form recognition straightforwardly mirrors a transition from partial to full-blown consciousness. We therefore hypothesized that this putative functional bottleneck to consciousness (i.e. the perceptual boundary between partial and full conscious perception) would emerge at a major key hub region for word-form recognition during reading, namely the left occipito-temporal junction. We applied a real-time staircase procedure and titrated subjective reports at the threshold between partial (letters) and full (whole word) conscious perception. This experimental approach allowed us to collect trials with identical physical stimulation, yet reflecting distinct perceptual experience levels. Oscillatory brain activity was monitored with magnetoencephalography and revealed that the transition from partial-tofull word-form perception was accompanied by alpha-band (7-11 Hz) power suppression in the posterior left occipito-temporal cortex. This modulation of rhythmic activity extended anteriorly towards the visual word form area (VWFA), a region whose selectivity for word-forms in perception is highly debated. The current findings provide electrophysiological evidence for a functional bottleneck to consciousness thereby empirically instantiating a recently proposed partial perspective on consciousness.

Moreover, the findings provide an entirely new outlook on the functioning of the VWFA as a late bottleneck to full-blown conscious word-form perception.

2. Keywords: Consciousness, VWFA, Reading, Neuronal oscillations, Alpha-band, MEG.

Introduction

Understanding how the brain creates conscious perception is probably one of the hardest challenges faced by modern science. Currently, it remains equivocal whether there can be a strict dissociation between unconscious and conscious processing. However, neuroimaging has recently shed important insight into that matter by providing sound evidence that such dissociation can be observed as a sharp nonlinear transition in brain activity [START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF][START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Dehaene | Experimental and theoretical approaches to conscious processing[END_REF]. Subjective reports of perception can be tightly reflected by such transitions to such an extent that individual trials, reported or not as consciously seen, can be categorically distinguished by their activation pattern. The bifurcation between unconscious and conscious processing follows the 'winner-takes-it-all' rule, and is therefore indicative of consciousness as an "all-or-none" phenomenon clearly distinct from the graded panel of possible prior sensory inputs.

Nevertheless, it has been very recently suggested that an important transition phase, namely partial consciousness, separates nil and full conscious perception [START_REF] Kouider | How rich is consciousness? The partial awareness hypothesis[END_REF]. Partial consciousness is described as a perceptual state with high subjective confidence about the informational content, but only at a limited level of the full percept. Accordingly, partial consciousness is assumed to be accessed independently from full (or nil) consciousness, thereby allowing low-level, not high-level representational contents to arise in conscious perception. Notably, partial consciousness may still reflect an all-or-none access, but in contrast to full consciousnessonly to a limited level of representation. Hence, the partial hypothesis is indicative of consciousness as a graded phenomenon, yet without compromising its all-or-none mechanism. Importantly, the partial consciousness hypothesis has not yet been demonstrated at the brain activation level. Here, we reasoned that in order to result in distinct, yet closely-matched subjective reports, the neural correlates of partial and full conscious perception ought to have different dynamic neural signatures. Namely, we assumed here that contrasting full to partial consciousness should recruit neuronal substrates, selectively reflecting the access to higher-level informational contents.

To investigate the neural substrates involved in conscious processes, authors have used a variety of paradigms (for reviews see [START_REF] Lamme | Towards a true neural stance on consciousness[END_REF][START_REF] Dehaene | Experimental and theoretical approaches to conscious processing[END_REF], of which a common strategy has been contrasting perceived versus unperceived words (e.g. [START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF]. Importantly, using single-words for probing consciousness can be particularly suitable in the case of partial consciousness due to the graded organization of words from partial (letters) to full (whole word) processing [START_REF] Selfridge | Pandemonium: a paradigm for learning[END_REF][START_REF] Mcclelland | An interactive activation model of context effects in letter perception: Part 1. An account of basic findings[END_REF] which is mapped onto separate localized functional cerebral clusters [START_REF] Levy | Piecemeal recruitment of left-lateralized brain areas during reading: a spatio-functional account[END_REF][START_REF] Mainy | Cortical dynamics of word recognition[END_REF][START_REF] Vidal | Long-distance amplitude correlations in the high γ band reveal segregation and integration within the reading network[END_REF]. Namely, the two perceptual levels involved in word reading (i.e. letter perception and whole word perception) can be exploited as two levels of conscious perception (i.e. partial and full blown perception, respectively). Thus, we probed the oscillatory neural responses marking the transition from partial to full conscious wordform perception, and tested whether this transition could be generally accounted as a functional bottleneck to consciousness. In the present context, a functional bottleneck to consciousness refers to the perceptual boundary separating partial and full conscious perception. We expected that contrasting full and partial word recognition should majorly recruit neuronal substrates selectively involved in word processing. Our major candidate was the left occipito-temporal junction, as it is by far considered as a key brain region to word processing, despite the ongoing debate running around its specificity and its exact coordinates [START_REF] Dehaene | The unique role of the visual word form area in reading[END_REF][START_REF] Price | The Interactive Account of ventral occipito-temporal contributions to reading[END_REF]. We reason that this neural substrate can be straightforwardly extrapolated as a putative functional bottleneck to consciousness.

During the experiment, participants viewed a sequence of foveally-presented masked words (Figure 1A) while oscillatory brain activity was monitored with magnetoencephalography. Participants reported the level of perception in a forced-choice procedure. The combinations of perceptual report yielded four possible outcomes: if a word was fully perceived, the report was through a semantic decision on meaning (animate/inanimate). If by contrast, a word was partially perceived, i.e. no meaning yet vague orthographic information was perceived, the report was through an orthographic detection task (letters/no-letters) (Figure 1A). Noteworthy, in the latter task, subjects were required to report whether they were able to recognize one or more letters. Subject were not required to report the identity of the letter(s), but rather to press 'yes' or 'no' for the presence of letters. Specifically, the instruction was (with the counterbalancing of fingers' order across individuals) as follows:

"Please press the button below your right middle finger whenever you fully see a word referring to an animate item; please press the button below your right index finger for a word referring to an inanimate item; please press the button below your left middle finger whenever you do not perceive any word, but instead you do perceive one letter or more; finally, please press the button below your left index finger whenever you do not perceive any word or any letter either. Alternatively, sample words were replaced by a blank screen in one sixth of the total trials (catch trials), for which subjects had to press the button corresponding to the absence of words/letters. The critical manipulation was the adjustment of masks' contrast as a function of participants' reports (see Figure 1B for a sample from one single experimental sequence). Such procedure is also referred to as "staircase" [START_REF] Levitt | Transformed up-down methods in psychoacoustics[END_REF], pointing to the real-time adaptation of physical stimulation to subjective reports.

Hence, this experimental strategy allowed us to obtain two types of subjective reports (partial or full) for an identical level of physical stimulation, thereby straightforwardly testing our hypothesis of a functional bottleneck to consciousness.

Materials and Method

Participants

Thirteen healthy right-handed and native-Dutch subjects (eight males, average age 23.92 ± 4.54 years) with normal or corrected-to-normal vision participated in the experiment. None of the participants had a history of neurological or psychiatric disorders. The study was approved by the local ethics committee, and a written informed consent was obtained from the subjects before the experiment according to the Declaration of Helsinki, and received monetary compensation.

Stimuli

Stimuli were generated using Presentation software (Neurobehavioral systems, Albany, USA) in a dimly lit room and subtended a horizontal visual angle of ∼2.5°. They were projected on an LCD monitor placed at a viewing distance of 70 cm. Responses were delivered by response pads. Stimuli were Dutch nouns (300 animate, 300 inanimate) between 3 and 9 letters in length with a lemma frequency between 100 and 300 occurrences per million [START_REF] Baayen | The CELEX lexical database [CD-ROM[END_REF]. Animate and inanimate words were balanced with respect to their word frequency of apparition and the number of letters they contained. The animacy character of the selected words was unequivocal. Words were presented only once to prevent automatic stimulus-response learning or repetition-suppression effect.

Experimental Procedure

Each trial began with a fixation cross presented for 600-933.33 ms (first presentation after pause presented for 2600-2933.33 ms), followed by the presentation of a forward mask (66.66 ms), a word (33.33 ms) and a backward mask (66.66 ms). The random time controlled for plausible anticipation of stimulus onset timing. In 16.67% of the totality of trials, we replaced the words with blank screens to avoid subject's habituation or anticipation and to control for the orthographic detection task (letters/no-letters). All stimuli were presented randomly. After stimuli presentation, a larger fixation crosshair prompted the subject to respond. Responses indicated trials' termination and lasted no longer than 2100 ms. A smaller subsequent crosshair spanned1000ms and signaled the interval linking the next trial. Participants were allowed to make eye movements or blinks during that inter-trial interval. In order to avoid guessing during the task, subjects were explicitly asked to categorize the word only when they had clearly recognized it, and were trained on the task with 90 training trials (different words than during the experiment) before measurement. During the task, subjects received visual feedback on the correctness of their response, so as to discourage them from guessing. Furthermore, in the unlikely event in which the semantic category (animate vs. inanimate) of a given word appeared equivocal, participants were instructed to avoid response, and hence exclude such trials from the analysis. After blocks of 30 trials, participants were allowed to pause. They initiated the start of each new trial block by a button press. In total, each subject performed 720 trials in a total measurement time of approximately 55 min. The reports were through button press on a response pad; they were counterbalanced across subjects between right/left hands and middle/index fingers (four response buttons in total, and hence eight possible combinations of response press, in the aim of controlling for differences in the motor representations of each finger.

To maximize and to balance attentional engagement, the subjects' task was to report (i) full or (ii) partial wordform perception by semantically categorizing the word (animate/inanimate) or if failed to perceive the word, by orthographically categorizing the percept (letters/none), respectively. Correctness in word detection was probed using the semantic categorization task (Animate or Inanimate), whereas correctness in letter detection was probed based on the ability to report the presence of letters during word trials (hit) or during blank trials (false alarm). To achieve maximal sensitivity of the neural signal and the statistical tests, the procedure aimed at collecting trials with a 50% distribution for each of the two reports. A modified "stair-case" version of the threshold estimation procedure described by Levitt [START_REF] Levitt | Transformed up-down methods in psychoacoustics[END_REF] was determined individually in real-time. The staircase effect was obtained by varying the contrast of a mask of scrambled characters (see Figure 1C) every six trials based on the given subjective response. Noteworthy, in order to obtain optimal parameters resulting in a 50/50 distribution of the two perceptual tasks, prior to this study we conducted pre-study piloting in which we obtained optimal stimulation/masking (i) latency and (ii) illumination, as well as (iii) optimal amplitude (masking intensity) of stair-case steps.

MEG recordings

Ongoing brain activity was recorded (sampling rate, 1200 Hz) using a whole-head CTF MEG system with 275 DC SQUID axial gradiometers (VSM MedTech Ltd., Coquitlam, British Columbia, Canada). Head position was monitored using three coils that were placed at the subject's left ear, right ear, and nasion. Bipolar EEG channels were used to record horizontal and vertical eye-movements as well as the cardiac rhythm for the subsequent artifact rejection.

Stratification

To minimize effects of fluctuations in response time on our analyses, we performed a post hoc stratification of the data based on the response time (RT) values. The goal of procedure was to assure that RT variance would not bias the tested contrast (i.e. full versus partial). We therefore obtained from each of the two perceptual reports (partial and full), a subset of trials with an identical distribution of RT values across the subsets in each of the two report conditions. This approach has been adjusted from Roelfsema et al. [START_REF] Roelfsema | Object-based attention in the primary visual cortex of the macaque monkey[END_REF] and a similar strategy has been successfully applied before to control for EMG fluctuations [START_REF] Schoffelen | Neuronal coherence as a mechanism of effective corticospinal interaction[END_REF]. For every participant, we binned the observations in each condition, while the bin centers were obtained by dividing the range of all RT values into four equally spaced bins with equal bin centers. In this way, each of the observations fell within one of the bins, for which we selected a subset of observations such that across the two conditions, the number of observations was identical. From the condition with the lowest number of observations in a given bin, all N observations constituting that particular bin were selected for the stratified sample. From the other condition, a subset of N observations was randomly drawn from the observations constituting that particular bin.

Spectral Analysis

The analysis was performed using Matlab 7 (MathWorks, Natick, MA, USA) and the FieldTrip toolbox [START_REF] Oostenveld | FieldTrip: Open source software for advanced analysis of MEG, EEG, and invasive electrophysiological data[END_REF]. Only correct trials with an identical contrast level were kept for spectral analysis (Figure 1C). They consisted in the largest fraction of report trials (Figure 1D) and were equally distributed (Figure 1E). Trials containing eye blinks, saccades, muscle artifacts and signal jumps were rejected from further analysis.

We applied tapers to each time window and calculated Time-Frequency Representations (TFRs) of power for each trial using a Fast Fourier Transform (FFT) applied to short sliding time windows; data was analyzed in alignment to stimulus onset. Power estimates were then averaged across tapers. Because the width of the frequency bands observed in physiological responses typically scales with the center frequency, we analyzed two frequency ranges separately: 2-40 and 40-150Hz. For the frequencies 2-40 Hz, we used a Hanning taper and applied a fixed time window of 0.5 s, resulting in a spectral resolution of slightly less than 2 Hz. For the higher-frequency band (40-150 Hz), we applied multitapers [START_REF] Percival | Spectral analysis for physical applications: multitaper and conventional univariate techniques[END_REF] using a fixed window length of 0.2 s, resulting in a frequency smoothing of 12.5 Hz.

The power values were calculated for the horizontal and vertical component of the estimated planar gradient and then summed [START_REF] Bastiaansen | Tangential derivative mappingof axial MEG applied to event-related desynchronization research[END_REF]. The horizontal and vertical components of the planar gradients were calculated for each sensor using the signals from the neighboring sensors thus approximating the signal measured by MEG systems with planar gradiometers. The planar field gradient simplifies the interpretation of the sensor-level data because the maximal signal typically is located above the source [START_REF] Hämäläinen | Magnetoencephalographytheory, instrumentation, and applications to noninvasive studies of the working human brain[END_REF]. The planar gradient power estimates were subsequently averaged over trials for a given condition.

Source analysis

For the analysis of the neuronal sources, we used an adaptive spatial filtering method called beamforming [START_REF] Gross | Dynamic imaging of coherent sources: Studying neural interactions in the human brain[END_REF] and relied on partial canonical correlations. Subjects' brain volume was divided into a regular grid. The grid positions were obtained by a linear transformation of the grid positions in a canonical 1cm grid. This canonical grid was based on a template brain (Montreal Neurological Institute), and for each subject, we computed the linear transformation optimally aligning the subject's brain volume to the template brain, using SPM8 (http://www.fil.ion.ucl.ac.uk/spm). We applied the inverse of this linear transformation to the grid positions of the canonical grid to obtain subject-specific dipole grids. This procedure facilitates the group analysis, because no spatial interpolation of the volumes of reconstructed activity is required.

For each grid position, we constructed spatial filters. These filters have the property that they optimally pass activity from the location of interest, while other activity, which is present in the data, is suppressed. To compute the lead field matrices, we used a single-shell volume conduction model, based on the shape of the inside of the skull [START_REF] Nolte | The magnetic lead field theorem in the quasi-static approximationand its use for magnetoencephalography forward calculation in realistic volume conductors[END_REF]. The inside of the skull was derived from each individual subject's structural magnetic resonance image, which was spatially aligned to the MEG sensors. Despite the fact that, in this volume conduction model, there is no truly magnetically silent direction, we excluded the most silent direction from the lead fields, since this direction potentially picks up a lot of noise.

To analyze the location of the sources accounting for the significant sensor level effects, we computed the CSD matrix between all MEG sensor pairs from the Fourier transforms of the tapered data epochs at the three frequency-bands where we had found significant sensor-level effects, i.e. the alpha, beta and gamma bands. For each subject, and within each of the frequency bands, we identified the frequency bin that yielded the most extreme t value (for power comparisons), averaged across sensors. Spatial filters were constructed for each grid location, based on the identified frequency bin, and the Fourier transforms of the tapered data epochs were projected through the spatial filters.

When analyzing at the coordinate level (virtual channel), we proceeded similarly with beamforming but this time we relied on a linear constrained minimum variance beamformer. Time frequency analysis was performed with the virtual channel at the coordinates of interest.

Statistical Analysis

Statistical significance of the power values was assessed similarly at the sensor and at the source levels. It was assessed using a randomization procedure [START_REF] Maris | Nonparametric statistical testing of EEG-and MEG-data[END_REF]. This nonparametric permutation approach does take the cross-subject variance into account, because this variance is the basis for the width of the randomization distribution. This approach was chosen as it does not make any assumptions on the underlying distribution, and it is unaffected by partial dependence between neighboring time-frequency pixels.

Specifically, the procedure was as follows: t-values representing the contrast between the conditions were computed per subject, channel, frequency and time. Subsequently, we defined the test statistic by pooling the tvalues over all subjects. Here, we searched time-frequency clusters with effects that were significant at the random effects level after correcting for multiple comparisons along the time and the frequency dimensions.

Testing the probability of this pooled t-value against the standard normal distribution would correspond to a fixed effect statistic. However, to be able to make statistical inference corresponding to a random effect statistic, we tested the significance of this group-level statistic by means of a randomization procedure: We randomly multiplied each individual t-value by 1 or by -1 and summed it over subjects. Multiplying the individual t-value with 1 or -1 corresponds to permuting the original conditions in that subject. This random procedure was reiterated 2000 times to obtain the randomization distribution for the group-level statistic. For each randomization, only the maximal and the minimal cluster-level test statistic across all clusters were retained and placed into two histograms, which we address as maximum (or minimum, respectively) cluster-level test statistic histograms. We then determined, for each cluster from the observed data, the fraction of the maximum (minimum) cluster-level test statistic histogram that was greater (smaller) than the cluster-level test statistic from the observed cluster. The smaller of the two fractions was retained and divided by 2000, giving the multiple comparisons corrected significance thresholds for a two-sided test. The proportion of values in the randomization distribution exceeding the test statistic defines the Monte Carlo significance probability, which is also called a p-value [START_REF] Nichols | Nonparametric permutation tests for functional neuroimaging: a primer with examples[END_REF][START_REF] Maris | Nonparametric statistical testing of EEG-and MEG-data[END_REF]. This cluster-based procedure allowed us to obtain a correction for multiple comparisons at all sensor and source analyses. At the virtual sensor level, however, such procedure was not possible given the analysis at a single coordinate; therefore, correction for multiple comparisons was performed by controlling the false discovery rate (FDR) [START_REF] Benjamini | Controlling the False Discovery Rate: A Practical and Powerful Approach to Multiple Testing[END_REF].

Results

The critical manipulation was the adjustment of masks' contrast as a function of participants' reports (see Figure 1B for a sample from one single experimental sequence). Such procedure is also referred to as "staircase" [START_REF] Levitt | Transformed up-down methods in psychoacoustics[END_REF], pointing to the real-time adaptation of physical stimulation to the subjective reports.

Mask contrasts fluctuated between three levels for most participants (eight), while the remaining five encountered only two levels. We selected for further analyses the middle luminance level, namely m, trials (Figure 1C) which accounted for the largest trial fraction (Figure 1D) and additionally presented a balanced distribution between the two perceptual reports (Figure 1E). Notably, the distribution error bar in the m contrast was virtually nil (±0.0044), thereby indicating low variance across subjects for the selected contrast. On average, 26.06 ±4.11% of the totality of trials were maintained for further analysis for each of the two conditions (full and partial). Mean response time after the stratification procedure was 999.84 ± 331.85 ms and 999.72 ± 343.45 ms for the full and partial conditions respectively, without significant (p = 0.61) difference (Figure 1,G). Mean accuracy for the two tasks was 97.38 ± 1.55 % and 98.16 ± 3.60 % in the full and partial conditions respectively, without significant (p = 0.39) difference (Figure 1,H). Furthermore, to test whether intrinsic bottom-up (word-length in letters) or top-down (frequency of apparition) features of the presented stimuli (i.e. words) had any significant influence on their probability of being perceived fully or partially, a non-parametric Spearman rank correlation was performed. Significant negative correlation was found between word length and perception (r = -0.08, p < 0.001), yet no significant correlation between word frequency of apparition and perception (r = -0.01, p = 0. 43). It was also found that full perception was biased towards longer words for 7-letter words (and longer) (Figure 1F). Nevertheless, the (theoretical) average word length (6.77 letters) yielded a balanced distribution (49.11/50.88%) in full/partial perception, respectively.

Frequency-localizer contrast

Neuronal network dynamics are characterized by rhythmic oscillatory activity [START_REF] Buzsaki | Neuronal oscillations in cortical networks[END_REF]). In the case of word-form perception, locally-organized cortical oscillations have been previously reported as reliable markers of full conscious processing [START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF]. Here, we measured brain activity with MEG and estimated neuronal oscillations during a double-task design of conscious perception. To constrain the coming analyses to functional rhythmic activity, we first quantified the post-stimulus neural response as compared to its pre-stimulus baseline response level. We therefore averaged power responses across all sensors (p<0.05, corrected for multiple comparisons across all time and frequency samples). Figure 2 These sensor-level patterns were subsequently investigated at the source level. Figure 3 illustrates source reconstruction maps applied to the four frequency bands by means of adaptive spatial filters: alpha power changes localized to the junction of the bilateral occipital, temporal and parietal cortices; beta power changes localized to bilateral parietal, bilateral primary somatosensory (S1), left primary motor (M1) and left premotor (PM) cortices; narrow-band gamma was expressed in right M1, left S1 and left PM; finally broad-band gamma was expressed in the middle occipital gyri. The functional localization of these four spectral sources suggests that they may be assigned to different processes. Globally, alpha and broad-band gamma oscillations may correspond to sensory processing whereas beta and narrow-band gamma may be more assigned to motor-related processing.

Full versus partial conscious perception

To probe the neural processing underlying the transition from partial to full conscious perception, we contrasted the two conditions while averaging across all channels in the frequency bands of interest revealed by the previous time-frequency analysis. Even though there was only an insignificant difference (p=0.27) in reaction time distributions between the two perceptual reports (partial and full), we further minimized the effects of reaction time variance on spectral power by applying a post hoc reaction time stratification (p=0.85, after stratification). Figure 4 illustrates the time-course of the resulting t-values during the course of the trial; the graph highlights a late yet sharp buildup of effect size followed by a sustained effect over the second part of the trial (see significance bars at the bottom of the panel) for the three frequency-bands. It also points to the opposite polarity of this effect in the low (<40 Hz) compared to the high (>40 Hz) frequencies. We did not include in this analysis the previously mentioned broad-band gamma (Figure 2) as it did not reach significance for this contrast (p>0.2). Time-frequency and source maps associated this effect with early visual processing (Figures 2 and3, respectively). Together with the low t-values during the early stages of trial processing in the three frequency bands (t = -0.18 cross-frequency averaged at 0 -0.4sec), this is suggestive of visual processing common to both reports (partial and full). T-values reached significance (p<0.05, corrected) around 0.5sec post stimulus onset (Figure 4). This late buildup of neural processing suggests that sensory evidence could be identical during the two reports. This result pattern is in line with our expectation based on the experimental strategy of maintaining bottom-up resources at steady-level stimulation. To further analyze the corresponding source level activity, we chose a time window (0.5-0.8 sec), which contained the perturbations in all three frequency bands (Figure 4). Scalp topographies revealed the (peak) sensors of interest of the full versus partial effects in the three frequency bands (Figure 5A). Source representations (Figure 5B) show differential activity in clusters expressed: at the left middle occipital gyrus and the left precuneus (Alpha), at the left cuneus (Beta), the left middle temporal gyrus (Beta) and at the left inferior parietal lobule (Beta), and finally, at the left S1 and the left PM (Gamma). Figure 6 illustrates the TFRs specific to the sensors of interest revealed in Figure 5A, together with their right hemispheric homologs and the sensors of interest revealed in the stimulus versus baseline analysis in the broad-band gamma (Figure 3A, right panel). These sensor-specific TFRs straightforwardly convey the activity in partial and full conditions, separately, as well as the full versus partial contrast (Figure 6). This supplementary analysis strengthens the previous findings by revealing full versus partial effects only in the left hemispheric alpha, beta and narrowband gamma (note, however, a weakened alpha effect in the right hemispheric sensors (Figure 6A, right panel)).

It also shows a certain spatial overlap in the alpha and beta effects (Figure 6AB, left panels). To further probe the observed cortical response patterns, we separated subjects who reported full conscious perception with their right hand (and therefore partial conscious perception with their left hand), and those who reported it with their left hand (and therefore partial conscious perception with their right hand). This strategy leans on the observation that modulation in neural oscillations is typically enhanced in the motor cortex contralateral to the responding hand relative to the ipsilateral one [START_REF] Donner | Buildup of choice-predictive activity in human motor cortex during perceptual decision making[END_REF]. Thus, a contralateral bias would be mainly indicative of motor-related activity, whereas spectral perturbations remaining unaffected as a function of response hand would be indicative of an effect which is largely unbiased by motor activity.

Clusters of activity at both the sensor and the source level were left-lateralized irrespective of response-hand at the alpha band (Figure 7A), in contrast to a significant contralateral bias in the beta (Figure 7B) and gamma (Figure 7C) bands. Furthermore, statistically significant clusters of sensors (contralateral to button press) and their symmetrical homologues in the other brain hemisphere (ipsilateral to button press) were averaged. The resulting t-values for the different bands are shown as insets and provided additional evidence that the alpha band modulation was always left lateralized, while the beta and gamma modulation was always contralateral to the response hand. Source reconstruction further determined that the alpha-power perturbation originated in the left occipito-temporal-parietal junction. By contrast, the beta-power perturbation originated in right S1, left M1 and left inferior parietal lobule, and the gamma-power perturbation in the right PM and left M1. Hence, neural activity peaked in regions with a known contralateral motor bias in the beta (except for the left inferior parietal lobule) and gamma bands. Such sensori-motor activity therefore exhibited robust lateralized effector selectivity of opposite polarity in the beta and gamma bands. By contrast, alpha-band suppression peaked in a region specific for word processing, namely, the left occipito-temporal cortical junction (LOT) and exhibited robust left-lateralized selectivity unaffected by response hand (Figure 7A).

Figure 6. Time-frequency representations (TFRs) across the sensors of interest

The left panels illustrate time-Frequency statistical maps averaged across the sensors of interest as revealed in figure 5 for the (A) alpha, (B) beta and (C) narrow-band gamma, and in figure 3 for the (D) broad-band gamma. The right panels illustrate the maps of the right hemispheric sensor homologs. The maps describe activity modulation for the full vs. partial contrast (left TFRs), the full vs. baseline (middle TFRs) and the partial vs. baseline (right TFRs). The analysis was performed with stimulus-aligned data and was stratified for reaction time. Color bars illustrate masked significant modulations (p<0.05, corrected).

Virtual sensor analysis in the LOT and in the VWFA

Although activity peaked in the posterior LOT (the left middle temporal gyrus, BA 39, MNI coordinates -38 -78 12), (Figure 8A, upper panel), it also reached statistical significance in the anterior LOT (Figure 8B, upper panel), in a region also known as the visual word form area (VWFA; the left fusiform gyrus, BA 37, MNI coordinates -42 -55 -9) [START_REF] Cohen | The visual word form area: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior splitbrain patients[END_REF]. We computed two virtual sensors in each one of the posterior and anterior LOT coordinates. TFRs revealed that the main effect peaked in the alpha-band in both partial and full conditions (Figure 8, lower panels). Temporal power curves (Figure 8, blue/red curves) additionally revealed that the full versus partial alpha effect became statistically significant at 0.5sec in the posterior LOT, and at 0.7sec in the anterior LOT, i.e. the VWFA. These analyses advocate that a transition from partial to full conscious (word) perception can be measured with the use of a staircase procedure. Specifically, the transition is highlighted as a neural signature expressed through a late suppression of alpha-oscillations in the LOT. The present experimental approach and the characteristics of the observed neural signature are discussed in the context of prior evidence and future directions.

Discussion

The present study tested the validity of a graded perspective on consciousness. This was achieved with the emblematic case of word reading. We hypothesized that the putative functional bottleneck to consciousness (from partial to full) would emerge at a major key hub region for reading, namely the left occipito-temporal junction (LOT). We report that the transition from partial to full conscious (word) perception is marked by alpha-band oscillation suppression in the LOT. We propose that the findings empirically support a recent theoretical proposal of an intermediate perceptual level between conscious and non-conscious perception [START_REF] Kouider | How rich is consciousness? The partial awareness hypothesis[END_REF]. Furthermore, the findings not only confirm that consciousness can be graded into a partial level, but additionally shed new light on the functioning of the LOT/VWFA as a bottleneck region to full-blown conscious word-form perception. Specifically, the bottleneck would first emerge (at 0.5sec) in the posterior LOT and would extend anteriorly later on (at 0.7sec) to the VWFA.

Figure 7. Full vs. Partial: the alpha, beta and gamma effects as a function of response-hand laterality

Scalp topographies and brain views (sagittal and axial planes) in the (A) alpha, (B) beta and (C) gamma bands illustrate statistically significant (p < 0.05, corrected) topographical and cortical clusters. Topoplots refer to a group of statistically significant clusters of sensors (contralateral to button press) and their symmetrical homologues in the other brain hemisphere (ipsilateral to button press) that were highlighted in the topography. Insets illustrate pooled t-spectra were averaged across the highlighted left-hemispheric (represented by L) and right-hemispheric (represented by R) clusters. The upper and lower panels illustrate results pooled across subjects reporting Full conscious perception with a right-hand button press (N=7) and with a left-hand button press (N=6), respectively. Alternately, subjects reported Partial conscious perception with a left-hand button press (N=7) and with a right-hand button press (N=6), respectively.

Previous studies commonly compared trials in which stimuli were consciously perceived (i.e. full consciousness) versus trials in which stimuli were not consciously perceived (i.e. nil consciousness) [START_REF] Supèr | Two distinct modes of sensory processing observed in monkey primary visual cortex (V1)[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF][START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Melloni | Synchronization of neural activity across cortical areas correlates with conscious perception[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF]. However, recent theoretical approaches posit that such experimental strategy highlights neural activity driven by various processes unspecific to conscious perception [START_REF] Aru | Distilling the neural correlates of consciousness[END_REF][START_REF] De Graaf | The 'correlates' in neural correlates of consciousness[END_REF]. Here, we attempted to bypass the bias of non-specificity by focusing on a near-threshold transition from partial to full word-form perception. Additionally, exogenous stimulation was adjusted to endogenous perceptibility in real-time, thereby achieving stable stimulation. Importantly, the putative transition was expected once sufficient sensory evidence was accumulated. Previous research has revealed that such accumulation of evidence is reflected through a gradual buildup of neural processing reaching its peak near the perceptual threshold [START_REF] Heekeren | The neural systems that mediate human perceptual decision making[END_REF].

Hence, under the current near-threshold stimulation, we expected the putative neural responses to reflect late high-level word processing rather than sensory processing. Accordingly, a notable aspect of the full versus partial contrast was its late rise of significance (approx. 0.5sec) in the three frequency bands (Figure 4). We therefore propose that the two reports (partial and full) reflect identical sensory processing and that a selective late neural process gates fully conscious perception of words under stimulation conditions of near perceptual threshold.

The present finding of a late neural processing marking the rise of full conscious perception is rather consistent with the predictions made by the global neuronal workspace (for a late review, see [START_REF] Dehaene | Experimental and theoretical approaches to conscious processing[END_REF] and with other experimental data showing a late divergence between conscious and nonconscious processing [START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF][START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Melloni | Synchronization of neural activity across cortical areas correlates with conscious perception[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF]. The global neuronal workspace, however, typically reports the divergence (between nil conscious and full consciousness) to arise at approximately 0.3sec post stimulus onset whereas the current experiment revealed an even later divergence (between partial and full consciousness), i.e. at approximately 0.5sec. Thus, the present finding sheds novel light on the temporal dynamics of conscious perception by suggesting that the gateway to fullyblown conscious is even later than what has been previously consideredif the intermediate (i.e. partial) conscious level is taken into account. Furthermore, the global neuronal workspace makes predictions also regarding the cortical signature of conscious access, namely involving the fronto-parietal network [START_REF] Dehaene | Experimental and theoretical approaches to conscious processing[END_REF]. In the present study the main non-motor activity (i.e. at the alpha-band) resulting from stimulus presentation in general (Figure 3, left panels), or from partial to full conscious perception in particular (Figure 5A, left panel) was not parieto-frontal, but rather occipital with temporal and parietal ramifications.

Nevertheless, the effect did extend towards left-hemispheric parietal and even frontal sensors (Figure 5A, left panel). The most plausible explanation for the lack of more robust frontal activity could be accounted for by several experimental parameters: the paradigm (near-threshold presentation of masked words), the nature of the tasks (semantic and orthographic decisions), the analysis applied (local time-frequency) and the neuroimaging technique (MEG). Although Gross and colleagues reported a fronto-parietal enhancement in the beta band [START_REF] Gross | Modulation of long-range neural synchrony reflects temporal limitations of visual attention in humans[END_REF], their paradigm relates to a different experimental paradigm, namely the attentional blink, consisting in triggering quite different sensorial and cognitive processes. By contrast, word presentation has been previously shown to rather trigger alpha/beta suppression in similar regions as illustrated in the present study [START_REF] Pammer | Visual word recognition: the first half second[END_REF][START_REF] Cornelissen | Activation of the left inferior frontal gyrus in the first 200 ms of reading: evidence from magnetoencephalography (MEG)[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF]. Another matter to consider is that the fronto-parietal recruitment is rather predicted [START_REF] Dehaene | Experimental and theoretical approaches to conscious processing[END_REF] and observed [START_REF] Melloni | Synchronization of neural activity across cortical areas correlates with conscious perception[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF] in terms of long-range neuronal synchrony; the current paper, however, investigates the effects in terms of local neural oscillations. Future investigations should also keep in mind the distinct nature of local oscillations and long-range synchrony when testing the predictions of the global neuronal workspace or of other frameworks. Finally, one should also acknowledge the difference between electrophysiological source localization and haemodynamic cerebral mapping: word processing often does not trigger frontal activations when measured by MEG in comparison to fMRI [START_REF] Vartiainen | Functional Magnetic Resonance Imaging Blood Oxygenation Level-Dependent Signal and Magnetoencephalography Evoked Responses Yield Different Neural Functionality in Reading[END_REF].

Despite the finding of insignificant processing modulation between the two perceptual reports during the first half second (Figure 4), we also probed activity in that early time window for both perceptual reports (Figure 2).

We found that local rhythmic activity (alpha, beta and gamma bands) emerged early (Figure 2) at bilateral occipito-temporo-parietal cortices (Figure 3). This early oscillatory activity could be taken as an indication of the elementary visual and language processes taking place at the first half second of word processing. This observation is in line with the scarce prior evidence of spatio-temporal oscillatory data during visual word processing [START_REF] Pammer | Visual word recognition: the first half second[END_REF][START_REF] Cornelissen | Activation of the left inferior frontal gyrus in the first 200 ms of reading: evidence from magnetoencephalography (MEG)[END_REF]. Considering the activity modulation observed here during the second half second (Figure 4), the report-driven divergence is suggestive of a critical threshold gating complete word-form perception.

As a consequence of the experimental strategy adopted here, contrasting full versus partial did not aim at including sensory processing. Furthermore, the counterbalanced response in both tasks and the post-hoc stratification for RT minimized the effect of eventual differences in reaction time distributions between conditions onto our data analysis. At the design level, response-hand laterality was task dependent and counterbalanced so as to highlight motor-independent effects; this strategy yielded the alpha power suppression observed here. Furthermore, the contralateral bias of the beta and gamma effects (of opposite polarity) suggested that they were related to motor response preparation (Figure 7BC); this observation is consistent with previous reports of the motor-driven gamma activation [START_REF] Gruber | Modulation of induced gamma band responses and phase synchrony in a paired associate learning task in the human EEG[END_REF][START_REF] Kranczioch | EEG gamma-band activity in rapid serial visual presentation[END_REF][START_REF] Kranczioch | Temporal dynamics of access to consciousness in the attentional blink[END_REF][START_REF] Donner | Buildup of choice-predictive activity in human motor cortex during perceptual decision making[END_REF] and beta deactivation [START_REF] Kaiser | Dynamics of oscillatory activity during auditory decision making[END_REF][START_REF] Donner | Buildup of choice-predictive activity in human motor cortex during perceptual decision making[END_REF] above central sensors. Notwithstanding the sound evidence of motor-related activity in the beta and gamma bands, the two hand-dependent sub-groups are most likely prone to reflect motor-driven effects whereas the whole group would rather reflect non-motor driven effects (e.g. semantic or conscious perception related effects). This could explain the presence of beta-band activity, at the whole group level, in cortical areas which are typically not assigned to motor activity, such as the left cuneus and the left middle temporal gyrus (Figure 5B, middle panels). By contrast, beta-band activity, at the hand laterality group level, rather emanated from motor-related cortical areas such as S1 or M1 (Figure 7B). We therefore recapitulate by stating that given the differential cortical mapping of the beta effect at the whole group level (Figure 5B, middle panels) versus at the hand laterality group level (Figure 7B), beta-band activity may have partially accounted for the partial to full conscious perception effect probed in this study. This is further corroborated by the beta effect detected at the level of the alpha-band sensors of interest (Figure 6A), and with previous reports of beta-band activity for higher linguistic functioning (for a review, see [START_REF] Bastiaansen | Oscillatory neuronal dynamics during language comprehension[END_REF]. Nevertheless, the robust lateralized effector selectivity at the beta-band (Figure 7B) compared to non-lateralized effector selectivity at the alpha-band (Figure 7A) convincingly argues for the alpha-band effect observed here as the main candidate for the partial to full conscious perception transition. The current results propose that the LOT alpha suppression was not driven by motor activity. This could be convincingly pointed out by its left-lateralized activity pattern irrespective of response-hand laterality (Figure 7A), and by the non-motor region in which it was generated, namely, the LOT. Noteworthy, a recently proposed framework postulates that alpha activity reflects functional inhibition [START_REF] Jensen | Shaping functional architecture by oscillatory alpha activity: gating by inhibition[END_REF]. This interpretation was derived from reports of alpha power suppression in task-engaged brain areas (van Gerven and [START_REF] Van Gerven | Attention modulations of posterior alpha as a control signal for two-dimensional braincomputer interfaces[END_REF][START_REF] Handel | Alpha oscillations correlate with the successful inhibition of unattended stimuli[END_REF], and alternatively of alpha power enhancement in non-engaged areas [START_REF] Haegens | Somatosensory working memory performance in humans depends on both engagement and disengagement of regions in a distributed network[END_REF]. Thus, alpha suppression as observed here could be indicative of the functional engagement of the LOT as a selective activation pattern gating fully conscious perception of words. Sharing the same logic, previous reports of alpha suppression highlight its crucial role in high-level language processing [START_REF] Roehm | The role of theta and alpha oscillations for language comprehension in the human encephalogram[END_REF][START_REF] Klimesch | Alpha and beta band power changes in normal and dyslexic children[END_REF][START_REF] Bastiaansen | Oscillatory neuronal dynamics during language comprehension[END_REF]. Finally, a most recent review is highly consistent with the findings shown here, as it features the role of alpha suppression in enabling conscious access to semantics [START_REF] Klimesch | Alpha-band oscillations, attention, and controlled access to stored information[END_REF].

Contemporary models of reading propose that the LOT cortex acts as a neural gateway from vision to language [START_REF] Jobard | Evaluation of the dual route theory of reading: a metanalysis of 35 neuroimaging studies[END_REF][START_REF] Price | A review and synthesis of the first 20 years of PET and fMRI studies of heard speech, spoken language and reading[END_REF]. Ample evidence points out that the functional role of this area is primarily constrained to specific word-form perception [START_REF] Dehaene | The unique role of the visual word form area in reading[END_REF] within an anterior segment of the LOT, namely the VWFA. Here, we note that the essence of the partial to full conscious perception effect emerged in the posterior LOT (Figure 8A). Yet, we also note that this activity extends towards the anterior LOT, namely the VWFA (Figure 8B) about 0.2sec later. Accordingly, our findings generate a novel outlook on word processing dynamics: the VWFA would not only be recruited at an early stage for allowing low-level (script processing) word processing as has been previously instantiated [START_REF] Pammer | Visual word recognition: the first half second[END_REF][START_REF] Dehaene | The unique role of the visual word form area in reading[END_REF], but also at a later stage for gating high-level (lexico-semantic) processing. Such late semantic gateway would not be selective to the VWFA but rather emerge in the posterior LOT and extend anteriorly to the VWFA. Notably, Cornelissen and colleagues have investigated the oscillatory activity at the alpha and beta bands during the first half second of passive word viewing [START_REF] Cornelissen | Activation of the left inferior frontal gyrus in the first 200 ms of reading: evidence from magnetoencephalography (MEG)[END_REF]. Their findings revealed significant activity suppression concentrated at the VWFA during the 0.2-0.5sec time window. Hence, the activity observed here as a gateway to higher language processing, namely, alpha band suppression in the VWFA starting at 0.7sec, probably accounts for a late stage of the processing cascade which is triggered in this area as early as at 0.2sec. Specifically, the present evidence of a gateway from lower to higher processing is congruent with the observation of simultaneous early activity in the VWFA and in frontal language areas [START_REF] Cornelissen | Activation of the left inferior frontal gyrus in the first 200 ms of reading: evidence from magnetoencephalography (MEG)[END_REF]. This "second go" in the VWFA is likely to be enhanced under the particular experimental conditions of the present paradigm, namely near-threshold conscious word perception. Further support for the outlook generated here can be drawn from the joint evidence that forward and backward connections are essential for allowing conscious perception [START_REF] Lamme | Towards a true neural stance on consciousness[END_REF] and for interfacing lower with higher language processes [START_REF] Price | The Interactive Account of ventral occipito-temporal contributions to reading[END_REF]. Future connectivity modeling should exploit the fine temporal sampling of MEG to further probe the neural dynamics gated through the alpha suppression in the LOT region.

Moreover, zooming into the LOT activation source, one could note not only an extension of activity towards the ventral pathway, but also towards dorsal pathways (Figure 5B, left panel). The crucial role of the ventral and dorsal pathways during reading has been highlighted before [START_REF] Pugh | Functional neuroimaging studies of reading and reading disability (developmental dyslexia)[END_REF]. Interestingly, in a previous effective connectivity (fMRI) study, it was shown that this same region was acting as a mediator of information trafficking to those two pathways, and thereby optimizing reading performance [START_REF] Levy | Testing for the Dual-Route Cascade Reading Model in the Brain: An fMRI Effective Connectivity Account of an Efficient Reading Style[END_REF]. Other connectivity studies also highlight the role of these pathways during reading [START_REF] Cao | Effective brain connectivity with reading difficulties during phonological processing[END_REF][START_REF] Richardson | Multiple routes from occipital to temporal cortices during reading[END_REF]. In the major endeavor of comprehending the complex neural dynamics sustaining reading, these findings propose that future studies should take particular note of the forward and backward connections emanating from the LOT towards the ventral and dorsal pathways.

The results reported here reflect the grading of consciousness to a partial level, between nil and full, without compromising its all-or-none mechanism. In other words, partial or full stimulus representations can trigger an all-or-none neural response [START_REF] Kouider | How rich is consciousness? The partial awareness hypothesis[END_REF]. Although it is impossible to rule out random attentional fluctuations during the course of experiment may have generated fluctuations between full and partial perception, our analyses showed that these were most probably due to a bottom-up feature, namely word-length (Figure 1F), in agreement with previous experimental psychology data [START_REF] Samuel | Length effects in word perception: we is better than i but worse than you or them[END_REF]. Furthermore, our data shows that fluctuations in perception could not have been significantly predicted by early baseline power activity (Figure 4) as previous studies near-threshold stimulus or a TMS-induced phosphene perception have reported [START_REF] Romei | Spontaneous fluctuations in posterior alpha-band EEG activity reflect variability in excitability of human visual areas[END_REF][START_REF] Van Dijk | Prestimulus oscillatory activity in the alpha band predicts visual discrimination ability[END_REF][START_REF] Mathewson | To see or not to see: prestimulus alpha phase predicts visual awareness[END_REF][START_REF] Dugue | The phase of ongoing oscillations mediates the causal relation between brain excitation and visual perception[END_REF]. Yet, these studies often refer to baseline phase change as a predictor to subsequent perceptual level. Although phase analysis is beyond the scope of the present study, future analyses may reveal a plausible link between baseline phase activity and a bottleneck to perceptual consciousness. Finally, the word-length effect found here should be taken into account when selecting word stimuli in future experiments testing for conscious perception.

Figure 1 .

 1 Figure 1. Experimental Design and behavioral results (A) Design. Experimental paradigm used to present masked words in a stair-case procedure resulting in two levels of conscious perception: Full and Partial. The two levels were reflected by either a forced-choice task of semantic decision or of orthographic detection, respectively. (B) A sample of mask luminance fluctuation during an entire experimental trial sequence. (C) Masks. The luminance of the forward and backward masks fluctuated between three levels (h: high, m: middle, l: low) and was individually determined for each participant depending on the reported percept. (D) Trial distribution across subjects in the three masking conditions. (E) Ratio between the two levels of reported percept (Full vs. Partial) in the three masking conditions. The dashed horizontal bar indicates the optimal balanced ratio of 0.5. (F) Trial proportion in the two perceptual conditions as a function of the presented stimuli's (i.e. words) length (in number of letters). (G) Mean response times and (H) accuracy in the two perceptual tasks. Error-bars represent ±1 SD. Vertical arrows pointing to the mask level 'm' illustrate the selected trials for further MEG analyses.

  illustrates timefrequency statistical maps of the MEG activity recorded over all sensors during the course of the trial, in the low (2-40 Hz) and high (40-150 Hz) frequency ranges. The neural response time-frequency statistics map highlights effects (p<0.001, corrected) in four standard frequency bands: Alpha (7-11 Hz), Beta (15-30 Hz), Narrow-band Gamma (60-90 Hz) and Broad-band Gamma (60-130 Hz). As trials lasted on average one second from stimulus onset to button press, one could note that in the lower frequency range (<40 Hz) spectral perturbations were down-regulated over the mid-part of the trial's epoch (approx. 500ms). In contrast, activity peaked in the first part of the trial (<500ms) in a transient broad-band gamma burst, and in the second part of the trial (>500ms) in a more sustained, narrow-band gamma burst. This clear-cut temporal division in the higher frequencies is suggestive of two distinct functional processes, one for sensory processing and the other for decision and response-related processing.

Figure 2 .

 2 Figure 2. Frequency-localizer contrast: Stimulus (Full and Partial) vs. baseline activity

Figure 3 .

 3 Figure 3. Frequency-localizer contrast: Stimulus (Full and Partial) vs. baseline sensor and source activity

Figure 4 .

 4 Figure 4. Full vs. Partial: temporal representation in the alpha, beta and gamma bands Time course of t-values of the group test statistic for Full vs. Partial, averaged over all sensors and separately in the alpha, beta and gamma bands (blue, turquoise and red colors, respectively). The gray shading indicates the time interval from stimulus onset to average reaction time. The horizontal dashed lines indicate significance levels (p = 0.05, corrected), while the three lower horizontal bars indicate significant values (p < 0.05, corrected) on the time axis for each of the three frequency bands. The time interval 0.5 -0.8 sec was significant for the three frequency bands and therefore selected for further analyses.

Figure 5 .

 5 Figure 5. Full vs. Partial: Source-localization in the alpha, beta and gamma bands (A) Scalp topographies and (B) overlaid cortical surface (MNI template) representations, both reveal topographic and cortical clusters (p < 0.05, corrected) in the selected time interval of interest (i.e. 0.5 -0.8 sec) as revealed in Figure4.

Figure 8 .

 8 Figure 8. Virtual sensor analyses Analyses were carried out on activity filtered from coordinates in (A, left panels) the posterior left occipito-temporal (pLOT) junction and in (B, right panels) the anterior LOT (aLOT), namely the visual word form area (VWFA). Source-localization (upper panels) illustrates on MNI templates statistical effects (Full vs. Partial) in the two coordinates. Underneath, plots represent the temporal evolution of power change vs. baseline of the two conditions averaged in the alpha band. Shades represent standard error of the mean power values, while green rectangles indicate statistically significant values (p < 0.05, corrected) on the time axis when contrasting Full vs. Partial. Below are shown TFRs of the Full and Partial conditions vs. baseline.

Acknowledgements

The present work was supported by both the European Science Foundation's European Young Investigator Award Program, and by the French allowance "Allocation de Recherche Ministérielle". We would like to thank Bram Daams and Sander Berends for technical support, as well as Jan-Mathijs Schoffelen and Stan van Pelt for their previous advice on the analysis scheme.


Chadi Barakat

Anshuman Kalla

Damien Saucez

Thierry Turletti

Minimizing Bandwidth on Peering Links with Deflection in Named Data Networking

Content dissemination is the primary usage of the Internet today, whereas the existing Internet architecture based on TCP/IP is mainly designed for point-to-point communications. Information-Centric Networking (ICN) has been proposed to conciliate Internet usage and technology. The idea behind ICN is to omit the notion of host and location and establish content as the first class citizen of the network. ICN advocates innetwork caching, i.e., to cache content on the path from content providers to requesters. This on-path caching achieves good overall performance but is not optimal as content may be replicated on routers so reducing the total volume of content that can be cached. To overcome this limitation, we introduce the notion of off-path caching where we allocate content to well defined off-path caches within the network and deflect the traffic off the optimal path toward these caches that are spread across the network. Off-path caching improves the global hit ratio by efficiently utilizing the network-wide available caching capacity and permits to reduce egress links' bandwidth usage.

I. INTRODUCTION

We are moving from an Internet used to connect us to well defined hosts (e.g., FTP, telnet, POP3) to an Internet where content delivery and retrieval are prominent. Our main focus nowadays is on finding contents and consuming them without caring about where they are located. Unfortunately, the TCP/IP architecture is designed for point-to-point communications, causing a gap between the technology and its usage and reducing the efficiency of communication.

To conciliate technology and usage, Information-Centric Networking has been proposed [START_REF] Ahlgren | A survey of information-centric networking[END_REF]. ICN removes the notion of end-to-end communication by accessing content directly with its name. Instead of delivering packets to hosts with well defined IP addresses, an ICN network fetches content with some universal name without bothering the end user with tasks as finding the host where the content is located and downloading it himself. Different copies of the content can be available and the ICN architecture automatically should find and download the most appropriate copy with a single request for the content made directly to the network. Several ICN proposals came up ( [START_REF] Ahlgren | A survey of information-centric networking[END_REF]) but even though their conception differs, they all rely on assigning a universal name (hierarchical or flat) to each content rather than identifying it by the IP address of its host. The name of a content is directly used to fetch it, while with "legacy Internet", the name is first resolved into the IP address of the hosting machine. In this paper, we focus on Content-Centric Networking (CCN) [START_REF] Jacobson | Networking named content[END_REF] but our study and findings can be applied to other ICN proposals.

CCN leverages in-network caching to place popular contents close to their consumers, hence reducing the load on the network and improving QoS. This opportunistic caching at routers on the path traversed by content reduces the overall content retrieval time and bandwidth consumption [START_REF] Muscariello | Bandwidth and storage sharing performance in information centric networking[END_REF], [START_REF] Fricker | A versatile and accurate approximation for LRU cache performance[END_REF] as requests for the same content only traverse a portion of the path until finding a cached copy. However, this on-path caching inside an Autonomous System (AS) (or a domain) is not optimal in terms of provider links' bandwidth usage. Indeed, on-path caching results in an uncontrolled content duplication within the AS, hence wasting cache space and reducing overall hit rate. Better overall hit rate can be obtained if caches within an AS coordinate their effort to avoid replicas and cache more contents. This raises the natural question of how to perform caching within an AS such that provider links' usage is minimized while keeping the AS links' usage below their nominal capacities. We address this question in this paper and propose a theoretical formulation together with a practically deployable solution.

Provider links' usage is minimized in a stub AS if it caches as many contents as possible, meaning that content replicas must be avoided. [START_REF] Chai | Cache "less for more" in information-centric networks[END_REF], [START_REF] Guo | Collaborative forwarding and caching in content centric networks[END_REF] propose to reduce the number of replicas inside an AS while keeping the on-path caching assumption. Our claim is to realize a better gain by allowing traffic deflection (i.e., indirect routing) within the AS in such a way that all requests for the same content traverse the same cache. Therefore, if the content is cached, then it is certain that whichever client (associated with the AS) asks for it, the request will eventually arrive at the cache handling the content. This avoids content replication and allows optimal usage of overall cache space, hence maximizing the overall hit rate and minimizing the usage of provider links. We assess the benefit of our approach with simulations on Rocketfuel topologies [START_REF] Mahajan | Inferring link weights using end-to-end measurements[END_REF]. We show that optimal off-path caching performs far better than on-path caching. However, the optimal is hardly computable in real environments so we propose a practically deployable placement heuristic called Caching All Contents by Hashing (CACH) that utilizes a hash function to deterministically determine in which cache content must be placed.

This paper is organized as follows. Sec. II provides background on CCN and overviews early work on in-network caching. Sec. III discusses optimal off-path caching and presents an optimization framework to minimize the provider links bandwidth usage in stub ASes, then defines CACH, a practical approximation of the optimal. Sec. IV illustrates the pros and cons of each approach with simulations on real topologies and the potential gain obtained when content is not replicated. Sec. V concludes this work.

II. STATE OF THE ART A. CCN in a nutshell

Content-Centric Networking proposed by Jacobson et al. in [START_REF] Jacobson | Networking named content[END_REF] is a future networking paradigm with the key idea of establishing the content, identified solely by its name, as self-sustaining entity while excluding the notions of host and location. Entire communications in CCN are carried out by two types of packets: (i) Interest, being generated and sent by a CCN client when requesting a content, and (ii) Data, containing the requested content, being sent in response to an Interest. Every CCN router maintains three data structures: (i) Pending Interest Table (PIT) that maintains a list of interested interfaces (called faces) for each unsatisfied content request, (ii) Forwarding Information Base (FIB) that holds a list of faces that can potentially serve a requested content, and (iii) Content Storage (CS) that retains a copy of content (in the limit of available storage) even after forwarding it to requesting faces. CS enables in-network caching transforming every CCN router into a legitimate source of data. An Interest packet is forwarded by intermediate CCN routers based on their FIBs while updating their PITs so that to be able to later match content to requesting faces. In return, a content is sent by the origin server or any CCN router encountered over the path followed by the Interest packet and holding a copy of the requested content. The Data packet back-tracks the path laid down by the Interest packet at each intermediate router and the carried content can be eventually cached at those routers.

B. In-network caching

Chai et al. in [START_REF] Chai | Cache "less for more" in information-centric networks[END_REF] show that it is possible to obtain a better hit rate, and hence less traffic on provider links, by constraining the location where content can be cached in CCN. In [START_REF] Chai | Cache "less for more" in information-centric networks[END_REF], content is cached on well chosen routers on the path between producers and consumers, improving so the hit rate. In addition, [START_REF] Rossi | On sizing ccn content stores by exploiting topological information[END_REF] studies the impact on performances of homogeneous and heterogeneous cache sizes in CCN and shows that allocating content store space heterogeneously across the CCN network brings very limited gain and that the simple degree centrality metric to decide where to store content provides good performance. Finally, in [START_REF] Guo | Collaborative forwarding and caching in content centric networks[END_REF], caching entities collaborate to ensure that globally popular contents are cached. However, in all these solutions contents are always cached on the path between consumers and providers, yielding content replication because of lack of collaboration between paths under utilization of the total caching capacity, and suboptimal traffic on provider links.

Even though caching problem is brought to the light today with the emergence of Information-Centric Networking, the problem of caching and more particularly the routing between caches, has been broadly studied for hierarchy of web caches [START_REF] Karger | Consistent hashing and random trees: distributed caching protocols for relieving hot spots on the world wide web[END_REF], [START_REF] Ross | Hash-routing for collections of shared web caches[END_REF], [START_REF] Fan | Summary cache: a scalable wide-area web cache sharing protocol[END_REF]. Among those proposals, the work from Ross in [START_REF] Ross | Hash-routing for collections of shared web caches[END_REF] is the closest to ours. Indeed, Ross discusses hashbased routing protocols which use hash functions to select web caches from where to retrieve objects in a hierarchy of caches.

Finally, Iyer et al. use a P2P overlay to build a decentralized web cache and the evaluation shows performance similar to centralized web caches [START_REF] Iyer | Squirrel: a decentralized peerto-peer web cache[END_REF]. We somehow confirm this result by showing that deflection to select caches in a stub network can outperform solutions without deflection.

III. ON OPTIMAL CACHING IN A NETWORK DOMAIN

Ubiquitous in-network caching offered by CCN at the router level [START_REF] Jacobson | Networking named content[END_REF] raises the question of the optimal placement of the different contents over caches available between content consumers and content producers. Generally speaking, for the set C of contents and the set R of caching routers, optimal content placement over caches amounts to finding a function f : C → {0, 1}

|R| that provides, for each content, a binary vector whose value at position i determines whether the content must be cached at router i or not. Function f must fulfill some optimization requirements and is thus determined by an objective function and a set of constraints. Our objective is to minimize the total bandwidth usage on the provider links of a stub AS (e.g., enterprise network, campus network) by means of in-network caching. This objective stems from the fact that the cost for a stub network of using its provider links is much higher than the one of operating local links, hence the benefit of satisfying the maximum of clients' interests locally.

To realize this objective, we must ensure that (i) routers cache the maximum number of contents by caching each content at most at one place inside the domain, and (ii) the cache hit rate is maximized by caching most popular contents. These two conditions can be formulated as in Proposition 1, see [START_REF]Minimizing Bandwidth on Peering Links with Deflection in Named Data Networking[END_REF] for proof and more details. Proposition 1: Assuming contents of equal size, the amount of inter-domain content-retrieval traffic of a stub AS operating CCN that can cache N contents is minimized if the N top most popular contents are cached.

It follows that on-path caching, the default behavior of CCN, is not optimal from a provider-link bandwidth-usage point of view. With on-path caching, contents are cached over paths between content consumers and providers. Since these paths are not the same for all consumers, contents can find themselves cached at different places inside the network, which violates Proposition 1.

To implement Proposition 1, each of the N most popular contents must be allocated to one and only one router and the remaining contents must be retrieved from outside the domain without being cached. Therefore, Interests for the N most popular contents must be redirected to the caching router associated to the content. As the caching router associated to the content might not be on the shortest path between the content consumer and producer, deflection has to be used. With deflection, Interest packets for a content follow an indirect path that passes by the associated router. Over this indirect path, they are only handled by the CCN router associated to the content and cross the other routers in a transparent way. We call caching that stems from deflection off-path caching.

Whereas [START_REF] Chai | Cache "less for more" in information-centric networks[END_REF] keeps caching contents over shortest paths and optimizes the placement of contents over them by correlating the demand on other paths, we target an optimal, and more generally, network-wide content placement. Note that several allocation matrices A can implement Proposition 1 leading to a minimal inter-domain traffic. The choice of A must account for two operational concerns. First, as deflection aims at concentrating traffic for a given content toward one deflection point, it might cause over-utilization of the links around the deflection point, especially that we are deflecting popular contents. Second, deflection induces path-stretch inside the AS and consequently, internal delays and traffic may increase. One can be interested in finding the subset of matrices A that limits the internal delays and makes the intra-domain traffic increase under some constraints. Note that internal delays are different from delays perceived by consumers, as the latter ones depend on the hit rate as well, which we are minimizing. Simulation results in [START_REF]Minimizing Bandwidth on Peering Links with Deflection in Named Data Networking[END_REF] indicate that consumers' delays can in overall decrease if one accounts for the gain in the hit rate and the external delays to reach the servers.

Next, we present one possible mathematical formulation of this optimization problem, which is NP-complete in general, but that can be reduced to a simple linear problem if internal links capacity is omitted. In Sec. III-B, we propose a practically doable solution that is a tradeoff between practical deployment issues and the theoretical optimal.

A. Optimal content placement

Denote by µ r the caching capacity in router r, µ r contents among the N most popular ones must be deflected to (and hence cached at) r. Moreover, the deflection must be such that link capacity is not exceeded and delay is minimized. Minimizing the delay can be seen as a way to minimize internal traffic as well. Our optimization problem thus aims to find an allocation matrix A such that: (i) only the N most popular contents are cached, (ii) the deflection does not cause internal link over-utilization and, (iii) the overall delay toward deflection points is minimized. Formally, this gives the following optimization problem, where the allocation matrix

A = (A r,c ), r ∈ R, c ∈ C is expected as output: minimize c∈C * e∈E λ c,e r∈R A r,c d e,r (1)
A r,c ∈ {0, 1} , c ∈ C * , r ∈ R, (2) A r,c = 0, ∀c ∈ C \ C * , ∀r ∈ R, ( 3
) r∈R A r,c = 1, ∀c ∈ C * , ( 4
) c∈C * A r,c = µ r , ∀r ∈ R, (5)
c∈C e∈E λ c,e • v c • δ l,e,c,A ≤ κ l , ∀l ∈ L. (6)
Here, C * ⊆ C is the set composed of the N most popular contents, C\C * being the complementary set. N is set equal to the overall network caching capacity, r∈R µ r = |C * | = N . λ c,e is the demand for content c in Interest packets/s seen at edge router e. E is the set of edge routers directly connected to the CCN clients and from where the requests enter the network. d e,r is the shortest path length between e and r. L is the set of network links and for each link, κ l is its capacity and δ l,e,c,A is a binary value that indicates whether the link l is on the path between the edge router e and the deflection point. Finally, v c models the size of content c. Eq. ( 1) models the mean internal delay of the top-N popular contents to be minimized in our search for the optimal placement among all those allocation matrices A that minimize the egress traffic. The other equations are the constraints for our optimization problem. Eq. ( 2) indicates that a popular content is either cached (i.e., 1) or not (i.e., 0). Eq. ( 3) indicates that non popular contents must not be cached by any router. Eq.( 4), combined with Eq. ( 2), imposes that a content in the list of top-N popular contents is cached once and only once in the network. Eq. ( 5) imposes that the caching capacity µ r of a router is never exceeded, and that the overall caching capacity of the network is exploited. Finally, Eq. ( 6) ensures that no link is used more than its capacity by both the deflected and non deflected contents (hence the sum of the set C).

Our optimization problem can be casted as a general problem of finding paths subject to multiple constraints, and hence is NP-complete [START_REF] Wang | Quality of service routing for supporting multimedia applications[END_REF]. If we remove the bandwidth constraint in Eq. ( 6) and assume similar popularity distribution over edges, the problem can be made of linear complexity and solved exactly as follows. First, we calculate for each router r its mean delay to all edges waited by the traffic demand per edge, i.e., e∈E λ e r∈R d e,r . Using this delay metric, routers are ranked from most central to least central, and contents are allocated to them by decreasing order of popularity (most popular on most central, least popular on least central, etc). We detail this heuristic the evaluation section.

B. CACH: Caching All Contents by Hashing

The cost of deploying optimal off-path caching requires tracking popular content, solving an NP-complete optimization problem, and running very large routing tables to account for the placement of each content. Instead of claiming to implement the optimal solution, we consider it as a reference solution that points to the maximum gain an operator can achieve by performing in-network caching. We start from it and construct a mechanism that requires no popularity knowledge and where routing table size is independent of N . To do so, content is still cached in one and only one place but non popular content can be cached as well. In other words, every content is deflected. As a result, caching routers will have to cache more content than their capacity, meaning that popular content might not be cached for some period of time and will have to be downloaded from the Internet. By doing so, we do not strictly respect Proposition 1 but we follow the same principle of one content, one replica and rely on cache eviction policies (e.g., Least Recently Used LRU) to keep most popular content cached at maximum one place inside the network.

To achieve the above goal, edge routers use a hash function to decide on the placement of every content on the routers, independently of its popularity. The hash function is applied to the content name and the returned value determines the position of the caching router for each content. If every router knows the complete list of caching routers in the network and uses the same hash function with the same seed, then deflection respecting the principle of one content, one replica can be performed on-line. Practically, when an Interest packet is received by a router, the router extracts the content name, hashes it, determines the deflection point associated to the content and forwards the Interest packet toward the chosen deflection point. Similarly, when a caching router receives a Data packet, it determines whether or not it must cache the content by looking at the hash value of the content name. We call this mechanism CACH, which stands for Caching All Contents by Hashing. CACH is a means to randomly select one single deflection point per content independently of its popularity and in a distributed and stateless way.

Note that routing tables with CACH are bound to the number of caching routers (O(|R|)) which is no more than what is observed in intra-domain routing today in stub networks. Moreover for each ICN packet, routers have to apply a hash function on the content name, which is already commonly done in traffic monitoring and engineering, e.g., ECMP [START_REF] Augustin | Measuring Multipath Routing in the Internet[END_REF].

IV. EVALUATION

In this section, we compare the different approaches in term of provider link bandwidth usage and discuss the impact of deflection on link utilization inside the network. For ease of presentation, we consider the variant of the problem with no limit on the capacity of internal links (the variant of linear complexity discussed in Sec. III-A). The comparison is done with an event-driven simulator we have built using Rocketfuel network topologies [START_REF] Mahajan | Inferring link weights using end-to-end measurements[END_REF]. Each node of the topology (i.e., router) is assigned a role, a caching capacity, and a forwarding table and a weight is set to every link. Nodes are either edges, routers, or servers. Edge nodes equally generate demands for content according to a Zipf law. Routers implement caching with the Least Recently Used (LRU) policy and forwarding follows shortest paths according to link weights.

We consider three scenarios: (i) on-path caching (in red in all figures), (ii) optimal off-path caching (in green), where the optimal placement of the N most popular content on the routers inside the AS is pre-determined, and (iii) CACH where hashing is invoked to place or retrieve the popular content on the fly (in blue). Simulations are run on several Rocketfuel topologies but only the results for the ASN 3,967 topology are presented as the others show similar trend. We randomly select edge routers such that every city in the topology has two edge routers. We then have a total of 44 edges, the 79 remaining nodes are taken as routers. Among the routers, 6 have been randomly selected to be attached to provider links. We consider that content can be retrieved via the closest provider link, which is a reasonable assumption for networks using default edge routes. We consider a total of 7,900 contents with a popularity distribution following a Zipf (0.8) law [START_REF] Fricker | A versatile and accurate approximation for LRU cache performance[END_REF]. We limit the caching capacity to 10 entries per router, meaning that total caching capacity of the network is 790 contents (i.e., 10% of content demand). Small cache size exacerbates competition between contents. Each scenario is simulated 11 times and each simulation generates 200,000 Interest packets.

A. Provider link bandwidth usage

The number of Interest packets that leave the AS determines the provider link bandwidth utilization. Fig. 1 shows the cumulative number of Interest packets that leave the AS as function of contents ordered by decreasing popularity. With on-path caching, 83% of Interest packets are forwarded outside the AS to fetch the requested contents, while for optimal offpath caching, only 35% of them are forwarded. CACH lies in between with 47% Interest packets forwarded. Fig. 1 also depicts that with on-path caching, 50% of the provider links traffic is generated by the first 5.5% of most popular contents while the same contents account for less than 0.7% and 22% of provider links traffic for optimal placement and CACH.

Fig. 2 complements Fig. 1 and gives per-content average hit ratio, ordered by content popularity (with 95 th confidence interval). With the optimal placement, the hit ratio for the most popular contents is close to one whereas it is close to zero for the remaining ones. Comparatively, the hit ratio is low with on-path caching. With CACH, the hit ratio is high for popular contents thanks to LRU but gracefully degrades when popularity diminishes. The large hit ratio variance observed for CACH is because the hash function seed is changed at every simulation, hence changing the contents allocation to routers. Depending on the allocation, contents with relatively equal popularity (e.g., many contents from the tail) might be deflected to the same router, increasing cache churn. Actually, the slight hit ratio increase for least popular contents is an artifact of the finite number of iterations. Interestingly, even though deflection potentially causes longest path within the domain, the higher hit rate reduces the overall delay. For instance, with an external delay of 100 ms, the average delay is 104 ± 0.05 ms with on-path, while it is only 34 ± 0.09 ms with optimal placement and 69 ± .11 ms with CACH.

B. Internal links bandwidth usage

We normalize the bandwidth usage of every internal link by the average bandwidth usage of the edge links. A value higher than one indicates that the link aggregates large amount of traffic from several edges making it central in the topology, and a value below one means that the link transports a small fraction of edge traffic. Internal links are over-provisioned to 10 times the capacity of edge links, which leaves enough margin to assess the increase in traffic they might experience. Fig. 3 shows, for every link in the topology, the normalized bandwidth for the different schemes. Links are ordered according to the normalized bandwidth in the optimal case. Many links are not utilized and present a normalized bandwidth of 0. These are redundant links that are not part of shortest paths. In the middle, we can see about fifty links having a normalized bandwidth below 1, indicating a reasonable bandwidth usage. Surprisingly, there is less usage of these links with deflection. This result, possibly specific to our topology, comes from the higher hit rate offered by deflection. Indeed, deflection increases the hit rate and reduces the traffic that leaves the AS, hence contributing to the decrease in the traffic on links close to the provider. Finally, we can see on the right-hand side of the figure, a few links that are heavily used because of their centrality in the topology and the fact they lay on several shortest paths. These can be also links close to caches storing highly popular content. Optimal placement seems improving the situation as it does its best in placing the most popular content on the most central routers having the maximum degree of connectivity, hence dispatching the traffic on the maximum number of links. CACH does as bad as on-path caching from this regard. To avoid the overload of these links, the best protection would be to take their capacity into account while placing content. This is what our constrained optimization is supposed to solve. We keep its evaluation under capacity constraints for a future research.

V. CONCLUSION

The existing CCN architecture is a future networking paradigm to cope with new Internet usages. We illustrate in this paper how the opportunistic caching technique advocated by CCN is not optimal in terms of inter-domain bandwidth usage as it results in wastage of limited caching space by duplicating content throughout the network. Following this observation, we propose a way to deploy an effective caching mechanism within a stub AS such that provider link usage is minimized. Our optimal off-path caching solution needs to deflect selectively the Interest packets corresponding to the cached popular contents toward optimally computed off-path routers (caches). With simulations on real topologies, we show that the cost of off-path caching in terms of intra-domain route inflation is compensated by the gain in terms of caching. When caching is optimized, a larger number of popular contents can be cached within the AS and less traffic has to exit the AS via inter-domain links. As the optimal off-path caching solution is NP-complete, we propose a hash-based mechanism named CACH as a practically implementable caching solution with low operating overhead to limit the number of replicas of popular content across the AS. We demonstrate by simulations that CACH is a good approximation of the optimal solution.

Fig. 1 .Fig. 2 .Fig. 3 .

 123 Fig. 1. Median of the aggregate traffic on provider links vs. content ordered by decreasing popularity