The informational correlates of conscious and nonconscious face-gender perception.

We used a face-gender repetition priming paradigm to precisely map the spatial frequencies (SFs) that influence observers' responses under different prime awareness conditions. A visible prime condition was set up by presenting the stimulus sequence mask-blank-prime-blank-mask-target and an invisible prime condition by switching the order of the masks and the blanks (see also Dehaene et al., 2001). The prime faces (~4.6° × 3.1°) were randomly filtered trial-by-trial according to the SF bubbles technique (Willenbockel, Fiset et al., 2010). Classification vectors, derived by summing the SF filters from each trial weighted by observers' transformed response times, revealed that SFs around 12 cycles per face width modulated responses in both prime awareness conditions. The significant SFs closely matched those optimal for accurate performance in a direct face-gender classification paradigm. Surprisingly, the significant SFs facilitated observers' responses in the visible prime condition, whereas they slowed responses in the invisible prime condition. Our findings suggest that SF tuning per se remains robust under different prime awareness conditions but that diagnostic visual cues might be utilized in a qualitatively different fashion as a function of awareness.

Audiovisual segregation in cochlear implant users.

It has traditionally been assumed that cochlear implant users de facto perform atypically in audiovisual tasks. However, a recent study that combined an auditory task with visual distractors suggests that only those cochlear implant users that are not proficient at recognizing speech sounds might show abnormal audiovisual interactions. The present study aims at reinforcing this notion by investigating the audiovisual segregation abilities of cochlear implant users in a visual task with auditory distractors. Speechreading was assessed in two groups of cochlear implant users (proficient and non-proficient at sound recognition), as well as in normal controls. A visual speech recognition task (i.e. speechreading) was administered either in silence or in combination with three types of auditory distractors: i) noise ii) reverse speech sound and iii) non-altered speech sound. Cochlear implant users proficient at speech recognition performed like normal controls in all conditions, whereas non-proficient users showed significantly different audiovisual segregation patterns in both speech conditions. These results confirm that normal-like audiovisual segregation is possible in highly skilled cochlear implant users and, consequently, that proficient and non-proficient CI users cannot be lumped into a single group. This important feature must be taken into account in further studies of audiovisual interactions in cochlear implant users.

Early- and late-onset blindness both curb audiotactile integration on the parchment-skin illusion.

It has been shown that congenital blindness can lead to anomalies in the integration of auditory and tactile information, at least under certain conditions. In the present study, we used the parchment-skin illusion, a robust illustration of sound-biased perception of touch based on changes in frequency, to investigate the specificities of audiotactile interactions in early- and late-onset blind individuals. Blind individuals in both groups did not experience any illusory change in tactile perception when the frequency of the auditory signal was modified, whereas sighted individuals consistently experienced the illusion. This demonstration that blind individuals had reduced susceptibility to an auditory-tactile illusion suggests either that vision is necessary for the establishment of audiotactile interactions or that auditory and tactile information can be processed more independently in blind individuals than in sighted individuals. In addition, the results obtained in late-onset blind participants suggest that visual input may play a role in the maintenance of audiotactile integration.

Event-related potentials to overlapping shapes: effects of saliency and interference.

Visual perception is often challenged by various difficulties that act concomitantly and whose respective impacts may therefore be hard to distinguish. We used event-related potentials to dissociate the impact of target saliency, generated by occlusion, from that of interference produced by incongruent nontargets. In one block, the target (a square) partially occluded another square tilted by 45 degrees. This nontarget square interfered only to a small extent with target perception. In another block, the target was the occluded stimulus, and interference from the nontarget was substantial. Blocks including two kinds of overlapping shapes (a cross and a square) were added to control for the interference effect. Block comparisons revealed that occlusion modulated an occipital N250 and reaction times. In contrast, interference modulated a parietal N380 but not reaction times.

Language tasks used for the presurgical assessment of epileptic patients with MEG.

Determining the language dominant hemisphere and the intrahemispheric localization of this function are imperative in the planning of neurosurgical procedures in epileptic patients. New noninvasive diagnostic techniques are being developed to reduce the risks associated with more invasive techniques. The aim of this paper is to review the different protocols for lateralizing and/or localizing language functions using magnetoencephalography (MEG), a noninvasive technique. The reviewed studies include control and patient populations using various protocols which employ different expressive and receptive language tasks. The overall findings reveal high concordance between MEG and the intracarotid amobarbital test (IAT). Moreover, MEG allows intrahemispheric localization of receptive and expressive language functions. However, the different language tasks used with MEG, whether receptive or expressive, appear to activate the left temporal more than frontal areas. The best task to assess language comprehension in both adults and children appears to be a word recognition task. A verbal fluency task could be used to test language production in children and a verb generation task in adults.

Functional reorganization of the auditory pathways (or lack thereof) in callosal agenesis is predicted by monaural sound localization performance.

Neuroimaging studies show that permanent peripheral lesions such as unilateral deafness cause functional reorganization in the auditory pathways. However, functional reorganization of the auditory pathways as a result of higher-level damage or abnormalities remains poorly investigated. A relatively recent behavioural study points to functional changes in the auditory pathways in some, but interestingly not in all, of the acallosal individuals that were tested. The present study uses fMRI to investigate auditory activities in both cerebral hemispheres in those same acallosal subjects in order to directly investigate the contributions of ipsilateral and contralateral functional pathways reorganization. Predictions were made that functional reorganization could be predicted from behavioural performance. As reported previously in a number of neuroimaging studies, results showed that in neurologically intact subjects, binaural stimulation induced balanced activities between both hemispheres, while monaural stimulation induced strong contralateral activities and weak ipsilateral activities. In accordance with behavioural predictions, some acallosal subjects showed patterns of auditory cortical activities that were similar to those observed in neurologically intact subjects while others showed functional reorganization of the auditory pathways. Essentially they showed a significant increase and a significant decrease of neural activities in the contralateral and/or ipsilateral pathways, respectively. These findings indicate that at least in some acallosal subjects, functional reorganization inside the auditory pathways does contribute to compensate for the absence of the corpus callosum.

Auditory pathways fail to re-establish normal cortical activation patterns in response to binaural stimulation following a unilateral lesion of the inferior colliculus.

The aim of this study was to investigate cortical activation in response to binaural stimulus presentations in an individual (FX) with a circumscribed traumatic hemorrhagic lesion of the right inferior colliculus. FX and control subjects were exposed to complex sounds while undergoing a functional magnetic resonance imaging assessment. Whereas normally-hearing individuals show well-balanced bilateral activation patterns in response to binaural auditory stimulation, the same stimuli produced stronger activation in the left hemisphere in FX. Combined with previous data, these findings reinforce the notion that the inferior colliculus is an essential auditory relay and that its loss cannot be significantly compensated.

Visual deprivation modifies auditory directional tuning in the inferior colliculus.

The aim of this study was to assess whether early visual deprivation could modulate the auditory directional tunings of single neurons in the central nucleus of the inferior colliculus of the rat. Extracellular recordings were carried out in normal and early bilaterally enucleated rats. Direction-specific auditory neurons were found in both groups, and no evidence was found for a topographical order of best azimuthal direction. Although the distribution of best azimuthal direction was unaltered in enucleated rats, our data suggest that early visual deprivation modifies the width of auditory directional receptive fields in the central nucleus of the inferior colliculus. This suggests that visual input plays a substantial role in refining auditory receptive fields in the inferior colliculus.

On the functional significance of the P1 and N1 effects to illusory figures in the notch mode of presentation.

The processing of Kanizsa figures have classically been studied by flashing the full "pacmen" inducers at stimulus onset. A recent study, however, has shown that it is advantageous to present illusory figures in the "notch" mode of presentation, that is by leaving the round inducers on screen at all times and by removing the inward-oriented notches delineating the illusory figure at stimulus onset. Indeed, using the notch mode of presentation, novel P1 and N1 effects have been found when comparing visual potentials (VEPs) evoked by an illusory figure and the VEPs to a control figure whose onset corresponds to the removal of outward-oriented notches, which prevents their integration into one delineated form. In Experiment 1, we replicated these findings, the illusory figure was found to evoke a larger P1 and a smaller N1 than its control. In Experiment 2, real grey squares were placed over the notches so that one condition, that with inward-oriented notches, shows a large central grey square and the other condition, that with outward-oriented notches, shows four unconnected smaller grey squares. In response to these "real" figures, no P1 effect was found but a N1 effect comparable to the one obtained with illusory figures was observed. Taken together, these results suggest that the P1 effect observed with illusory figures is likely specific to the processing of the illusory features of the figures. Conversely, the fact that the N1 effect was also obtained with real figures indicates that this effect may be due to more global processes related to depth segmentation or surface/object perception.

Knowledge inhibition and N400: a within- and a between-subjects study with distractor words.

We tested whether the N400 event-related potential (ERP) indexes the integration of semantic knowledge in the context or whether it indexes the inhibition of activated, but inappropriate, knowledge. A distractor-prime-target word sequence was presented in each trial. Subjects had to make semantic relatedness judgments on prime-target pairs. In the first experiment, subjects had an additional task. They either had to ignore or to attend to distractors. In critical conditions, that is, when distractors were related to targets, the times to make the prime-target semantic relatedness judgments were longer when subjects had to attend to distractors than when they had to ignore them. In accordance with the inhibition hypothesis, the amplitudes of the N400 elicited by distractors were larger in the ignore than in the attend task. In the second experiment, the same distractor-prime-target triplets were used. However, there was no additional task. Subjects only had to make the prime-target semantic-relatedness judgment. They were then split in two subgroups: the good ignorers, who did not take much longer to make the judgment in critical than in control conditions, and the poor ignorers, that is, those who did take much longer. Results were again consistent with the inhibition idea. The amplitudes of the N400s evoked by distractors were larger in the good than in the poor ignorers [corrected]. The results of these two studies are taken together to support the idea that N400 index a semantic inhibition rather than an integration effort.

Effects of early binocular enucleation on auditory and somatosensory coding in the superior colliculus of the rat.

The present study aimed at investigating the effects of early visual deprivation (bilateral neonatal enucleation) on auditory and somatosensory coding in the polysensory deep layers of the superior colliculus of the rat. The proportion of cells responding to auditory and somatosensory stimulation and the receptive field properties of single neurons were assessed in both normal and enucleated rats. As expected, in enucleated rats there was a drastic increase in the number of unresponsive units and visual responses could no longer be evoked. Most importantly, the proportion of cells that responded to auditory stimulation was drastically reduced. However, the few cells that remained responsive to auditory stimulation were well tuned to noise stimuli presented in both azimuth and elevation, principally in the contralateral hemifield. Enucleation also increased the proportion of cells responding to somatosensory stimulation, particularly to the vibrissae. Implications in terms of neural plasticity and functionality are discussed.

Speech and non-speech audio-visual illusions: a developmental study.

It is well known that simultaneous presentation of incongruent audio and visual stimuli can lead to illusory percepts. Recent data suggest that distinct processes underlie non-specific intersensory speech as opposed to non-speech perception. However, the development of both speech and non-speech intersensory perception across childhood and adolescence remains poorly defined. Thirty-eight observers aged 5 to 19 were tested on the McGurk effect (an audio-visual illusion involving speech), the Illusory Flash effect and the Fusion effect (two audio-visual illusions not involving speech) to investigate the development of audio-visual interactions and contrast speech vs. non-speech developmental patterns. Whereas the strength of audio-visual speech illusions varied as a direct function of maturational level, performance on non-speech illusory tasks appeared to be homogeneous across all ages. These data support the existence of independent maturational processes underlying speech and non-speech audio-visual illusory effects.

Internal surface representations approximated by reverse correlation.

We presented two nai ve observers with 20,000 random-dot stereograms. On each trial, the observers had to indicate the presence or absence of a complex 3D pattern (a large '+' sign in relief). However, unbeknownst to them, the stereograms did not contain any signal, but only disparity noise. Responses and verbal reports indicate that the observers 'saw' the suggested 3D surface configuration in roughly half the trials even though structured local low-level signal was never presented. Using reverse correlation, we derived an approximation of the internal surface-based representations, or templates, that best accounted for the observers' responses. These templates were shown to be spatially well defined and temporally stable. We propose that the 3D surface-based representations that we derived are the first approximations and depictions of the intermediary process that allows the visual system to successfully link degraded, bottom-up signal and high-level, top-down object recognition.

Neural correlates of shape from shading.

Psychophysical studies have shown that human observers resolve shape-from-shading ambiguities by assuming that light is coming from above-left. Using event-related potentials (ERPs), we measured the processing time of the perception of an ambiguous shaded pattern. We found that the N2 component followed the change of perceived shape with stimulus orientation. We also found that the P1 component in occipital and temporal areas was correlated with the observers' idiosyncratic bias for light source position. The precocity of the correlated ERP components suggests that the light source is represented early in the visual system. Altogether, our results indicate that shape-from-shading is a mostly bottom-up mechanism.

Amodal completion and the perception of depth without binocular correspondence.

Half-occlusions and illusory contours have recently been used to show that depth can be perceived in the absence of binocular correspondence and that there is more to stereopsis than solving the correspondence problem. In the present study we show a new way for depth to be assigned in the absence of binocular correspondence, namely amodal completion. Although an occluder removed all possibility of direct binocular matching, subjects consistently assigned the correct depth (convexity or concavity) to partially occluded 'folded cards' stimuli. Our results highlight the importance of more global, surface-based processes in stereopsis.