Benoit A. Bacon

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.

Spatial disparity coding in the superior colliculus of the cat

Abstract Cells in the superficial layers of the superior colliculus of the cat have mainly binocular receptive fields. The aim of the present experiment was to investigate the sensitivity of these cells to horizontal spatial disparity. Unit recordings were carried out in the superficial layers of the superior colliculus of paralyzed and anesthetized cats. Centrally located receptive fields were mapped, separated using prisms, and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory region of the receptive fields. Only binocular cells were tested, and 65% of these units were found to be sensitive to spatial disparities. Some cells (20%) were clearly insensitive to spatial disparity and the remaining 15% showed complex, unclassifiable interactions. The sensitive cells could be divided into four classes based on their disparity-sensitivity profiles: 38% showed excitatory interactions, whereas 9% showed inhibitory interactions. Moreover, 11% and 7% of the cells responded, respectively, to crossed or uncrossed disparities, and were classified as near cells and far cells. Whereas the general shapes of the sensitivity profiles were similar to those of cells in areas 17–18, selectivity in the superior colliculus was significantly coarser. The superficial layers of the superior colliculus project topographically to the deep layers of the superior colliculus, which are known to contain circuits involved in the control of ocular movements. The results thus suggest that disparity-sensitive cells of the superior colliculus could feed information to these oculomotor neurons, allowing for the localization and fixation of objects on the appropriate plane of vision.

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.

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.

Functional reorganization of the human auditory pathways following hemispherectomy: an fMRI demonstration.

The present study investigated the functional reorganization of ipsilateral and contralateral auditory pathways in hemispherectomized subjects. Functional reorganization was assessed using functional Magnetic Resonance Imaging (fMRI) and stimulation with complex sounds presented binaurally and monaurally. For neurologically intact control subjects, results showed that binaural stimulations evoked balanced activity in both hemispheres while monaural stimulations induced strong contralateral activity and weak ipsilateral activity. The results obtained from hemispherectomized subjects were substantially different from those obtained from control subjects. Specifically, activity in the intact hemisphere showed a significant decrease in response to contralateral stimulation but, concomitantly, an increase in response to ipsilateral stimulation. The present findings suggest that a substantial functional reorganization takes place in the auditory pathways following an early hemispherectomy. The exact nature of this functional reorganization remains to be specified.

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.

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.

Evidence for a Generic Process Underlying Multisensory Integration

It has been shown repeatedly that the various sensory modalities interact with each other and that the integra- tion of incongruent percepts across two modalities, such as vision and audition, can lead to illusions. Different individual cognitive features (i.e., attention, linguistic experience, etc.) have been shown to modulate the level of multisensory integ- ration. As such, it may be hypothesized that an intra-individual generic process underlies parts of illusory perception, irre- spective of illusory material. One simple way to address this issue is to assess whether observers experience multisensory integration to a similar degree when the illusory material varies with respect to its sensory features. Here, performance on two distinct audio-visual illusions (McGurk effect, illusory flash effect) was tested in a group of adult observers. Results show a positive within-subject correlation between both illusions indirectly supporting the existence of a generic process for multisensory integration that could include individual differences in attention.

Neurons in the posteromedial lateral suprasylvian area of the cat are sensitive to binocular positional depth cues.

Abstract. Single units in the posteromedial lateral suprasylvian area of the cat are known to be very sensitive to movement. A proportion of these cells can encode movement in depth, but it is unclear whether posteromedial lateral suprasylvian cells only rely upon motion cues to evaluate stimulus depth or whether they can also code for spatial cues. The present study aims at assessing the sensitivity to spatial disparity of binocular cells, in the posteromedial lateral suprasylvian area, in order to determine whether these units are tuned to positional depth cues. A total of 126 single cells located in the posteromedial lateral suprasylvian area of anesthetized, paralyzed cats were examined. As recordings were performed in the central visual field representation, receptive fields were small. A third of the receptive fields were surrounded by an inhibitory region and almost three-quarters of the cells were direction-selective. Most cells (110/114) were binocular, and a large proportion of single neurons responded to stimuli appearing on the fixation plane by increasing (tuned excitatory cells, 43%) or decreasing (tuned inhibitory cells, 14%) their response rate. A smaller proportion of cells increased their firing rate in response to crossed (near cells, 10%) or uncrossed (far cells, 6%) spatial disparities, hence demonstrating respective preference for stimuli presumably appearing in front of or behind the fixation plane. As compared to primary visual cortex, the proportion of disparity-sensitive cells in posteromedial lateral suprasylvian area is similar, but selectivity is significantly coarser. As the posteromedial lateral suprasylvian area can code for both spatial and temporal aspects of stimuli, this area might be involved in the spatiotemporal integration of depth cues, a process that may also participate in the control of accommodation and vergence.

Binocular interactions and spatial disparity sensitivity in the superior colliculus of the siamese cat

Abstract In Siamese cats, a genetically determined massive misrouting of retinal ganglion cells toward the contralateral hemisphere, as well as an accompanying strabismus, is believed to underlie the extreme paucity of binocular cells in the primary visual cortex. However, binocular cells have been shown to be present in more important numbers at the collicular level. The present study aims at investigating binocular interactions and sensitivity to spatial disparity in the superior colliculus of the Siamese cat. The activity of single units was recorded in the superficial layers of paralyzed and anesthetized Siamese cats. Although most collicular cells were monocularly driven, a significant proportion could be driven through both eyes (34/216 or 16%). Upon isolation of a binocular cell, the receptive fields were separated, then simultaneously stimulated with two light bars. A temporal delay was introduced between the arrival of the bars in the receptive fields to generate spatial disparities (–3° to +3°, in 0.5° or 1° steps). Results showed that some binocular cells presented disparity tuning profiles similar to the tuned excitatory (12/34), tuned inhibitory (2/34), near (2/34) and far (3/34) cells found at various cortical levels in the normal cat. These interactions might allow for coarse binocular fusion as well as play a role in the initiation of vergence and the fixation of the eyes upon the appropriate plane of vision.