Unilateral right parietal damage leads to bilateral deficit for high-level motion.

Patients with right parietal damage demonstrate a variety of attentional deficits in their left visual field contralateral to their lesion. We now report that patients with right lesions also show a severe loss in the perception of apparent motion in their "good" right visual field ipsilateral to their lesion. Three tests of attention were conducted, and losses were found only in the contralesional fields for a selective attention and a multiple object tracking task. Losses in apparent motion, however, were bilateral in all cases. The deficit in apparent motion in the parietal patients supports previous claims that this relatively effortless percept is mediated by attention. However, the bilateral deficit suggests that the disruption is due to a bilateral loss in the temporal resolution of attention to transient events that drive the apparent motion percept.

Complete sparing of high-contrast color input to motion perception in cortical color blindness.

It is widely held that color and motion are processed by separate parallel pathways in the visual system, but this view is difficult to reconcile with the fact that motion can be detected in equiluminant stimuli that are defined by color alone. To examine the relationship between color and motion, we tested three patients who had lost their color vision following cortical damage (central achromatopsia). Despite their profound loss in the subjective experience of color and their inability to detect the motion of faint colors, all three subjects showed surprisingly strong responses to high-contrast, moving color stimuli--equal in all respects to the performance of subjects with normal color vision. The pathway from opponent-color detectors in the retina to the motion analysis areas must therefore be independent of the damaged color centers in the occipitotemporal area. It is probably also independent of the motion analysis area MT/V5, because the contribution of color to motion detection in these patients is much stronger than the color response of monkey area MT.

Impaired fronto-temporal processing of emotion in schizophrenia.

AIMS: Abnormal emotion processing in schizophrenia affects social and functional outcome. Spatiotemporal brain mechanisms underlying this deficit are unclear. MATERIALS AND METHODS: Event-related potential (ERP) responses to emotional and neutral face processing during an implicit (gender detection) and an explicit (expression detection) task were compared between a group of healthy volunteers (n=10) and a group of patients with schizophrenia (n=10). RESULTS: Whereas patients had normal primary visual cortex responses, the early modulation of occipital, temporal, and frontal responses by emotional expression observed in controls was absent in patients. The occipito-temporal N170 amplitude was reduced in patients relative to controls during expression detection, but not during gender detection. Frontal activity within 180-250ms was reduced in patients compared to controls. As opposed to controls, no significant difference was seen in patients at the right temporal electrode (T6) between amplitudes of long-latency ERPs elicited by distinct emotions during the expression detection task. CONCLUSION: In patients with schizophrenia, abnormal early extraction of expression-related information in the occipito-temporal cortex (before 170ms) impairs structural encoding of facial expressions (N170) and may disrupt motivation- and task-dependent context processing (180-250ms time window) of expression-related facial features. Moreover, top-down neuromodulation from frontal and limbic structures to visual occipito-temporal cortex may not be sufficient to optimize the extraction of expression-specific face features.

[Part I: Face recognition]. / Les visages et leurs émotions.

Faces represent a crucial vector of interhuman communication. The message transmitted by the face has multiple features. Recognition of each feature can be impaired independently or in combination with others. In order to understand the behavioral consequences of such impairments, which can be a major social handicap, we first must specify the neural networks involved in face recognition. We propose in this first part to present the systems involved in face recognition, in particular the question of identity and prosopagnosia. Different neural networks are indeed implicated in the recognition of invariant facial features such as identity, gender, ethnicity, and recognition of variant features like facial expression and eye gaze. This paper is illustrated by some of our scalp and intracranial electrophysiological studies performed in humans allowing us to describe some aspects of face recognition dynamics combining an excellent spatial and temporal resolution. Intracranial recordings were performed in drug refractory epileptical patients implanted with depth electrodes. These studies demonstrate that numerous deep brain and cortical structures participate early and sometimes in a sustained manner in face recognition.

[Part II: Recognising facial expressions]. / Les visages et leurs émotions.

In this second part, we address particularly the question of the neural mechanisms and structures involved in the recognition of facial emotional expressions that are crucial in social cognition. Emotion recognition in others can be critically impaired in some neurodegenerative and neurovascular diseases. That dysfunction sometimes correlated to disabling behavioural disorders and interpersonal communication impairment must be further understood. The results of a series of scalp and intracranial event related potential recordings, as well as recent advances in the literature, are reported. ERPs to facial emotional expressions were thus recorded in multiple subcortical and cortical areas in drug refractory epileptical patients implanted with depth electrodes. The roles of amygdala, insula and prefrontal cortex located at crossroads between perceptive analysis and emotional conceptual knowledge are particularly underlined. Altogether, these studies demonstrate that facial expressions are widely processed in space and time, some structures reacting very early and automatically, others providing a sustained reaction depending on the attention.

Decision-making in a changing world: a study in autism spectrum disorders.

To learn to deal with the unexpected is essential to adaptation to a social, therefore often unpredictable environment. Fourteen adults with autism spectrum disorders (ASD) and 15 controls underwent a decision-making task aimed at investigating the influence of either a social or a non-social environment, and its interaction with either a stable (with constant probabilities) or an unstable (with changing probabilities) context on their performance. Participants with ASD presented with difficulties in accessing underlying statistical rules in an unstable context, a deficit especially enhanced in the social environment. These results point out that the difficulties people with ASD encounter in their social life might be caused by impaired social cues processing and by the unpredictability associated with the social world.

Unmasking of visual deficits following unilateral prestriate lesions in man.

The primate visual cortex is a mosaic of different areas which are roughly organized into two major pathways, a dorsal one along an occipito-parietal channel and a ventral one along an occipito-temporal channel. It is known that visual stimuli are processed differently within these two systems and one might expect therefore that behavioral deficits would differ according to the channel damaged by cerebral lesions. Two patients with left prestriate lesions presented perceptual deficits in the right visual field. These deficits involved reading, recognition of line drawings and colour perception and would be compatible with a dysfunction of the ventral system. Magnetic resonance imaging analysis confirmed that this was the case.

Two different readers in the same brain after a posterior callosal lesion.

Two different types of reading, one in each hemifield, were exhibited by a patient with a lesion of the posterior half of the corpus callosum. The patient read normally when words and non-words were presented to his right visual field. However, with left visual field presentations, the patient could not read non-words and vocalized real words very slowly, especially abstract words, inflected verbs and function words. He often replaced concrete words by semantic associates. Such an abnormal reading pattern is similar to that known as deep dyslexia. This unilateral deficit reveals the competence of the right hemisphere to initiate some semantic processing and its inability to manage phonological coding. The hypothesis that deep dyslexia is due to right hemisphere reading is reinforced by the present case.

Able to name, unable to compare: the visual abilities of a posterior split-brain patient.

A patient with a lesion of the posterior half of the corpus callosum correctly named simple (e.g. colors) and complex (e.g. faces) stimuli whether presented to one or both hemifields. Although proficient at these tasks, and also able to make same/different judgements for stimuli within an hemifield, he failed dramatically when required to compare stimuli between the two hemifields. These results indicate that, while the posterior portion of the corpus callosum may not be essential for naming visual stimuli, it is essential for comparing stimuli between the two visual fields.

Failure to identify the target does not prevent the attentional blink effect.

The attentional blink has been attributed to capacity limitations at a central level of processing. We tested whether failure to identify the target would eliminate the blink. Two agnostic patients were presented with streams of letters, which they were able to identify, and streams of pictures, which they were unable to identify. The dual-task involved identification of a target and detection of a probe. With letters the duration of the blink was equivalent to that of the control subjects. A prolonged blink was observed in both patients for pictures irrespective of whether the target was identified. This result indicates that failure to identify the target does nevertheless mobilize attentional resources sufficiently to prevent detection of a second target stimulus.

Dependence of color on context in a case of cortical color vision deficiency.

Color constancy depends on sensitivity to change in both illumination spectral properties and object position. We investigated this latter form of color constancy by asking a cerebral achromatopsic to name the colors of papers that were presented atop black, gray or white backgrounds under identical illumination. Comparison of color names across background conditions reveals poor constancy, characterized by a contrasting of foreground and background values that is not corrected by proper anchoring.

Brain regions involved in the perception of gaze: a PET study.

Mutual gaze may be described as a psychological process during which two persons have the feeling of a brief link between their two minds. In the monkey, specific cell assemblies in the superior temporal cortex of the brain are responsive to gaze. This suggests that the brain may have evolved mechanisms for interpreting direct eye contact. These mechanisms could depend on the activation of specific brain regions. Positron emission tomography was used to measure activity in brain regions in healthy volunteers while they were looking at faces featuring, respectively, eye contact, averted gaze, or no gaze. As expected a region known to be involved in face processing was found to be activated in the ventral occipito-temporal region, especially in the right hemisphere. Averted gaze and mutual gaze triggered blood flow responses in similar areas which were different from those involved in face processing. These areas included the occipital part of the fusiform gyrus, the right parietal lobule, the right inferior temporal gyrus, and the middle temporal gyrus in both hemispheres. These results are consistent with the hypothesis that perception of eyes regardless of the direction of the gaze is subserved by a distributed network. However, no conclusive evidence was found for specific area(s) devoted to mutual gaze processing.

The visual word form area: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients.

A standard model of word reading postulates that visual information is initially processed by occipitotemporal areas contralateral to the stimulated hemifield, from whence it is subsequently transferred to the visual word form (VWF) system, a left inferior temporal region specifically devoted to the processing of letter strings. For stimuli displayed in the left visual field, this transfer proceeds from the right to the left hemisphere through the posterior portion of the corpus callosum. In order to characterize the spatial and temporal organization of these processes, reading tasks with split-field presentation were performed by five control subjects and by two patients suffering from left hemialexia following posterior callosal lesions. The subjects' responses were studied using behavioural measures and functional brain imaging techniques, providing both high spatial resolution (functional MRI, fMRI) and high temporal resolution (high-density event-related potentials, ERPs). Early visual processing was revealed as activations contralateral to stimulation, located by fMRI in the inferior occipitotemporal region and presumably coincident with area V4. A negative wave occurring 150-160 ms post-stimulus, also strictly contralateral to stimulation, was recorded over posterior electrodes. In contrast with these hemifield-dependent effects, the VWF system was revealed as a strictly left-hemispheric activation which, in control subjects, was identical for stimuli presented in the left or in the right hemifield and was located in the middle portion of the left fusiform gyrus. The electrical signature of the VWF system consisted of a unilateral sharp negativity, recorded 180-200 ms post-stimulus over left inferior temporal electrodes. In callosal patients, due to the inability of visual information to pass across the posterior part of the corpus callosum, the VWF system was activated only by stimuli presented in the right visual field. Similarly, a significant influence of the word/non-word status on ERPs recorded over the left hemisphere was discernible for either hemifield in controls, while it affected only right-hemifield stimuli in callosal patients. These findings provide direct support for the main components of the classical model of reading and help specify their timing and cerebral substrates.