Jean-René Duhamel

The Mental Representation of Hand Movements After Parietal Cortex Damage

Recent neuroimagery findings showed that the patterns of cerebral activation during the mental rehearsal of a motor act are similar to those produced by its actual execution. This concurs with the notion that part of the distributed neural activity taking place during movement involves internal simulations, but it is not yet clear what specific contribution the different brain areas involved bring to this process. Here, patients with lesions restricted to the parietal cortex were found to be impaired selectively at predicting, through mental imagery, the time necessary to perform differentiated finger movements and visually guided pointing gestures, in comparison to normal individuals and to a patient with damage to the primary motor area. These results suggest that the parietal cortex is important for the ability to generate mental movement representations.

Promoting social behavior with oxytocin in high-functioning autism spectrum disorders

Social adaptation requires specific cognitive and emotional competences. Individuals with high-functioning autism or with Asperger syndrome cannot understand or engage in social situations despite preserved intellectual abilities. Recently, it has been suggested that oxytocin, a hormone known to promote mother-infant bonds, may be implicated in the social deficit of autism. We investigated the behavioral effects of oxytocin in 13 subjects with autism. In a simulated ball game where participants interacted with fictitious partners, we found that after oxytocin inhalation, patients exhibited stronger interactions with the most socially cooperative partner and reported enhanced feelings of trust and preference. Also, during free viewing of pictures of faces, oxytocin selectively increased patients’ gazing time on the socially informative region of the face, namely the eyes. Thus, under oxytocin, patients respond more strongly to others and exhibit more appropriate social behavior and affect, suggesting a therapeutic potential of oxytocin through its action on a core dimension of autism.

Reference frames for representing visual and tactile locations in parietal cortex

The ventral intraparietal area (VIP) receives converging inputs from visual, somatosensory, auditory and vestibular systems that use diverse reference frames to encode sensory information. A key issue is how VIP combines those inputs together. We mapped the visual and tactile receptive fields of multimodal VIP neurons in macaque monkeys trained to gaze at three different stationary targets. Tactile receptive fields were found to be encoded into a single somatotopic, or head-centered, reference frame, whereas visual receptive fields were widely distributed between eye- to head-centered coordinates. These findings are inconsistent with a remapping of all sensory modalities in a common frame of reference. Instead, they support an alternative model of multisensory integration based on multidirectional sensory predictions (such as predicting the location of a visual stimulus given where it is felt on the skin and vice versa). This approach can also explain related findings in other multimodal areas.

Motor and Visual Imagery as Two Complementary but Neurally Dissociable Mental Processes

Recent studies indicate that covert mental activities, such as simulating a motor action and imagining the shape of an object, involve shared neural representations with actual motor performance and with visual perception, respectively. Here we investigate the performance, by normal individual and subjects with a selective impairment in either motor or visual imagery, of an imagery task involving a mental rotation. The task involved imagining a hand in a particular orientation in space and making a subsequent laterality judgement. A simple change in the phrasing of the imagery instructions (first-person or third-person imagery) and in actual hand posture (holding the hands on the lap or in the back) had a strong impact on response time (RT) in normal subjects, and on response accuracy in brain-damaged subjects. The pattern of results indicates that the activation of covert motor and visual processes during mental imagery depends on both top-down and bottom-up factors, and highlights the distinct but complementary contribution of covert motor and visual processes during mental rotation.

Visual–vestibular interactive responses in the macaque ventral intraparietal area (VIP)

Self‐motion detection requires the interaction of a number of sensory systems for correct perceptual interpretation of a given movement and an eventual motor response. Parietal cortical areas are thought to play an important role in this function, and we have thus studied the encoding of multimodal signals and their spatiotemporal interactions in the ventral intraparietal area of macaque monkeys. Thereby, we have identified for the first time the presence of vestibular sensory input to this area and described its interaction with somatosensory and visual signals, via extracellular single‐cell recordings in awake head‐fixed animals. Visual responses were driven by large field stimuli that simulated either backward or forward self‐motion (contraction or expansion stimuli, respectively), or movement in the frontoparallel plane (visual increments moving simultaneously in the same direction). While the dominant sensory modality in most neurons was visual, about one third of all recorded neurons responded to horizontal rotation. These vestibular responses were typically in phase with head velocity, but in some cases they could signal acceleration or even showed integration to position. The associated visual responses were always codirectional with the vestibular on‐direction, i.e. noncomplementary. Somatosensory responses were in register with the visual preferred direction, either in the same or in the opposite direction, thus signalling translation or rotation in the horizontal plane. These results, taken together with data on responses to optic flow stimuli obtained in a parallel study, strongly suggest an involvement of area VIP in the analysis and the encoding of self‐motion.

Representation of the visual field in the lateral intraparietal area of macaque monkeys: a quantitative receptive field analysis

Abstract. The representation of the visual field in the primate lateral intraparietal area (LIP) was examined, using a rapid, computer-driven receptive field (RF) mapping procedure. RF characteristics of single LIP neurons could thus be measured repeatedly under different behavioral conditions. Here we report data obtained using a standard ocular fixation task during which the animals were required to monitor small changes in color of the fixated target. In a first step, statistical analyses were conducted in order to establish the experimental limits of the mapping procedure on 171 LIP neurons recorded from three hemispheres of two macaque monkeys. The characteristics of the receptive fields of LIP neurons were analyzed at the single cell and at the population level. Although for many neurons the assumption of a simple two-dimensional gaussian profile with a central area of maximal excitability at the center and progressively decreasing response strength at the periphery can represent relatively accurately the spatial structure of the RF, about 19% of the cells had a markedly asymmetrical shape. At the population level, we observed, in agreement with prior studies, a systematic relation between RF size and eccentricity. However, we also found a more accentuated overrepresentation of the central visual field than had been previously reported and no marked differences between the upper and lower visual representation of space. This observation correlates with an extension of the definition of LIP from the posterior third of the lateral intraparietal sulcus to most of the middle and posterior thirds. Detailed histological analyses of the recorded hemispheres suggest that there exists, in this newly defined unitary functional cortical area, a coarse but systematic topographical organization in area LIP that supports the distinction between its dorsal and ventral regions, LIPd and LIPv, respectively. Paralleling the physiological data, the central visual field is mostly represented in the middle dorsal region and the visual periphery more ventral and posterior. An anteroposterior gradient from the lower to the upper visual field representations can also be identified. In conclusion, this study provides the basis for a reliable mapping method in awake monkeys and a reference for the organization of the properties of the visual space representation in an area LIP extended with respect to the previously described LIP and showing a relative emphasis of central visual field.

Space Coding in Primate Posterior Parietal Cortex

Neuropsychological studies of patients with lesions of right frontal (premotor) or posterior parietal cortex often show severe impairments of attentive sensorimotor behavior. Such patients frequently manifest symptoms like hemispatial neglect or extinction. Interestingly, these behavioral deficits occur across different sensory modalities and are often organized in head- or body-centered coordinates. These neuropsychological data provide evidence for the existence of a network of polymodal areas in (primate) premotor and inferior parietal cortex representing visual spatial information in a nonretinocentric frame of reference. In the monkey, a highly modular structural and functional specialization has been demonstrated especially within posterior parietal cortex. One such functionally specialized area is the ventral intraparietal area (VIP). This area is located in the fundus of the intraparietal sulcus and contains many neurons that show polymodal directionally selective discharges, i.e., these neurons respond to moving visual, tactile, vestibular, or auditory stimuli. Many of these neurons also encode sensory information from different modalities in a common, probably head-centered, frame of reference. Functional imaging data on humans reveal a network of cortical areas that respond to polymodal stimuli conveying motion information. One of these regions of activation is located in the depth of human intraparietal sulcus. Accordingly, it is suggested that this area constitutes the human equivalent of monkey area VIP. The functional role of area VIP for polymodal spatial perception in normals as well as the functional implications of lesions of area VIP in parietal patients needs to be established in further experiments.

Heading encoding in the macaque ventral intraparietal area (VIP).

We recorded neuronal responses to optic flow stimuli in the ventral intraparietal area (VIP) of two awake macaque monkeys. According to previous studies on optic flow responses in monkey extrastriate cortex we used different stimulus classes: frontoparallel motion, radial stimuli (expansion and contraction) and rotational stimuli (clockwise and counter‐clockwise). Seventy‐five percent of the cells showed statistically significant responses to one or more of these optic flow stimuli. Shifting the location of the singularity of the optic flow stimuli within the visual field led to a response modulation in almost all cases. For the majority of neurons, this modulatory influence could be approximated in a statistically significant manner by a two‐dimensional linear regression. Gradient directions, derived from the regression parameters and indicating the direction of the steepest increase in the responses, were uniformly distributed. At the population level, an unbiased average response for the stimuli with different focus locations was observed. By applying a population code, termed ‘isofrequency encoding’, we demonstrate the capability of the recorded neuronal ensemble to retrieve the focus location from its population discharge. Responses to expansion and contraction stimuli cannot be predicted based on quantitative data on a neurons frontoparallel preferred stimulus direction and the location and size of its receptive field. These results, taken together with data on polymodal motion responses in this area, suggest an involvement of area VIP in the analysis and the encoding of heading.

Multisensory Integration in the Ventral Intraparietal Area of the Macaque Monkey

The goal of this study was to characterize multisensory interaction patterns in cortical ventral intraparietal area (VIP). We recorded single-unit activity in two alert monkeys during the presentation of visual (drifting gratings) and tactile (low-pressure air puffs) stimuli. One stimulus was always positioned inside the receptive field of the neuron. The other stimulus was defined so as to manipulate the spatial and temporal disparity between the two stimuli. More than 70% of VIP cells showed a significant modulation of their response by bimodal stimulations. These cells included both bimodal cells, i.e., cells responsive to both tested modalities, and seemingly unimodal cells, i.e., cells responding to only one of the two tested modalities. This latter observation suggests that postsynaptic latent mechanisms are involved in multisensory integration. In both cell categories, neuronal responses are either enhanced or depressed and reflect nonlinear sub-, super-, or additive mechanisms. The occurrence of these observations is maximum when stimuli are in temporal synchrony and spatially congruent. Interestingly, introducing spatial or temporal disparities between stimuli does not affect the sign or the magnitude of interactions but rather their occurrence. Multisensory stimulation also affects the neuronal response latencies of bimodal stimuli. For a given neuron, these are on average intermediate between the two unimodal response latencies, again suggesting latent postsynaptic mechanisms. In summary, we show that the majority of VIP neurons perform multisensory integration, following general rules (e.g., spatial congruency and temporal synchrony) that are closely similar to those described in other cortical and subcortical regions.

A Selective Impairment of Hand Posture for Object Utilization in Apraxia

This study reports the case of an apraxic patient who was impaired in all aspects of gestural behavior following bilateral posterior parietal cortex lesions. The main impairment concerned manual prehension of objects during their utilization. The deficit contrasted with both normal movement trajectories of the arm during execution of such gestures, and with accurate manual prehension in the context of simple reaching movements. Although recognition of gestures and pantomimes made by the examiner was preserved, the patient showed a striking inability to visually discriminate or describe manual prehension associated with object utilization. We thus propose the existence of specialized cortical mechanisms for the representation and activation of the postural schemata of the hand required for complex actions.