Contrôle du mouvement du regard (1) : Le rôle du cortex pariétal

Abstract

Les mouvements des yeux constituent un mode d’accès privilégié au monde qui nous entoure. Ils permettent, en plaçant les objets d’intérêt dans la partie centrale du champ visuel, d’explorer les scènes visuelles, d’en identifier les composants significatifs et d’acquérir les informations nécessaires pour pouvoir agir sur eux (préhension, évitement…). De nombreuses étapes de traitement se succèdent entre l’arrivée des photons sur la rétine et la contraction des muscles oculaires. Dans cet article, nous étudions la place du cortex pariétal dans cet enchaînement de mécanismes neurophysiologiques. Nous proposons que celui-ci soit impliqué dans la représentation de l’espace et la sélection des objets pertinents dans l’environnement, c’est-à-dire après le traitement visuel perceptif et avant l’élaboration du signal moteur.

Eye movements constitute one of the most basic means of interacting with our environment, allowing to orient to, localize and scrutinize the variety of potentially interesting objects that surround us. In this review we discuss the role of the parietal cortex in the control of saccadic and smooth pursuit eye movements, whose purpose is to rapidly displace the line of gaze and to maintain a moving object on the central retina, respectively. From single cell recording studies in monkey we know that distinct sub-regions of the parietal lobe are implicated in these two kinds of movement. The middle temporal (MT) and medial superior temporal (MST) areas show neuronal activities related to moving visual stimuli and to ocular pursuit. The lateral intraparietal (LIP) area exhibits visual and saccadic neuronal responses. Electrophysiology, which in essence is a correlation method, cannot entirely solve the question of the functional implication of these areas : are they primarily involved in sensory processing, in motor processing, or in some intermediate function ? Lesion approaches (reversible or permanent) in the monkey can provide important information in this respect. Lesions of MT or MST produce deficits in the perception of visual motion, which would argue for their possible role in sensory guidance of ocular pursuit rather than in directing motor commands to the eye muscle. Lesions of LIP do not produce specific visual impairments and cause only subtle saccadic deficits. However, recent results have shown the presence of severe deficits in spatial attention tasks. LIP could thus be implicated in the selection of relevant objects in the visual scene and provide a signal for directing the eyes toward these objects. Functional imaging studies in humans confirm the role of the parietal cortex in pursuit, saccadic, and attentional networks, and show a high degree of overlap with monkey data. Parietal lobe lesions in humans also result in behavioral deficits very similar to those that are observed in the monkey. Altogether, these different sources of data consistently point to the involvement of the parietal cortex in the representation of space, at an intermediate stage between vision and action.