[Control of visual exploration: anatomic and physiologic data].

Abstract

The idea that saccadic eye movements necessary for visual space exploration could be under the control of the frontal eye field was based mainly upon Ferrier's early experiments of electrical stimulation of the cortex. More recent anatomical as well physiological data have demonstrated in monkeys that the sensori-motor processing needed to locate visual targets and to program an accurate eye movement to them is achieved in the superior colliculus, the frontal prearcurate and the posterior parietal cortices. Cells in those three structures have been identified by microelectrode studies that modify their response to a visual target before any eye movement when a saccade directed to this stimulus is to occur. Motivation to move the eyes towards the stimulus is under the control of the limbic cortex of the cingulate gyrus. Lesions to any of these structures lead to a neglect phenomenon characterized by the paucity of spontaneous saccades towards the half of space opposite to the lesion. In primates and in man neglect is observed only after cortical or thalamic lesions but not, or very transiently, after collicular lesion. The prearcuate frontal, the cingulate, and the posterior parietal cortices are interconnected via direct cortico-cortical fibres and also by a cortico-thalamo-cortical circuit, the thalamic relay of which is the medial pulvinar. The medial pulvinar is known to receive polymodal sensory informations from the intermediate layers of the superior colliculus. Thus a complete network involving cortical, thalamic and subthalamic structures is responsible for a very secure and redundant control of gaze.