RECENT RESULTS ON THE PHYSIO-PATHOLOGY OF GAZE

This paper presents a summary of results which concern the neuronal mechanisms underlying the control of gaze. The development of techniques of recording from alert animals and combining functional analysis of neuronal activity with intracellular marking by horse-radish peroxidase has allowed a study of the neuronal mechanisms underlying the vestibulo-ocular reflex and the generation of saccades. The results of these analysis show that there is a close interaction at the level of the brainstem between this two elements of the oculomotor repertoire. This interaction involves both the action of premotor burst neurons involved in the generation of the saccade but also ponto-bulbar reticulo-spinal neurons belonging to the tecto-reticulo-spinal system and involved in the coordination between eye and head movements. This interactions has a very important consequence in the field of recuperation from vestibular lesions. It has been suggested that after vestibular lesions, in addition to local plasticity at the level of the vestibular nuclei it has been suggested that recuperation of function can be due to a functional creation of pseudo-vestibular reflexes by a sequence of blended saccades induced by the saccadic generator mechanisms and with the contribution of the reticulo-spinal system. These interactions have been studied in humans. We have also shown that after prism adaptation the non functional vestibulo-ocular reflex could be replaced by saccades confirming our previous studies in animals. In order to try to approach the neuronal mechanisms underlying these cortical influences we have been able to delineate by position emission tomography techniques the areas of the brain contributing to the generation of volontary or remembered saccades (parietal cortex, frontal eye fields, supplementary motor eye field, cingulate gyrus, etc.). We have also shown that in patients with lesions of these areas the major deficit in the execution of saccades derived from vestibular information about body motion was the prefrontal cortex and SMA.