Cortical Control of Vestibular Memory‐Guided Saccades a

By measuring the magnitude of subjective angular velocity in humans, Guedry had shown that human subjects can correctly evaluate the amplitude of a passive angular whole-body displacement,’ from the angular acceleration signal transduced into angular velocity signal by the mechanical properties of the semicircular The exact mechanism of this evaluation of displacement is not known. However, it confirmed the old proposal of Beritoff and the more recent ideas of Potegal‘ concerning an involvement of the vestibular system in “path integration.” Recently, quantitative studies on the reproduction, with eye movements, of passive horizontal whole-body angular displacements in darkness have been performed in humans, for angular movements by Bloomberger aLML (who have proposed a “vestibular memory contingent saccade” task) and Israel et ~ l . , ~ and for linear displacements by Israel et al.’”s” The results showed that humans can correctly match the amplitude of a preceding head displacement with a voluntary ocular saccade of equal but opposing amplitude. This oculomotor return after passive transport is possible only if the vestibular system is intact.“ These authors not only showed that the amplitude of a passive body displacement in darkness is correctly estimated by the brain, but also that this information can be adequately stored, retrieved, and used by the oculomotor (saccadic) system. I t has also recently been suggestedl2-I4 that the vestibuloocular reflex (VOR) is not a simple vestibular-induced reflex, but proceeds from an internal reconstruction of the spatial relations between subject and target, or of a mental “reference.” Such a hypothesis is supported by the recent finding by Ventre and Faugier-Grimaud of cortical afferences on the vestibular nuclei and VOR gain modification after parietal and also by the discovery of a vestibular area in the parietoinsular cortex.” Segal and Katsarkas have furthermore suggested that the VOR is goal-directed, and that it cooperates with the saccadic system.” This vestibulosaccadic cooperation, or functional coupling which had been suggested by Berthoz,” has been confirmed by Bloomberg et ~ l . ’ , ~ The latter further suggested that this synergistic gaze compensation of slow-phase and saccadic components could arise from vestibular perception of body rotation in space.