EVALUATION OF VESTIBULAR AND VISUAL OCULOMOTOR FUNCTION

The visual system interacts synergistically with the vestibular system . A normally functioning vestibule-ocular reflex is necessary but not sufficient for optimum visual acuity during head motion. Studies of dy namic visual acuity, the acuity achieved during relative motion of vis ual targets or of the observer, indicate that motion of images on the retina markedly compromises vision. The vestibulo-ocular reflex normal ly provides a substantial measure of stabilization of the retina durin g head movements, but purely vestibular compensatory eye movements are not sufficiently precise for optimal vision under all circumstances. Other mechanisms, including visual tracking, motor preprogramming, pre diction, and mental set, interact synergistically to optimize the gain (eye velocity divided by head velocity) of compensatory head movement s. All of these mechanisms are limited in their capacity to produce ef fective visual-vestibular interaction at higher rotational frequencies and velocities. It Is under these conditions that vestibular deficits give rise to symptoms of oscillopsia. Patients having vestibular lesi ons exploit mechanisms of visual-vestibular interaction to compensate by substitution for deficient vestibular function. Thus, for accurate topographic clinical diagnosis of vestibular lesions, testing conditio ns should isolate purely vestibular responses. This may be done by tes ting reflex eye movements during passively generated rotations in dark ness, or perhaps by testing during other types of motion under conditi ons of extreme frequency and velocity sufficient to attenuate the effe cts of visual-vestibular interaction. This article reviews clinical te sts of vestibular function in relation to synergistic interactions wit h vision.