OCULOMOTOR SYSTEM--NEURAL STRUCTURE AND FUNCTION

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DESCRIPTION: (Adapted from the applicant's abstract.) The vestibulo-ocular reflex (VOR) is essential for normal visual acuity because it acts to stabilize visual images on the retina against movements of the head and body. Head and body movements are sensed by the semicircular canals (angular accelerometers) and the otolith organs (linear accelerometers). The shortest neural pathway is a 3-neuron arc consisting of primary receptor neurons, interneurons in the vestibular nuclei, and motoneurons that drive the muscles of the eye. The reflex is extremely fast, exhibiting a latency as short as 8 ms in the monkey. In general, if one is viewing a target in the distance, the compensatory eye movements should be equal in magnitude but opposite in direction to the perturbing head movement (Gain - -1). Frequently, however, one views an object in extrapersonal space, e.g., something held in the hands. For near targets such as these, there is no single correct gain for the VOR, since the axis of head rotation cannot be coincident with the rotational axes of the eyes. Normally the center of rotation is behind the eyes (e.g., above the spinal cord for left or right head turns), and geometric considerations show that the gain of the VOR must be greater than one to fully stabilize a visual image. Head movements about axes displaced with respect to the eye cause translation as well as rotation of the eyes in space, and give rise to otolith as well as canal afferent signals. However, the otolith and canal signals are insufficient to determine the correct VOR gain; information about target location is also required. The central goal of this proposal is to identify the site(s) where the computation of VOR gain occurs, and then to analyze the neural pathways and physiological mechanisms involved in producing compensatory eye movements. This work relates to issues of how we determine spatial location, and to neural mechanisms involved in binocular control of eye movements.