Compensation from the loss of function of one labyrinth is an important clinical problem, and an excellent example of sensorimotor adaptation and learning. Patients who undergo acute unilateral vestibular deafferentation (UVD), as a surgical intervention for acoustic neuroma or Meniere's disease, suffer from a variety of vestibular, oculomotor, and postural disorders. Initially, they have a vigorous spontaneous nystagmus due to the sudden imbalance of vestibular tone, which largely resolves itself after a few weeks. Associated with this static imbalance is a dynamic disturbance: the gain (eye velocity / head velocity) of the vestibular- ocular reflex (VOR), in response to head rotations, is significantly decreased, and is lower for rotations toward the lesioned side. This leads to problems of postural stability, disorientation, dizziness, and blurred vision during head motion. The gain of the VOR gradually increases and becomes more symmetric. However, tests of VOR gain with rapid, high- acceleration steps of head rotation show a decreased and asymmetric gain even up to a year after nerve section. We propose a series of studies designed to enhance our knowledge of the process of compensation to UVD. These studies will help us better to understand vestibular adaptation in normals, and will aid us in the development of appropriate exercises for the most rapid and effective rehabilitation of patients after UVD. This project has four major aims. First, to study adaptation to asymmetric vestibular stimuli in normal humans. Second, to measure the ability of patients to adapt to these same stimuli, and determine if adaptive capability in this situation enhances their ability to compensate for UVD. Third, to design visual-vestibular stimuli that provide the most rapid and complete increases in the gain of the VOR in normals, as a preliminary to applying these same stimuli to exercises for UVD patients. Fourth, to promote recovery of vestibular function -- even if the gain of the VOR can not be enhanced -- by a program of rapid combined eye and head movements to visual targets. This task reproduces those circumstances that UVD patients find troubling during recovery; its study in the clinic and laboratory can help us to determine those strategies that patients employ in order to augment an insufficient VOR, so that we can specifically target these strategies in programs of physical therapy.
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