These studies will concentrate on three adaptive processes that play an important role in recovery of stable vision in human patients who have lost the function of one or both vestibular labyrinths. These processes are: voluntary modification of the vestibuloocular reflex (VOR), long-term plastic modification of the VOR (a form of unconscious learning) and plastic increase in the cervicoocular reflex (COR) which involves reflex eye movements elicited by rotation of the neck. Our primary focus will be on the first two processes, which involve adjustment of the VOR. Their properties will be quantitatively measured in normal human subjects instructed to voluntarily suppress or enhance their VOR and in subjects wearing magnifying or reducing lenses that cause plastic VOR changes. The resulting characterization of voluntary and plastic processes will be used in a subsequent analysis of the extent to which these processes contribute to restoring gaze stability in a group of patients who have lost the function of one labyrinth. We will also measure the effects of these two processes on oculomotor and neck motor responses produced by combinations of visual and vestibular stimuli. The resulting data will aid us in understanding the neuronal mechanisms that mediate these adaptive processes and will also provide needed information on the properties of oculomotor and neck motor reflexes, especially those that stabilize vertical gaze. Our secondary goal is to characterize the properties of the human COR and to determine its role in restoring gaze stability in the same group of labyrinthine defective patients. These experiments will also provide information about the properties of vestibular and neck reflexes acting on neck muscles (vestibulocollic and cervicocollic reflexes), which have not been studied previously in man. This information will permit a quantitative analysis of the role of these two reflexes in stabilizing the head. The analysis and other information about human gaze control will then be correlated with the results of ongoing animal studies to obtain a comprehensive picture of the mechanisms involved in reflex stabilization of gaze.
