Foveal smooth pursuit is unique to the only foveated mammals, the primates. Its control requires the participation of higher nervous functions, such as visual attention to the target, and depends on the processing of signals representing the velocities of the eye, of the head, and of retinal image slip. The flocculus of the cerebellum is one of the few brain structures where all of these velocity signals are known to converge. Discharges of Purkinje cells (output signals) and those of mossy fibers (input signals) will be recorded from the flocculus of monkeys trained to fixate and track a moving visual target. These discharges will be analyzed in paradigms designed to dissociate their visual, vestibular, and oculomotor components. In paradigms where a combination of two, or all three kinds of velocity signals are involved, their interactions will be investigated. Discharge rates of floccular Purkinje cells are known to be strongly correlated with various types of oculomotor behavior. By isolating a Purkinje cell unit which responds to a certain parameter of eye movements and applying pulse trains to the same location, we will investigate whether the oculomotor behavior can be modified by the counterfeit impulse patterns. Retinal slip has a velocity of zero when eye velocity perfectly matches target velocity. In such a case, the visual input will be insignificant in maintaining smooth pursuit. The smooth pursuit performance of trained monkeys will be evaluated quantitatively, before and after monocular laser coagulation of a localized region of the fovea. The surgery will be repeated at intervals of approximately 4 weeks, increasing the size of the central scotoma in small steps. The size and side of the scotoma with respect to the foveola will be correlated against smooth pursuit performance during the periods between treatments. Finally, when the scotoma covers the entire foveal region, visual input signals to floccular Purkinje cells will be investigated in the lesioned eye. Responses to the visual stimulation of the lesioned eye will be compared with responses of the intact eye.
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