The goal of the proposed studies is to elucidate the physiological characteristics of the pathways in the brain that are involved in the generation and voluntary control of the vestibulo-ocular reflex (VOR). Previous studies showed that monkeys utilize two mechanisms to voluntarily cancel their VOR (VORC). One mechanism involves a reduction in the head movement sensitivity of the secondary vestibular neurons that project to the abducens nucleus, the position-vestibular-pause (PVP) neurons. A second mechanism involves the generation of a smooth pursuit eye movement in the opposite direction to the VOR.
The specific aims of the present proposal are to study the behavioral circumstances and neural mechanisms that are involved in the use of each of the mechanisms for voluntarily controlling the VOR. There are four specific aims. 1) In order to more precisely determine the behavioral circumstances in which the non-pursuit mechanism is recruited, behavioral experiments will be carried out in which the head will be passively perturbed during head free gaze pursuit of a visual target that is moving predictably and unpredictably. The gain of the VOR evoked by passive head perturbations will be studied as a function of head, gaze and target movement parameters. 2) The firing behavior of PVP neurons will be recorded in head free monkeys during gaze pursuit using the chronically implanted microwire in order to determine what signals are generated in secondary VOR pathways during head free gaze pursuit. 3) The possibility that inputs from the cerebellum or the reticular formation cause the head velocity sensitivity of secondary PVP neurons to decrease during VORC will be tested by recording from Purkinje cells in the flocculus and nodulus and neurons in the reticular formation near the abducens nucleus during unpredictable steps in head acceleration that are generated during VORC and during the VOR. 4) Several observations suggest that the abducens nucleus must receive inputs from cells whose activity is specifically related to smooth pursuit eye movements in order for the effects of residular head movement signals in VOR pathways to be canceled. An attempt will be made to determine whether the smooth pursuit neurons in the propositus nucleus and the adjacent medial vestibular nucleus whose activity is closely related to smooth pursuit eye movements project to the abducens nucleus by using the technique of spike triggered averaging of lateral rectus or abducens potentials combined with antidromic stimulation of the abducens nucleus.
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