MULTIDIMENSIONAL DYNAMICS OF THE VESTIBULO-OCULAR REFLEX

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A new conceptual approach to model visual-vestibular interaction and velocity storage in three dimensions has been developed. The basis for the approach is the characterization of velocity storage by the eigenvalues and eigenvectors of its system matrix. This characterization is an objective measure of spatial orientation and enables a direct comparison between model prediction and data of eye velocity as a function of gravity. The model has been tested against experiments on optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) in both monkeys and humans. From the result, it has been possible to link orientation of velocity storage to human perception of the spatial vertical. Model-based experiments are proposed that will enable us to determine the orientate of velocity storage when moving in a gradational environment, characterize its orientation following adaption of the gain of the vestibulo-ocular reflex (VOR), and find the neural basis for the spatial orientation. Eye velocity following stopping after off- verticals-axis-rotation (OVAR) will be analyzed to determine how spatial orientation is affected by activation of the semicircular canals while titled and being subjected to a prior continuous rotating gravity vector. We will also examine how orientation of velocity storage is affected by centrifugation. This stimulus rotates the direction of the force fields relative to the head and also increases its magnitude. The orientation parameters of velocity storage following stopping after OVAR and centrifugation will be compared to those obtained during OKAN. The affects of adaptation of the VOR on the orientation of velocity storage will also be studied. Gain eye velocity is rotated relative to the stimulus axis. It will be determined whether velocity storage maintains its gravity dependent orientation or adapts coincident with the gain adaption. The neural basis for velocity saccade (VPS) units in the vestibular nuclei (VN). Recent work has shown that the frequency of firing of VO and VPS neurons during OKN, OKAN, VOR, and OVAR has dynamic characteristic of velocity storage. The frequency of firing of lateral, anterior, and posterior canal related VO and VPS units during identical vestibular and optokinetic stimuli will be recorded. Average eigenvalues and eigenvectors associated with the response curves will be compared to those obtained from eye velocity. Our hypothesis is that the orientation parameters obtained by analyzing the unit activity will be the same as the orientation parameters obtained by analyzing the unit activity will be the same as the orientation parameters associated with eye velocity. This research should give a better understanding of spatial orientation of compensatory gaze movement during locomotion and how orientation is affected by gain adaptation of the VOR. We will also have a clearer idea about the neural organization in the vestibular system that produces velocity storage and the coding for spatial orientating.