Defense Date


Document Type


Degree Name

Doctor of Philosophy


Physical Therapy

First Advisor

Dr. Mary S. Shall


Introduction: The Vestibulo-ocular reflex (VOR) is quantified by computing the ratio of head angular velocity and eye angular velocity (VOR Gain). This measure only includes head angular movements; linear translations are not accounted for. These investigations postulate an alternative method of VOR quantification, one that assesses retinal image stability during head angular and linear movements (Foveal fixation (FF)). This method was used to assess the role of vision in balance reactions. Methods: Experiment 1 : Ten Young subjects were fitted with an eye tracker linked to an EM kinematic recording system. This allowed for the recording of head, trunk and eye kinematics during the performance of gaze stabilization tasks. Subjects fixated an LED target while performing head flexion and extension exercises at four frequencies. Point-of-gaze analysis was performed by transforming eye-in-head and head-in-space data into eye-in-space data, which were compared to the known location of the targets. The distance between the eye vector target plane intersection and the target location provided an error that could be used to calculate the estimated image location on the retina. FF and VOR gain were compared with head angular velocities to determine correlations. Experiment 2: Balance was assessed in Young and Elderly following a series of perturbations. Dependent variables included: step latency, head and trunk angular velocity, VOR Gain, FF. Results: Correlations between head angular velocity and FF showed that retinal image stability degraded as head angular velocity increased. Elderly showed a more rapid degradation of FF with higher overall head angular velocities. Comparisons between rate of change of VOR and FF over velocity spectrum indicated a greater change in FF response. A negative linear correlation between FF and Step Latency was observed: there was no relationship between VOR gain and Step Latency. Conclusion: FF is a more sensitive measure of VOR than Gain as it accounts for angular and translational head movements. Its correlation with Step Latency suggests the importance of image stability in formulating responses following perturbation.


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