ASYMMETRY OF VESTIBULAR FUNCTION INDUCED BY UNIDIRECTIONAL VISUAL-VESTIBULAR CONFLICT

We describe an attempt to model unilateral vestibular dysfunction in n ormal man by inducing vestibular asymmetry with exposure to long-term, unidirectional, visual-vestibular conflict. Subjects were exposed to pseudo-random (0.13, 0.2, 0.25, 0.3Hz; 77 degrees/s peak) oscillation in Yaw for 30 min whilst viewing a surrounding, whole field optokineti c drum which rotated with them when they were rotating rightwards and remained earth stationary when they rotated leftwards. Adaptation to t his stimulus was assessed by combined tests of ''goal-directed'' vesti bular-ocular reflex (VOR) and vestibular memory contingent saccades (V MCS) in 5 subjects and in a further 4 subjects by combined tests of pe rception of reorientation (a ''navigation'' task) and sinusoidal VOR a t 0.1 and 0.32 Hz. The exposure induced a reduction in the gain of the VMCS and an underestimation of perceived amplitude of displacement wh en subjects were turned rightwards. VOR gain for rightwards movement w as reduced more markedly at 0.1 Hz. No change was found in the goal-di rected VOR gain. Thirty minutes after adaptation, the asymmetry of the VOR gain remained at 0.1 Hz, but vestibular perception recovered to n ormal. Asymmetrical adaptation can be achieved with short exposures an d is more marked for low frequency stimuli. Modification reflex of ves tibular functions endures longer than of perception of reorientation.