Effect of Target Velocity Upon Horizontal Axis Otolith-Visual Interactions

Abstract

Visual-vestibular interactions were assessed for eleven human subjects during earth-horizontal axis rotation. The apparatus consisted of a rotating chair and an independently controlled rotating optokinetic surround. Subjects underwent ten different test runs where vestibular and optokinetic stimuli were given independently and in combination. The resultant nystagmus slow component velocity was analyzed. When vestibular stimuli were given, the typical slow component velocity response consisted of an exponential decay to a non-zero baseline value (bias component). Superimposed on this was a cyclic modulation of the slow component velocity whose period was equal to the time required for one complete revolution. Our data indicate that the addition of visual input to otolith input does not affect the slow component velocity modulation component during earth horizontal axis rotation. The average bias component during otolith stimulation alone was much lower than the stimulus velocity. The bias component during optokinetic stimulation produce velocity dependent saturation. Thus, neither input alone was adequate to produce a bias component that matched the higher stimulus velocities. In contrast, the average bias component during otolith-visual interaction runs produced responses that were nearly equal to the relative target velocity. This occurred despite large individual variability of the otolith alone and optokinetic response alone. Thus. the brain compensates to match the target velocity when otolith and visual stimuli are presented together.

Key Words:

• visual-vestibular interactions
• vestibuloocular reflex
• otolith organs
• gravity
• optokinetic nystagmus
• sensory interactions.