EFFECT OF TARGET VELOCITY UPON HORIZONTAL-AXIS OTOLITH-VISUAL INTERACTIONS

Visual-vestibular interactions were assessed for eleven human subjects during earth-horizontal axis rotation. The apparatus consisted of a r otating chair and an independently controlled rotating optokinetic sur round. 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 vestibu lar stimuli were given, the typical slow component velocity response c onsisted of an exponential decay to a non-zero baseline value (bias co mponent). Superimposed on this was a cyclic modulation of the slow com ponent velocity whose period was equal to the time required for one co mplete revolution. Our data indicate that the addition of visual input to otolith input does not affect the slow component velocity modulati on component during earth horizontal axis rotation. The average bias c omponent during otolith stimulation alone was much lower than the stim ulus velocity. The bias component during optokinetic stimulation produ ce velocity dependent saturation. Thus, neither input alone was adequa te to produce a bias component that matched the higher stimulus veloci ties. In contrast, the average bias component during otolith-visual in teraction runs produced responses that were nearly equal to the relati ve 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 stimu li are presented together.