Unidirectional vestibular habituation in the squirrel monkey. The time course and its influence on optokinetic nystagmus.

Twenty clockwise (CW) angular stimulations were given daily to squirrel monkeys over a 14-calendar day period. The gain of vestibulo-ocular reflex (VOR) to CW angular acceleration decreased after each daily session. Particularly, the maximum reduction of VOR was found after the first day's session. The reduction was retained until the time of the next day's session. But, the across-day course of the VOR reduction showed up-and-down sinusoidal patterns in all animals. With respect to the effect on optokinetic nystagmus (OKN), an increase of counter-clockwise (CCW) OK responses after repeated CW vestibular stimulations was found; however, there was no clear retention of this response modification across daily sessions.