Reverse optokinetic after-nystagmus generated by gaze fixation during optokinetic stimulation.

Gaze fixation during optokinetic stimulation generates an after-nystagmus with a slow component towards the reverse direction of the optokinetic stimulation. The duration and maximum slow component velocity (SCV) of this "reverse OKAN" were observed by changing the duration, velocity and direction of the optokinetic stimulation in nine normal volunteers. The duration of reverse OKAN increased with increasing stimulation time but was unaffected by changes in the stimulation velocity. The maximum SCV of reverse OKAN decreased with an increase in the stimulation velocity but was not significantly affected by changes in the optokinetic stimulation time. There was no directional difference among the horizontal, upwards and downwards reverse OKANs. The reverse OKAN was thought to be generated by a mechanism different from the velocity storage mechanism which produced optokinetic nystagmus and the first phase of OKAN. Retinal slip during the optokinetic stimulation was considered to be an input to the mechanism which generated the reverse OKAN. We hypothesize that the mechanism causing the reverse OKAN may be a generator of the second phase of OKAN, which was also intimately connected with self-motion sensation during the optokinetic stimulation.