Effects of static tilt about the roll axis on horizontal and vertical optokinetic nystagmus and optokinetic after-nystagmus in humans.

Horizontal and vertical OKN and OKAN were recorded in four conditions using the EOG technique. Instructions to subjects were aimed at obtaining a "look" type OKN. Two optokinetic stimulators, a stationary sphere and a binocular portable model, were compared with the subject in the upright condition. Three posture orientations, upright, 90 degree roll (horizontal), and upside-down, were then compared using the portable stimulator to determine the effect of roll-axis tilt on OKN at three velocities and on OKAN. Vertical OKN asymmetry was found to increase in the 90 degree roll position and to tend toward a reversal in the upside-down position. The time constant of vertical OKAN with slow phase up increased in both the 90 degree roll and upside-down positions. And finally, cross-coupled vertical eye movements during and after horizontal OKN were clearly observed. These results confirm the data obtained in monkeys, and are in accordance with the hypothesis of a three-dimensional organization of the velocity storage mechanism.