Visual suppression of torsional vestibular nystagmus in rhesus monkeys

Juvenile rhesus monkeys, placed on a motorized turntable, were rotated at constant velocity and then decelerated about an Earth-vertical axis. The animals were implanted with dual search coils to measure eye movements in three dimensions. By changing the monkey's body position (upright, ear-down, supine), postrotatory nystagmus was elicited in the horizontal, vertical, or torsional direction. Peak slow phase eye velocity and time constant of velocity decay were compared between decelerations in the dark and in the light. In all nystagmus directions, illumination reduced the time constant (T_c) to values around 5 sec. Peak velocity (V_p) was markedly attenuated in the horizontal and vertical directions (around 50%), but the effect of light on V_p in the torsional direction was small (< 20%). These findings were independent of the velocity step size. Our hypothesis is that the two dynamic components of optokinetic nystagmus, as they interact with postrotatory nystagmus during visual suppression, differ in their dimensionality: the early component (fast component, direct pathway, pursuit system) is mainly activated in the horizontal and vertical directions, while the late component (slow component, indirect pathway, optokinetic system) effectively operates in all three dimensions.