Optokinetic nystagmus under prolonged alteration in the direction of gravity

Abstract

Optokinetic Nystagmus (OKN) exhibit different responses depending on the direction of gravity, with respect to head and body. In 13 normal healthy volunteers (10 experimental subjects and 3 controls), we studied the OKN evoked by horizontal and vertical stimuli, in upright, side-lateral, and repeated upright positions during a long time course of six test stages. Stage 1 was an upright sitting position. Stage 2 was a 90-degree recumbent lateral position immediately after Stage 1. Stage 3 was the same position as in Stage 2, but 2h later. Stage 4 was the same position, another 2h later (total of 4h). Stage 5 was a return to the upright position (the same as Stage 1) immediately after Stage 4. Stage 6 was a return to the same upright position, 2h after Stage 5. The results showed continuous modifications of OKN. A lateral tilt produced different effects on the horizontal and vertical optokinetic oculomotor systems. The gain in horizontal OKN (HOKN) decreased during Stages 2-4 with time, and returned to normal in Stages 5 and 6. As for vertical OKN (VOKN), the gain of upward (the direction by slow phase) OKN increased during Stages 2-4, and returned to normal in Stages 5 and 6. But the gain of downward OKN did not show any clear modulation. We showed that the change in gravity direction has a clear effect on OKN, and that it takes some time to reach the maximal and desirable level of modification. The difference in time-dependent modifications between HOKN and VOKN should be due to the difference in OKN-producing mechanisms and velocity storage systems.