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.Stage6 was a teturn 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 oculamotor systems.The gain in horizontal OKN(HOKN)decreased during Stages2-4with time, and returned to normal in Stage5end6.As for vertical OKN(VOKN), the gain of upward (the direction by slow phase)OKN increased during Stages2 4.and returned to normal in Stages5 and 6.But the gain of dowwnward 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 velacity storage systems.
