title: [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.
authors.affiliation: Department of Otolaryngology, Nihon University School of Medicine, Tokyo.
authors.forename: Y
authors.initials: Y
authors.lastname: Watanabe
authors.forename: M
authors.initials: M
authors.lastname: Igarashi
mesh_terms: D000293:Adolescent; D000328:Adult; D006112:Gravitation; D006801:Humans; D008297:Male; D017120:Nystagmus, Optokinetic / Q000502:physiology*; D011187:Posture; D013997:Time Factors