Affiliations: Departments of Physiology and Otolaryngology, University of Manitoba, Winnipeg, Canada
Note: [*] Present address: Department of Physiology and Biophysics, NYU Medical Center, 550 1st Avenue, New York, NY 10016 USA.
Note: [**] Reprint address: Ralph Jell, Department of Physiology, University of Manitoba, 770 Bannatyne Avenue, Winnipeg, MB R3E OW3, Canada. Tel: (204) 787-4916; Fax: (204) 787-4920; E-mail: [email protected].
Abstract: Characteristics of human vertical optokinetic nystagmus (OKN) and afternystagmus (OKAN) were examined by ISCAN imaging system on the ground and on board NASA's KC-135 aircraft in a parabolic flight study. The aircraft produced alternating periods of micro- (ca. 10−2 G) and hyper- (ca. 1.8 G) gravitoinertial forces. Each phase lasted approximately 20 to 25 s. As compared to the baseline data on the ground, there was a significant increase of downward OKN/OKAN, whereas the upward OKN/OKAN did not change significantly in either micro-or hyper-G. As a consequence, the asymmetry of vertical OKN/OKAN, normally seen on the ground, was lost. It is postulated that the increase of downward OKN/OKAN in non-1-G conditions was governed by two different mechanisms. In micro-G, the normal inhibition effect in 1 G on the downward eye movement mediated by otolith organ activity to vertical velocity storage was removed. In hyper-G, downward OKN/OKAN was facilitated by augmented otolith-ocular reflex because of increased gravitoinertial force, so as to produce enhanced compensatory downward eye movement.
Keywords: OKN, OKAN, microgravity, gaze, linear VOR
