Effects of gravitoinertial force variations on optokinetic nystagmus and on perception of visual stimulus orientation.

Recordings of horizontal and vertical eye movement were obtained with subjects exposed to vertical, horizontal, and oblique optokinetic stimulation during parabolic flight. When the optokinetic stimulation was vertical, the upward slow phase eye velocity increased during transition from high force level to free-fall, and decreased during transition from free-fall to high force level. During optokinetic stimulation in the horizontal and oblique plane, the gravitoinertial forces of parabolic flight induced changes in the velocity of the vertical component of the eye movements, and, therefore, changes in the plane of the eye movements. Some subjects also perceived modifications in the apparent orientation of the visual motion. These findings are in agreement with previous observations on the presence of a vertical nystagmus induced by changes in plane vertical acceleration. They also suggest a close interaction of reflexive eye movements induced by graviceptor inputs and visual inputs for visual stabilization during variations of gravitoinertial force level.