Effects of gravitoinertial force variations on vertical gaze direction during oculomotor reflexes and visual fixation

Recordings of horizontal and vertical eye movement were obtained on eight subjects exposed to repeated patterns of vertical and horizontal optokinetic stimulation, visual fixation with a fixed or unseen target, and voluntary head oscillation in the high force and free-fall periods of parabolic flight. The downward shift of the beating field of vertical optokinetic nystagmus (OKN) observed in previous experiments was confirmed in the present study. The same directional shift was also noticed during optokinetic after-nystagmus (OKAN). Vertical direction of gaze clearly shifted downward during the decreased gravitoinertial force level when subjects were exposed to horizontal optokinetic stimulation, or when they attempted to track an unseen target in the dark with the head stationary or actively moved up and down. A vertical nystagmus with slow phases directed upward was observed during transition from high force level to free-fall when subjects were fixating their gaze on a stationary target. These findings are in agreement with those showing a general downward drive of the eyes on the first exposure to microgravity during orbital flight and an upward drive on the first day of return. Although this drive could be a consequence of a disorientation experienced by subjects undergoing parabolic flight or by astronauts, the phenomenon also supports the hypothesis of a tonic influence exerted by the otoliths on the postural and extra-ocular musculature in order to compensate for the downward pull by the gravitoinertial forces.