Modifications of gain asymmetry and beating field of vertical optokinetic nystagmus in microgravity.

Optokinetic nystagmus (OKN) was measured in human subjects before, during and after exposure to microgravity induced by either parabolic flight or space flight. The downward (slow phase up) OKN gain was greater than upward gain in normal gravity. On first exposure to microgravity this asymmetry was reversed. In addition, the beating field of OKN tended to shift downward, and the vertical optokinetic after nystagmus (OKAN) time constant was increased. This reversed asymmetry disappeared after 3 days of space flight. On return to 1 g gravity, there was a general drift of the eye in the upward direction during either spontaneous eye movements or OKN. This suggests that the sacculus normally influences mean vertical eye position and the perception of the subjective horizontal direction, both of which are gravity dependent.