Effects of hypergravity on optokinetic after-nystagmus and perceived direction of optokinetic stimulation

Previous observations made in parabolic flight and centrifuge studies have shown the presence of a vertical nystagmus (Lz-nystagmus) induced by changes in gravitoinertial forces, and its interaction with oculomotor reflexes.

This Lz-nystagmus is also responsible for the changes in optokinetic after-nystagmus (OKAN) and the subjective perception of optokinetic stimulation direction during hypergravity.

OKAN was recorded during the 1.8-g phase of parabolic flight after exposure to horizontal or vertical optokinetic stimulation during the preceding 1.0-g or 0-g phases. Changes in the apparent direction of image motion in subjects presented with an optokinetic stimulus were investigated in another experiment where longer exposure to hypergravity was generated by flying an airplane along a spiral path.

In upright subjects, the time constant of OKAN with slow phase up decreased during 1.8 g, whereas the horizontal OKAN showed no change in 1.8 g compared with OKAN recorded in 1.0 g. When the subjects were lying on their left side, the OKAN with slow phase right (slow phase up with respect to gravity) decreased in 1.8 g. The subjects tested showed larger error in setting the optokinetic stimulus in a pure horizontal plane in 1.8 g than in 1.0 g. The error was also larger for oblique stimulus in 1.8 g than in 1.0 g, but no differences were seen for the vertical stimulation.

The changes in OKAN can be explained by an interaction between slow phase eye movements generated by OKAN and the Lz-nystagmus generated by change in the gravitational force level. The error of the perceived direction of the optokinetic stimulus measured during horizontal and oblique stimulation is also presumably due to the interaction between the visual system and the Lz-nystagmus generated by hypergravity.