Dynamics of torsional optokinetic nystagmus under altered gravitoinertial forces

The purpose of the present study was to investigate the influence of varying gravitoinertial forces on torsional optokinetic nystagmus during parabolic flights. Using the scierai search-coil technique, we measured the gain and phase lag of torsional optokinetic nystagmus (OKN) induced by a hemispherical visual display rotating about the roll axis either at constant velocity or sinusoidally at various frequencies during level flight, hypogravity, and hypergravity. Compared with level flight, there was a significant increase in slowphase eye velocity during hypogravity and an increase in nystagmic frequency. An absence of well-developed torsional optokinetic afternystagmus was observed in all three gravity conditions. Other characteristics included a lack of a slow rise component. These data suggest that otolith inputs do affect torsional OKN. The absence of well-developed torsional optokinetic afternystagmus suggests that the velocity storage pathways do not contribute significantly to the torsional OKN system in humans.