European vestibular experiments on the Spacelab-1 mission: 5. Contribution of the otoliths to the vertical vestibulo-ocular reflex

The gain of the vestibulo-ocular reflex in the sagittal plane may be due to a cooperation between otoliths and the vertical semi-circular canals. The present space experiment was aimed at studying the influence of the absence of gravity stimulation on the otoliths, by comparing VOR gain and phase in space and on ground. Measurements were taken the 5th and the 7th day of flight, the subject being asked to perform, eyes closed, active head oscillations in pitch while fixating an imaginary target in front of him. No significant decrease of the VOR gain was found in space, but a change in phase was noted. A significant increase of the VOR gain was found 14 h after landing. Control experiments have been done on ground on several subjects. They indicate that pitch VOR gain during active head movements is about one, with eyes open in darkness at 1 Hz.