Semicircular canal influence on the visually perceived eye level during gondola centrifugation

When exposed to an increased gravitoinertial force, a subject, sitting upright, experiences an illusion of being tilted backwards. This so-called "G-excess illusion" is generally ascribed to the otolith organs. The present study aimed at elucidating how stimulation of the semicircular canals may influence the development of the G-excess illusion.

The visually perceived eye level (VPEL) was measured by means of a visual indicator in a large swing-out gondola centrifuge. The roll position of the gondola was controlled so that the subject was always upright with respect to the resultant vector of the Earth gravity force and the centrifugal force. Subjects (n = 8) underwent four centrifuge runs (2 G, 5 min), sitting in different positions, i.e., heading forwards, backwards, centripetally, and centrifugally.

At the 2-G plateau there was a depression of the VPEL which was initially small but increased with a time constant of 90 +/- 30 s toward an asymptote of -22.0 +/- 6.9 degrees (mean and 1 SD for all positions). The initial depression was significantly smaller for the centripetal (+2.0 +/- 14.6 degrees) than for the centrifugal position (-14.5 +/- 10.4 degrees). However, there was no difference between the forward (-5.6 +/- 4.8 degrees) and backward (-4.0 +/- 4.5 degrees) positions. Initially after deceleration of the centrifuge to 1 G there was still a significant depression of the VPEL (-13.5 +/- 7.9 degrees), decreasing with a time constant of 100 +/- 46 s.

The considerable delay in the otolith-mediated changes in the VPEL is interpreted as due to the absence of adequate canal information for a change in head position. The difference in VPEL between the centripetal and centrifugal positions suggests an influence of canal change-in-position information. However, pitch-plane angular velocity, being of considerable magnitude but of opposite sign for the forward and backward positions, did not influence the VPEL.