Perceptual thresholds of radial accelerations as indicated by visually perceived eye level.

The purpose of this study was to determine whether very low gravito-inertial forces produced by centrifugation affect the visually perceived eye level (VPEL) in the same way as the oculagravic illusion. Eleven subjects in total darkness were instructed to set a luminous target to the VPEL, either while they were motionless or undergoing very low centrifugation. Results showed a significant effect on VPEL at 0.01 m/s2 radial acceleration, which corresponds to a resultant gravito-inertial equal to 9.81001 m/s2. This radial acceleration value is lower than the lowest perception thresholds previously measured for a linear acceleration (about 0.05 m/s2). Thus, as previous results have shown that the oculogyral illusion indirectly decreases perceptual thresholds for the perception of angular acceleration in darkness, the lowering of the VPEL indirectly decreases thresholds for perception of radial acceleration produced by centrifugation. Moreover, there is a logarithmic relationship between very low centrifugation and the positioning of the VPEL at a lower level. This relationship is explained as a direct and sole effect on the sensory utricular otolithic membrane by the radial acceleration of centrifugation.