Effects of supine body position and low radial accelerations on the visually perceived apparent zenith

The visually perceived eye level (VPEL) has been shown to shift toward the lower part of the body in upright subjects facing toward the axis of rotati on on a centrifuge. This shift occurs in the same direction as the shift in the gravito-inertial forces (Gis) produced by very low radial acceleration (centrifugation) combined with gravity. The purpose of this study was to d etermine whether the same phenomenon affects the visually perceived apparen t zenith (VPAZ) in subjects in a supine position. Twelve supine subjects we re instructed to set a luminous target to the VPAZ, either while they were in total darkness and motionless or while undergoing very low centrifugatio n. Data showed that Gis induced a VPAZ shift similar to that observed for t he VPEL. Thus, as is the case for the VPEL, the corresponding logarithmic p sychophysical function of the VPAZ may be considered to be a type of oculog ravic it lusion phenomenon with differences in the subjects ' that differs from subject to subject, depending on the subject's sensitivity to low radi al accelerations. Data on VPEL and VPAZ support the notion that the subject ive perception of eye level in total darkness takes into account changes-ev en if extremely slight-in the direction of the gravito-inertial forces prod uced by the combination of gravity and low radial accelerations, although s ubjects are unaware of the Gi shift. However, depending on the intensity of the radial acceleration and the angular deviation of Gi relative to G, the shift of the VPEL and the VPAZ can be either amplified or attenuated. More over, differences between VPEL and VPAZ responses suggest two explanatory a ssumptions-namely, that this is (1) a peripheral phenomenon dependent on th e neurophysiological anisotropy of the otolithic system or (2) a central ph enomenon dependent on the relevance assigned to the peripheral information by the integrative sensory functions and the associative processes.