ILLUSIONS OF VERTICALITY IN WEIGHTLESSNESS

In weightlessness most subjects feel themselves, and see the visual su rroundings, in either an upright or an upside-down orientation althoug h the gravitational force vector is missing. According to a theory of gravity perception, these illusions of positional and visual verticali ty are assumed to be caused by the force-independent z-axis bias of ve stibular and somatic graviceptors. This hypothesis is tested by compar ison of measurements of the joint bias in normal gravity with reports of probands in space flight. The expected correlations between the sig n of the biases and the occurrence of the respective illusions appear in fact to exist, as well as a negative correlation to incidences of s pace sickness. If confirmed in a larger sample, the presumed dependenc y may eventually afford a predictive test of both phenomena.