Geometric illusions in astronauts during long-duration spaceflight.

In our previous studies, we have shown that the occurrence of geometric illusions was reduced in vestibular patients who presented signs of otolith disorders and when healthy observers were tilted relative to gravity. We hypothesized that the alteration in the gravitational (otolith) input was responsible for this change, presumably because of a connection between vestibular and visual-spatial cognitive functions. In this study, we repeated similar experiments in astronauts during long-duration spaceflight. In agreement with the data of otolithic patients, the inverted-T geometric illusion was less present in the astronauts in 0 g than in 1g. In addition, the vertical length of drawings made by astronauts in orbit was shorter than that on the ground. This result is also comparable with the otolithic patients who perceived the vertical length of line drawings to be smaller than healthy individuals. We conclude that the impairment in the processing of gravitational input in long-duration astronauts affects their mental representation of the vertical dimension similar to the otolithic patients. The astronauts, however, recover to baseline levels within 1 week after returning to Earth.