Spatial reference in weightlessness: perceptual factors and mental representations.

The role of gravity in spatial coordinate assignment and the mental representation of space were studied in three experiments, varying different perceptual cues systematically: the retinal, the visual background, the vestibular, and proprioceptive information. Verbal descriptions of visually presented arrays were required under different head positions (straight/tilt) and under different gravitational conditions (gravity present/gravity absent). The results of two experiments conducted with 2 subjects who participated in a space flight revealed that subjects are able to adequately assign positions in space in the absence of gravitational information, and that they do this by using their head-retinal coordinates as primary references. This indicates that they cognitively adapted to the perceptually new situation. The findings from a third experiment conducted with a larger group of subjects under a condition in which the gravitational information was present but irrelevant to the task being solved (subjects were in a horizontal supine position) show that subjects, in general, are flexible in using cues other than gravitational ones as references when the latter cannot serve as a referential system. These findings, together with the observation that consistent spatial assignment is possible even immediately after first exposure to the perceptually totally novel situation of weightlessness, seem to suggest that the mental representation of space, onto which given perceptual information is mapped, is independent of a particular percept.