Perceived body orientation in microgravity: effects of prior experience and pressure under the feet.

Human activities often involve sensing body orientation using cues from gravity. Astronauts in microgravity are deprived of those cues and may have difficulty with certain tasks. We theorized that experience in microgravity combined with mechanically induced pressure under the feet (foot pressure) would improve the accuracy of a subject's perception of the body's z-axis as indicated by pointing to the subjective horizon (SH). Method: Experiments were conducted during parabolic flights using five experienced subjects and five novices. Subjects were required to raise their arm to point to their SH with eyes closed. Measurements were made on Earth and in microgravity, with or without foot pressure. Both pointing accuracy and the kinetics of the movement were analyzed. Results: Performance by experts was stable under all conditions. However, novices in microgravity pointed to a significantly lower SH (16.5degrees below the 1-G SH) and slowed their movements (mean angular velocity of movement: 16.8degrees (.) s(-1) less than in 1 G). Foot pressure improved the performance of the novices so that it was closer to that observed at 1 G (8.9degrees below the 1-G SH). Discussion: These results suggest that pressure cues under the feet activated the internal model of gravity in the novices, and thus improved the accuracy of their perception of their z-axis. Subjects with prior experience in microgravity correctly perceived their z-axis without the supplementary input.