Perception of longitudinal body axis in microgravity during parabolic flight.

It has been proposed that an internal representation of body vertical has a prominent role in spatial orientation. This investigation investigated the ability of human subjects to accurately locate their longitudinal body axis (an imaginary straight body midline running from head to toes) while free-floating in microgravity. Subjects were tested in-flight, as well as on ground in normal gravity in both the upright and supine orientations to provide baseline measurements. The subjects wore a goggle device and were in total darkness. They used knobs to rotate two luminous lines until they were parallel to the subjective direction of their longitudinal body axis, in the roll (right/left) and the pitch (forward/backward) planes. Results showed that the error between the perceived and the objective direction of the longitudinal body axis was significantly larger in microgravity than in normal gravity. This error in this egocentric frame of reference is presumably due to the absence of somatosensory cues when free-floating. Mechanical pressure on the chest using an airbag reduced the error in perception of the longitudinal body axis in microgravity, thus improving spatial orientation.