Perception of Body Longitudinal Axis in Microgravity during Parabolic Flight

On Earth, gravity and an internal sense of the body vertical help to maintain balance in every situation. This experiment investigated the ability of human subjects to accurately locate their trunk axis while free-floating in microgravity during parabolic flight. In this study we have compared the conditions where the head is aligned with the trunk, with the conditions where the head was tilted to the side or forward in combinations with tactile pressure applied to the chest. The goal was to identify the respective role of otolith, proprioceptive, and tactile inputs for the location of the body vertical. The subjects wore a mask and were in total darkness. They had to align two luminous lines presented inside the mask with their perceived trunk vertical: one in the roll plane (right/left) and one in the pitch plane (forward/backward). Results showed that the subjects' ability to locate their body longitudinal axis was considerably impaired in microgravity, compared to normal gravity, but that pressure on the chest much restored this ability.