Perceived head-trunk angle during microgravity produced by parabolic flight

INTRODUCTION: Neck proprioceptors are essential for orienting the head relative to the trunk. However, it has been shown that the available information about the relationship of gravity to different body parts would augment the clues about their relative orientation. In weightlessness, the absence of relevant body position signals from the otoliths and other inertial graviceptors requires the substitution of other sensory information. The aim of the present study was to investigate the ability of humans to accurately locate the head relative to the trunk in microgravity. METHODS: Experiments were conducted during two separate sessions: on Earth and during parabolic flights. Volunteers were asked to adjust a visual rod until it looked parallel to their head or trunk axis in two different segmental configurations: head and trunk aligned or head tilted. RESULTS: There was no effect of microgravity when the head and trunk were aligned. However, when the head was tilted with respect to the trunk, the orientation of the visual rod relative to the head or the trunk (visual egocentric coordinates) was deviated toward the head tilt, although the orientation between the body parts themselves (head-trunk angle) was correctly estimated. DISCUSSION: These results suggested that, in microgravity, the proprioceptive signals from neck muscles seem sufficient to provide accurate head on trunk information. However, the representation of orientation in visual space was modified. This experiment provides evidence for the role of gravity on the visual perception of head- and trunk-based egocentric coordinates.