Listing's plane and the otolith-mediated gravity vector

The orientation of Listing's plane (LP) was examined under one-g and zero-g conditions during parabolic flight and during prolonged spaceflight. Ten healthy subjects participated in the parabolic flight study. In zero-g, the orientation of LP was consistently altered. LP elevation was tilted backwards by approximately 10 degrees (p=0.003). The azimuth angles of the left and right eyes also diverged in zero-g, with a statistically significant change (p=0.04) in the vergence angle between 6.1 degrees and 11.8 degrees. A discernible dissociation in torsional eye position was also observed, which proved to be statistically significant (p=0.03). The thickness of LP was found to be of the order of 1 degree, and was not significantly altered by the transitions between one-g and zero-g. The results of the spaceflight measurements demonstrate that this LP re-orientation persists over 6-month periods in microgravity, and that re-adaptation after returning to one-g earthbound conditions proceeds over several days to weeks. The findings demonstrate that in contrast to repositioning of the head in one-g, the elimination of gravity represents a qualitative change for the vestibular and oculomotor systems. It appears that given the lack of voluntary control of ocular torsion, the tonic otolith inputs are instrumental in the stabilization of torsional eye position and consequently of LP. The observed torsional divergence also provides support for the so-called otolith asymmetry hypothesis.