Adaptation of otolith responses assessed by off-vertical axis rotation

Abstract

Objectives: During rotation in darkness at constant velocity about an axis tilted relative to gravity (OVAR) the otolith organs are stimulated by the change in head orientation relative to gravity. The frequency of the linear acceleration stimulus during OVAR is determined by the speed of rotation. The magnitude of the linear acceleration stimulus during OVAR is demonstrated by how far the axis of rotation is tilted (20 degrees = 0.34 g). Methods: Tilt angles used were 10 degrees and 20 degrees. Rotational frequencies were 0.125 Hz and 0.5 Hz. Directions were clockwise and counterclockwise. The subjects were normative controls (n = 14), unilateral vestibular deficient patients (n = 3) and astronauts pre- and postflight (n = 14). Results: In normal subjects during OVAR, there is a clear correspondence between eye movements and motion perception data. In patients with unilateral vestibular loss, OVAR responses were not substantially different than healthy control subjects (compensation from the intact side and/or proprioceptive inputs). Astronauts returning from spaceflight experienced a larger sense of both tilt and translation during OVAR at low and high frequency, respectively. Torsion at 0.125 Hz and modulation of horizontal SPV at 0.5 Hz appear similar to preflight values. Conclusion: The overestimation of tilt and translation during OVAR in returning astronauts is in agreement with previous results. It is due to a decrease in the weight of the inertial body vertical after adaptation to microgravity. We have isolated the otolith inputs from the semicircular canals, proprioceptive, and visual inputs. Eye movements from otolith stimulation were not altered after spaceflight.