Ocular Reflex Phase during Off-Vertical Axis Rotation in Humans is Modified by Head-Turn-On-Trunk Position

Samantha B Douglas,Scott J Wood,Pierre Denise,Gilles Clément

doi:10.1038/srep42071

Abstract

Constant velocity Off-Vertical Axis Rotation (OVAR) imposes a continuously varying orientation of the head and body relative to gravity, which generates a modulation of horizontal (conjugate and vergence), vertical, and torsional eye movements. We introduced the head-turn-on-trunk paradigm during OVAR to examine the extent to whether the modulation of these ocular reflexes is mediated by graviceptors in the head, i.e., otoliths, versus other body graviceptors. Ten human subjects were rotated in darkness about their longitudinal axis 20° off-vertical at a constant velocity of 45 and 180°/s, corresponding to 0.125 and 0.5 Hz. Binocular responses were obtained with the head and trunk aligned, and then with the head turned relative to the trunk 40° to the right or left of center. The modulation of vertical and torsional eye position was greater at 0.125 Hz while the modulation of horizontal and vergence slow phase velocity was greater at 0.5 Hz. The amplitude modulation was not significantly altered by head-on-trunk position, but the phases shifted towards alignment with the head. These results are consistent with the modulation of ocular reflexes during OVAR being primarily mediated by the otoliths in response to the sinusoidally varying linear acceleration along the interaural and naso-occipital head axis.

Highlights

There is evidence that the ocular reflexes during Off-Vertical Axis Rotation (OVAR) are mediated by the otoliths of the vestibular system and not the semicircular canals
Our results clearly demonstrate that ocular reflexes are modified by HTOT during constant velocity OVAR
Since the somatosensory inputs from the trunk are not modified when the subjects turned their head to the left or right, the change in phase with head turn confirm that ocular reflexes are primarily mediated by the otoliths

Summary

Introduction

There is evidence that the ocular reflexes during OVAR are mediated by the otoliths of the vestibular system and not the semicircular canals. Entirely dismissed[14,15,16], there is strong evidence that the modulation of eye movements induced during constant velocity OVAR primarily reflects otolith function. Some studies have indicated that somatosensory inputs may play a role in the eye movements induced by constant velocity OVAR. Whether somatosensory inputs or the otoliths primarily mediate the eye movements observed during constant velocity OVAR in humans has yet to be clarified. Because the somatosensory inputs from the trunk remain essentially the same when the head is turned to the right or the left, a shift in the phase of these ocular reflexes to remain aligned with the head in these conditions would reflect the extent to which these responses are otolith-driven

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