Torsional vestibulo-ocular reflex during whole-body oscillation in the upright and the supine position I. Responses in healthy human subjects

In rhesus monkeys, the dynamic properties of the torsional vestibule-ocular reflex (VOR) are modified by otolith input: compared with torsional oscill ations about an earth-vertical axis (canal-only stimulation), the phase lea d observed at frequencies below 0.1 Hz is cancelled when the animals are ro tated about an earth-horizontal axis (canal-and-otolith stimulation); the g ains of the torsional VOR, however, are nearly identical in both conditions . To test whether or not canal-otolith interaction in humans is similar to that in rhesus monkeys, we examined ten healthy human subjects on a three-a xis servo-controlled motor-driven turntable. The subjects were oscillated i n upright or supine position in complete darkness over a similarly wide ran ge of frequencies (0.05-1.0 Hz) with peak velocities <40 degrees/s. Eye mov ements were recorded using the three-dimensional search coil technique. Com pared with the torsional vestibuloocular gains during canal-stimulation onl y (earth-vertical axis), the gains obtained during combined canal-otolith-s timulation (earth-horizontal axis) were significantly higher throughout the entire frequency range (P<0.05). The gain increased by 0.100+/-0.074 (SD), independent of frequency. During the earth-horizontal axis stimulation, th e phase remained always around zero, which is in contrast to the canal-stim ulation only, during which one finds an increasing phase lead as frequency decreases. We conclude that, in healthy humans as in rhesus monkeys, the ph ase lead from the canal signals at low frequencies is effectively cancelled by the otolith input. In contrast to rhesus monkeys, however, otolith sign als in healthy humans increase the gain of the torsional VOR at frequencies from 0.05 to 1.0 Hz. This normal database is crucial for the interpretatio n of results obtained in patients with vestibular disorders.