Position and velocity responses from the otoliths and the canals: results from ESA's parabolic flights.

Background: Transient (i.e., phasic) bell-shaped vestibularly evoked potentials (VESTEP's) were recorded from the human scalp during whole body rotation about the vertical z-axis using a multi-axis rotary chair (yaw-motion). For pitch-motions about the interaural y-axis, however, a sustained (i.e., tonic) VESTEP was recorded, presumably because of the additional otolithic stimulation.

Hypothesis: During microgravity, only phasic VESTEP's were recorded because pitch-motions stimulate only the vertical semicircular canals without otolithic contamination.

Methods: The motion profile applied simulated the form of a natural smooth head movement ("raised cosine"). It was designed to minimize all possible sources of mechanical, electrical, and physiological artifacts. With this motion profile, seated subjects were tilted 90 degrees nose-down or 90 degrees nose-up about their interaural y-axis ("pitch") thus stimulating the vertical semicircular canals and the otoliths. In addition, the raised cosine velocity profile was applied during the microgravity phases of a total of 90 parabolic flight maneuvers, thus stimulating the vertical canals without additional otolithic stimulation.

Results: A transient (phasic) bell-shaped VESTEP was found in the Earth-bound laboratory for backward-tilts corresponding to the velocity profile used. For nose-down tilts, however, a sustained (tonic) negativity was found which matched the applied position profile. In microgravity, only the transient bell-shaped responses could be recorded irrespective of tilt direction.

Conclusion: These results are interpreted in terms of both a position response generated by pitch-down and a velocity response caused by pitch-up motions. This differentiation might prove to be a useful electrophysiological tool in oto-neurological diagnosis to distinguish between otolithic and canal disorders.