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ARE THERE TWO MECHANISMS UNDERLYING SPACE MOTION SICKNESS?    D.G.D. Watt.   Aerospace Medical Research Unit, McGill University, Montreal, Canada

                Motion sickness likely occurs whenever there is a conflict between actual vestibular (and other) inputs and those predicted by the CNS based on past experience. Conflicts can be caused by (1) damage or disease, (2) acceleration environments that go beyond normal physiological range, and (3) altered processing of labyrinthine signals resulting from prolonged vestibular suppression. Previous studies have assumed that the main cause of space motion sickness is abnormal vestibular signals generated during movement in microgravity (mechanism 2). This study’s objective was to determine if altered central processing (mechanism 3) also contributes.  Each in-flight experiment lasted 43 min. and consisted of monitoring eye, head and upper torso rotation while 4 astronauts went about their normal activities. Post-flight, the data were filtered to reduce noise, artifacts were identified and removed, and eye position was converted to eye velocity. Finally, eye velocity was plotted relative to head velocity during periods of pure, yaw-axis head movement.  During spontaneous, self-generated head rotations, the vestibulo-ocular reflex should keep the eyes stabilized relative to inertial space. Except during quick gaze refixations, eye velocity should be equal and opposite to head velocity. Analysis of Flight Day 1 data demonstrated a statistically significant decrease in eye stabilization, however. Furthermore, head velocities were mostly below 100є/sec, where visual tracking alone should have been able to maintain gaze on target, suggesting deliberate suppression of both visual and vestibular mechanisms. A more normal pattern was seen on Flight Days 7, 12, 13 and 15, and after landing.  Results suggest that the astronaut has much in common with a passenger reading in the back seat of a car. In both cases, vestibular systems operate in altered acceleration environments beyond the normal range defined by evolution. This produces motion sickness. In addition, both seem to suppress vestibular function. The passenger does this to keep his or her gaze stable relative to the book. Why the astronaut does it is unclear. Whatever the reason, excessive and prolonged suppression leads to temporary changes in vestibular function and this second mechanism also produces motion sickness.

(Supported by the Canadian Space Agency.)