Abstract:

The three-dimensional representation of body orientation by the central nervous system is dependent on both afferent and efferent information about ongoing and intended body motion. The range of factors that contribute to this representation and the range of types of computations involved are much wider than is generally believed. Some of this complexity becomes apparent from attempts to understand how oculomotor and vestibular function are affected by exposure to weightlessness in orbital flight and in the free-fall phases of parabolic flight. For example, on earth, during body rotation, if head movements are made out of the axis of rotation, Coriolis cross-coupling stimulation is produced; this stimulation evokes nystagmus and an aberrant sense of head and body motion, and, if repeated, rapidly elicits symptoms of motion sickness. By contrast, when astronauts were exposed to Coriolis cross-coupling stimulation in orbital flight (the Skylab M-131 experiment), they experienced little or no aberrant head motion and no symptoms of motion sickness, even though their preflight subjective and motion sickness responses were normal.

We have attempted to determine whether these changes were due in part to immediate changes in vestibulo- and sensory-motor function that occurs in weightlessness or whether they were attributable to gradual adaptive changes. The latter possibility arises because the Skylab astronauts were first tested in the M-131 experiment on or after Mission Day 6. Our approach has been to use parabolic flight maneuvers as a way of generating variations in gravitational force level, g (1 g is equivalent to the terrestrial gravitational force level). Repeated alternating periods of free fall, 0 g, and high-force level, approximately 1.8 g peak, of 20-25 sec duration were generated. Experimental observations were collected during these steady state force level periods.

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