Basic and Applied Aspects of Vestibular Function

20. MIT/Canadian Spacelab experiments on vestibular adaptation and space motion sickness

Basic and Applied Aspects ofVestibular Fwncrion le. Hwang, N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 MIT/CANADIAN SPACELAB EXPERIMENTS ON VESTIBULAR ADAPTATION AND SPACE MOTION SICKNESS C.M. Oman*, L.R. Young*, D.G.D. Watt**, K.E. Money**, B.K. Lichtenberg***, R.V. Kenyon*+ and A.P. Arrott*** *Man Vehicle Laboratory, Massachusetts Institute ofTechnology, Cambridge, MA 02139, U.SA.; **Defense and Civil Institute ofEnvironmental Medicine, Downsview, Ontario, Canada; and ***Payload Systems, Inc., Wellesley, MA, 02181, U.SA. Abstract Experiments on sensory-motor adaptation to weightlessness and re-adaptation to 1 g were conducted on Space Shuttle/Spacelab Missions 1 and D-l by a team of investigators from MIT and Canada. Results from both missions are reviewed in the context of a sensory re-interpretation hypothesis and the conflict theory for motion sickness. Introduction The microgravityenvironmentofspaceflightprovideschallengingopportunitiesforresearch on sensory-motor adaptation. This paper summarizes results obtained in a series of interrelated experiments performed on Spacelab Missions 1 (November, 1983) and D-l (October, 1985) by a team of investigators from MIT and Canada. Methods and results are presented in more detail in a series of papers (Money et ai., 1984; Oman et ai., 1984, 1986; Young et al., 1984, 1986a; Arrottand Young, 1986; Kenyon and Young, 1986; Oman, 1986a, b; Watt et al., 1986). The experiments were designed to assess human sensory-motor adaptation to weightlessness and re-adaptation to earth's gravity, and simultaneously to examine the question: is space sickness a motion sickness? A sensory re-interpretation hypothesis forms the basis for ourexperiments, and provides a framework for interpreting many of the results. We assume that the functionally appropriate physiological adaptation to weightlessness should involve a re-interpretation of afferent signals originating in the gravireceptors, particularly in the otolith organs. Our working hypotheses (Young et al., 1976, 1986a, b, c; Oman, 1983) were: + Present address of Dr. R.V. Kenyon is Department of Electrical Engineering and Computer Science, University of Illinois, Chicago, IL 60680, U.S.A. Keywords: vestibular, spatial orientation, motion sickness, space medicine 184 Oman et al. 1. thatas aresultofsensory-motorexperienceinweightlessness,otolith afferent signals are centrally inhibited or re-interpreted as indicating head translation rather than tilt, 2. that longitudinal (Z axis) reflex sensitivity to 'falling' is decreased, 3. that visual and tactile cues should play an increasing role in spatial orientation. We believe these central adaptations will prove to underlie the amelioration of space sickness, as predicted by a sensory-motor conflict theory for motion sickness (Reason, 1978; Oman, 1982).On these missions, we wanted to identify the specific stimuli which cause space sickness. What are typical symptoms and signs? Is space sickness a motion sickness? What is the role of fluid shift? What can be done to improve prediction, prevention and treatment? Prior to Spacelab 1, detailed space sickness case histories were not available in the open scientific literature. Results obtained to date demonstrate that adaptation to weightlessness is a complex process, but are generally consistent with our working hypotheses. Responses to Linear Acceleration - Sled Experiments Oculomotor and perceptual responses to linear acceleration were explored using a rail mounted, servo-controlled linear acceleration sled. On both missions, pre-flight and postflight experiments were conductedina ground-basedlaboratory with the subjects uprightand acceleration applied laterally. On the D-l mission, in-flight experiments were conducted on two crew members on the first and sixth days of the seven-day mission utilizing the ESA Vestibular Sled. Both lateral and longitudinal acceleration stimuli were used. In-flight sled experiments showed no dramatic change in time to detect linear acceleration onset. Small step accelerations (0.001 g to0.08 g) were applied inrandomized directions, Time to Detect Step Accelerations j lnfllgnt .. tPn,fl1gflt eas.lm. 3 2 0.1 0.02 0.01 0.006 Acceleration Step lsI Fig. 1. 0-1 Sled Experiments: Time to detect a step of linear acceleration pre-flight (upper line) and in-flight (lower line). Results of both subjects for both Yand Z axes are grouped. Bars indicate +/- one standard deviation from the mean. MIT/Canadian Spacelab Experiments on Vestibular Adaptation 185 and the correctness and latency ofdetection were recorded. The average time to detect a step oflinear acceleration vs. acceleration level is shown in Figure 1for two D-I mission subjects. Although the in-flight average time to detect is never greater than the pre-flight average at any level ofacceleration, the...