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Basic and Applied Aspects ofVestibular Function I.C. Hwang. N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press. Hong Kong. 1988 VESTIBULAR FACTORS INFLUENCING THE BIOMEDICAL SUPPORT OF HUMANS IN SPACE B.K. Lichtenberg Payload Systems, Inc., 66 Central St., Wellesley, MA 02181, U.S.A. Abstract This paper will describe the biomedical support aspects of humans in space with respect to the vestibular system. The vestibular system is thought to be the primary sensory system involved in the short-term effects of space motion sickness, although there is increasing evidence that many factors playa role in this complex set of symptoms. There is the possibility that an individual's inner sense of orientation may be strongly coupled with the susceptibility to space motion sickness. A variety of suggested counter-measures for space motion" sickness will be described. Although there are no known ground-based tests that can predict space motion sickness, the search should go on. The long-term effects of weightlessness on the vestibular system are still relatively unknown. Some preliminary data have shown that the otoconia are irregular in size and distribution following extended periods of weightlessness. The ramifications of these data are not yet known, and because the data were obtained on lower order animals, definitive studies and results must wait until the space station era when higher primates can be studied for long durations. This leads us to artificial gravity, the lasttopic of this paper. The vestibular systemis intimately tied to this topic since ithas beenshown onearth that exposure to a slow rotating room causes motion sickness for some period of time before adaptation occurs. Ifthe artificial gravity is intermittent, will this mean thatpeople will getsickevery timetheyexperience it?Thedatafrom many astronauts onreturn to earth indicate that a variety ofsensory illusions are present, especially immediately upon return to a I-g environment. Oscillopsia, or apparent motion of the visual surround during head motion along with inappropriate eye motions for a given head motion, indicates that there is much to be studied about the vestibular and eNS reaction to a sudden application of a steady state acceleration field like 1 g. From the above information it is obvious that the vestibular system has unique requirements when it comes to the biomedical support of spaceflight. This is not to say that other areas such as cardio-vascular, musculo-skeletal, immunological Keywords: vestibular system, space biomedicine, space adaptationsyndrome, artificial gravity, space station 176 Lichtenberg and hematological systems do not have their own unique requirements, but that possible solutions to one system can provide continuing problems to another system. For example, artificial gravity might be helpful for long-term stabilization of bone demineralization or cardio-vascular deconditioning, but might introduce a new set of problems in orientation, vestibular conflict and body motion in a rotating space vehicle. Introduction The topic of biomedical support of humans is very broad and encompasses such areas as environmental control systems, radiation protection, cardio-vascular stimulation, musculoskeletal maintenance, immunological system responses, human factors, behavior and psychology , and the vestibular system. This paper will address only the vestibular system and will divide the effects of weightlessness on the system into short-term (several weeks to several months) and long-term (over about six months) effects. These dividing lines are not firm, but rather stem from our current knowledge that there are short-term vestibular adaptation effects that take on the order of 3-4 days, after which the system seems to reach a new operating condition. For the long-term designation, we have very littleinformation, but it appears that any potential change in the system over the long-term must take months to years to occur. The vestibularsystem exists primarily to provideorientation ability, to stabilize oureyes during head and body motion, and to help control posture and locomotion. It consists of two sub-systems, the semicircularcanals which senseprimarily angular accelerations (yaw, pitch and roll) of the head; and the otolith organs which sense primarily linear accelerations, including the earth's gravity. It is impossible for the otoliths to distinguish between gravity and other linear accelerations. For example, to distinguish between a head tilt (left or right) with respect to earth's vertical and an acceleration to the left or right, one must use the semicircular canals to sense the rotation of the head which gives the cue that a head tilt has taken place. When a person goes into orbital flight'(microgravity), the net linear acceleration (gravity minus the centripetal acceleration of the...

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