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Basic and Applied Aspects olVestibularFWlCtio" I.C. Hwang, N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 MACULAR BIOACCELEROMETERS ON EARTH AND IN SPACE M.D. Ross*, L. Cutler*, G. Meyer**, P. Vaziri* and T. Lam** *NASA-Ames Research Center, Moffett Field. CA 94035. U.S.A.; and **Sterling Federal Systems. Palo Alto. CA 94303. U.S.A. Abstract Spaceflight offers the unique opportunity to study linear bioaccelerometers (vestibular maculas) in the virtual absence of a primary stimulus, gravitational acceleration. Macularresearchinspaceisparticularly importanttoNASAbecause the bioaccelerometers are proving to be weighted neural networks in which information is distributed for parallel processing. Neural networks are plastic and highly adaptive to new environments. Combined morphological-physiological studies of maculas fIxed in space and following flight should reveal macular adaptiveresponses to microgravity, and their time-course. Ground-basedresearch, already begun, using computer-assisted, 3-dimensional reconstructions of macular tenninal fields will lead to development of computer models of functioning maculas. This research shouldcontinue inconjunction with physiological studies, including workwithmultichannelelectrodes.Theresults ofsuch acombinedeffort could usherin anew erainunderstanding vestibularfunctiononearthandinspace. They can also provide a rational basis for counter-measures to space motion sickness, whichmayprove troublesome as space voyagers encounter new gravitational fields on planets, or must re-adapt to 1 g upon return to earth. Introduction Before humans actually flew in space, there was greatfear thatspace would prove too hostile an environment to withstand. Among the predicted effects of weightlessness were those related to the balance end organs of the inner ear, which are finely tuned to earth's gravitational field. Some medical scientists believed that humans would become severely disoriented and unable to function in space (see Deitlein, 1977). The vestibular system was, therefore, looked upon as a prime candidate for study in space and high priority was given Keywords: macula, 3-D reconstructions, neural network, space adaptation 220 Ross et al. to it. These fears, however, did not materialize and interest waned. During the first suborbital flights, when astronauts were closely confined within the vehicle, there were no vestibularrelatedproblemsatall . Itwasonlylaterwhen spacecraftbecamelarger,permittingmovement (particularly of the head), that the syndrome called space motion sickness, or spaceadaptation syndrome, appeared. Even then, statistics compiled have shown that only about half the astronauts and cosmonauts become ill, and that the syndrome tends to disappear during the first 2-4 days of flight (Nicogossian and Parker, 1982). It is unclear in the remaining cases whether prophylaxis accounts for some of the resistance to the syndrome, or if some individuals are simply immune. A further interesting fact is that there does not appear to be a relationship between motion sickness in earth's gravitational field and proneness to space motion sickness. Thus, there is currently no means of assessing an individual's inclination to become sick in space through ground-based testing. Upon return to earth, there is a re-adaptation to the 1 g field which varies somewhat in symptoms and in severity from one individual to another (Homickand Miller, 1975; Hornick et ai., 1977). The period of re-adaptation is roughly proportional to that of the flight (Komilova et ai., 1979) as is also known to occur in sea-sickness. The finding that symptoms of re-adaptation are more universal in astronauts and cosmonauts than are those of space sickness would suggest that all individuals experience an adaptation to weightlessness, whether or not frank illness is a complement of the process. Thispaperwill consider recentfindings which indicate thatthe vestibularsystem is most worthy ofregaining its place ofimportancein theNASA spaceprogram. The system is highly significant because of its functional organization as a parallel processing, weighted neural network. Such networks are considered to be plastic and highly adaptive to new environmental factors. Maculas, the linear bioaccelerometers of the vestibular system, should, therefore, undergo adaptive changes in response to microgravity. Once adapted, they must re-adapt to earth's gravity, or adapt again to a different, partial gravitational field if that is encountered. This has long-term implications as we look forward to permanent stations on the moon, or to exploration of Mars. Of equal importance is the fact that macular tuning to various gravitational environments can serve as a relatively simple model in the study of central neural adaptation, which must occur as the brain resolves conflicts between visual, kinesthetic and vestibular information in space. Finally, maculas are amenable to computer modelling to predict, simulate, and test adaptive processes. The Peripheral Receptor as a Parallel Processor Recent research with long series of sections through rat maculas has...

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