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Journal of Vestibular Research, Vol. 8, No. I, pp. 57-59, 1998 Copyright CO 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0957-4271/98$19.00 ELSEVIER + .00 Pll S0957-4271(97)00039-6 Review THE RELATIVE ROLES OF THE OTOLITH ORGANS AND SEMICIRCULAR CANALS IN PRODUCING SPACE MOTION SICKNESS Donald E. Parker Department of Otolaryngology-HNS, University of Washington, Seattle, Washington Reprint address: Donald E. Parker, Box 357923, Department of Otolaryngology-HNS, University of Washington, Seattle , WA 98195-7923 . Tel : (206) 285-7528; Fax: (206) 616-1828; E-mail: [email protected] o Abstract Intljgbt and post.lauding ''immu nity" to the "coriolis sickness susceptibility test", observed during the Skylab M131 experiment, suggests that the otolith organs play a major role in space motion sickness (SMS). This view is supported by the report that ocular counter-torsion asymmetries correlate with SMS incidence and severity. Further data indicate that sensory-motor adaptation to microgravity includes a process whereby central interpretation of otolith signals is biased from ''tilt'' toward translation. However, unexpected responses to linear acceleration suggest the importance of graviceptors distributed throughout the body in addition to the vestibular otolith organs. Research is needed to assess distributed graviceptor effects. © 1998 Elsevier Science Inc. o Keywords - '. ine effeet5 of pwionged micrograviLy ollIIlotion sickness produced by this procedure. The surprising finding from this experiment was that astronauts who reported motion sickness during CSSI testing preflight on earth were nearly "immune" in orbit. Further, this immunity persisted for up to 18 days postflight before preflight susceptibility returned. The main importance of these findings was to focus attention on the otolith receptors. There was no reason to expect that, in contrast to the otolith organs, canal stimuli or function would be radically altered by microgravity. This suggests that a canal-otolith conflict, absent or much reduced in microgravity, is the main contributor to the motion sickness elicited by CSSI testing on earth. The M131 results also suggested revision of the view that orbital flight produced "physiologicaJ deafferentation of the otoliths." 1L should be noted that CSSI testing was not begun until 5 da~'s in orbit hac elapsed : conse- graviceptors. Orbital Flight Reduced Motion Sickness Elicited by Coriolis Sickness Susceptibilit~,· Index Testin~ i I I I .I l I I opment was nOl observed. Further. po~tt1ighi CSSI leSi immunity 'v\'a~ not (lb~e"ved during informal observations following the 6-day STS-S Space Shuttle mission. The Coriolis Sickne~~ Susceptibility Index (CSS1) motion sicknes~ lest developed at the Pensacola Naval Aeromedical Laboratory during the 1960s requires subjects to move their heads in pitch and roll while seated upright on a chair rotating around an earth-vertical axis. This procedure produces "cross-coupled" stimulation to the canals, which was thought to cause the motion sickness produced by the CSSI test. The Skylab Ml31 experiment (1) was undertaken to exam- Ocular Counter-Torsion Predicts Space Motion Sickness Diamond and Markham (2) developed procedures to assess precisely ocular counter-torsion '. 57 D. E. Parker 58 elicited by both static and dynamic roll-axi s stimulation. To examine differences in otolith responses to altered gravity, they recorded torsional eye movements during parabolic flight on NASA's KC-135 aircraft. Subjects were placed in a rotatable chair upright, left-ear down or right-ear down (with respect to the gravito-inertial force vector). Torsional eye movements were determined during the hypogravity (approximately 0 G) and hypergravity (aboLlt 1.8 G) segments of the parabolas. Oi fferences in the mag.iilL.!tie j: L,r:<l-, t: L.l ~ ~ L'L' 1~; :.i; :..:: 1~~ 0- ,.\';~ ,: ~h e hy pogra\ It:' ,1I1ci l y pergr ~l\ ny :tJl1Jil!On:-, ;prrelaleLi with SiVlS ·,e':erir~..11lt.! !l.Irauoll rc,:nned by [he S astronaLlts during orbitai rliglH. :-./1) correlation between SMS .tnd torsional Jisconjugacy evoked by roll on earth was noted. These findings support the hypothesis that SMS is p:.u1ly due to otolith asymmetry. von Baumgarten (3) suggested that the left and right otolith receptors would ordinarily differ in mass and other response properties, and that these asymmetries would normally be compensated by central processes. Alteration of ambient gravitoinertial force may disturb this compensation and result in SMS. respect to gravity. In microgravily, central interpretation of graviceptor signals as indicating tilt would lead to inappropriate compensatory responses. Consequently, sensory-motor adaptation to microgravity appears to include a reinterpretation of graviceptor signals such that all are taken to indicate translational motion (4,5). This has become known as the "tilt-translation reinterpretation . hypothesis" of s~nsory-motor adaptation [0 l11icrogravity . Responses to Linear A~c~jeration hl-mghi:~md Post-night Do Confirm Predictions ~O{ As part of the Microgravity Vestibular Investigations (MVI), astronauts reported perceived selforientation and self-motion during and after exposure to sustained constant angular velocity stimulation produced by a rotator. Due to space constraints, the subjects were placed on the rotator eccentrically with their heads about 0.5 m from the axis of rotation; this resulted in a maximum linear acceleration at the otoliths of 2.2 rnIs 2 aligned with the longitudinal (Z) head/body axis. Based on the stimuli at the otoliths, self-inversion was predicted. A major finding from MVI was Sensory-Motor Adaptation to the lack of reported self-inversion inflight durWeightlessness Includes Reinterpretation ing "pitch" and "roll" ramp stimulus profiles (6). of Graviceptor Signals Failure to report self-inversion may have resulted from the non-uniform force gradient pro-----------~s possible changes in Ijnear:mQti~du_c.e_cLby the eccentric placement of the astrodetection thresholds following orbital flight, nauts on the rotator. In microgravity, the vector subjects were exposed to translational oscilla- directions at the head and feet were in opposi- pected finding from this experiment was that pure roll stimulation elicited a complex perceived self-motion path that included a strong translation component immediately post-landing but not preflight (4). The basic finding of translational scene or self-motion evoked by roll head motion has repeatedly been confirmed by observations performed in the Space Shuttle middeck during entry and immediately after landing. Signals from graviceptors, including the otoliths, are fundamentally ambiguous: a signal may indicate accelerated translation or tilt with ceptors. The MVI findings are reports by Mittelsteadt (7) that graviceptors located in the trunk, and perhaps associated with displacement of the viscera, may have an even stronger influence on perceived self-orientation than do the otolith graviceptors. Conclusion As reported elsewhere in this issue, vestibular receptors affect autonomic function as well as eye movements, posture, and locomotion, and Otolith Organs and Semicircular Canals in SMS perceived self-orientation and self-motion. Development of "countermeasures" for the many disturbances experienced during transitions between different gravito-inertial force environments requires more complete understanding than is currently available of linear acceleration effects in microgravity as well as on earth. This understanding should also inform research and intervention strategies for falling and other agerelated disturbances. Future research should focus on graviceptors distributed throughout the body as well as those in the vestibule. REFERENCES I . Graybiel A. Miller EF. Hornick JL. Experiment M 131. Human vestibular function. In: Johnson RS , Dietlein LF, 59 editors. Biomedical results from Skylab (NASA SP377). Washington: National Aeronautics and Space Administration; 1977. p74-103. 2. Diamond SG, Markham CH. Validating the hypothesis of otolith asymmetry as a cause of space motion sickness. In : Cohen B. Tomko DL, Guedry F. editors. Sensing and controlling motion. Ann N 'Y Acad Sci 1992: 656. p725-31. 3. von Baumganen RJ. Thumler RA. A model for vestibular function in altered gravitational states. Life Sci Space Res 1978; 15:161-70. 4. Parker DE, Reschke MF, Arrot! AP, Hornick JL. Lichtenberg BK. Otolith tilt-translation reinterpretation following prolonged weightlessness: implications for preflight-training. Aviat Space Environ Med 1985:56: 601-6. aptation to earth's 6. Benson AJ . Guedry FE. Parker DE. Reschke ME Microgravi ty vestibular investigations: perception of self-orientation and self-motion. J Vestib Res (in press). 7. Mittelstaedt H. Somatic versus vestibular gravity reception in man. In: Cohen B. Tomko DL. Guedry F. editors. Sensing and controlling motion. Ann N Y Acad Sci 1992;656. p 124-39. "i I· I',. ~·I