NeurophysiologyDora Angelaki and William H. Paloski, Co-Chairs

Platform Presentations � Salons A, B, C, January 18

File# Title Authors308 Neurophysiology D. Angelaki and W. H. Paloski

180 Synaptic Ribbon Plasticity in Utricular and SaccularMaculae: New Clues to Functions?

M. D. Ross and J. Varelas

181 Orthostatic Intolerance and Autonomic CardiovascularChanges After Parabolic Flight

T. T. Schlegel, T. E. Brown, S. J. Wood,E. W. Benavides, R. L. Bondar, F. Stein,P. Moradshahi, D. L. Harm, J. V. Meck, P.A. Low

182 Vestibular Control of Sympathetic Activity H. Kaufmann, I. Biaggioni, A.Voustianiouk, A. Diedrich, F. Costa, M.Gizzi, S. Moore, T. Raphan, B. Cohen

183 The Role of Gravitoinertial Force Background, SpatialOrientation and Contact Cues in Perturbations ofReaching Movements by Coriolis Forces

P. DiZio and J. R. Lackner

184 Self-Motion System Frequency Response: Implicationsfor Cybersickness

D. E. Parker, H. H. L. Duh, J. O. Phillips,T. A. Furness

185 STS-90 Neurolab Experiments on the Role of Visual Cuesin Microgravity Spatial Orientation

C. M. Oman, I. P. Howard, T. Smith, A.C. Beall, A. Natapoff, J. E. Zacher, H. L.Jenkin

186 Visual Orientation in Unfamiliar Gravito-InertialEnvironments

C. M. Oman, I. P. Howard, W. L.Shebilske, J. S. Taube

187 Visually-Induced Adaptation of the TranslationalVestibulo-Ocular Reflex

M. Wei, H.-H. Zhou, D. E. Angelaki

188 Otolith and Vertical Canal Contributions to DynamicPostural Control

G. D. Kaufman, F. O. Black, C. C.Gianna, W. H. Paloski, S. J. Wood

189 Characterization of Sensory Integration and ControlStrategies That Regulate Human Postural Control inChanging Conditions

R. J. Peterka

190 Spatial Reorientation and Sensory-Motor Balance Controlin Altered Gravity

W. H. Paloski, S. J. Wood, G. D.Kaufman, F. O. Black, M. F. Reschke

191 Perception of Tilt (Somatogravic Illusion) in Response toSustained Linear Acceleration During Space Flight

B. Cohen, G. Clement, S. T. Moore, T.Raphan

192 Context-Specific Adaptation of Gravity-DependentVestibular Reflex Responses (NSBRI NeurovestibularProject 1)

M.Shelhamer, J. Goldberg, L. B. Minor,W. H. Paloski, L. R. Young, D. S. Zee

193 Locomotion After Long-Duration Spaceflight: AdaptiveModulation of a Full-Body Head and Gaze StabilizationSystem

J. J. Bloomberg, A. P. Mulavara, C.miller, P. V. McDonald, C. S. Layne, J.Houser, H. Cohen, I. B. Kozlovskaya

194 Recovery Trajectories to Perturbations DuringLocomotion

C. Wall and L. Oddsson

195 Maintaining Neuromuscular Contraction UsingSomatosensory Input During Long Duration Spaceflight

C. S. Layne, A. P. Mulavara, P. V.McDonald, C. J. Pruett, J. J. Bloomberg

196 Effect of Microgravity on Afferent Innervation C. D. Fermin, R. F. Garry, Y-P. Chen, D.Zimmer

197 The Effect of Spaceflight on the Ultrastructure of AdultRat Cerebellar Cortex

G. R. Holstein and G. P. Martinelli

198P Developing Future Countermeasures for the DetrimentalEffects of Space Flight: Role of Otolith Systems andResolution of Tilt/Translation

F. O. Black, S. J. Wood, C. C. Gianna,W. H. Paloski

199P Use of the Neurologic Function Rating Scale FollowingSpace Shuttle Flights

J. B. Clark and J. U. Meir

200P Varied Practice and Response Generalization as theBasis for Sensorimotor Countermeasures

H. S. Cohen, J. J. Bloomberg, C. Roller,A. Mulavara

201P Somatosensory Suppression of Re-Entry Disturbances P. DiZio and J. R. Lackner

202P Neurovestibular Aspects of Artificial Gravity H. Hecht and L. R. Young

203P Responses of Eye Movement Related Vestibular Neuronsto Linear Acceleration

W. M. King, W. Zhou, B. Tang

204P Influence of Sensory Integration on the NeuralProcessing of Gravito-Inertial Cues

D. Merfeld

205P Inflight Centrifugation as a Countermeasure forDeconditioning of Otolith-Based Reflexes

S. T. Moore, G. Clement, A. Diedrich, I.Biaggioni, H. Kaufmann, T. Raphan, B.Cohen

206P The Influence of Visual Rotational Cues on HumanOrientation and Eye Movements

L. Zupan, D. M. Merfeld, K. King

NEUROPHYSIOLOGYDora E. Angelaki, Ph.D.

William H. Paloski, Ph.D.

INTRODUCTION

The terrestrial gravitational field serves as an important orientation reference for human perception and movement,being continually monitored by sensory receptors in the skin, muscles, joints, and vestibular otolith organs. Cuesfrom these graviceptors are used by the brain to estimate spatial orientation and to control balance and movement.Changes in these cues associated with the tonic changes in gravity (gravito-inertial force) during the launch and entryphases of space flight missions result in altered perceptions, degraded motor control performance, and in some cases,�motion� sickness during, and for a period of time after, the g-transitions. In response to these transitions, however,physiological and behavioral response mechanisms are triggered to compensate for altered graviceptor cues and/or toadapt to the new sensory environment.

Basic research in the neurophysiology discipline is focused on understanding the characteristic features of and theunderlying mechanisms for the normal human response to tonic changes in the gravito-inertial force environment.These studies address fundamental questions regarding the role of graviceptors in orientation and movement in theterrestrial environment, as well as the capacity, specificity, and modes for neural plasticity in the sensory-motor andperceptual systems of the brain. At the 2001 workshop basic research studies were presented addressing:neuroanatomical responses to altered gravity environments, the neural mechanisms for resolving the ambiguitybetween tilting and translational stimuli in otolith organ sensory input, interactions between the vestibular system andthe autonomic nervous system, the roles of haptic and visual cues in spatial orientation, mechanisms for trainingenvironment-appropriate sensorimotor responses triggered by environment-specific context cues, and studies ofsensori-motor control of posture and locomotion in the terrestrial environment with and without recent exposure tospace flight.

Building on these basic research studies are more applied studies focused on the development of countermeasures tothe untoward neurophysiological responses to space flight. At the 2001 workshop applied research studies werepresented addressing issues related to the use of rotational artificial gravity (centripetal acceleration ) as a multi-system (bone, muscle, cardiovascular, and, perhaps, neurovestibular) countermeasure. Also presented was a clinicalstudy reporting on a new rating system for clinical evaluation of postflight functional neurological status.

The neurophysiology group met first on Wednesday evening in a two hour session moderated by Chuck Oman, JonClark, Tom Marshburn, and Jason Richards. The focus of this session was a discussion of the experiences of USastronauts participating in the Phase 1 (Mir Station) long-duration flight program. A draft manuscript on the subjectauthored by the moderators was distributed to all participants, and some novel video footage from the Mir station aswell as the ISS was presented. The main platform session, which is summarized in the next section, lasted throughoutmost of the day on Thursday. The session was very busy, with little time for questions following each presentation,and insufficient scheduled break time. Nevertheless, the session was well attended and achieved its primary goal ofpresenting the scope and depth of the entire NASA-NSBRI neurophysiology research program to all of itsparticipants.

SUMMARY OF PRESENTATIONS

Neuroanatomical StudiesRoss, MD and J Varelas. Synaptic Ribbon Plasticity in Utricular and Saccular Maculae: New Clues to Functions?

Ross and Varelas presented cellular evidence of vestibular plasticity during space flight. They reported that utricularand saccular maculae differ completely in their responses to weightlessness. Their results confirmed previousfindings of increased numbers of synapses in Type II hair cells of the utricular maculae, with Type I cells showinglesser increases. In the saccular maculae, however, they found that synapses in Type II cells remain relatively stablethroughout flight and postflight, while Type I cells fluctuate. From Washington University School of Medicine, Department of Anatomy and Neurobiology, St. Louis, MO (D.Angelaki) and NASA Johnson Space Center, Life Sciences Laboratories, Houston, TX (W. Paloski).

Holstein, GR and GP Martinelli. The Effect of Spaceflight on the Ultrastructure of Adult Rat Cerebellar Cortex.

Holstein and Martinelli also presented cellular evidence of vestibular plasticity during space flight. They reportedthat changes observed in the ultrastructure of Purkinje cells from the adult rat cerebellar cortex harvested 24 hrs aftershuttle launch suggest that space flight induces excitotoxic responses in Purkinje cells.

King, WM, W Zhou, and B Tang. Responses of Eye Movement Related Vestibular Neurons to Linear Acceleration(poster).

King et al. reported that specific classes of neurons were identified in the brainstem of awake behaving monkeys thatselectively process and transform otolith sensory inflow into an occulomotor command for gaze stabilization duringtranslation. These central vestibular neurons, characterized by their discharge pattern and anatomical connections,transmit otolith signals that are modulated by gaze. They point out that unlike semicircular canal reflexes, eyemovements produced by otolith-ocular reflexes depend on gaze direction and are inherently disjunctive.

Fermin, CD, RF Garry, YP Chen, and D Zimmer. Effect of Microgravity on Afferent Innervation.

Fermin et al. reported that some members of S100 Calcium Binding Proteins (CBP) family are expressed investibular afferents of chicken at 1g, and that the mRNA of certain mammalian CBP S100 isoforms sharedistribution characteristics with chicken isoforms. They further reported that the expression pattern changesfollowing mechanical injury of vestibular afferents and suggested that gene expression and protein distribution ofS100 CBP may be affected by altered gravity.

Otolith Ambiguity StudiesZupan, L, DM Merfeld, and K King. The Influence of Visual Rotational Cues on Human Orientation and EyeMovements (poster).

Zupan et al. investigated how the central nervous system separates the otolithic measurement of gravito-inertial forceinto estimates of gravity and linear acceleration. They measured eye movements and subjective roll in humansubjects during and after roll optokinetic stimulation about the subject's naso-occipital axis. They reported that, inaddition to a torsional optokinetic after-nystagmus observed for all orientations, a horizontal after-nystagmus wasobserved to the right following clockwise stimulation and to the left following counterclockwise stimulation. Theysuggest that these observations are in agreement with the GIF resolution hypothesis that suggests that subjective tiltillusion will induce a non-zero estimate of interaural linear acceleration, and therefore a horizontal translational VOReven in the absence of "true" linear acceleration.

Wei, M, HH Zhou, and DE Angelaki. Visually-Induced Adaptation of the Translational Vestibulo-Ocular Reflex.

Wei et al. reported on an experiment addressing the issue of visually-induced learning in the resolution of gravito-inertial forces. Preliminary results during cross-axis adaptation of the translational vestibulo-ocular reflex in primatessuggest that learning effects in the resolution of gravito-inertial forces are limited and probably more complex thanoriginally thought.

Merfeld, D. Influence of Sensory Integration on the Neural Processing of Gravito-Inertial Cues (poster).

Merfeld reported on plans for a future flight project (currently in definition phase) to investigate how certain neuralprocesses of sensory integration adapt when astronauts experience weightlessness. The specific processes to bestudied are those underlying the use of rotational cues to interpret ambiguous gravito-inertial cues via internalmodels.

Vestiblo-Autonomic Studies

Schlegel, TT, TE Brown, SJ Wood, EW Benavides, RL Bondar, F Stein, P Moradshahi, DL Harm, JM Fritsch-Yelle,and PA Low. Orthostatic Intolerance and Autonomic Cardiovascular Changes after Parabolic Flight.

Schlegel et al. reported that syndromes of orthostatic intolerance resembling those occurring after space flight wereinduced by a brief (2 hr.) parabolic flight. The mechanisms differed between vomiters and non-vomiters, andvomiting was associated with increases in R-R interval variability and carotid-cardiac baroreflex responsiveness,suggesting that the emetic reflex transiently increases resting fluctuations in efferent vagal-cardiac nerve traffic.

Moore, ST, G Clément, A Diedrich, I Biaggioni, H Kaufman, T Raphan, and B Cohen. Inflight Centrifugation as aCountermeasure for Deconditioning of Otolith-Based Reflexes (poster).

Moore et al. reported on plans for a future flight study to confirm results from the Neurolab mission suggesting thatin-flight centripetal accelerations may protect subjects from postflight orthostatic intolerance.

Kaufmann, HC, I Biaggioni, B Cohen, A Diedrich, M Gizzi, R Clark, F Costa, and D Saadia. Vestibular Influenceson Autonomic Cardiovascular Control.

Kaufman et al. reported that forward acceleration in the naso-occipital axis (as sensed by the otoliths) increasessympathetic efferent activity in the peroneal nerve.

Spatial Orientation Studies

Cohen, B, G Clément, ST Moore, and T Raphan. Perception of Tilt (Somatogravic Illusion) in Response to SustainedLinear Acceleration During Space Flight.

Cohen et al. reported results from the Neurolab mission suggesting that the somatogravic illusion induced bycentrifugation is maintained in space. They found that the illusion of tilt increased as flight continued and dependedon the magnitude of linear acceleration, suggesting that astronauts continue to assign the gravito-inertial accelerationas the spatial upright after adaptation to altered gravity.

Parker, DE, HBL Duh, JO Phillips, and TA Furness. Self-Motion System Frequency Response: Implications forCybersickness.

Parker et al. examined the frequency response of the visual self-motion system and found a motion frequency wherethe summed response of the visual and inertial self-motion systems was maximized. Their data support thehypothesis that conflicting visual and inertial motion cues at this "cross-over" frequency would be more likely toelicit sickness than conflicting cues at a higher frequency.

Oman, CM, IP Howard, T Smith, AC Beall, A Natapoff, JE Zacher, and HL Jenkin. STS-90 Neurolab Experimentson the Role of Visual Cues in Microgravity Spatial Orientation

Oman et al. reported using a Virtual Environment to record the subjective vertical in 4 astronauts during flight. Theyfound that astronauts became more dependent on dynamic visual cues, and some also on static visual cues. He alsoreported that the subjective vertical is labile and can influence figure recognition and shading interpretation.

Oman, CM, IP Howard, WL Shebilske, and JS Taube. Visual Orientation in Unfamiliar Gravito-InertialEnvironments.

Oman et al. also described results of a collaborative NSBRI research project on human visual orientation cues inhumans and animals. They reported that performance of humans in 3D spatial memory experiments correlates withability to mentally rotate 2D and 3D objects, and improves with training. They also reported that parabolic flightstudies of rat head direction cells show that cells continue to respond in 0-G, and occasionally show changes inpreferred direction that correspond to visual reorientation illusion onset in humans.

DiZio, P and JR Lackner. The Role of Gravitoinertial Force Background, Spatial Orientation and Contact Cues inPerturbations of Reaching Movements by Coriolis Forces.

DiZio and Lackner reported that rapid adaptation to rotating artificial gravity environments is possible. They alsoreported that the motor effects of the rate of adaptation to Coriolis force perturbations are equivalent in 2g, 1g, and0g force backgrounds, but disorientation and motion sickness elicited by head movements during body rotation areless severe in low force backgrounds. Finally they reported that non-supportive contact with the environment duringvoluntary movement is a critical orientation cue driving adaptation.

DiZio, P and JR Lackner. Somatosensory Suppression of Re-Entry Disturbances (poster).

DiZio and Lackner also reported on plans for a future flight study to assess the role of non-supportive contact withthe environment (haptic inputs) during voluntary movement in spatial orientation and adaptation to altered gravito-inertial environments.

Context-Specific Adaptation Studies

Shelhamer, M, J Goldberg, LB Minor, WH Paloski, LR Young, and DS Zee. Context-Specific Adaptation of Gravity-Dependent Vestibular Reflex Responses (NSBRI Neurovestibular Project 1).

Shelhamer et al. reported that during parabolic flight the magnitude of gravito-inertial force can be used as a contextcue for switching between adapted saccade states. They found evidence for retention of this adaptation after 8months. They also reported that the gain of the translational LVOR can be made context-specific, using head tilt as acontext cue and that saccadic eye movements can be adapted in a context-specific manner, using a number ofdifferent context cues. For interaural translations, they found head roll to be a more effective context cue than headpitch. Finally, they reported that sensorimotor adaptation to head movements during short-radius centrifugation (23rpm, 1 g at the feet) can be induced and retained for at least a week.

Cohen, HS, JJ Bloomberg, C Roller, and AP Mulavara. Varied Practice and Response Generalization as the Basisfor Sensorimotor Countermeasures (poster).

Cohen et al. presented preliminary data demonstrating that variable context adaptation may result in more rapidadaptive responses to novel environments through response generalization. They also described a future ISS studybeing planned to develop sensorimotor training regimens that promote adaptive generalization of locomotor functionas a means of facilitating the adaptive transition between gravitational environments.

Paloski, WH, SJ Wood, GD Kaufman, FO Black, and MF Reschke. Spatial Reorientation and Sensory-MotorBalance Control in Altered Gravity.

Paloski et al. described a future flight study that will examine the fragility of the postural readaptation response bychallenging crewmembers during postflight recovery with unusual z-axis acceleration created using a short radiuscentrifuge.

Posture and Locomotion Studies

Peterka, RJ. Characterization of Sensory Integration and Control Strategies that Regulate Human Postural Controlin Changing Conditions.

Peterka reported that the human balance control system relies predominantly on proprioceptive cues during quietstance, but becomes increasingly reliant on graviceptor (vestibular) cues when balance is perturbed. He also showedthat loss of vestibular function profoundly alters and limits a subject�s ability to utilize his/her remaining visual andproprioceptive cues, suggesting that reinterpretation of vestibular cues following space flight might be expected toproduce similar deficits.

Bloomberg, JJ, AP Mulavara, C Miller, PV McDonald, CS Layne, J Houser, H Cohen, and IB Kozlovskaya.Locomotion After Long-Duration Space Flight: Adaptive Modulation of a Full-Body Head and Gaze StabilizationSystem.

Bloomberg et al. reported that after space flight, astronauts show reductions in dynamic visual acuity duringlocomotion. These data, along with supporting ground-based studies, reveal the existence of a full-body, gazestabilization system that exploits the multiple degrees of freedom available during locomotion to help maintain clearvision during body movement.

Layne, CS, AP Mulavara, PV McDonald, CJ Pruett, and JJ Bloomberg. Maintaining Neuromuscular ContractionUsing Somatosensory Input During Long Duration Spaceflight.

Layne et al. reported that application of pressure to the feet of free-floating astronauts enhanced the stereotypicallower limb neuromuscular activation associated with rapid arm movements. This suggests that the plantar sensoryinputs can be used to enhance motor unit activation and may therefore be used as a countermeasure to inflight muscledegradation.

Wall, C and L Oddsson. Recovery Trajectories to Perturbations During Locomotion.

Wall and Oddsson described a novel method to study perturbations of gait during steady locomotion by giving small(5�10 cm) disturbances to one foot during the support phase while recording the response with optical trackers.Preliminary results demonstrated a vestibular dependence by showing that normal subjects recover from theperturbations in 3�4 steps, while a pilot Labyrinthine Deficient subject required a much longer recovery time.

Artificial Gravity Precursor Studies

Kaufman, GD, FO Black, CC Gianna, WH Paloski, and SJ Wood. Otolith and Vertical Canal Contributions toDynamic Postural Control.

Kaufman et al. reported that 90-minute hypergravity (1.4g) stimulus (roll plane centripetal acceleration) inducespostural and subjective vertical changes in normal and some vestibular deficient subjects.

Black, FO, SJ Wood, CC Gianna, and WH Paloski. Developing Future Countermeasures for the Detrimental Effectsof Space Flight: Role of Otolith Systems & Resolution of Tilt/Translation (poster).

Black et al. described a new study that will establish the extent to which otolith-mediated tilt and translationresponses can be adapted at different stimulus frequencies, and then examine whether subjects can 'dual adapt' toaltered sensory environments using the orientation of gravity to provide context.

Hecht, H and LR Young. Neurovestibular Aspects of Artifical Gravity (poster).

Hecht and Young described a new multicenter study investigating whether head and body movements during highrate artificial gravity are tolerable and how such artificial gravity can be implemented most efficiently. The study willalso investigate methods to minimize the undesirable side-effects of neurovestibular adaptation associated withintermittent artificial gravity.

Clinical Studies

Clark, JB and JU Meir. Use of Neurologic Function Rating Scale Following Space Shuttle Flights (poster).

Clark and Meir reported that a Neurological Function Rating Scale has been designed for clinical assessment ofneurological dysfunction associated with space flight. Over 100 crewmembers have now been rated on landing day,and the most severe deficits observed were in gait station and occulomotor disturbances.

IMPLICATIONS FOR FUTURE RESEARCH

The critical path research plan defines the following five risks (in order of importance) for neuro-vestibulardiscipline:

1. Disorientation and inability to perform landing, egress, or other physical tasks, especially during/after g-levelchanges.

2. Impaired neuromuscular coordination and/or strength.3. Impaired cognitive and/or physical performance due to motion sickness symptoms or treatments, especially

during/after g-level changes4. Vestibular contribution to cardioregulatory dysfunction.5. Possible chronic impairment of orientation or balance function due to microgravity or radiation.

The current NASA-NSBRI research program addresses each of these risks to some degree, and of the 24 criticalquestions posed beneath these risks, the current program addresses 21. Thus, the current program appears to havereasonable breadth.

The neuroanatomical results from the peripheral vestibular and cerebellar regions presented at the workshop areextremely important, providing limited anatomical evidence for the long-noted behavioral adaptive responses.Further work should be supported both in tracking the anatomical changes that occur as a function of space flight andin correlating the observed anatomical changes to concomitant behavioral changes.

The vestibulo-autonomic studies presented at the workshop just scratch the surface of the field. Nevertheless, theydemonstrate that the vestibular system may exert important influences on both the cardioregulatory system and themotor control system. Further work should be supported in both areas, preferably by cross-disciplinary teams.

The context-specific adaptation studies and related artificial gravity precursor studies are providing importantbackground evidence, in humans, supporting the development of intermittent, rotational artificial gravity as a multi-system countermeasure. NASA-NSBRI should begin funding multi-disciplinary groups to develop and test specificartificial gravity protocols in ground-based studies, and, concomitantly, should begin developing the hardware andinfrastructure required for flight testing of an artificial gravity countermeasure.

SYNAPTIC RIBBON PLASTICITY IN UTRICULAR AND SACCULAR MACULAE:NEW CLUES TO FUNCTIONS?

*Muriel D. Ross and **Joseph Varelas, *The University of New Mexico Health Sciences Center, Albuquerque,N.M. ** Lockheed-Martin and NASA Ames Center for Bioinformatics, Moffett Field, CA

INTRODUCTION

Research into the effects of weightlessness on rat vestibular maculae has consistently shown that ribbon synapses inhair cells of the utricular maculae exhibit statistically significant changes in number, kind, and distribution when ratsare exposed to space flight (Ross, 1993, 1994, 2000). Synaptic plasticity was most evident when all type II hair cellsof these maculae were considered and was confined to type II cells in just the complete hair cells. Analysis of theco-variance of the multiple variables (number, rod or sphere, pairs and groups) by the MANOVA feature ofSuperANOVATM software demonstrated further that day and weightlessness both had statistically significant effectson type II hair cells (Ross, 2000). These results were obtained from the posterior portion of the utricular macula anddid not include the striola. For Neurolab, the striola and parastriolar area internal to the striola (pars interna) werestudied. The Neurolab striolar data in general support the findings in hair cells of the utricular maculae, but effects inthe saccular maculae differed. In saccular maculae, ribbon synapses in type I cells fluctuated while synapses in typeII hair cells remained relatively stable throughout the flight and up to postflight day 2. Only differences betweenflight days 2 and 14 in ribbon synapses of type I hair cells were statistically significant. It cannot be argued that thelack of significant differences in type II hair cells was due to inner ears utilized since utricular and saccular sampleswere matched for the same rats in two cases.

The results in the saccular maculae raise the interesting question whether the anatomical findings signify verydifferent functions for the two maculae, even though both are subject to stimulation by gravitoinertial forces. Thisreport provides the data for the utricular and saccular maculae of the Neurolab experiment and compares thefindings with those previously obtained on SLS-1 and SLS-2. It also discusses the findings in light of previousphysiological data that indicated different functions for the two maculae (Fernandez et al., 1976a,b).

CURRENT STATUS OF RESEARCH

MethodsThe Fisher strain of rats was used for this experiment. Rats were euthanized by decapitation on the ground on daytwo of flight (Basal), in flight on flight days 2 (FD2) and 14 (FD14), and postflight on days 2 (PF2) and 14 (PF14).Labyrinths were quickly removed from dissected temporal bones, fixed by immersion, and prepared for electronmicroscopy as described previously (Ross, 2000). Unfortunately, only one of the four FD2 utricular maculae madeavailable to us proved useful for electron microscopic study. Study of the utricular maculae was limited, therefore,to one Basal, one FD2 and one FD 14 sample. Attention then turned to the saccular macula. Study of the saccularmaculae is incomplete at this time. Two maculae from Basal, FD2, FD14 and PF2 will be analyzed. This Abstract isbased on two samples from FD2 and FD14 and one from a Basal and another from a PF2 rat which have beenstudied thus far in 50 serial sections cut at 150 nm. The striolar area is identified in the rat by the presence ofmyelination to the calyx in some afferents. In the case of a FD2 utricular and a FD2 saccular sample, an additionalset of 50 sections that crossed from the striola into pars interna was studied, to learn whether differences in synapticcounts and other properties would be evident. Analysis of statistical significance was accomplished using theANOVA features of SuperANOVATM software. All procedures used in this experiment were reviewed by theAnimal Use Committee established at NASA Ames Research Center and are in compliance with the Guide for theUse of Laboratory Animals and the Animal Welfare Act.

ResultsUtricular macula: Mean values for number of ribbon synapses in type I cells of the utricular striolar area were asfollows: Basal; 2.034; FD2, 3.512; and FD 14, 2.400. The increment in synaptic mean value in ribbon synapses fromthe Basal to FD2 was significant (p< 0.0044) as were differences in values for sphere-like ribbons (p< 0.0002). FD2mean values for total synapses (p< 0.0408) and spheres (p< 0.0379) differed from FD14 values. For the parastriolar

area, pars interna, the mean value of synaptic ribbons was 3.543, differing significantly from the basal (p< 0.0013)as did also sphere-like ribbons (p< 0.0014). Set two of FD2 also differed significantly from FD14 in total synapses(p< 0.0209). and groups (p< 0.0447).

For type II hair cells, the Basal mean value for ribbon synapses at the striola was 4.744. For FD2, the mean valuewas 9.000 and for FD14 it was 6.884. The differences between synaptic means between the Basal and FD2 weresignificant for total synapses (p< 0.0001), spheres (p< 0.0034), rods (p< 0.0016) and pairs (p< 0.0003). FD14differed from the Basal in total synapses (p< 0.0306), spheres (p< 0.0093), and pairs (p< 0.0074). For set two FD2,the mean value of total synapses was 7.929. This value differed from the Basal (p< 0.0023) as did sphere-likeribbons (p< 0.0008) and pairs (p< 0.0012). The two series from FD2 were essentially similar.

Saccular macula: In type I cells of the saccular macula, mean values were: Basal, 3.545; FD2, 2.646; FD14, 3.788;PF2, 3.250. FD2 differed significantly from FD14 in total synapses (p< 0.0277) and in spheres (p< 0.0074). In typeII cells, the mean values of synaptic number were as follows: Basal: 6.939; FD2, 6.317; FD14, 6.382, PF2, 6.480.None of these or other mean values (pairs, etc.) differed significantly.

ConclusionsFindings in the striolar area of the utricle were essentially similar to those obtained from the posterior part of themacula on SLS-1 and SLS-2. That is, synapses increased in the hair cells, but changes were greater overall in thecase of type II cells. Nevertheless, the results in the striolar area of both the utricle and the saccule differ from thoseobtained previously; i.e., in type II hair cells, synapses had doubled to 11.4±7.2 on day 13 of a 14 day flight (Ross,2000). This is not surprising since the striola differs morphologically from other portions of the macula inorganization of hair cells, afferents and processes. In the saccular samples of this experiment, in contrast to theutricular, ribbon synapses in type I cells had declined by FD2 but fluctuated on FD14 and PF2. In type II hair cells, aslight decline in synaptic mean occurred by FD2 and was maintained through PF2. None of the changes in type IIcells was statistically significant. The fact that in two cases saccular and utricular maculae came from the same innerears precludes a difference in findings due to sampling. The conclusion thus far is that saccular and utricularmaculae differ in responses to weightlessness, with utricular type II hair cells showing the greater plasticity inweightlessness. This result may be related to functional differences described by Fernandez and Goldberg (1976a,b).Their experimental findings indicated that utricular afferents were most responsive to static tilt in the X direction(left-right) while saccular afferents were primarily sensitive to Z direction (vertical or dorsoventral) tilt. Saccularmaculae had a lower sensitivity to static tilt. Present and previous findings (Ross, 2000) additionally show thatweightlessness has differing effects on striolar and posterior portions of the utricular macula. This is likely due tovarying morphological features of receptive fields. The resultant of gravitoinertial vectors acting on type I and typeII hair cells of a receptive field, plus parallel processing by many afferents, determines the message delivered tocentral sites. Type II cells may be particularly sensitive to static stimuli and gravity while type I cells may be moresensitive to phasic stimuli and translational accelerations (Ross, 2000). According to this hypothesis, the maculaeuse two kinds of receptors as comparators to determine the afferent’s signal, with morphological variations fromlocation to location (including macular geometry and otoconial loading) contributing to the outcome.

FUTURE PLANS:Work is to be completed on postflight (PF2) and Basal samples. All data will be subjected to analysis of variance.

REFERENCESFernandez C and JM Goldberg (1976a) Physiology of peripheral neurons innervating otolith organs of the squirrelmonkey. I. Response to static tilts and to long-duration centrifugal force. J Neurophysiol 39:970-984.Fernandez C and JM Goldberg (1976b) Physiology of peripheral neurons innervating otolith organs of the squirrelmonkey. II. Directional selectivity and force-response relations. J Neurophysiol 39:985-995.Ross MD (1993) Morphological changes in rat vestibular system following weightlessness. J Vestib Res 3:241-251.Ross, MD (1994) A spaceflight study of synaptic plasticity in adult rat vestibular maculas. Acta Otolaryngol(Stockh) Suppl 516:1-14.Ross MD (2000) changes in ribbon synapses and rough endoplasmic reticulum of rat utricular macular hair cells inweightlessness and 1-g environments. Acta Otolaryngol (Stockh.) 120:490-499.

INDEX TERMS

Ribbon synapses, vestibular system, hair cells, saccule, utricle, macula, synaptic plasticity, Neurolab

Orthostatic Intolerance and Autonomic Cardiovascular Changes after ParabolicFlight

Todd T. Schlegel1, Troy E. Brown2, Scott J. Wood3, Edgar W. Benavides2, Roberta L. Bondar4, Flo Stein5,Peyman Moradshahi4, Deborah L. Harm1 , Janice V. Meck1 and Phillip A. Low6

1Life Sciences Research Laboratories, National Aeronautics and Space Administration, Johnson Space Center,Houston, Texas; 2Wyle Laboratories, Houston, Texas; 3Baylor College of Medicine, Houston, Texas; 4RyersonPolytechnic University, Toronto, Ontario, Canada; 5Maple Lake Non-Invasive Laboratory, Farmington, New Mexico;and 6Autonomic Reflex Laboratory, Mayo Foundation, Rochester, Minnesota. E-mail: tschlege@ems.jsc.nasa.gov

INTRODUCTION: It is not clear that orthostatic intolerance (OI) in returning astronauts is strictly

contingent upon prolonged exposure to microgravity. To gain insight into acute conditions that may

exacerbate postspace-flight OI, we investigated the effects of brief parabolic flights—and of parabolic

flight-induced vomiting—on orthostatic tolerance and autonomic cardiovascular function. CURRENT

STATUS OF RESEARCH: Methods: R-R interval and arterial pressure power spectra, carotid-cardiac

baroreflex and Valsalva responses, and tolerance to 30 min of 80-degree head-up tilt (HUT) were measured

in 16 healthy subjects both before and after brief (2 hr) parabolic flights in the seated position. Results:

After parabolic flight: 1) the incidence of OI increased fourfold, with 8 of 16 subjects unable to tolerate 30-

min of HUT, compared to 2 of 16 subjects before flight; 2) 6 of 16 subjects vomited; 3) new intolerance to

HUT was associated with exaggerated falls in total peripheral resistance (P<0.05), whereas vomiting was

associated with increased supine R-R interval variability and carotid-cardiac baroreflex responsiveness

(P<0.05); and 4) the mode of new OI differed in subjects who did and did not vomit, with newly-intolerant

Vomiters experiencing comparitively isolated upright hypocapnia and cerebral vasoconstriction and newly-

intolerant non-Vomiters developing signs and symptoms reminiscent of the clinical postural tachycardia

syndrome. Conclusions: Results suggest first, that syndromes of OI resembling those occurring after space

flight can occur after a brief (2 hr) parabolic flight; and second, that recent vomiting can influence not only

responses to HUT, but also the results of supine tests of autonomic cardiovascular function commonly

measured in returning astronauts. FUTURE PLANS: An investigation of autonomic cardiovascular

responses to rapid HUT (in both pitch and roll) in labyrinthine-deficient vs. age- and gender-matched

healthy individuals. INDEX TERMS: Postural tachycardia syndrome, vomiting, microgravity,

hypergravity, vestibular, autonomic, baroreflex.

1

VESTIBULAR CONTROL OF SYMPATHETIC ACTIVITY

Horacio Kaufmann*, Italo Biaggioni †, Andrei Voustianiouk*, André Diedrich†, Fernando Costa†, Martin Gizzi‡,Stephen Moore *,Theodore Raphan,* & Bernard Cohen* * Department of Neurology, Mount Sinai School ofMedicine, New York, New York 10029, USA † Department of Medicine, Vanderbilt University, Nashville,Tennessee 37232, USA ‡ New Jersey Neuroscience Institute, JFK Medical Center, Edison, New Jersey 08818, USA

The otolith organs sense head position with regard to gravity and initiate compensatory ocular and posturalreflexes that maintain upright posture. In the cat, these receptors also regulate sympathetic efferent vasoconstrictoractivity, which contributes to blood pressure maintenance during orthostatic stress. To test whether vestibulo-sympathetic reflexes are present in humans, we stimulated otolith receptors along different head axes with off-vertical axis rotation (OVAR) and recorded eye movements, pupillary diameter, beat-to-beat blood pressure, heartrate, respiratory rate and sympathetic efferent activity in the peroneal nerve with a miniaturized microneurographydevice. During OVAR, pupillary diameter, blood pressure, respiratory rate and sympathetic efferent activity toskeletal muscle (MSNA) were entrained at the frequency of rotation,. MSNA increased in the nose-up position anddecreased when nose-down while the pupil constricted during nose up and dilated during nose down. MSNA wasclosely correlated with blood pressure when nose-down, while arterial pressure was increasing, with a latency of1.38 s, indicating that, in this position, it was driven by baroreflex afferents. MSNA was not correlated with bloodpressure but was tightly correlated with the gravito-inertial acceleration vector during the nose-up position, whenblood pressure was decreasing, at a latency of 0.6 s and was not affected during transient voluntary apnea. Hence, inaddition to the baroreflex, a gravitoinertial-sympathetic reflex most likely originating in otolithic vestibular neurons,is present in humans and may contribute to blood pressure maintenance during forward linear acceleration, such asexperienced upon standing. Impairment of vestibular-sympathetic reflexes may contribute to orthostatic intolerancein returning astronauts.

THE ROLE OF GRAVITOINERTIAL FORCE BACKGROUND, SPATIALORIENTATION AND CONTACT CUES IN PERTURBATIONS OF REACHINGMOVEMENTS BY CORIOLIS FORCES

P. DiZio and J.R. Lackner . Ashton Graybiel Spatial Orientation Laboratory, BrandeisUniversity MS033, Waltham, MA 02545

INTRODUCTIONThe objective of the proposed research is to provide a technical base for evaluating the feasibilityof a rotating "artificial gravity" environment for long-duration space missions. Our focus is onsensory-motor control and spatial orientation, and our goal is to understand how Coriolis forcesthat are generated by body movements in a rotating environment disrupt movement coordinationand how to alleviate or prevent these disruptions. Previous studies showed that Coriolis forcesgenerated by arm reaching movements in a slow rotation room (SRR) disrupt the paths andendpoints of the reaches but complete adaptation at 10 rpm is possible within 20 movementseven without visual feedback (Lackner & DiZio, 1994). The studies summarized here examined1) whether studies conducted in the SRR in 1g accurately predict Coriolis perturbations andadaptation in different force backgrounds, 2) whether neuromotor compensation for Coriolisforces in the SRR involves neuromotor mechanisms that subserve reaching in a normal terrestrialenvironment, and 3) the role of fingertip contact cues in calibration of reaching movementendpoint.

CURRENT STATUS OF RESEARCH

Coriolis Perturbations and Adaptation in Non-terrestrial Force BackgroundsBlindfolded subjects seated over the axis of a rotating chair attempted to reach forward from amidline start button to a location 35 cm straight ahead also on their midline. Movements beganand ended on a waist-high, smooth desktop, and the fingertip was tracked with a motion analysissystem. Five subjects were each tested in the 0 g and 1.8 g phases of parabolic flight as well asin 1 g, on separate days. Cued to move in the desired force level of successive parabolas, theywere able to complete 20 movements while stationary, 40 movements during 10 rpmcounterclockwise rotation, and 20 post-rotation, in one 40 parabola mission. Per-rotation reacheswere delayed until 2 minutes after rotation onset in order to allow semicircular canal activity toequilibrate. The movements generated transient rightward Coriolis forces on the reaching arm,which deviated reaching endpoints and movement paths to the right, relative to pre-rotationbaseline. Comparable errors occurred in 1 g, 0 g and 1.8 g. Complete adaptation of endpointand of path curvature were achieved during rotation, but adaptation was most rapid in 1 g.Symmetric aftereffects were present when rotation stopped in all force backgrounds, and re-adaptation occurred most quickly in 1 g.

Coriolis Effects During Active Turning and ReachingCoriolis forces generated during passive, constant velocity rotation in either the SRR or arotating chair perturb reaching endpoint and trajectory until adaptation occurs. In theseconditions, there is no physiological signal indicating that the body is rotating. In a naturalenvironment, self-generated Coriolis forces are created whenever a reaching movement is madeand the torso is simultaneously turning. Subjects actively plan and control voluntary torso

rotation and receive sensory-motor feedback during their rotation. We measured the magnitudeof the Coriolis interaction torques and the accuracy of reaching movements directed towardtarget locations that required synergistic, voluntary turning of the torso and extension of the arm.These movements generated Coriolis torques about twice as large as in our previous 10 rpm SRRexperiments. However, reaching movement paths and endpoints were not deviated, compared toperformance in trials involving target locations requiring extension of the arm with no torsorotation.

The Role of Fingertip Contact Cues in Calibration of Reaching Movement Endpoint.An unexpected result in previous studies was that fingertip contact with a surface at the terminusof target-directed reaches is essential for adaptive elimination of endpoint errors caused bytransient Coriolis force perturbations in a rotating room (Lackner & DiZio, 1994). Therefore,we measured how fingertip landing forces relate to reaching endpoint on a horizontal surfacewhen no perturbations are present. Four subjects stood in front of a 40 by 60 cm force plate(Kistler Model 9268) resting on a desktop at waist height, in a normal stationary environment. Alaser dot was projected in various locations (7 or 22 cm distance, and 11 lateral locations at 5 cmintervals), and subjects were instructed to reach and touch the target position using comfortablearm and torso motions at a natural speed and to hold the finger in the touchdown position for 2sec. The resultant forces 30-50 ms after initial contact peaked at ∼ 6 N and settled at ∼ 4 N by 200ms. Horizontal shear forces decayed rapidly in this interval and then hovered around zero, thevertical component remained constant over time. There were significant variations in peakhorizontal impact force as a function of target distance and lateral position. The direction ofhorizontal impact force was highly correlated with target direction relative to the shoulder.

ConclusionThese three studies show that: 1) Studies performed in a rotating artificial gravity environment ina 1 g force background predict accurately motor disruptions due to Coriolis force perturbations in0 g and 1.8 g. Rapid adaptation to 10 rpm rotation is possible in 0g, 1 g and 1.8 g. 2) Motorcompensations for and adaptation to Coriolis perturbations is part of our natural neuromotorrepertoire. Central nervous system registration of self-rotation is an important context cue incontrol of Coriolis interaction torques. 3) Terminal fingertip contact forces provide a spatial mapof reaching movement endpoint that contribute to calibration of limb position and sensorimotorcontrol.

SELF-MOTION SYSTEM FREQUENCY RESPONSE: IMPLICATIONS FORCYBERSICKNESS

D. E. Parker1,2, H.B. L. Duh2, J. O Phillips1 and T. A. Furness2

1Department of Otolaryngology – HNS and 2Human Interface Technology Laboratory, University of Washington,Seattle, WA 98195-7923

INTRODUCTIONUsing a postural stability measure, we determined the frequency response of the visual self-motion system.

Based on the results, we proposed a visual-vestibular cross-over frequency range and hypothesized that conflictingvisual and inertial self-motion cues at the frequency of maximum cross-over would be more likely to evokesimulator sickness (SS) / cybersickness than conflicting cues at a higher frequency. This hypothesis was supportedexperimentally. Implications for alleviation of SS are discussed.

CURRENT STATUS OF RESEARCHThree experiments are described. Experiments 1 and 2 determined postural disturbance evoked by visual

scene oscillation at different frequencies. Experiment 3 recorded SS at 2 frequencies of visual-vestibular conflict.

Methods – Experiments 1 and 2Experiment 1. 11 subjects stood on a balance platform while viewing a scene that oscillated in roll. The

Chattecx balance platform automatically calculated dispersion around the center-of-balance. The visual scene, awaterfall on the island of Maui, was presented on a VR4 head-mounted display that has a nominal 48° x 36° field-of-view (FOV).

Frontal visual scene roll oscillation was presented at 5 frequencies: 0.8, 0.4, 0.2, 0.1, 0.05 Hz. Peak scenevelocity was constant across frequencies (70°/sec). Subjects in a sharpened Rhomberg stance attempted to standsteady during 10-sec data collection periods. Baseline data were collected in darkness before and after the visualstimulus trials. The following data were collected for each trial: subjective difficulty rating (1-10 scale) anddispersion of the center-of-balance.

Experiment 2 replicated Experiment 1 with the following changes. The visual scene was a simple black andwhite radial pattern, similar to a propeller. This image was back-projected by a video projector onto a 36 inchcoated plastic dome. 10 subjects stood on the balance platform leaning forward so that their heads in the dome. TheFOV was about 180° x 180°.

Results - Experiments 1 and 2Because of large inter- and intra-subject variability, difficulty ratings and balance dispersion scores were

‘standardized’: each visual trial score was divided by the average baseline performance for that subject. The resultsfrom Experiments 1 and 2 illustrated in Fig. 1 show that balance disturbance was inversely related to sceneoscillation frequency. Statistical analysis (ANOVA) indicated that the effects of frequency were highly significant.

Conclusions - Experiments 1 and 2The visual self-motion system exhibits low-pass filter characteristics. At frequencies below 5 Hz, the

vestibular self-motion system operates as a high-pass filter. Combining the data from Experiments 1 and 2 with apreviously reported vestibular frequency response (Melvill Jones & Milsum, 1965), we suggest that maximumoverlap between the 2 self-motion systems occurs at about 0.04 Hz. Conflicting motion cues at this frequency shouldevoke SS.

Methods – Experiment 3Experiment 3. Using a rotator, 10 subjects were oscillated around their yaw axis at low frequency (0.07 or

0.08 Hz) or high frequency (0.20 or 0.18 Hz). Simultaneously they viewed a scene comprised of black and whitevertical stripes and also oscillated around the their yaw axis at low frequency (0.03 or 0.05 Hz) or high frequency(0.20 or 0.25 Hz). The images were presented on the VR4. Peak chair angular velocity was about 60º/sec.

To generate conflicting motion signals, visual and inertial oscillation were presented at slightly differentfrequencies. For example, rotator oscillation at 0.07 Hz was combined with visual scene oscillation at 0.05 Hz.Consequently, the phase relationship between the self-motion cues from the visual and inertial signals changedcontinuously. Each subject received a maximum of 20 trials alternating between low and high frequency. The initialtrial was always low frequency so that carry-over effects worked against our hypothesis. SSQ symptoms [Kennedyet al. (1993) International Journal of Aviation Psychology, 3, pp 203-220] were recorded before the experiment

started as well as during after each trial. The experiment was terminated if stomach awareness persisted for longerthan 1 minute following a trial, if “moderate” nausea was reported, or at the subject’s request.

Results – Experiment 3Useful data were obtained from 8 subjects. Fig. 2 illustrates data from a moderately susceptible subject who

completed the full set of trials. Note that SS symptoms gradually increased across trails Note also that low frequencyoscillation evoked more SS than high frequency oscillation. Mean Total Sickness (TS) scores were significantly (p= 0.012) larger for low frequency chair / scene oscillation (195.5) than for high frequency oscillation (128.8). MeanNausea scores also were significantly (p = 0.014) larger for low frequency oscillation (22.0) than for high frequencyoscillation (13.4).

Conclusions - Experiment 3Our hypothesis is that simulator sickness may be most readily evoked by visual-inertial conflicts in the

frequency range where both the visual and the inertial self-motion systems are active. As expected, subjects reportedmore motion sickness for low frequency conflicting motion stimuli than for higher frequency stimuli.

FUTURE PLANSWe are currently developing interventions to alleviate SS / cybersickness that we are evaluating using theprocedures described in the studies presented here.

INDEX TERMSMotion sickness, simulator sickness, cybersickness, sensory conflict, frequency response

AcknowledgementsSupported by a Contract with Eastman Kodak and National Aeronautics and Space Administration Grants NAG5-4074 and NCC 9-56. We thank M. F. Reschke for loaning us the rate table, D. L. Harm for loaning us the balanceplatform and Ryan McCaskey Cameron Lee, and Hillary Cummings for their assistance.

Fig. 1. Visual-Vestibular Cross-Over. Solid line:vestibular response. Dashed line: visual response --combined normalized data from Experiments 1 and 2.Maximum overlap appears be about 0.04 Hz.

Fig.2 . SSQ Scores for low frequency chair / sceneoscillation (dashed line) and high frequency out-of-phase oscillation (solid line).

STS-90 NEUROLAB EXPERIMENTS ON THE ROLE OF VISUAL CUES INMICROGRAVITY SPATIAL ORIENTATION.

CM Oman1, IP Howard2, T Smith1, AC Beall1, A Natapoff1, JE Zacher2, and HL Jenkin2.1Man Vehicle Laboratory, MIT, Cambridge, MA ; 2Human Peformance Lab, York University, Toronto.

Purpose. Since gravitational "down" cues are absent in weightlessness, astronauts rely on vision andproprioception for spatial orientation. Many maintain a local "subjective vertical" (SV), as evidenced byreports of inversion illusions and visual reorientation illusions (VRIs). Instability in the SV direction inmicro-gravity can trigger space motion sickness.

Methods. On Neurolab, a Virtual Environment Generator (WinNT Pentium II PC driving a color 68degx45deg FOV stereo HMD) was used to quantify the direction of the SV when viewing static and rotatingvisual scenes, susceptibility to circular and linear vection, and effects of SV manipulation on figurerecognition and shading interpretation. Results. Both linear and circular vection illusion magnitudeincreased on flight day 4 in 2 subjects, and by day 16 for a third, when tested free floating vs. preflighterect and supine. Both linear and circular vection were reduced in flight when a harness was worn whichheld the subjects firmly to the deck with a 88 lb. force. Static visual scene orientation strongly influencedthe SV of one subject in flight and early postflight, but the SV of the other three was congruent with theirbody axis. VRI frequency and onset angle in supine testing preflight and in flight were similar for mostsubjects. We hypothesized that even in micro-gravity, complex figure recognition and interpretation ofshape from shading would show effects of SV manipulation. Three subjects had response biases orperformed the task inconsistently, which may have masked the effect. However, one demonstrated botheffects consistently, showing that choice of SV can have important perceptual consequences.Conclusions. Results indicate that most astronauts become more dependent on dynamic visual andproprioceptive cues, and some also respond to static visual orientation cues. The direction of the SV islabile, and can influence figure recognition and shading interpretation.

Supported by NASA Contract NAS9-19536 and Canadian Space Agency 9F007-5-8515.

Index Terms: Neurovestibular, Neurolab, Spatial Orientation, Vection, Virtual Reality, Vision, SpacePhysiology, Space Sickness

VISUAL ORIENTATION IN UNFAMILIAR GRAVITO-INERTIALENVIRONMENTS

Charles M. Oman1, Ian P. Howard2, Wayne L. Shebilske3 and Jeffrey S. Taube4

Massachusetts Institute of Technology1, York University2, Wright State University3, and Dartmouth College4

INTRODUCTION: The most overt change affecting an astronaut in space flight is the immediate response of theneurovestibular system to changes in gravity level. NASA’s Critical Path Roadmap defines spatial disorientation andreduced performance on cognitive and physical tasks as one of the primary biomedical risks of spaceflight. Onearth, gravity provides a convenient “down” cue. Large body rotations normally occur only in a horizontal plane.In space, the gravitational down cue is absent. When astronauts roll or pitch upside down, they must recognizewhere things are around them by a process of mental rotation which involves three dimensions, rather than just one.While working in unfamiliar situations they occasionally misinterpret visual cues and experience striking “visualreorientation illusions”, in which the walls, ceiling, and floors of the spacecraft exchange subjective identities. VRIscause disorientation, reaching errors, trigger attacks of space motion sickness. MIR crewmembers say that 3Drelationships between modules - particularly those with different visual verticals - are difficult to visualize. Crewmembers learn routes, but their apparent lack of survey knowledge is a concern should fire, power loss, ordepressurization limit visibility.

CURRENT STATUS OF RESEARCH:Human visual orientation. We used an 8 foot tumbling room at York to investigate how the perception of selforientation with respect to the vertical is dominated by gravity, the visual frame of reference provided by the room’srealistic interior, or by the relative orientation of the subject’s body. There is a natural tendency to perceive the feetas “down”. It has long been known that moving visual scenes can produce compelling illusions of self motion, butit was not understood that motionless visual scenes could produce large sensations of static tilt under somecircumstances. We showed that when gravitationally supine subjects view a furnished room interior that wassimilarly tilted 90 degrees with respect to gravity, so that it appeared upright with respect to their body, a majority ofsubjects felt gravitationally upright. We call this a “Levitation Illusion”. If subjects extended their limbs abovetheir supine body, their limbs felt weightless. The strength of the illusion has been systematically studied in a largegroup of subjects with the room and the subject in all the different possible orientations, modulo 90 deg. In certainother relative orientations, subjects experienced VRIs– for example they perceived the floor of the room as a ceiling.Susceptibility to the levitation illusion consistently increased with age. Vestibular function is known to degradewith age, and the association between the orientation of familiar visual objects and gravity (which we refer to as“visual polarity”) is probably a learned phenomenon. In a related experiment, we constructed a “mirror bed” device,which allowed us to quantify how “visual polarity”. A subject lying gravitationally supine in the bed views thelaboratory through a mirror mounted at 45 degrees over his head. When strongly polarized objects are in view, thesubject interprets the view as horizontal, and feels subjectively almost upright. When weakly polarized objects areseen, the subject feels nearly supine. Intermediate tilt perceptions can be created by manipulating the polarity (typeand arrangement) of objects in the visual scene. Understanding how the relative orientation of gravity, body axisand the visual scene interact is potentially important for astronaut training, and also in entertainment and clinicalapplications. Strongly polarized objects and pictures may prove useful in reducing the incidence of disorientingVRIs in space station modules. Placing strongly polarized pictures in staircases might help some elderly people beless prone to falling.

Three dimensional spatial memory and learning. What limits human ability to orient and navigate in a 3Dweightless environment ? Can spatial abilities in such 3D environments be improved by preflight training ? Mostnavigation and spatial memory research has addressed only the terrestrial situation. We designed several 3D spatialtasks (Oman, et al, 1999, 2000; Shebilske et al, 2000; Richards, 2000; Richards et al, in preparation) analogous tothose confronting astronauts trying to learn the spatial relationships between the six entrance hatches in a spacestation node module of a space station. Experiments were conducted in both real and virtual environments. After abrief period training, many subjects were able to perform the spatial tasks in any relative orientation to the visualenvironment. Gravitational body position (erect vs. supine) had little effect. Subjects chose to remember therelationships amongst objects as they would appear with the room in a specific “baseline” orientation, andmemorized opposite pairs of objects. Formal training with these concepts helped. Performance also correlated withconventional paper-and-pencil tests of figure rotation ability. Subjects trained in two different environments

successively learned faster in the second, suggesting they “learned how to learn”. Ability was retained one day, oneweek, and even one month after initial training. Another experiment showed that learning with randomly chosenrather than grouped (blocked) sets of room orientations enhanced ultimate performance. We are currently extendingthe paradigm to measure spatial memory across two previously learned modules, one of which is unseen. We wantto know if coalignment of the baseline memorized module orientations is critical for performance. Our ultimateobjective is to develop a methodology/pedagogy for generic and mission specific ISS preflight visual orientationtraining. Another application of our paradigms is in the design and evaluation of emergency escape route markingsand systems of visual landmarks within modules that help crewmembers keep track of the principal axes of the ISS.

Neural coding of spatial orientation in an animal model. We conducted experiments in a Long-Evans rat model tobetter understand how the human sense of place and direction may be coded in 3 dimensions. In rats and primates,limbic “head direction” cells appear to code head direction in a gravitational horizontal plane, independent of theanimal’s location, and roll or pitch of the head up to 90 degrees. The maximum response (“preferred direction”) liesin a fixed direction which varies from cell to cell. In 1-G, moving a prominent background visual landmark resultsin a corresponding re-orientation of the preferred directions of all HD cells by the corresponding angle. Until now,HD cells response has been studied only in a gravitationally horizontal plane. We trained rats to crawl up a wall,across a ceiling and down the opposite wall, in an apparatus that allows us to verify the 3D response characteristicsof HD cells in 1-G, and infer whether the response sensitivity remains anchored by gravity or whether the responsecoordinate frame of the cell re-orients to the animal’s locomotion plane. Cells in some animals show robustdirection specific firing in the same world-centered reference frame when the animal is walking upside down, andresponse on the walls depends on the wall and whether the animal was going up or coming down, as expected. Inother animals, the cells lose their direction specific firing on the ceiling. and there is a significant increase inbackground firing, suggesting that the animals may be disoriented. We have also studied HD cell responses inparabolic flight in a test chamber that was visually symmetrical in an up-down direction. All cells HD cells studiedmaintained their direction specific discharge when the animal was on the floor or the wall of the chamber. However,when placed on the ceiling of the chamber, HD cell directional specificity was frequently lost. In some cases, thepreferred direction of HD cell response reversed across the visual axis of symmetry of the cage, as expected if thecell’s response coordinate frame had reoriented to the ceiling. When humans roll inverted in parabolic flight and puttheir feet on the ceiling of the aircraft, they experience a VRI in which the ceiling seems like a “floor”, and the left-right axis is reversed. We believe this is the first demonstration of the limbic correlate of a human 0-G spatialorientation illusion. Our experiments provide insights on the role played by gravireceptors in stabilizing the humansense of place and direction not only in astronauts, but also in vestibular and Alzheimer’s disease patients.

REFERENCES:Allison, R., Howard, I.P., and Zacher, J. (1999) The effect of field size, head motion and rotational velocity on rollvection and illusory self-tilt in a tumbling room. Perception, 28, 299-306.Calton JL, Tullman ML, Taube JS (2000) Head direction cell activity in the anterodorsal thalamus during upside-down locomotion. Soc Neurosci Abstr, Vol 26, Part 1, p. 983Howard, I and Hu G. (1999) Visually induced reorientation illusions. Submitted to Perception.Howard, I.P. and Jenkin, H.L. and Hu, G. (2000) Visually induced reorientation illusions as a function of age.Aviation, Space and Environmental Medicine, 71(9), A87-A91.Oman C, Shebilske W, Richards J, Tubre T, Beall A, Natapoff A. (2000) Three dimensional spatial memory andlearning in real and virtual environments. J. Spatial Cog. and Comp. (submitted)Richards, JT (2000) Three dimensional spatial learning in a virtual space station node. SM Thesis, Dept. ofAeronautics and Astronautics, MIT, Cambridge, MA September, 2000.Shebilske, WL., Goettl, BP., & Garland, D. (in press). Situation Awareness, Computer-Automation, and Training. InM. R. Endsley & D. Garland (eds.), Situation awareness analysis measurement. Mahwah, NJ: Lawrence Erlbaum.Shebilske, W. L., Tubre, T., Willis, T., Hanson, A., Oman, C., and Richards, J. (2000). Simulating Spatial MemoryChallenges Confronting Astronauts. Proceedings of the Annual Meeting of the Human Factors and ErgonomicsSociety, July 30, 2000.Stackman RW, Tullman ML, Taube JS (2000) Maintenance of rat head direction cell firing during locomotion in thevertical plane. Journal of Neurophysiology 83: 393-405.Taube JS, Stackman RW, Oman CM (1999) Rat head direction cell responses in 0-G. Soc Neurosci Abstr 25: 1383.

Supported by NASA Cooperative Agreement NCC9-58 with the National Space Biomedical Research Institute

Visually-induced Adaptation of the Translational Vestibulo-Ocular ReflexMIN WEI, HUI-HUI ZHOU AND DORA E. ANGELAKI

Dept. of Neurobiology, Washington University School of Medicine, St. Louis, MO 63110

The adaptive plasticity of the translational vestibulo-ocular reflex has beeninvestigated in rhesus monkeys after two hour exposure to either vertical or torsionaloptic flow stimulation accompanied by lateral translation stimuli (0.5 Hz). Because of theinherent ambiguity in the otolith system for the detection of linear accelerations, wehypothesized that cross-axis adaptation of the translational VOR during lateral motionwould be preferentially selective for a torsional optic flow stimulus that would mimic aroll tilt movement. However, the present results do not support this hypothesis. Instead,there was a selective and significant increase in the amplitude of the orthogonal eyemovement component after exposure to both vertical and torsional optic flow stimulation.Moreover, there was no difference in the size of adaptive changes for opposite directionsof torsional flow stimuli, including those in phase and out of phase with linearacceleration. These results suggest that, at least at 0.5 Hz, there seems to be nopreferential visually-induced adaptive capacity of the otolith system for tilt/translation re-interpretation during motion. Similarly as with the rotational VOR, translational VORexhibits a general form of cross-axis adaptation that operates for different directions ofoptic flow stimulation.

OTOLITH AND VERTICAL CANAL CONTRIBUTIONS TO DYNAMICPOSTURAL CONTROL

Kaufman, G.D. 2, F. O. Black1, C. C. Gianna1, W. H. Paloski2 and S. J. Wood1

1 Legacy Health System, Portland, OR; 2 Johnson Space Center, Houston, TX

INTRODUCTIONOur experiments focus on issues related to the effect of artificial gravity on vestibulospinal adaptation. Adaptation tomicrogravity requires a re-organization of central nervous system (CNS) processing of the three major sources ofspatial information on earth: visual, vestibular and somatosensory (proprioceptive). Adaptation to microgravity ischaracterized by a combination of perceptual and physiological responses to changes in gravitational acceleration.Experimental results to date support the hypothesis that the absence of a gravity vector leads to adaptive changes inneural strategies used for resolving ambiguous linear accelerations detected by the otolith systems. In the absence ofgravitational vertical, normally ambiguous visual references on earth become critical for orientation on orbit. Inmicrogravity, contact surfaces determine initial and final orientation references. The scientific goal of this ground-based research is to understand how canal and otolith-mediated responses to linear translation and roll tilt can beadapted by altered sensory environments. One question for effective sensorimotor countermeasure development isthe extent to which otolith-mediated responses can be adapted such that stimulation normally inducing tilt responseswill instead induce translation responses (and vice versa). Our first set of ground based experiments have addressedthis problem by studying postural responses to altered gravito-inertial vectors.

The postural instability immediately after shuttle egress observed in all crewmembers studied to date appears lesssevere with increasing flight experience. This observation suggests that experienced astronauts may be able tomaintain contextually dependent adaptive states. Our results also support the hypothesis that exposure to artificialgravity induced by centrifugation could be employed as a sensorimotor countermeasure during long duration spaceflight missions.

CURRENT STATUS OF RESEARCHOur project has studied postural control adaptation to dynamic linear acceleration stimuli delivered using a short armvariable radius human centrifuge. Efficacy of adaptation protocols was measured using otolith-mediated roll-tiltperception and postural stability in normal and vestibular deficient subjects. In all experiments, response variabilityof each subject was determined with reference to baseline measures of each response. The adaptation period, from30 to 90 minutes, consisted of constant velocity centrifugation. Short-arm centrifuge devices were driven by directdrive motors (300 ft-lb at NASA, 80 ft-lb at Legacy). Subjects were seated upright with the long body (z) axisparallel to, but 80 cm offset from, the Earth vertical axis of rotation. A snug four-point safety harness, and vacuum-forming cushions, restrained the subject as comfortably as possible. The subject's head was free to rotate in all threeplanes, but a cushioned stop provided relief from neck strain caused by the centrifugal force. The subjects facedforward tangentially with the left (LEO) or right (REO) ear directed radially away from the axis of rotation.Unilateral vestibular deficient subjects were tested in both orientations while all others were tested in only oneorientation. At the NASA facility, a one meter diameter hemisphere dome in front of the subject, and shroudsattached around the subject chair, minimized rotation-related wind and sound cues. At the Legacy facility, no domeor shroud was used, and the LED targets were presented on a thin aluminum bar.

Following a 20 s ramp-up period at a constant angular acceleration of 10 deg/s2, the centrifuge angular velocitywas maintained constant for 90 min (in some cases, the adaptation period was shortened at the subjects’ request) at200 deg/s, resulting in a gravito-inertial force (GIF) vector of 1.4 g tilted 45 deg in roll with respect to the subject.After the adaptation period, the centrifuge was decelerated at 10 deg/s2 to a complete stop.

Our results showed that: 1) a 1.4 g artificial gravity stimulus (roll plane centripetal acceleration) over a period of 90min is sufficient to induce postural and subjective vertical changes in normal subject s; 2) these changes aredependent on both the orientation and magnitude of the applied gravito-inertial force (GIF), and perhaps on cross-coupling forces; and 3) the changes require intact vestibular (presumably otolith) input.

FUTURE PLANSAfter establishing the extent to which otolith-mediated tilt and translation responses can be adapted at differentstimulus frequencies and GIF magnitudes, our next series of experiments will examine whether subjects can 'dualadapt' to altered sensory environments using the orientation of gravity to provide context. We will investigatecontext specific adaptation by varying visual and somatosensory references. Demonstration of ‘dual-adaptation’ toaltered sensory environments will provide insight into the feasibility of using intermittent exposures to artificialgravity induced by centrifugation as an in-flight sensorimotor countermeasure.

INDEX TERMSotolith, posture, adaptation, centrifuge, countermeasure

CHARACTERIZATION OF SENSORY INTEGRATION AND CONTROL STRATEGIESTHAT REGULATE HUMAN POSTURAL CONTROL IN CHANGING CONDITIONS

R. J. PeterkaNeurological Sciences Institute, Oregon Health Sciences University, Portland, Oregon 97006

INTRODUCTIONMost researchers agree that human postural control requires active regulation via feedback

mechanisms to maintain balance. It is of interest to understand (1) how different components inthe feedback system influence overall postural stability, (2) how the postural control system usesvisual, proprioceptive, and graviceptive sensory cues, and (3) how the system compensates forexternal perturbations, altered sensory environments, and sensory deficits. A basicunderstanding of the postural control system should contribute to determining why astronautshave balance problems following space flight, and perhaps how compensation can be facilitated.

CURRENT STATUS OF RESEARCHMethods

We performed experiments that perturbed quiet stance using support surface and/or visualsurround rotational motions that evoked anterior/posterior (AP) body sway in subjects withnormal sensory function and with bilateral vestibular loss. The rotational motion followed apseudorandom waveform (60.5 s period) whose velocity amplitude spectrum was approximatelyconstant up to about 2.5 Hz. Each trial presented six to eight cycles of the pseudorandomwaveform with a peak-to-peak stimulus amplitude ranging from 0.5° to 8°. The response wasconsidered to be AP center-of-mass (COM) body sway angle. A cross spectral analysis betweenthe stimulus and response provided transfer function gain, phase, and coherence data thatcharacterized the dynamic behavior of the postural control system, and indicated the linearity ofthe stimulus-response relationship. By convention, a gain value of 1 and phase of 0° at aparticular stimulus frequency indicate that the subject’s COM sway angle was perfectly alignedwith the stimulus (in amplitude and timing) at that frequency. A gain of zero indicates that thesubject’s COM sway angle remained oriented to earth vertical. Transfer function curve fits,based on a simple feedback control model, were made to the gain and phase data. These curvefits provided estimates of model parameters including a stiffness factor (corrective torqueproportional to COM position), a damping factor (corrective torque proportional to COMvelocity), response time delay, and a sensory integration factor. The sensory integration factorprovided information about the relative contributions of visual, proprioceptive, and graviceptivesensory information to stance control.

ResultsBody sway responses generally followed the stimulus waveform indicating that subjects

tended to orient to the moving support surface and/or visual surround. For a given test condition,there was a remarkably linear relationship between the stimulus and response indicated bycoherence function values which were large and changed little with changing stimulusamplitudes. However, transfer function gains decreased with increasing stimulus amplitudeindicating that the overall behavior of the postural control system was a nonlinear function ofstimulus amplitude. These apparently conflicting results can be reconciled if one hypothesizes

that the relative contribution (sensory channel weighting) of visual, proprioceptive, andvestibular sensory cues changed as a function of the stimulus amplitude.

For example, when support surface and/or visual stimuli were close to perceptual thresholdlevels (0.5° peak-to-peak stimulus amplitude), our model-based analysis indicated that visualcues contributed about 35%, proprioceptive cues about 50%, and graviceptors about 15% of thesensory orientation information used by the postural control system. However, with the largestamplitude stimuli (8°), the postural control system used about 5% visual, 15% proprioceptive,and 80% graviceptive information. The increased utilization of graviceptive cues produced aproportionally greater orientation toward earth vertical and less towards the visual or supportsurface stimulus, consistent with the observed reduced gains.

In contrast to normals, subjects without vestibular function showed little or no ability toreduce gain with increasing stimulus amplitude. As a result, body sway increased linearly withincreasing stimulus amplitude, resulting in falls at larger stimulus amplitudes. This is consistentwith a limited ability to alter the proportion of visual and proprioceptive cues used for posturalcontrol. Additionally, the results indicated that the vestibular system was the primary source ofgraviceptive information used for postural control over the frequency range tested.

When the stimulus was provided by support surface rotations, other system factors alsochanged with increasing perturbations. Subjects increased their stiffness but did not change theirdamping. Modeling work demonstrated that an increase in stiffness without an accompanyingincrease in damping can lead to resonant behavior (~1 Hz) and eventually to instability.However, experimental results showed that the increased stiffness was compensated by anapparent decrease in response time delay from about 190 ms to 100 ms. Response time delay isusually considered to be a fixed factor which adversely affects a feedback control system.However, our results suggest the overall dynamic behavior of stance control is maintained, inpart, by actively regulating the response time delay.

ConclusionsThe simple task of maintaining quiet stance was shown to involve a complex sensory

integration process where the relative contribution of different sensory systems changes as afunction of environmental and stimulus conditions. Additionally, subjects appear to regulate thedynamic behavior of their postural control system by changing their stiffness and altering thetiming of corrective responses. Subjects without vestibular function have limited ability to altertheir use of available sensory cues when stimulus conditions change. This suggests that thevestibular system plays an important role in regulating the sensory integration process. Perhapsthis sensory integration regulation is disrupted in returning astronauts who have altered theirinterpretation of vestibular sensory cues to accommodate the free fall environment of space.

FUTURE PLANSOur current model of the postural control system is descriptive in nature. It provides a means

of quantifying experimental results in terms of simple dynamic factors (stiffness, damping, timedelay, sensory channel weighting), and characterizing how these factors change as a function ofstimulus and environmental conditions. The next step is to develop a predictive model thataccurately reflects the true structure and function of the human postural control system.

INDEX TERMSPostural Control, Balance, Sensory Integration, Vestibular, Vision, Proprioception, Human.

SPATIAL REORIENTATION AND SENSORY-MOTOR BALANCE CONTROL INALTERED GRAVITY

W. H. Paloski1, S. J. Wood2, G. D. Kaufman3, F. O. Black2, and M. F. Reschke1

1Johnson Space Center, Houston, TX; 2Legacy Health Systems, Portland, OR; 3UT Medical Branch, Galveston, TX

INTRODUCTIONThis space flight investigation will examine changes in spatial processing of sensory-motor function following

adaptation to microgravity and will explore the feasibility of forcing changes between motor program sets optimizedfor different gravito-inertial environments. Pre- and post-flight measurements will be made on ten astronaut subjects(first-time fliers) selected from several short-duration Shuttle missions. Our first specific aim is to examine adaptivechanges in the spatial reference frame used for coding orientation and motion as a function of space flight. Byexamining the effects of head tilt on balance control, one of the most significant post-flight manifestations ofsensory-motor adaptation to microgravity, we will test the hypothesis that there is a reorientation of central vestibularprocessing from a gravitational frame of reference to a head-centric frame of reference as a function of adaptation tomicrogravity. Our second specific aim is to examine the feasibility of altering the readaptation process followingspace flight by providing discordant canal-otolith-somatosensory stimuli using short-radius pitch axis centrifugation.Previous observations by our laboratory suggest that exposure to this stimulus during, or soon after, sensory-motorreadaptation to the terrestrial environment will trigger a switch in central vestibular processing from the external(gravitational) reference frame used on earth to the internal (head-centered) frame of reference used in microgravity.Demonstration of centrifuge-induced switching between sensory-motor program sets learned for differentgravitoinertial environments will provide insight into the feasibility of using intermittent exposures to artificialgravity induced by centrifugation as an inflight sensory-motor countermeasure during long duration space flightmissions. Furthermore, confirmation of these earlier results will have important operational implications forprotecting crew safety by constraining postflight activities of returning astronauts.

CURRENT STATUS OF RESEARCHMethods

The influence of head tilt on the control of postural equilibrium will be studied using a modified NeuroComcomputerized dynamic posturography system. We have previously employed this system’s sensory organization tests(SOT) to demonstrate postflight reductions in the effectiveness of vestibular control of posture and increasedreliance on visual inputs for posture control. In the present experiments, we propose to use the six standardNeuroCom SOT conditions, which combine two proprioceptive conditions (fixed-support, sway-referenced support)with three visual conditions (eyes open, eyes closed, sway-referenced vision). We also propose to incorporate twoadditional test conditions to examine the influence of head tilt on balance control with absent vision and inaccurate(sway-referenced) proprioceptive inputs. The two new conditions will modify the standard SOT 5 by adding dynamicvoluntary head movements and static head tilts. The dynamic head movements will consist of 0.33 Hz sinusoidalhead oscillations in the roll or pitch planes. A modulated auditory tone will will aid the subject in maintaining aconstant frequency of oscillation while operator feedback will be provided to maintain the same magnitude(approximately 30° in each direction). The static head tilts will consist of fixed 30° right or left head tilts. The statictilts will be made in the roll plane to avoid changing the antero-posterior center-of-gravity locations and becausenormal subjects are capable of performing this maneuver without altering postural stability. We expect that thechanges in head tilt conditions will not significantly affect postural stability in crewmembers preflight; however,immediately after flight we expect that head tilts will further exacerbate postural instability by decoupling the head-centered spatial reference frame from the gravity reference frame.

The pitch axis centrifugation stimuli will be provided by the JSC short-arm centrifuge. Pitch rotation will beprovided about an earth-vertical axis with subjects in a left-side down position. The subjects will be oriented withtheir interaural axes positioned 50 cm from the axis of rotation (i.e., body center of mass approximately over the axisof rotation), and thus will be exposed to eccentric rotation (i.e. short-radius centrifugation). The nominal centrifugerotation profile will be: 1. Accelerate at 25 deg/sec2 to constant velocity of 140 deg/sec; 2. Maintain constantvelocity rotation at 140 deg/sec for 120 sec (subject in dark); 3. Superimpose a 0.33 Hz, 40 deg/sec sinusoidalvelocity profile upon the 140 deg/sec constant velocity for 20 cycles (subject in dark); 4. Continue the constant plussinusoidal profile for 20 more cycles, with matched full field optokinetic stimulus; 5. Return to constant velocity

rotation for 60 sec (subject in dark); 5. Decelerate at 25 deg/sec2 to stop. After the stop the subjects will remainstationary for two minutes and then the rotation profile will be repeated in the CCW direction. Binocular threedimensional eye movements will be measured throughout using video-oculography techniques. Perceptual reports ofself-motion will be recorded during the pitch centrifugation. Immediately prior to and following the pitchcentrifugation trials, postural control will be assessed using the sensory organization tests described above.

Subjects will be exposed to the pitch axis centrifugation during two preflight test sessions and two postflightrecovery test sessions, as shown in the figure. The R+3 days time frame was selected for the postflight session basedon our previous posturography studies showing that balance control is nearly recovered in all short durationcrewmembers by this time frame. We do not expect this centrifugation exposure to significantly affect the dependentmeasures during the preflight testing or the R+90 days session. However, according to our second hypothesis,exposure to these discordant stimuli during postflight recovery (R+3) will trigger a return to spatial processing usinga head-centered frame of reference, and will therefore transiently disrupt sensorimotor control. This hypothesis willbe tested by comparing the postural measurements obtained after centrifugation with those obtained immediatelybefore centrifugation and with those obtained immediately following space flight. Note that crewmembers will testedagain on R+4 days (the day following the centrifuge exposure) and R+8 days to determine whether any disruption topostflight recovery persists. To comply with the JSC Institutional Review Board recommendations, the subjects willalso be tested at R+90 days. This final test will ensure that participating crewmembers have no long term sensitivityto the centrifuge stimuli or disruption of sensorimotor function.

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Figure 1: The measurement timeline per mission consists of three preflight sessions, and postflight sessions on R+(recovery days) 0, 2, 3, 4, 8 and 90. Each test session (indicated by open circles) will require tests of posturalequilibrium. The anticipated readaptation curve for the postural performance is depicted, showing a disruption inrecovery on R+3 following the pitch centrifugation exposure (solid bar).

ResultsN/A

CONCLUSIONA better understanding of the nature of sensory-motor recovery from short space flights will help us determine

the mechanisms of adaptation of balance control. The use of the postflight centrifugation stimulus will allow us tobetter understand what sensory stimuli may retard or reverse the readaptation process, and therefore know whichactivities may also contribute to decompensation during recovery from vestibular pathology. New understandinggained in our research on mechanisms of vestibular system conditioning will be fundamental to further developmentof artificial gravity countermeasures and potentially to new vestibular rehabilitation techniques.

FUTURE PLANSDuring the initial phase of funding, experiment definition was completed and the experiment was approved for

implementation as a pre-/post-flight Detailed Supplemental Objective (DSO 635). An experiment crew briefing wasrecently completed, resulting in the first flight crew member agreeing to participate during the STS-104 mission inmid 2001. A preliminary ground-based study has been initiated to refine the posture protocol by examining the effectof head movement frequency and plane of motion on balance control. In addition, the JSC Short-Arm CentrifugeFacility is being reconfigured to accommodate the proposed eccentric pitch visual-vestibular interaction paradigm.

INDEX TERMSBalance control, posture, sensory-motor adaptation, vestibular, centrifuge, artificial gravity

PERCEPTION OF TILT (SOMATOGRAVIC ILLUSION) IN RESPONSE TOSUSTAINED LINEAR ACCELERATION DURING SPACE FLIGHT

Bernard Cohen, Gilles Clément, Steven T. Moore and Theodore Raphan

From the Departments of Neurology and Physiology, Mount Sinai School of Medicine, New York, NY(BC, STM), the Centre de Recherche Cerveau et Cognition, CNRS/UPS,Toulouse, France (GC) and theDepartment of Computer and Information Science, Brooklyn College, Brooklyn, NY, USA (TR, STM).

Four astronauts were exposed to interaural and head vertical (dorsoventral) linear accelerations of 0.5-g and1-g during constant velocity rotation on a centrifuge, both on Earth and during the 1998 Neurolab (STS-90)orbital space flight. Subjects were oriented either left-ear-out or right-ear-out (Gy centrifugation), or laysupine along the centrifuge arm with their head off-axis (Gz centrifugation). Pre-flight centrifugation,producing linear accelerations of 0.5-g and 1-g along the Gy (interaural) axis, induced illusions of roll-tiltof 20° and 34° for gravito-inertial acceleration (GIA) vector tilts of 27° and 45°, respectively. Pre-flight0.5-g and 1-g Gz (head dorsoventral) centrifugation generated perceptions of backward pitch of 5º and 15º,respectively. In the absence of gravity during space flight, the same centrifugation generated a GIA thatwas equivalent to the centripetal acceleration and was aligned with the Gy or Gz axes. Perception of tiltwas underestimated relative to this new GIA orientation during early, in-flight Gy centrifugation, but wasclose to the GIA after 16 days in orbit. During the course of the mission, in-flight roll tilt perception duringGy centrifugation increased from 45° to 83° at 1-g and from 42° to 48° at 0.5-g. Toward the end of theflight, subjects reported that they felt as if they were lying-on-side'. Subjects felt 'upside-down' during in-flight –Gz centrifugation from the first in-flight test session, which reflected the new GIA orientation alongthe head dorsoventral axis. The different levels of in-flight tilt perception during 0.5-g and 1-g Gycentrifugation suggests that other non-vestibular inputs, including an internal estimate of the body verticaland somatic sensation, were utilized in generating tilt perception. Interpretation of data by a weighted sumof body vertical and somatic vectors with an estimate of the GIA from the otoliths, suggests 1) thatperception weights the sense of the body vertical more heavily early in-flight, 2) that this weighting fallsduring adaptation to microgravity, and 3) that the decreased reliance on the body vertical persists earlypost-flight, generating an exaggerated sense of tilt. Graviceptors respond to linear acceleration and not tohead tilt in orbit, and it has been proposed that adaptation to weightlessness entails reinterpretation ofotolith activity, causing tilt to be perceived as translation. Since linear acceleration during in-flightcentrifugation was always perceived as tilt, not translation, the findings do not support this hypothesis.Supported by NASA Contract NAS 9-19441

CONTEXT-SPECIFIC ADAPTATION OF GRAVITY-DEPENDENT VESTIBULAR REFLEXRESPONSES (NSBRI NEUROVESTIBULAR PROJECT 1)

M. Shelhamer1, J. Goldberg2, L.B. Minor1, W.H. Paloski3, L.R. Young4, and D.S. Zee1

1Johns Hopkins University School of Medicine, Baltimore MD, 2Baylor College of Medicine,Houston TX, 3NASA Johnson Space Center, Houston TX, 4MIT, Cambridge MA

INTRODUCTIONImpairment of gaze and head stabilization reflexes can lead to disorientation and reduced performance in

sensorimotor tasks such as piloting of spacecraft. Transitions between different gravitoinertial force (gif)environments – as during different phases of space flight – provide an extreme test of the adaptive capabilities ofthese mechanisms. We wish to determine to what extent the sensorimotor skills acquired in one gravity environmentwill transfer to others, and to what extent gravity can serve as a context cue to assist in maintaining the appropriatesensorimotor responses in different environments.

We use the general approach of adapting a response (such as the VOR) in a particular manner (e.g. gain increase)in one context, adapting in a different manner (e.g. gain decrease) in another context, and then seeing if the contextcue itself can cause switching between the previously-learned adapted responses.

Various experiments investigate the behavioral properties, neurophysiological bases, and anatomical substrate ofcontext-specific learning, emphasizing otolith (gravity) signals as a context cue. The following is an outline of themethods and major results for each experiment which is a part of this project.

CONTEXT-SPECIFIC ADAPTATION IN PARABOLIC FLIGHT (MS)This experiment studies the ability of human subjects to switch between two adapted saccade gains based on

various context cues. Saccadic gain is adaptively increased (using a standard double-step paradigm) in one context,and decreased in the other context, then tested to see if the context cue can cause switching between the two adaptedgains.

Results show that saccades can be adapted in a context-specific manner, using vertical eye position, horizontal eyeposition, head tilt, and upright/supine orientation as cues. The effectiveness of a cue appears to depend on itsrelevance to the response being adapted: horizontal eye position is a more effective cue for horizontal saccadeadaptation than is vertical eye position. Gravity magnitude (0g vs. 1.8g) during parabolic flight can also be used as acontext cue. Some adaptation appears to be retained after 8 months. Lunar and Martian g levels can recalladaptations imposed during 0 g.

CONTEXT-SPECIFIC ADAPTATION OF THE HUMAN LVOR (MS, DSZ) This experiment studies the ability of subjects to switch between two adapted LVOR gains based on the context

cue of head tilt (gravity orientation). Subjects are translated laterally at 0.7 Hz, 0.3 g. During adaptation, for 5 min avisual display moves so as to ask for a gain of ×0 with the head rolled left or pitched up, then for 5 min a gain of ×2is asked for with the head rolled right or pitched down. This is repeated for 1 hr. Sine and step translations beforeand after adaptation determine if head orientation alone causes switching between the two adapted gains.

Results show that the orientation of gravity with respect to the head can serve as a context cue. For inter-auraltranslations, head roll is a more effective context cue than is head pitch. This is analogous to the situation withsaccade adaptation: the closer the context cue is to the response being adapted, the more effective it is.

CONTEXT-SPECIFIC ADAPTATION OF RESPONSES TO CENTRIFUGATION (LRY)This experiment studies context-specific adaptation in human subjects during repeated exposure to short-radius

centrifugation, so that they will have the appropriate oculomotor responses and subjective orientation in both therotating and non-rotating environments and be able to switch between them. Subjects make head movements whilerotated at 23 rpm (1g gradient from head to feet), while eye position, subjective orientation, and motion sickness areassessed.

Yaw head movements during rotation initially provoke disorientation and inappropriate vertical eye movements.Repeated head movements in this situation reduce (adapt out) the noncompensatory eye movements. Adaptation tothe centrifugation does occur; three 10-min adaptation sessions produced adaptation that was retained (at reducedlevel) a week later. Adaptation to head movements to one side did not generalize to head movements in otherdirections. While motion sickness disappears after 10 adaptation sessions, vertical nystagmus and illusory tilt do not.

2

Context-specificity of the adaptation is apparent since subjects did not experience motion illusions when off thecentrifuge between test sessions.

PROPERTIES AND CONTEXT-SPECIFICITY OF VESTIBULOCOLLIC REFLEX (JG, WHP)This experiment quantifies and models the contributions of canal and otolith feedback to head movements induced

by trunk rotations and translations in 3 dimensions. The head/neck system is inherently unstable in 1 g and requirestonic neck activity, mediated via the vestibular system, for upright posture. Rotations (centered and eccentric) areapplied to human subjects while upright or supine, to assess the contributions of gravity and tangential acceleration.

Properties of the head-neck control system (VCR) in three dimensions can be adequately modeled by a relativelysimple, 2nd-order linear system, plus a single dead-zone nonlinearity. Adaptation of this system to changes in headinertia can be induced. This adaptation can be made dual-state, such that the appropriate neural control mechanismsfor head stabilization change modes immediately upon a change in head inertia.

CEREBELLAR CONTRIBUTION TO CONTEXT-SPECIFIC ADAPTATION (DSZ, LBM)These experiments determine the role of the vestibulocerebellum in otolith-ocular reflexes and their adaptation,

and the relationship between the translational LVOR and pursuit.In rhesus monkey, bilateral removal of the flocculus and paraflocculus produced almost complete loss of the

horizontal LVOR (even after the angular VOR had recovered). Likewise, human cerebellar patients have comparabledefects in pursuit and the LVOR, while the AVOR appears to be controlled independently. This suggests that thevestibulocerebellum plays a critical role in the generation of the LVOR, and that there is a tight relationship betweenthe generation of the LVOR and smooth pursuit. A separate experiment showed systematic variations in the axis ofeye rotation at different vertical elevations, during pursuit, AVOR, and LVOR. Axis tilts for pursuit and LVOR werealmost identical, and different from that for the AVOR, again showing a close relationship between neural processingfor pursuit and the LVOR.

Context-specific adaptation of smooth pursuit eye movements has been demonstrated in both humans and rhesusmonkeys. Using vertical eye position as a context cue, the initial acceleration of the eyes, when presented with amoving target, can be made to decrease with the eyes elevated, and to increase with the eyes depressed.

LVOR gain adaptation has been induced in squirrel monkeys, and was specific to the frequency used foradaptation. Following adaptation of LVOR gain, there was no significant change in the torsional eye movements tohead tilt, suggesting that the responses to head tilt and head translation are not tightly coupled.

CONCLUSIONSDuring extended space flight crew members may live in artificial gravity and make transitions to and from

weightlessness for planetary exploration and return to Earth. If they learn sensorimotor skills such as piloting in thenormal gravity of Earth, will they be able to perform them adequately in the weightless or the artificial gravityenvironment? We have convincing evidence for context-specificity in various sensorimotor responses. Such context-specific adaptation is a potential countermeasure to the performance decrements seen during these transitions. Inaddition, experiments on the relationship between pursuit and LVOR have implications for countermeasures basedon adapting translation versus tilt responses mediated by the otoliths.

INDEX TERMSSensorimotor adaptation, vestibular, oculomotor, saccade, VOR, LVOR, otolith, Coriolis, VCR, cerebellum,

parabolic flight.

LOCOMOTION AFTER LONG-DURATION SPACEFLIGHT: ADAPTIVEMODULATION OF A FULL-BODY HEAD AND GAZE STABILIZATION SYSTEM

J.J. Bloomberg1, A.P. Mulavara2, C. Miller2, P.V. McDonald2, C.S. Layne3, J. Houser3, H. Cohen4, I.B.Kozlovskaya5

1NASA Johnson Space Center, Houston, TX 77058, 2Wyle Life Sciences Inc., Houston, TX, 3University of Houston,Houston, TX, 4Bobby R. Alford Department of Otorhinolaryngology and Communicative Sciences, Baylor Collegeof Medicine, Houston, TX, 5Institute for Biomedical Problems, Moscow, Russia

INTRODUCTIONFollowing short and long duration spaceflight, crewmembers experience impairment of postural equilibrium, gazecontrol and locomotor function. Our previous studies indicate that astronauts returning from spaceflight experiencedisturbances in head-trunk coordination, lower limb muscle activation patterning, kinematics and alterations in theability to coordinate effective landing strategies during jump tasks after spaceflight (Glasauer, et al., 1995;Bloomberg, et al., 1997; McDonald, et al., 1996; Layne, et al., 1997; Newman, et al., 1997; Layne, et al., 1998).

Traditionally, gaze stabilization has been studied almost exclusively as a problem of eye-head or eye-head-trunkcoordination (Bloomberg, et al, 1997; Crane and Demer, 1997; Moore, et al., 1999; Hirasaki, et al., 1999).However, coordination between the eye and head or among the eye, the head and the trunk may not be the onlymechanism aiding gaze stabilization during activities like locomotion that involve physical impact with theenvironment (McDonald, et al., 1997). One hypothesized constraint on the coordination between segments duringlocomotion is the regulation of energy flow or shock-wave transmission through the body at high impact phases withthe support surface like that which occurs at heel-strike (McDonald, et al., 1997; Lafortune, et al., 1996). Excessivetransmission of energy to the head may compromise gaze stability, leading to oscillopsia and decreased dynamicvisual acuity (Griffin, 1990; Pozzo, et al., 1990; Crane and Demer, 1997; Hillman, et al., 1999). Several studieshave shown that changing: lower limb joint configurations (Perry, et al., 1993), degree of flexion at the knee duringheel-strike (McMahon, et al., 1987; Lafortune, et al., 1996), and head-trunk-pelvis configurations (Cappozzo, et al.,1978; Thorstensson, et al., 1984) all contribute to attenuating shock-waves to reduce head perturbations duringlocomotion. Thus, stabilized gaze during natural body movement results from full-body coordination of the eye-head and head-trunk systems combined with the lower limb apparatus.

From this point of view, the whole body is an integrated gaze stabilization system, in which several subsystemscontribute to gaze stabilization and accurate visual acuity during body motion. Therefore, the goal of this study wasto investigate how exposure to long-duration spaceflight impacts this full-body gaze stabilization system.

CURRENT STATUS OF RESEARCHMethodsSix astro/cosmonauts performed a locomotor task that entailed walking on a motorized treadmill whilesimultaneously fixating gaze on a visual target. This task was performed before and after 3-6 month missions on theMIR Space Station. Head, trunk and lower limb kinematic data were collected with a video-based motion analysissystem. Triaxial accelerometers mounted on the shank and the head measured the shock-wave transmission throughthe body during locomotion.

ResultsCompensatory pitch head movements were determined and the power in this signal was summed in the frequencyrange of 1.5-2.5 Hz reflecting the contributions of reflexive head stabilization mechanisms. (Keshner and Peterson,1995). Subjects showed a reduction in power in this frequency range during postflight locomotion reflecting achange in the dynamics of head movement control after spaceflight.

During postflight locomotion the peak shock at the shank and the head were significantly reduced. To determine thesource of this shock-wave modulation we characterized the lower limb response to heel-strike by calculating the totalangular displacement of knee and ankle angles within the epoch from heel-strike to the first peak of knee flexion.

The knee and ankle total angular displacement was increased after spaceflight indicating increased lower limbflexion subsequent to the heel-strike event. This change in modulation of lower limb flexion during locomotion willresult in the reduction of the axial stiffness of the lower limb complex and thus may be an active strategy designed toreduce the shock-wave transmitted to the head in response to oscillopsia and reduced dynamic visual acuityexperienced by these returning crewmembers.

CONCLUSIONTaken together these data suggest that the lower limbs can play an active role in head and gaze stabilization bymodulating energy transmission to the head during locomotion. Importantly, this full-body pattern of intersegmentalcoordination is modified after long-duration spaceflight leading to multiple deficits in performance.

FUTURE PLANSOur new study titled, “Promoting Sensorimotor Response Generalizability: A Countermeasure to MitigateLocomotor Dysfunction After Long-Duration Spaceflight”, was selected by NASA for development in May, 2000.The goal of this study is to develop a countermeasure to mitigate postflight locomotor disturbances. Rather thandevelop a separate countermeasure, with commensurate demands on valuable crew time, we have developed anapproach that can be easily integrated with the existing International Space Station treadmill exercise procedures.This approach will lead to a unified, multi-disciplinary countermeasure system designed to enhance postflightadaptive locomotor function. The countermeasure we are developing is based on the concept of adaptivegeneralization training. In these training regimens practice is varied about some parameter, so that the subject learnsto solve a class of motor problems, rather than a specific motor solution to one problem, i.e., the subject learnsresponse generalizability or the ability to "learn to learn" under a variety of environmental constraints. Using thistechnique, we have proposed a countermeasure built around inflight treadmill exercise countermeasure activities. Bymanipulating the sensorimotor conditions during exercise we will systematically and repeatedly promote dynamicsensorimotor transitions in locomotor behavior, during the usual treadmill exercise. We anticipate that this trainingregimen will enhance locomotor response generalizability, facilitating locomotor adaptive transition frommicrogravity to partial (Mars) or unit (Earth) gravity environments following spaceflight.

In support of exploration class missions we envision that the next generation treadmill will incorporate the use of avirtual reality system coupled with a multi-direction treadmill that will allow the user to walk or run in any directionimmersed in a varied and interesting virtual environment. This type of fusion interface, which integrally incorporatesboth virtual and non-virtual devices across sensory modalities, produces multi-sensory, virtually augmented,synthetic environments. These synthetic environments can serve as pre and inflight training tools providing sufficientsensorimotor and locomotor challenge to crewmembers to maximize their motor response adaptability, facilitatingthe adaptive transition to partial or unit gravity.

INDEX TERMSLocomotion, gaze, long-duration spaceflight, coordination, sensorimotor

RECOVERY TRAJECTORIES TO PERTURBATIONS DURING LOCOMOTIONConrad Wall1 , Lars Oddsson2

1 Harvard Medical School, 2 Boston UniversityINTRODUCTIONAdapting to microgravity is not the only balance difficulty astronauts face. Major postflight problems includedifficulties with standing, walking, turning corners, climbing stairs and other activities that require stability of uprightposture and gaze. These difficulties inhibit astronauts’ ability to stand up, bail out, or escape from the vehicle duringemergencies and to function effectively when leaving the space/shuttlecraft after flight. Any developedcountermeasure must be tested to determine its effect on gait stability, particularly under those conditions that aremost troublesome following spaceflight. These include recovering from perturbations during walking, turningcorners and climbing stairs. systems The development of an experimental paradigm that introduces a calibrateddisturbance to the foot during the support phase of normal locomotion provides a means for the objectivequantification of locomotor response dynamics that are known to be altered in astronauts upon return from exposureto microgravity but for which no current test exists. These responses to perturbations can be characterized by theirRecovery Trajectory Duration (RTD). RTD is a measure of the number of paces after the disturbance that it takesfor the subject to return to the unperturbed baseline. Returning astronauts whose orientation mechanism has beendistorted and patients having vestibulopathies that may well affect their orientation mechanism are expected to havelonger RTD’s than healthy normals.

CURRENT STATUS OF RESEARCHMethods. We investigate locomotor stability by applying a disturbance or perturbation. This approach is ananalogue of examining the impulse response often used in the determination of stability of linear and non-linearsystems. The perturbation stimulus is generated by a custom-built moveable balance disturber (BALDER) platform,which can be programmed while the motion of their body segments is optically tracked. Four different perturbationsand one control case (no perturbation) are delivered to the right foot using a randomized Latin squares design. Theperturbations are applied in the X-Y (horizontal) plane at two different amplitudes (5 and 10 cm). Two differentdirections are used: (1) a 45° angle forward and to the right, and (2) a minus 135° angle rearward and to the leftrelative to the direction of walking. To ensure that the subject's left leg is in its swing phase, the onset of perturbationis programmed to occur 200 ms after the detection of the right heel-strike.

Kinematic data are collected using two ganged Optotrak 3020's (Northern Digital, Waterloo, Ont.). They areplaced at a distance of 7.6 m and 13.7 m from the beginning of the walkway respectively, which allows the viewingarea of each 3020 to overlap on the BALDER platform itself. This arrangement provides viewing for the arrays overapproximately 12 m of walkway, depending on the height of the subject.

Two basic quantities of interest are the mediolateral leg separation and mediolateral torso sway, when sampledat points when the displacements of the right and left legs along the line of march (Y-axis) are equal. Torso sway isanalyzed by determining the differential torso sway. That is, we analyze the amount of medio-lateral translation of amarker array placed on the subject's sternum. First, we determine the difference in sternum sway betweenconsecutive steps (X(n+1) - X(n)). Then we divide by the differences for the control trials. This is also done on astep by step basis.

Results. The mediolateral displacement of the legs and torso (Fig. 1) show an underlying periodic component thatcoincides with pacing. Superimposed is a large deviation to the right (downward direction in figure) that occurs afterthe perturbation, marked by the arrow, is applied. This deviation is followed by a partial recovery toward theoriginal line of march, but typically with a small change in direction we call drift. Less obvious is a transientreduction in the separation distance between the two legs. Our preliminary analysis of these data only considers theresponses on a once-per-pace basis that samples the position of both feet and the sternum at the time that one foot isin its support phase and the anterioposterior position of both feet are equal. We further analyze the difference inresponse between successive paces in order to eliminate the slight drift mentioned above, but to still capture thedynamics of the response trajectory.

Fig. 1. Mediolateral leg and torsodisplacements for a 10 cm forward-rightperturbation delivered to the right footduring steady locomotion. Arrow showsdirection of applied disturbance, whilehorizontal bar shows its duration. Solidcircles show locations of the stance foot attimes when both legs have sameanteroposterior position. These events arethe samples that are used for subsequentanalysis.

Fig. 2A shows the averaged difference inresponse for 12 subjects to a 10 cm rightforward perturbation. There is a largeright (downward in the figure) responsethat occurs at the first step after thedisturbance. The trajectory subsequentlycrosses the baseline at the second step toshow a slight underdamped response at thethird step with a return to, or near, thebaseline by the fourth step. The shape andnumber of paces needed to recover istypical for the other three perturbations.In contrast, the recovery trajectory for a

pilot vestibulopathic patient (Fig.2B)takes several more paces to recover. .

Fig. 2. A) Mean normalized, differencedmediolateral sternum displacement for agroup of 12 healthy subjects in responseto a 10 cm forward-right perturbationdelivered to the foot. Errorbars mark ±1 standard error. Recovery to baselineis in three paces. B) Response ofvestibulopathic subject to sameperturbation. Recovery to baseline is inabout five paces.

Conclusion. The vestibulopathic subjecthas normal computerized dynamicposturography scores. This wouldindicate a fairly subtle deficit, and wouldalso suggest that the neuromuscularcomponents of her postural responses arenormal. Thus, one conclusion is that thissubject has a distortion of her orientationmechanism which does not permit her torecover her trajectory in response to aperturbation as rapidly as healthy subjectcan recover.

FUTURE PLANSBy comparing the existing data from normals with the data of vestibulopathic subjects taken over the next year, wewill develop a quantitative, parametric approach for establishing the limits needed to apply this paradigm fordetecting subtle deficits to disturbances of locomotion. We will also coordinate with our JSC counterpart in thecontinued development of the recovery trajectory approach.

MAINTAINING NEUROMUSCULAR CONTRACTION USING SOMATOSENSORYINPUT DURING LONG DURATION SPACEFLIGHT

C.S. Layne1, A. P. Mulavara2, P.V.McDonald2, C.J. Pruett3, and J.J. Bloomberg4

1Department of Health and Human Performance, University of Houston, Houston, TX 77024, 2Wyle Life Sciences,3Tecmath, 4Life Sciences Research Laboratories, NASA-Johnson Space Center

INTRODUCTIONWith the development of the International Space Station (ISS), crewmembers will be spending more time in spacethan ever before. Previous work has indicated that increases in mission duration may lead to greater increases inmuscle decrements than observed after short duration missions. Providing efficient measures to counter the negativeimpact of weightlessness upon the neuromuscular system is important to insure optimal utilization of the ISS. Onepotentially useful technique is the application of pressure to the feet which has been shown to increase inflight lowerlimb muscle activation above that normally observed without foot pressure (Layne, et al., 1998). A foot pressureapplication system that provides controlled patterns of foot pressure could be used to ‘drive’ orderly neuromuscularactivation throughout the course of an ISS mission, thus serving to maintain muscle function. A pressure ‘shoe’could be worn during many inflight activities without interference and serve as a supplement to aerobic and resistiveexercises. For a foot pressure device to be an effective countermeasure, the applied pressure must continue togenerate the increased neuromuscular activation that has been observed during the course of short duration missions.Therefore, the purpose of this investigation is to evaluate the neuromuscular activation data obtained from twocrewmembers who were exposed to foot pressure repeatedly throughout their long duration mission aboard Mir.

CURRENT STATUS OF RESEARCHMethodsTwo male cosmonauts (mean age 44), whose mission lasted 196 days, served as subjects for this investigation. Bothsubjects provided informed consent as approved by the NASA Institutional Review Board for Human Research. Thesubjects were tested four times throughout the mission (Flight Days 13, 56, 141, 188). The subjects performed 15rapid, right-arm 900 shoulder flexions while freefloating in two conditions: with or without foot pressure. Prior toeach trial, subjects adopted a body configuration that mimicked that of upright 1-g stance by aligning their bodysegments in the sagittal plane. The subjects performed the arm movements at a self-selected time with their eyesclosed. After a movement trial, subjects opened their eyes, realigned their body, and repeated the movement untilthe block of movement trials was complete. Pressure to their feet was applied with a pair of ‘boots’ that containedair bladders inflated with a hand-held sphygmomanometer pump. The thin aluminum boots, packed withcomfortable, high-density foam, approximated a men’s size 13 shoe, and weighed 2.2 kg in 1-g. Within the boots,individualized inserts were positioned above the bladders. The inserts had slightly elevated areas under the heels andballs of the feet that when used in combination with the inflated bladders, generated a level and distribution ofpressure that imitated that experienced during 1-g stance.

Surface electromyography was collected from the anterior deltoid (RAD), biceps femoris (RBF), rectus femoris(RRF), right gastrocnemius (RGA), tibialis anterior (RTA) and from the left biceps femoris (LBF) and paraspinals(LPA). A uniaxial accelerometer attached to a wrist splint was used to measure arm tangential acceleration in thesagittal plane.

In the laboratory, the EMG data records were first aligned relative to the initiation of arm motion, as determined by achange in accelerometer voltage. A data “window” was then obtained consisting of 300 ms prior to arm movementinitiation until 50 ms after the completion of arm motion. The data were then averaged and the resulting waveformsreduced to 50 epochs, each epoch representing between 6 and 8 ms of EMG data. The average voltage value of thereduced waveform for each muscle in the no pressure condition was used as the normalizing value. That is, each ofthe 50 voltage values comprising the reduced waveform for each muscle were divided by the normalization valueand then multiplied by 100. The procedure served to set the average voltage value in the no pressure condition to100 percent. Peak arm acceleration was amplitude normalized in a similar manner with the exception that the voltagevalue associated with peak acceleration in the no pressure condition was used as the normalization value. Student t

tests were then used to assess potential amplitude differences between the no pressure and with pressure conditions.Pearson r correlation coefficients were developed between each muscle’s EMG waveform, for each test session, todetermine if the addition of foot pressure during spaceflight altered the phasic features of the waveforms.

ResultsThe data indicate that the enhancements in neuromuscular activation with the addition of foot pressure observedduring the initial inflight testing session were preserved throughout the long duration flight. Additionally, thecorrelation coefficients between the EMG waveforms obtained in the two experimental conditions generallyremained unchanged across the four testing sessions.

CONCLUSIONThe results indicate that the application of foot pressure throughout the course of a long duration spaceflighteffectively increases lower limb neuromuscular activation when accompanying a voluntary arm movement. Thissuggests that it may prove useful to explore more sophisticated forms of delivering foot pressure during spaceflightas a countermeasure to muscle degradation.

FUTURE PLANSThe efficacy of using foot pressure to enhance neuromuscular activation will be explored in a ground-based studyutilizing rats experiencing hindlimb unloading. This project will allow the assessment of cellular features that maybe modified in response to the application of foot pressure during the unloading period.

INDEX TERMS muscle atrophy, neuromuscular activation, counter measures, EMG, humans, somatosensory

EFFECT OF MICROGRAVITY ON AFFERENT INNERVATION

C.D. Fermin1, R.F. Garry2, Y-P. Chen3, and D. Zimmer4

1Depts. of Pathology, 2Microbiology, 3Cell & Mol. Biol. Tulane, New Orleans, LA 70112-2699,and 4Pharmacology, University of South Alabama. Mobile, Al 36688.

INTRODUCTIONTo evaluate bipolar neurons of Aves at 1.0g (ground) and under the effects of microgravity(MIR) on vestibular afferents, fertile quail eggs were flown to MIR on the space Shuttle.Hardware malfunction in MIR did not yield sufficient specimens for analysis and comparison toground controls. We emphasize results from ground experiments, those obtained with fundingduring the last year of the NASA grant, and data obtained since NASA funds ended.

CURRENT STATUS OF RESEARCHThe number of specimens returned from MIR for analysis was not sufficient, and the brain andinner ear tissues were not optimally fixed to allow the morphological and immuno-histochemicalevaluation proposed. Thus, the ground approved portion of the project was expanded, andmolecular evaluation of ground controls was added to the original plan.

RESULTSVestibular neurons (VNs) provide a bridge of communication between the brain and the innerear. VNs are pleomorphic, and analysis of terminal branching by others suggests that the VNsmorphological variation may be related to VNs connection to 5 different end organs. Survival ofVNs after peripheral deafferentation (labyrinthectomy) is longer for VNs than for adjoiningbipolar neurons of the auditory (ANs) or statoacoustic ganglion (SAG) in the VIIIth cranialnerve. Expression of S100 proteins was evaluated after a double immuno-histochemical stainingof VNs cytosol and nuclei in ANs and VNs (Fig. 1). The staining patterns of an antibody to asynthetic peptide of S100B suggested that VNs and ANs expressed S100 differently. Laterattempts for in situ hybridization analysis of S100 mRNA in Aves was hindered by the lack ofavian S100 probes. After identification of an avian S100 mammalian homologue (clone M126),we constructed an antisense riboprobe and corroborated previous observations about VNproperties and diversity. Similar to the differential distribution of S100 proteins over VNs andANs (Fig. 2), there was a differential probe hybridization to the message over VN and ANafferent neurons. S100 mRNA for these interesting calcium binding proteins varies in differentregions of the VG and may reflect a functional and/or survival strategy after injury, orfunctionally induced changes due to external stimulus such as hyper- and micro-gravity.

CONCLUSIONThe results suggest that M126 may be an avian homologue of the mammalian S100 family ofgenes, which were shown by many as excellent neuronal markers in the central and peripheralnervous system. M126 and other avian S100 genes and their proteins may play important rolesin remodeling, structural integrity and/or function of vestibular neurons.

FUTURE PLANSTo compare the hybridization patterns (during development and in mature chickens) of the M126probe with additional avian genes that were recently cloned by other investigators.

INDEX TERMSCalcium binding proteins, S100B avian homologue, chicken, in situ hybridization, immuno-histochemistry, vestibular afferents, vestibular dysfunction, mRNA, gravity, deafferentation.

Figure 1. Panel A.terminal node ofRanvier (arrowheads)of an afferent fiberexpressing S100B.Neurofilament (Gray)pass the habenula(arrow) and over-shadows the brownS100B tint better seenin B & C. Panel B.GABA (B) andS100B (S) positiveafferents in the utricleover boutons (red)and chalice (brown)fibers. Note thatmyelin (positive built-in control) is brown

(arrow). Differential expression of calcium binding proteins such as S100B and neurotransmitter such asGABA were shown to be important markers of neuronal diversity in the CNS and may be important in theperiphery. Panel C. S100B positive chalice terminal of a horizontal crista. Panel D. Vestibular neuronsextending bouton and chalice dendrites are pleomorphic. Their phenotype may be related to innervationpatterns and survival strategies after injury. Large green (asterisk), medium red (arrow) and small yellow(arrowhead) neurons may have different terminals. Panel E. In fact, expression of S100B is restricted tomyelin (asterisk), to only some neurons cytoplasm (arrows) and nucleus (arrowhead), whereas adjacentneurons are completely unreactive, and the staining pattern may be related to neuronal phenotype and/orfunction. Nuclear staining of VNs is common, but still unexplained. Panel F. S100 mRNA is found atthe chalice ends of the vestibular neurons contacting three hair cells (1,2,3).

Figure 2. PanelA. Vestibularganglion (VG) ofpost hatchedchick with M126mRNA positiveneurons. PanelB. Hybridizationof the M126probe over thestatoacousticganglion (SAG)resembles S100Bstaining pattern,in that VGneurons maycontain moremRNA thanSAG neurons atthe age, betterillustrated on the insert (arrow). Panel C. Enlargement of box in A showing that the mRNA variesamong neurons. Panel D. Contrary to the VG neurons, the mRNA is more evenly distributed over SAGneurons. Panel E. Neurons of the medial vestibular nucleus in the brain stem (BS) also had abundantm126 mRNA. These results and those above, suggest that M126 may be an avian homologue of the

mammalian S100 family of proteins which were shown by many as excellent neuronal markers in thecentral and peripheral nervous system.

THE EFFECT OF SPACEFLIGHT ON THE ULTRASTRUCTURE OFADULT RAT CEREBELLAR CORTEX

G.R. Holstein1,2 and G.P. Martinelli3Departments of 1Neurology, 2Cell Biology/Anatomy and 3Surgery, Mount Sinai School of Medicine, New York,NY 10029

INTRODUCTIONExposure to microgravity causes irregularities in posture, locomotion and oculomotor function. The vestibularabnormalities experienced by astronauts entail immediate reflex motor responses, including postural illusions,sensations of rotation, nystagmus, dizziness and vertigo. Behavioral adaptation to the microgravity environmentusually occurs within one week. Although the mechanism(s) underlying this adaptation process remain unclear,current evidence favors some type of sensory conflict theory. The present study was conducted to identify themorphologic alterations in adult rat cerebellar cortex that correlate with short-term adaptation to spaceflight.

CURRENT STATUS OF THE RESEARCHMethodsHindbrain tissue was obtained from rats flown on the Neurolab shuttle mission (STS-90). Tissue for the presentreport was obtained from four adult Fisher 344 rats sacrificed on orbit during flight day 2 (FD2), 24 hr after launch.Equal numbers of vivarium controls and control rats housed in flight-type cages maintained on Earth were sacrificed48 and 96 hr, respectively, after the flight dissections. Following decapitation, hindbrains were immersion-fixed for18 days at 4°C, and then the cerebellum was dissected away from the ventral portion of the brainstem. Allexperiments were performed in accordance with the Principles of Laboratory Animal Care Guide (NIH Pub. 85-23)and were IACUC-approved.

The entire cerebellum of each rat was cut by Vibratome into 100 µm thick parasagittal sections. These sections werecollected serially, processed for electron microscopy, and embedded as tissue wafers in resin between plasticcoverslips. The entire series of wafers from each animal was examined to identify those containing the nodulus. Allsuch wafers were traced, in order to reconstruct the full parasagittal extent of the nodulus for each subject. Thisreconstruction was used to identify the tissue wafer containing the midsagittal section. Using this as the zero point,the wafers from zones located 13-1400 µm and 16-1700 µm from the midline were identified for each rat, sincethese regions receive otolith-related signals. These wafers were dissected, coded, thin-sectioned, and examined usingan Hitachi 7500 transmission electron microscope. No post-microtomy stained was performed.

ResultsCisterns of smooth endoplasmic reticulum that are normally present in Purkinje cells were substantially enlarged andmore complex in the otolith-recipient zones of the nodulus from the FD2 flight animals. The increased complexityof the cisterns resulted in the formation of long, stacked lamellar bodies that were observed throughout entirePurkinje cells, including the somata, dendrites, thorns, and axon terminals. In addition, profoundly enlarged ordistended mitochondria were apparent in some Purkinje cells of the nodulus from FD2 flight animals. The presenceof gigantic mitochondria in Purkinje cells has been suggested by other investigators to serve as an ultrastructuralsign of early cell degeneration reflecting an underlying process of synaptic remodeling. Inter-neuronal cellularprotrusions were also observed in the neuropil of otolith-recipient zones of the nodulus from the FD2 rats. Suchprotrusions suggest enhanced membrane fluidity, and may also possibly reflect an underlying process of neuronalplasticity. Surprisingly, electron-dense degeneration was apparent in Purkinje cell dendrites. Such profiles containedincreased numbers of lysosomses and degenerated mitochondria, but maintained apparently healthy synapticcontacts with normal-appearing axon terminals. In addition to the degeneration apparent in the postsynapticelements, evidence of a separate process of synaptic remodeling was obtained. Synapses involved in this processwere characterized by retraction of the synaptic vesicles away from the region of membrane contact, and theinsertion of an electron-lucent subjunctional organelle between the vesicle cluster and the presynaptic membrane.

The cerebellar molecular layer from the otolith-recipient zones of the nodulus were also examined in control ratsthat were treated identically to the flight animals, but were not exposed to spaceflight. Sets of electron micrographswere taken at random in the molecular layer of each control animal. These random fields were used to evaluate the

tissue condition, and the incidence and prevalence of enlarged mitochondria, neuronal protrusions, and lamellarbodies in the control animals. In this tissue, subsurface cisterns, but few lamellar bodies, were present in thePurkinje cells. In addition, no gigantic mitochondria, no neuronal protrusions, and no evidence for degeneration orsynapse retraction were present in the control tissues. Similarly normal-appearing ultrastructure was present in themolecular layer of cerebellar cortex obtained from a non-vestibular portion of the cerebellum (semilunar lobule)from the FD2 flight rats.

CONCLUSIONSUltrastructural signs of neuronal plasticity and degeneration were observed in the otolith-recipient zones of the adultrat nodulus after 24 hrs of spaceflight, but were not observed in this region in cage-matched ground control animals,or in non-vestibular regions of cerebellum from the FD2 flight rats.

FUTURE PLANSThe immersion fixation protocol developed for the Neurolab experiment provided acceptable overall tissuepreservation. However, vascular perfusion is far superior to immersion fixation for ultrastructural tissuepreservation. Since the results obtained from the Neurolab tissue are unique and intriguing, they require replicationusing traditional ultrastructural methods. This will be accomplished through the examination of cerebellar tissuefrom rats flown on STS-107, and perfusion-fixed immediately following return to Earth.

ACKNOWLEDGEMENTSThe authors are grateful to Dr. Louis Ostrach and Ms. Lisa Baer of NASA for supporting this project, and Dr. EwaKukielka, Ms. Rosemary Lang, and Mr. E.D.MacDonald at Mount Sinai Medical School for invaluable assistancewith the research. The work was supported by NASA grant NAG2-946 and NIH grant DC02451 from the NIDCD.

INDEX TERMSSpaceflight, microgravity, ultrastructure, synaptology, cerebellum, vestibular, otolith, anatomy, morphology,nodulus

DEVELOPING FUTURE COUNTERMEASURES FOR THE DETRIMENTALEFFECTS OF SPACE FLIGHT:

ROLE OF OTOLITH SYSTEMS & RESOLUTION OF TILT/TRANSLATION

F. O. Black1, S. J. Wood1, C. C. Gianna1 and W. H. Paloski2

1 Legacy Health System, Portland, OR; 2 Johnson Space Center, Houston, TX

INTRODUCTIONAdaptation to microgravity drives a re-organization of central nervous system (CNS) processing of three majorsources of spatial information: visual, vestibular and somatosensory (proprioceptive). The nature of this adaptationis reflected by tilt-translation perceptual disturbances and sensorimotor coordination problems experienced byastronauts post-flight. We hypothesize that the absence of a gravity vector leads to adaptive changes in neuralstrategies used for resolving ambiguous linear accelerations detected by the otolith systems. Visual referencesconsequently become critical for orientation on orbit due to the lack of spatial references from graviceptors andcontact surfaces. The scientific goal of this ground based proposal is to understand how otolith-mediated responsesto linear translation and roll tilt can be adapted by altered sensory environments. One question for effectivesensorimotor countermeasure development is the extent to which otolith-mediated responses can be adapted suchthat stimulation normally inducing tilt responses will instead induce translation responses (and vice versa). Our firstset of experiments will address this by utilizing altered visual feedback during dynamic linear acceleration stimuli atlow (0.01 Hz) and high (1 Hz) frequencies normally interpreted as tilt and translation, respectively.

The observation that postural instability immediately after egress appears less severe with increasing flightexperience suggests that experienced astronauts may be able to maintain contextually dependent adaptive states. Thissupports the hypothesis that intermittent exposures to artificial gravity induced by centrifugation can be employed asa sensorimotor countermeasure during long duration space flight missions. Our later set of experiments will addresstwo issues related to the use of artificial gravity as a countermeasure: (1) the extent to which adaptive states areretained (i.e., does the amount of adaptation increase with repeated exposure to specific adaptive stimuli?), and (2)the contextual specificity of the adaptation (i.e., can gravity or an equivalent linear acceleration be used as a cue totrigger which set of possible responses to use?). We will specifically examine if adaptive changes of the humanvestibulo-ocular and vestibulo-spinal systems can be made contextually dependent, the context being provided by theorientation of gravity. Development of effective countermeasures is dependent upon a thorough understanding ofsuch adaptive changes.

CURRENT STATUS OF RESEARCHThis grant was recently funded, and preparations are underway for the initial set of experiments. Our project willstudy the ability of the CNS to adapt to altered visual feedback during dynamic linear acceleration stimuli deliveredusing a short arm variable radius human centrifuge. Two motion profiles will be used to deliver linear translation androll-tilt with subjects seated in an upright position. During one motion profile the radial drive of the centrifuge willbe used as a linear sled to provide pure translation (no rotation) at a frequency of 1 Hz. During the other profile, wewill accelerate the centrifuge at 20°/s2 to a constant angular velocity of 200• /s with the subject at the center. Thisconstant velocity will be held for at least 5 minutes to allow canal effects to decay to zero (or near zero), after whichwe will begin to sinusoidally vary the radius of the centrifuge at a frequency of 0.01 Hz with a peak sinusoidaldisplacement of 0.293 m. Both profiles will yield a sinusoidal interaural shear force with a peak amplitude of 0.364G (20° tilt). These motion profiles will be used for measurement stimuli (pre- and post-adaptation) and alsoselectively used for adaptation stimulation.

We will determine the efficacy of our adaptation protocols with several different measurements, including otolith-mediated vestibulo-ocular reflexes (VOR), roll-tilt perception, and postural stability in the roll-plane. In allexperiments, response variability of each subject will be determined from baseline measures of each response (3-DVOR, perception & postural control) on two separate days prior to the adaptation day and just prior to adaptationstimulation. The adaptation period, from 30 to 90 minutes, will consist of oscillatory motion stimuli accompanied bycorrelated visual stimuli. In some cases the visual stimuli will translate, enhancing translation responses. In othercases the visual display will tilt, enhancing tilt responses. Immediately after the adaptation period, we will

simultaneously measure eye movements and perceptual responses on the centrifuge using the exact same protocol asbefore the adaptation. Pre and post adaptation tests will always be conducted in the following order: measurement ofperception and eye movements at the adapting frequency (low or high) followed immediately by the non-adaptingfrequency (high or low). The subject will then be blindfolded and transported to another room where posturalstability in the roll-plane will be measured. The post-adaptation measurements will be repeated following 2 hours toevaluate the time course of recovery.

Data from our initial experiments will attempt to confirm previous findings which indicate otolith-mediatedtranslation responses can be adaptively increased at high frequencies by concomitant translation of a visual display,and otolith-mediated tilt responses can be adaptively increased at low-frequencies by tilt of a visual display. Twodisplay distances (0.25 and 1m) will allow us to examine the effect of fixation distance on the level of adaptationobtained. Subsequent experiments are designed to induce an interpretation of tilt at high frequencies and translationat low frequencies. In other words, we will use a tilt display to enhance tilt responses at the high frequency, or thetranslation display to enhance translation responses at the low frequency. Measurements across 3 consecutiveadaptation days will quantify the degree of retained adaptation.

FUTURE PLANSAfter establishing the extent to which otolith-mediated tilt and translation responses can be adapted at differentstimulus frequencies, our final series of experiments will examine whether subjects can 'dual adapt' to altered sensoryenvironments using the orientation of gravity to provide context. We will investigate context specific adaptation byusing the tilt display to enhance tilt responses when subjects are upright and using the translation display to enhancetranslation responses when the same subjects are supine. Demonstration of ‘dual-adaptation’ to altered sensoryenvironments will provide insight into the feasibility of using intermittent exposures to artificial gravity induced bycentrifugation as an in-flight sensorimotor countermeasure.

INDEX TERMSotolith, posture, adaptation, centrifuge, countermeasure

1

USE OF THE NEUROLOGIC FUNCTION RATING SCALE FOLLOWING SPACESHUTTLE FLIGHTS

J. B. Clark, J. U. MeirMedical Operations Branch, NASA Johnson Space Center, Houston, Texas 77058

INTRODUCTIONThe most common neurologic difficulties encountered in spaceflight are space motion sickness (SMS) and post-flight neurovestibular symptoms. Understanding neurologic difficulties of spaceflight will allow for missionobjectives to be met in full, increase productivity on short duration flights, ensure that readaptation problems willnot jeopardize the safe landing of pilot controlled spacecraft or the safety of astronauts during emergency egress,and maintain fitness after long duration missions. In order to assess various factors related to SMS in-flight andneurovestibular dysfunction post-flight, an extensive database was created incorporating astronaut medical debriefforms, astronaut aeromedical summary information, and the Neurological Function Rating Scale form from shortduration U.S. Space Shuttle missions. The Neurological Function Rating Scale form was implemented in Novemberof 1996 with STS-80 as a means of assessing any existing neurological dysfunction associated with spaceflight andis part of pre-flight (L-10 and L-2) astronaut physicals, landing day (R+0) physical, and post-flight (R+3) physicalassessment completed by the NASA flight surgeon, as are the astronaut medical debrief forms. The NeurologicalFunction Rating Scale assessment is conducted by the NASA flight surgeon one to four hours post landing onlanding day. The Neurological Function Rating Scale form consists of a series of eleven categories of neurologicalsymptoms and signs or performance measurements scored between 1 (no symptoms or normal performance) and 4(persistent symptoms or severe performance decrement) as determined by the NASA flight surgeon. A total scorebetween 11 (all 1s) and 13 is regarded as normal, 14-15 as suspect, and a score greater than 15 is considered forreferral to the neurovestibular lab for posturography, gaze, and locomotion testing (see Appendix for form example).The subsets of the Neurological Function Rating Scale tests may correspond to operational skills. The first subsetevaluates subjective neurological symptoms (headache, dizziness/faintness, and vertigo/spinning) which coulddistract crewmembers from their tasks and duties. The next subset deals with motor performance skill, which couldinfluence vehicle control, particularly reentry and landing phases. Proper functioning of gaze and ocular movementsis critical to the acquisition and interpretation of visual displays. Neurological disturbances associated withspaceflight can cause delays or incorrect interpretation in the acquisition and processing of visually acquiredinformation. The third subset of Neurological Function Rating assesses gait and station, which are vital toemergency egress.

RESULTSData within the database created and analyzed in this study is solely from U.S. Space Shuttle missions launchedbetween the dates of November 19, 1996 and February 11, 2000 (STS-80 through STS-99). The database accountsfor 112 astronauts, 88 of which are male and 24 of which are female. One individual was outside the generalastronaut population in terms of age and was consequently excluded from any affected analyses. Statistical analysisof database parameters included age; sex; height; crew position; mission activities; mission duration; prophylactic,in-flight, and post-flight medication use for space motion sickness or neurovestibular symptoms; episodes ofvomiting in-flight; orthostatic intolerance upon landing and associated medication use; previous duration ofspaceflight experience; time lapsed since last spaceflight; severity of space motion sickness and neurovestibulardisturbances in previous flight; and flight surgeon rating of likelihood of successful egress.

CONCLUSIONSThe most severe neurological spaceflight deficits on the Neurological Function Rating Scale are the Gait and Stationsubset. Commanders and pilots may have a more stable landing day performance than other crewmember positionsfor total score on the Neurological Function Rating Scale tests. Gaze and ocular movement function is affected afterspaceflight. The neurovestibular rating score from previous flight is a good predictor for the probability ofdistribution of failure index scores in this database. Previous flight experience may result in less performance deficiton post-flight Neurological Function Rating Scale test scores, particularly within the Gait and Station subset. Spacemotion sickness and neurovestibular symptom scores from previous flights are likely to be good predictors for spacemotion sickness, though both may be contributing through separate mechanisms.

VARIED PRACTICE AND RESPONSE GENERALIZATION AS THE BASIS FORSENSORIMOTOR COUNTERMEASURES

Helen S. Cohen, EdD1, Jacob J. Bloomberg, PhD2, Carrie Roller, MD1, Ajitkumar Mulavara,PhD3

1) Bobby R. Alford Department of Otorhinolaryngology and Communicative Sciences, BaylorCollege of Medicine, Houston, TX

2) Neuroscience Laboratory, NASA/ Johnson Space Center, Houston, TX3) Wyle Laboratories, Houston, TX

IntroductionFollowing exposure to long duration space flight astronauts have a variety of motor

control problems readapting to the terrestrial 1G environment. These problems include ataxia,disequilibrium, oscillopsia and deficits in spatial orientation. These sensorimotor deficits cansignificantly affect mission safety. This issue will be particularly problematic on early missionswhen no established base camp, staffed by astronauts who will have become adapted to thatenvironment, will be available.

The evidence from patients with severe bilateral and unilateral vestibular impairmentssuggests that ataxia, disequilibrium, oscillopsia and spatial orientation deficits cause functionallimitations. In ordinary conditions, when performing well-overlearned motor tasks, theseproblems have mild to moderate impact, depending on the presence of other health conditionsand the individual’s ability to adapt or modify his or her motor skills. These problems haveserious functional significance in patients when their motor skills are challenged by unpredictableor unusual requirements, as when walking over uneven terrain, negotiating around unpredictableobstacles, or operating complex equipment that requires cognitive multi-tasking, such as drivinga car in fast-moving traffic or under degraded visual conditions. Parallels can be made to routinedaily life tasks of astronauts, such as walking rapidly over uneven and unpredictable terrain, andoperating high-performance aircraft or landing the Orbiter.

Given the known occurrence of sensorimotor problems in the acute re-adaptation periodin Mir astronauts, following a transit to Mars crewmembers will probably experience similarproblems. One approach to dealing with this issue is to develop preventative countermeasuresthat astronauts will practice during the transit. These countermeasures will be constrained byseveral factors, including space and equipment within the vehicle, and astronauts’ time. Sincenormal movement is goal-directed and clinical rehabilitation strategies are most effective whengoal-directed activities are incorporated into the regimen, training tasks should be goal-directed.Countermeasures should require the participant to solve sensorimotor problems and thosesolutions should involve developing adaptive motor strategies. No countermeasure can specify asingle strategy for all astronauts, and no countermeasure can duplicate the unique conditions thatastronauts will experience on a new planetary surface. Therefore, one approach to thedevelopment of a sensorimotor countermeasure is to develop a training paradigm aimed not atdeveloping specific splinter skills or modifying specific reflexes in mechanistic ways, but atfacilitating adaptive plasticity in motor planning so that the individual will be able to develop

unique adaptive strategies that can be applied under novel conditions in response to newsensorimotor demands.

The goal of the present series of ground-based studies is to develop the basis forsensorimotor countermeasures that utilize adaptive generalization as a training paradigm. Futurespaceflight studies will then apply these techniques and will test the efficacy of a specific trainingregimen that uses practices variability.

General Methodology and AnalysesSubjects, normal adults or patients with specific vestibular impairments, are randomized

to sham, variable practice and blocked practice groups in which they wear lenses with no specialoptical properties (sham), only one set of lenses or (blocked), or X 2.0 magnifying, up/ downreversing, and 20° left shift lenses (variable). After training while performing various eye-handor eye-foot coordination tasks post-test transfer trials are performed with a novel set of lenses.Retention tests may also be performed, depending on the specific experiment being performed.

Several dependent measures are used, depending on the specific experiment. Thesemeasures include obstacle avoidance, dynamic visual acuity, gait kinematics, and target accuracy.To compare across groups within each experiment data are then analyzed with the appropriatelevel of statistical analyses. This poster will present specific results from several experiments, allusing the practice variability paradigm.

Supported by NIH grant DC04167 and the Clayton Foundation for Research.

SOMATOSENSORY SUPPRESSION OF RE-ENTRY DISTURBANCES

P. DiZio and J.R. Lackner . Ashton Graybiel Spatial Orientation Laboratory, BrandeisUniversity MS033, Waltham, MA 02545

INTRODUCTIONPost-flight derangements of posture and locomotion are experienced by astronauts. Thesereentry disturbances are operationally significant after flights lasting several days and are greatlyexacerbated after flights lasting months. In the future, astronauts may be required to perform notonly in weightless environments but also in artificial gravity environments and differentplanetary environments, such as Mars gravity. Transitions between these various forceenvironments will severely tax sensory-motor control of movement, posture, and orientation.Significant disturbances can be anticipated until adaptation to the new force milieu is achieved.

CURRENT STATUS OF RESEARCHBackgroundWe have developed a way of stabilizing perceived orientation and balance that we hypothesizewill minimize sensory-motor reentry disturbances exhibited by astronauts. In normal, blind, andlabyrinthine defective subjects under 1g conditions, we have demonstrated that contact of theindex finger with a stationary surface has a powerful stabilizing effect on postural control. Suchhaptic contact or precision touch at mechanically non-supportive force levels provides sensoryinformation about body position and sway that is more effective than visual or vestibularinformation in stabilizing the body. We propose to test the role of haptic contact in stabilizationof posture and gait in a series of systematic studies using parabolic flight and slow rotation roomenvironments as analogs of space flight environments.

Preliminary ResultsPreliminary results from a parabolic flight experiments show that 1) patterns resembling re-entrydisturbances are briefly experienced after exposure to parabolic flight maneuvers; 2) light touchcues from the finger can be used both to suppress the aftereffects of exposure to theseenvironments and to hasten re-adaptation to the normal force environment.

Postural stability of eight individuals was tested immediatelybefore and after parabolic flight missions. During parabolicflight (40 parabolas of alternating 1.8 and 0 g, 25 sec of eachper parabola), the subjects were required to stand or moveabout. They remained seated after the last parabola untiltested post-flight in the aircraft, within 3 minutes after theplane stopped on the runway. For both pre- and post-flighttesting the subjects attempted to stand, eyes closed, heel-to-toe, on the aircraft deck. They alternated precision touch(<100g) and no touch trials, 30 sec in duration. AnOPTOTRAK system monitored head (H) and center of mass(CM) position, and mean sway amplitudes (MSAs) of H andCM were computed.

0

0.4

0.8

1.2

1.6

2

Pre-flight Post-flight

(cm

)

- No touch- Touch

MSA of Lateral CM

Post-flight, most subjects could not stand for more than a few seconds without grasping a safetyrail when denied touch of the hand, all could do so pre-flight. MSAs of H and CM weresignificantly elevated post-flight relative to pre-flight without touch. With precision touch, theMSAs did not differ significantly pre-flight versus post-flight. In the four post-flight trialswithout touch, the MSAs of H and CM decayed linearly from elevated levels toward the pre-flight baseline.

ConclusionThese results indicate that parabolic flight is an effective model for studying reentrydisturbances, and suggest that precision touch attenuates aftereffects and possibly hastens re-adaptation.

NEUROVESTIBULAR ASPECTS OF ARTIFICIAL GRAVITY

HEIKO HECHT & LAURENCE R. YOUNGMassachusetts Institute of Technology

INTRODUCTIONTraditional countermeasures against the adverse effects of prolonged weightlessness, such as exercise, resistive

garments and lower-body negative pressure, appear to be insufficient in practice and are often too inconvenient forastronauts. AG represents a potential countermeasure that is unique. It promises salutary effects on bone, muscle,cardiovascular and vestibular function. Rather than alleviating the symptoms, it attempts to remove their cause.Although long a favorite topic of scientists and science fiction authors, it is only now receiving serious attention forspace flight experiments and validation. Spacecraft size dictates that any AG centrifuge tested in the foreseeable futurebe of limited radius (on the order of 1-3 m). The largest diameter human centrifuge being considered for installation onSpacehab, is under 2.5m in diameter, thus permitting a short astronaut only to sit or bicycle, but not to stand up.Centripetal accelerations on the order of 1 g (9.8 m/sec2) at the rim will therefore require relatively high angularvelocities (on the order of 30 rpm). At these speeds, AG will create disruptive sensory effects as soon as the astronautstarts to move, and movement is mandatory during long-term centrifugation (e. g. in a rotating spacecraft) and it isdesirable during intermittent centrifugation (e. g. when combined with exercise). Thus, AG for in-flight gravityreplacement therapy requires that crewmembers be capable of rapidly adapting to the unexpected canal inputs withminimal side- or after-effects. Furthermore, it will be essential for astronauts to retain the adaptation to the 0-g state inorder to avoid “Space Adaptation Syndrome” each time they transition from the centrifuge to weightlessness.

The funded research will addresses some of the most important questions requiring answers prior to AGimplementation for a long mission. We will investigate if head and body movements during high rate AG are tolerableand how such AG can be implemented most efficiently. We further plan to investigate methods to minimize theundesirable side-effects of multiple neurovestibular adaptation associated with intermittent AG. The co-investigatorson the AG project are Bernard Cohen (Mount Sinai Medical Center), Malcolm M. Cohen (NASA Ames ResearchCenter), Mingjia Dai (Mt. Sinai), Paul DiZio (Brandeis University), James Lackner (Brandeis), Fred Mast(Harvard/MIT), Charles M. Oman (MIT), William H. Paloski (NASA Johnson Space Center), Lee Stone (NASAAmes), Robert B. Welch (NASA Ames).

CURRENT STATUS OF RESEARCHOur (MIT) experiments on the Short-Radius Centrifuge (SRC) encourage the use of a SRC as a viable

countermeasure. Inappropriate eye movements (vestibulo-ocular reflexes), motion sickness and perceptual illusions areall reduced after several adaptation periods. Short daily exposures to head movements while rotating appear to yieldsignificant adaptation. Additionally, our (Mt Sinai) experience with intermittent off-axis rotation on the Neurolabrotator demonstrated tolerance to high rotation rates and centrifugation in space. Our (Brandeis) Slow Rotating Room(SRR) has yielded a wealth of information concerning the process of sensorimotor adaptation to movements in arotating framework. Our (JSC) experiments show important adaptive and maladaptive changes in head and bodycontrol following centrifugation.

FUTURE PLANSAG feasibility may be limited by the potential side-effects that accompany adaptation to a rotating environment.

The negative experiences of the IML-1 crew to in-flight rotation advise caution and thorough ground-based research.We currently lack a full understanding of the mechanism and the limits of adaptation. For instance, we do not know ifintermittent or continuous AG works best, and AG has not yet been put to a serious test with humans in a 0-genvironment. Since very few studies have investigated adaptation to short-radius, high-rate centrifugation, we willextend this knowledge to the particular case of short-radius centrifugation.

Altered sensory environments (such as generated by weightlessness) generate disturbing motor-sensory feedbackwhenever movements are made. If the altered environment is rotating, as on a centrifuge, these sensory effects arecomplicated by Coriolis forces and inappropriate signals from the semicircular canals. People adapt to such sensoryrearrangement changes, but they normally adapt slowly over the course of several days or even weeks. Short-radiusAG as a countermeasure is designed to deal with space missions in a very particular fashion. Our senses and motorsystem still need to function in 0-g. Thus, the astronaut must adapt to function effectively in two environments,centrifugation and 0-g. This includes exercise and probably recreation during centrifugation. And consequently headand limb movements will have to be made during centrifugation. AG will work only if the sensorimotor system can befunctional in different g-environments while requiring very little or no time to switch between adaptive states. Suchstate changes need to be made smoothly and with minimal adverse effects (e. g. without motion sickness). That is,context-specific adaptation has to be acquired and maintained over longer periods. The goals our research are to gaininsights into how the motor and perceptual systems are able to adapt in this context-specific manner and to use theseinsights to develop practical AG countermeasure protocols. We will present our unified research program that consistsof two categories. The first (basic aims) attempts to understand the basic mechanisms underlying context-specificadaptation. The second (countermeasure development) involves applied questions related to optimizing the conditionsfor adaptation.

INDEX TERMS: Countermeasure, Artificial Gravity, Neurovestibular, Motion Sickness,Centrifugation, Sensory Illusions, Coriolis

Structure of the AG proposal

Underst anding the mechanism( basic aims)

Count ermeasure Development( applied aims)

Opt im iz ing the adapt at ion schedule (Aim 4 )

Inf luence of grav icept ive inf ormat ion ( Aim 5 )

Adapt ive generalizat ion ( Aim 6 )

Task-relat ed measures of adapt at ion ( Aim 7)

Mot ion-s ickness drugs and adapt at ion ( Aim 8)

Role of ext ra-vest ibular s ignals (Aim 2)

Intersensory integrat ion ( Aim 3 )

Cont ex t cues f or adaptat ion ( Aim 1)

RESPONSES OF EYE MOVEMENT RELATED VESTIBULARNEURONS TO LINEAR ACCELERATION.

W. M. King1, Wu Zhou

2, and Bingfeng Tang

3 Departments of Neurology

1,2, Anatomy

1 &

Surgery2, University of Mississippi Medical Center, Jackson, MS 39216

The linear vestibulo-ocular reflex (LVOR) stabilizes a point of interest, not the whole visualfield. Thus, the amplitude and direction of the LVOR are not fixed, but are determined by targetdistance and location. To study the neural mechanisms that transform head translation signalsinto eye movement commands, eye movement related vestibular neurons in monkeys wereidentified using head rotations and eye movement tasks. Based on eye movement and headrotation on-directions, vestibular neurons were classified as position-vestibular-pause (PVP,n=51; eye and head on-directions opposite) and eye-head velocity (EHV, n=42; eye and headon-directions the same) units. Neurons were tested during sinusoidal angular rotation (freq. =02.5 Hz, 30 d/s peak) and linear translation (freq. = 1Hz, 0.2g peak) and/or transient linearaccelerations (peak 0.4g, 100ms). Only 1 of 51 PVP neurons exhibited a response to linearacceleration during this paradigm. Of 5 PVP neurons tested with transient acceleration, only oneexhibited a response to head translation. Nine of the 42 EHV units displayed responses to linearacceleration during the cancellation paradigm. However, 8 EHV units that were unresponsiveduring steady state acceleration at 1Hz responded to transient linear accelerations. Furthermore,8 of 11 EH units exhibited modulation of their firing rates with viewing distance during transientlinear accelerations. These data suggest that most PVP units do not carry signals directly relatedto linear acceleration. In contrast, many EHV units encode linear acceleration signals, and inmany EHV cells, these signals are modulated by viewing distance. Supported by NEI 04045and NSBRI.

INFLUENCE OF SENSORY INTEGRATION ON THENEURAL PROCESSING OF GRAVITO-INERTIAL CUES

Dan MerfeldJenks Vestibular Physiology Laboratory, Mass. Eye and Ear Infirmary/Harvard Medical School

INTRODUCTIONOur sense of spatial orientation results from a complex set of neural processes of sensory

integration; these processes utilize information from many different physiological systems.Evidence suggests that extended exposure to micro-gravity yields adaptive changes in theseneural processes. For the most part, these adaptive changes appear functionally relevant forspaceflight but yield significant transient problems when astronauts return to gravitationalenvironments (Earth, Mars, Moon, etc.). In-flight countermeasures that reinforce the normalsensory interactions encountered in a gravitational environment (e.g., rotations about any axisnot aligned with gravity are accompanied by changes in the relative orientation of gravity) willalmost certainly help astronauts function adequately when they transition to a gravitationalenvironment. This will be important to pilot astronauts landing spacecraft as well as tocrewmembers trying to rapidly exit a spacecraft. Furthermore, artificial gravity has beenproposed as a countermeasure for numerous problems associated with spaceflight (includingcardiovascular deconditioning, bone decalcification, etc.). One undesired side effect of artificialgravity will be additional alterations in these processes of sensory integration. For all of thesereasons, it is essential that we understand the predominant sensory interactions that are altered byexposure to micro-gravity.

CURRENT STATUS OF RESEARCHProject is in definition phase.

FUTURE PLANSWe proposed to address these problems by extending and expanding previous preflight-

postflight studies investigating adaptive changes in sensory interactions using astronaut subjects.Specifically, we proposed preflight/postflight investigations of the sensory interactions betweenthe semicircular canals and graviceptors. In parallel with these flight studies, we proposed aseries of ground experiments that seek a better understanding of sensory interactions between thesemicircular canals and graviceptors. We proposed to perform these investigations using severaldifferent measurement techniques including manual control methods, measures of reflexiveresponses (including eye movements), as well as psychophysical measures of perception.

INDEX TERMSVestibular, Semicircular Canals, Otolith Organs, Vestibulo-ocular Reflex, Eye

Movements, Human, Perception, Psychophysics, Manual Control, Spaceflight

INFLIGHT CENTRIFUGATION AS A COUNTERMEASURE FORDECONDITIONING OF OTOLITH-BASED REFLEXES

Steven T. Moore1, Gilles Clement2, Andre Diedrich3, Italo Biaggioni3, Horacio Kaufmann1,Theodore Raphan4, Bernard Cohen1

1Neurology Dept., Mount Sinai School of Medicine, New York NY, 2CNRS, Toulouse, France,3Vanderbilt University, Nashville, TN, 4Brooklyn College, City University of New York,Brooklyn, NY.

INTRODUCTIONThe human balance system comprises of the semi-circular canals, which sense rotation of the head, and the otoliths,which act as linear accelerometers. On Earth, the otoliths sense the constant linear acceleration of gravity, and thisinformation is used by the brain to determine the spatial vertical, and the orientation of the head with respect to thevertical. This information is critical in controlling our posture and eye movements during everyday activities such aswalking and driving an automobile. In addition, recent studies have suggested that the otoliths play a role in theactivation of sympathetic outflow in response to changes in posture, triggering a vestibulo-sympathetic reflex whichproduces changes in heart rate and vascular tone that contributes to maintain blood flow to the brain duringorthostatic stress.During our 1998 Neurolab (STS-90) experiment, four payload crewmembers were exposed to artificial gravity (a1-g or 0.5-g centripetal acceleration) generated by in-flight centrifugation. In contrast to previous post-flight studies,both in-flight and post-flight measures of otolith-ocular function were unimpaired. Post-flight tests also indicated nosymptoms of orthostatic intolerance (an inability to maintain blood flow to the brain) in all four payload crew. Thisis an unlikely occurrence if the finding that 64% of astronauts experience profound symptoms of post-flightorthostatic intolerance (Buckey et al. J Appl Physiol 1996; Fritsch Yelle et al. J Appl Physiol 1996) is a generalphenomenon. In addition, preliminary data suggests that sympathetically-mediated vasoconstriction was bettermaintained in the payload crew compared to two other crewmembers not exposed to in-flight centrifugation. Apossible explanation for these results is that intermittent exposure to artificial gravity during the 16-day mission hadprevented deconditioning of otolith-ocular and vestibulo-sympathetic reflexes in the microgravity environment.

The aim of the current proposal is to obtain control measures of otolith and orthostatic function followingshort duration missions, utilizing techniques developed for the Neurolab flight, from astronauts who have not beenexposed to in-flight centrifugation. This will enable a direct comparison with data obtained from the Neurolab crew.Deficits in otolith-mediated responses, specifically ocular counter-rolling and spatial orientation of the angularvestibulo-ocular reflex, would support the hypothesis that intermittent exposure to in-flight centripetal accelerationis a countermeasure for otolith deconditioning. Furthermore, a correlation between post-flight otolith deconditioningand orthostatic intolerance would establish an otolithic basis for this condition.

CURRENT STATUS OF RESEARCH

MethodsWe plan to perform pre- and post-flight testing on astronauts after short duration shuttle missions. The techniquesused to assess vestibular function and orthostatic tolerance are similar to those developed for the Neurolab mission,and are summarized below.

Vestibular TestingSubjects will be tested pre- and post-flight during off-axis centrifugation. Subjects will be oriented tangentially andface the direction of motion, and we will measure the resultant three-dimensional (3D) eye movements as well as tiltperception (the somatogravic illusion). At constant angular velocity, a 1-g interaural centripetal acceleration will begenerated at the otoliths, which when added to the 1-g dorsoventral gravity component, tilts the gravito-inertialacceleration (GIA) vector 45° relative to head vertical. This will induce rotation of the eyes towards the GIA (ocularcounter-rolling or OCR), an otolith-mediated reflex (Fig. 1). Off-axis rotation also activates the angular vestibulo-ocular reflex (aVOR) during angular acceleration and onset of constant velocity rotation, which generates ahorizontal eye velocity. An otolith-dependent vertical eye velocity component also develops that tends to align the

eye velocity axis with the tilted GIA (we term this ‘spatial orientation of the aVOR’). Subjects will also be presentedwith horizontal and vertical optokinetic stimuli during constant velocity centrifugation. Optokinetic nystagmus(OKN) also exhibits spatial orientation. That is, a vertical component appears during yaw optokinetic stimulationthat tends to orient the axis of eye velocity towards the tilted GIA. The gain of OCR, and spatial orientation of theaVOR and OKN toward a tilted GIA, allows us to assess the effect of microgravity exposure on otolith-ocularreflexes.

Fig. 1. Short-arm centrifugation as a means to assess otolith function. A. The subject is shown in the left-ear-outorientation. Constant rotation at 254 °/s generates a centripetal linear acceleration, Ac, of 1-g, which adds with the1-g of gravity, Ag, to tilt the GIA vector 45° with respect to the head. B. Subjects perceive the GIA as the spatialvertical, and feel a strong sense of tilt in the opposite direction, termed the somato-gravic illusion. The eyes tend torotate about the line of sight towards the GIA, which is an otolith-mediated reflex (ocular counter-rolling or OCR).Thus, post-flight changes in OCR gain would suggest deconditioning of otolith-based reflexes.

Orthostatic ToleranceAstronauts will be tested pre- and post-flight. Orthostatic tolerance will be ascertained by monitoring heart rate andblood pressure during a standardized tilt test as previously used for the Neurolab mission. Segmental impedance willbe used to estimate fluid shifts and stroke volume, from which vascular resistance can be calculated and used as anestimate of sympathetic activation. We will also use spectral analysis of heart rate and blood pressure as anadditional parameter to estimate autonomic responses. Any changes in these parameters produced by post-flighthead-up full body tilt will be correlated with measures of post-flight otolith-ocular function obtained duringcentrifugation.

FUTURE PLANS

This project is currently in the flight definition phase. We plan to test approximately 12 astronaut subjects both priorto and upon return from short-duration missions during the assembly stage of the International Space Station. Wewill compare the post-flight function of otolith-ocular reflexes with pre-flight data to gauge the effect ofmicrogravity exposure on otolith function. Any deficits in these otolith-mediated eye movements would support thehypothesis that centrifugation during the Neurolab flight helped to maintain otolith function. We will also attempt tocorrelate any deficits in sympathetically-mediated vasoconstriction during head-up tilt with otolith-ocular function.This may provide an otolithic basis for post-flight orthostatic intolerance.

INDEX TERMSartificial gravity, centrifugation, countermeasure, eye movements, vestibulo-ocular reflex, orthostatic intolerance

G I A Αg

Αc

A B

GIA

subject's percept

THE INFLUENCE OF VISUAL ROTATIONAL CUESON HUMAN ORIENTATION AND EYE MOVEMENTS

Lionel Zupan, Daniel M. Merfeld, Kellin KingJenks Vestibular Physiology Laboratory, Mass. Eye and Ear Infirmary/Harvard Medical School

INTRODUCTIONSensory systems often provide ambiguous information. For example, otolith organs

measure gravito-inertial force (GIF), the sum of gravitational force and inertial force due tolinear acceleration. However, according to Einstein's equivalence principle, gravitational force isindistinguishable from inertial force. Therefore, the central nervous system (CNS) must useother sensory cues to distinguish tilt from translation. For example, the CNS can use visual cuesproviding motion information. The GIF resolution hypothesis states that the CNS estimatesgravity and linear acceleration such that the difference between these estimates match themeasured GIF. Due to sensory interactions, the hypothesis predicts that inaccurate estimates ofgravity and linear acceleration can occur. Specifically, the hypothesis predicts that illusory tiltcaused by roll optokinetic cues should lead to a horizontal VOR compensatory for a interauralneural representation of linear acceleration, even in the absence of true interaural linearacceleration.

CURRENT STATUS OF RESEARCHMethods

To investigate this prediction, we measured eye movements (binocularly using infraredvideo methods) in 17 subjects during and after roll optokinetic stimulation about the subject'snaso-occipital axis (60º/s, clockwise or counterclockwise). The optokinetic stimulation wasapplied for 60s followed by 30s in darkness. We simultaneously measured subjective roll tiltusing a somatosensory bar. Each subject was tested in 5 different orientations: upright, pitchedforward 5º or 10º, pitched backward 5º or 10º.

ResultsEight subjects reported subjective roll tilts (>10º) in directions consistent with the

direction of the optokinetic stimulation . Besides the torsional optokinetic afternystagmus, weobserved for all orientations a horizontal afternystagmus to the right following clockwisestimulation and to the left following counterclockwise stimulation. These observations are inagreement with the GIF resolution hypothesis that suggests that a subjective tilt in the absence ofreal tilt should induce a non-zero estimate of interaural linear acceleration, and therefore ahorizontal VOR. On the contrary, an axis-shift component toward alignment with gravity doesnot account for these observations since it would reverse between pitched forward and backwardorientations.

FUTURE PLANSWe plan to continue to investigate sensory integration in humans. Specifically, we plan

to continue to investigate how rotational cues influences the neural processing of ambiguousgravito-inertial cues.

INDEX TERMSVestibular, Semicircular Canals, Otolith Organs, Vestibulo-ocular Reflex, Eye Movements,Human, Perception, Psychophysics