Virtual reality as a countermeasure for astronaut motion sickness during simulated post-flight water landings

Entry motion sickness (EMS) affects crewmembers upon return to Earth following extended adaptation to microgravity. Anticholinergic pharmaceuticals (e.g., Meclizine) are often taken prior to landing; however, they have operationally adverse side effects (e.g., drowsiness). There is a need to develop non-pharmaceutical countermeasures to EMS. We assessed the efficacy of a technological countermeasure providing external visual cues following splashdown, where otherwise only nauseogenic internal cabin visual references are available. Our countermeasure provided motion-congruent visual cues of an Earth-fixed scene in virtual reality, which was compared to a control condition with a head-fixed fixation point in virtual reality in a between-subject design with 15 subjects in each group. We tested the countermeasure's effectiveness at mitigating motion sickness symptoms at the end of a ground-based reentry analog: approximately 1 h of 2Gx centrifugation followed by up to 1 h of wave-like motion. Secondarily, we explored differences in vestibular-mediated balance performance between the two conditions. While Motion Sickness Questionnaire outcomes did not differ detectably between groups, we found significantly better survival rates (with dropout dictated by reporting moderate nausea consecutively over 2 min) in the visual countermeasure group than the control group (79% survival vs. 33%, t(14) = 2.50, p = 0.027). Following the reentry analogs, subjects demonstrated significantly higher sway prior to recovery (p = 0.0004), which did not differ between control and countermeasure groups. These results imply that providing motion-congruent visual cues may be an effective mean for curbing the development of moderate nausea and increasing comfort following future space missions.

Influence of gravitoinertial force level on the subjective vertical during recumbent yaw axis body tilt

We tilted recumbent subjects at various angles about their yaw (foot to head) axis and had them indicate the direction of their subjective vertical and apparent head midline about the same axis. One set of tests was conducted during parabolic flight maneuvers where the background gravitoinertial acceleration varied from 0 to 1.8g. The blindfolded subjects (n = 6) were tested supine and at tilts of 60 degrees and 30 degrees left and right about their horizontal long body axis. They used a gravity neutral joystick to indicate their subjective vertical or their head midline continuously from the high force through the 0g portions of parabolas. In 0g, all subjects felt supine and oriented the joystick perpendicular to their body when indicating the subjective vertical. This points to strong influences of the symmetric somatic touch and pressure cues from the apparatus on orientation when the otolith organs are unloaded. In contrast to the settings in 0g, settings of the subjective vertical in 1g and 1.8g varied as a function of body orientation. However, the settings did not differ between 1g and 1.8g test conditions. Subjective vertical judgments were also made by subjects (n = 11) in the Brandeis slow rotation room, with the room stationary and rotating at a speed that produced a 2g resultant of gravitational and centrifugal acceleration. There were no differences between settings of the subjective vertical made in 1g and 2g. The similarity of 1g and hyper-g settings during recumbent yaw tilts, both in parabolic flight and in the rotating room, contrasts with the previously observed, strong influence which force levels above 1g have on settings of the subjective vertical during tilt of the body in pitch or roll. The findings for all three axes are consistent with a recently developed model of static spatial orientation.

Mechanisms of human static spatial orientation

We have developed a tri-axial model of spatial orientation applicable to static 1g and non-1g environments. The model attempts to capture the mechanics of otolith organ transduction of static linear forces and the perceptual computations performed on these sensor signals to yield subjective orientation of the vertical direction relative to the head. Our model differs from other treatments that involve computing the gravitoinertial force (GIF) vector in three independent dimensions. The perceptual component of our model embodies the idea that the central nervous system processes utricular and saccular stimuli as if they were produced by a GIF vector equal to 1g, even when it differs in magnitude, because in the course of evolution living creatures have always experienced gravity as a constant. We determine just two independent angles of head orientation relative to the vertical that are GIF dependent, the third angle being derived from the first two and being GIF independent. Somatosensory stimulation is used to resolve our vestibular model's ambiguity of the up-down directions. Our otolith mechanical model takes into account recently established non-linear behavior of the force-displacement relationship of the otoconia, and possible otoconial deflections that are not co-linear with the direction of the input force (cross-talk). The free parameters of our model relate entirely to the mechanical otolith model. They were determined by fitting the integrated mechanical/perceptual model to subjective indications of the vertical obtained during pitch and roll body tilts in 1g and 2g force backgrounds and during recumbent yaw tilts in 1g. The complete data set was fit with very little residual error. A novel prediction of the model is that background force magnitude either lower or higher than 1g will not affect subjective vertical judgments during recumbent yaw tilt. These predictions have been confirmed in recent parabolic flight experiments.

Vertical linear self-motion perception during visual and inertial motion: More than weighted summation of sensory inputs

We evaluated visual and vestibular contributions to vertical self motion perception by exposing subjects to various combinations of 0.2 Hz vertical linear oscillation and visual scene motion. The visual stimuli presented via a head-mounted display consisted of video recordings of the test chamber from the perspective of the subject seated in the oscillator. In the dark, subjects accurately reported the amplitude of vertical linear oscillation with only a slight tendency to underestimate it. In the absence of inertial motion, even low amplitude oscillatory visual motion induced the perception of vertical self-oscillation. When visual and vestibular stimulation were combined, self-motion perception persisted in the presence of large visual-vestibular discordances. A dynamic visual input with magnitude discrepancies tended to dominate the resulting apparent self-motion, but vestibular effects were also evident. With visual and vestibular stimulation either spatially or temporally out-of-phase with one another, the input that dominated depended on their amplitudes. High amplitude visual scene motion was almost completely dominant for the levels tested. These findings are inconsistent with self-motion perception being determined by simple weighted summation of visual and vestibular inputs and constitute evidence against sensory conflict models. They indicate that when the presented visual scene is an accurate representation of the physical test environment, it dominates over vestibular inputs in determining apparent spatial position relative to external space.

Vertical linear self-motion perception during visual and inertial motion: more than weighted summation of sensory inputs

We evaluated visual and vestibular contributions to vertical self motion perception by exposing subjects to various combinations of 0.2 Hz vertical linear oscillation and visual scene motion. The visual stimuli presented via a head-mounted display consisted of video recordings of the test chamber from the perspective of the subject seated in the oscillator. In the dark, subjects accurately reported the amplitude of vertical linear oscillation with only a slight tendency to underestimate it. In the absence of inertial motion, even low amplitude oscillatory visual motion induced the perception of vertical self-oscillation. When visual and vestibular stimulation were combined, self-motion perception persisted in the presence of large visual-vestibular discordances. A dynamic visual input with magnitude discrepancies tended to dominate the resulting apparent self-motion, but vestibular effects were also evident. With visual and vestibular stimulation either spatially or temporally out-of-phase with one another, the input that dominated depended on their amplitudes. High amplitude visual scene motion was almost completely dominant for the levels tested. These findings are inconsistent with self-motion perception being determined by simple weighted summation of visual and vestibular inputs and constitute evidence against sensory conflict models. They indicate that when the presented visual scene is an accurate representation of the physical test environment, it dominates over vestibular inputs in determining apparent spatial position relative to external space.

Analysis of human postural responses to recoverable falls

We studied the kinematics and kinetics of human postural responses to "recoverable falls." To induce brief falling we used a Hold and Release (H&R) paradigm. Standing subjects actively resisted a force applied to their sternum. When this force was quickly released they were suddenly off balance. For a brief period, approximately 125 ms, until restoring forces were generated to shift the center of foot pressure in front of the center of mass, the body was in a forward fall acted on by gravity and ground support forces. We were able to describe the whole-body postural behavior following release using a multilink inverted pendulum model in a regime of "small oscillations." A three-segment model incorporating upper body, upper leg, and lower leg, with active stiffness and damping at the joints was fully adequate to fit the kinematic data from all conditions. The significance of our findings is that in situations involving recoverable falls or loss of balance the earliest responses are likely dependent on actively-tuned, reflexive mechanisms yielding stiffness and damping modulation of the joints. We demonstrate that haptic cues from index fingertip contact with a stationary surface lead to a significantly smaller angular displacement of the torso and a more rapid recovery of balance. Our H&R paradigm and associated model provide a quantifiable approach to studying recovery from potential falling in normal and clinical subjects.

Stabilization of posture by precision touch of the index finger with rigid and flexible filaments

Light touch of the index finger with a stationary surface at non-mechanically supportive force levels (<100 g) greatly attenuates the body sway of standing subjects. In three experiments, we evaluated the properties of finger contact and of the contacted object necessary to produce postural stabilization in subjects standing heel-to-toe with eyes closed, as well as how accurately hand position can be controlled. Experiment 1 involved finger contact with flexible filaments of different bending strengths, a flat surface, and an imagined spatial position. Contact with the flat surface was most effective in attenuating sway; the flexible filaments were much less effective but still significantly better than imagined contact. Experiment 2 compared the effectiveness of finger contact with a flexible filament, a rigid filament of the same diameter, a flat surface, and an imagined spatial position. The rigid filament and flat surface conditions were equally effective in attenuating body sway and were greatly superior to contact with the flexible filament, which was superior to imagined contact. Experiment 3 included five conditions: arms by sides; finger "contact" with an imagined spatial position; finger contact with a flat surface; finger contact with a flexible filament attempting to maintain it bent; and contact with the flexible filament attempting not to bend it. The arms by sides and finger "contact" with an imagined position conditions did not differ significantly; all three conditions involving actual finger contact showed significantly less center of pressure and hand sway, but contact with the flat surface was most effective in attenuating both postural and hand displacement. In all three experiments, the level of force applied in fingertip contact conditions was far below that necessary to provide mechanical stabilization. Our findings indicate that: (1) stimulation of a small number of receptors in the fingertip is adequate to allow stabilization of sway, (2) fingertip force levels as low as 5-10 g provide some stabilization, (3) contact with a stationary spatial referent is most effective, and (4) independent control of arm and torso occurs when finger contact is allowed.

Gravitoinertial force magnitude and direction influence head-centric auditory localization

We measured the influence of gravitoinertial force (GIF) magnitude and direction on head-centric auditory localization to determine whether a true audiogravic illusion exists. In experiment 1, supine subjects adjusted computer-generated dichotic stimuli until they heard a fused sound straight ahead in the midsagittal plane of the head under a variety of GIF conditions generated in a slow-rotation room. The dichotic stimuli were constructed by convolving broadband noise with head-related transfer function pairs that model the acoustic filtering at the listener's ears. These stimuli give rise to the perception of externally localized sounds. When the GIF was increased from 1 to 2 g and rotated 60 degrees rightward relative to the head and body, subjects on average set an acoustic stimulus 7.3 degrees right of their head's median plane to hear it as straight ahead. When the GIF was doubled and rotated 60 degrees leftward, subjects set the sound 6.8 degrees leftward of baseline values to hear it as centered. In experiment 2, increasing the GIF in the median plane of the supine body to 2 g did not influence auditory localization. In experiment 3, tilts up to 75 degrees of the supine body relative to the normal 1 g GIF led to small shifts, 1--2 degrees, of auditory setting toward the up ear to maintain a head-centered sound localization. These results show that head-centric auditory localization is affected by azimuthal rotation and increase in magnitude of the GIF and demonstrate that an audiogravic illusion exists. Sound localization is shifted in the direction opposite GIF rotation by an amount related to the magnitude of the GIF and its angular deviation relative to the median plane.

Coriolis-force-induced trajectory and endpoint deviations in the reaching movements of labyrinthine-defective subjects

When reaching movements are made during passive constant velocity body rotation, inertial Coriolis accelerations are generated that displace both movement paths and endpoints in their direction. These findings directly contradict equilibrium point theories of movement control. However, it has been argued that these movement errors relate to subjects sensing their body rotation through continuing vestibular activity and making corrective movements. In the present study, we evaluated the reaching movements of five labyrinthine-defective subjects (lacking both semicircular canal and otolith function) who cannot sense passive body rotation in the dark and five age-matched, normal control subjects. Each pointed 40 times in complete darkness to the location of a just extinguished visual target before, during, and after constant velocity rotation at 10 rpm in the center of a fully enclosed slow rotation room. All subjects, including the normal controls, always felt completely stationary when making their movements. During rotation, both groups initially showed large deviations of their movement paths and endpoints in the direction of the transient Coriolis forces generated by their movements. With additional per-rotation movements, both groups showed complete adaptation of movement curvature (restoration of straight-line reaches) during rotation. The labyrinthine-defective subjects, however, failed to regain fully accurate movement endpoints after 40 reaches, unlike the control subjects who did so within 11 reaches. Postrotation, both groups' movements initially had mirror image curvatures to their initial per-rotation reaches; the endpoint aftereffects were significantly different from prerotation baseline for the control subjects but not for the labyrinthine-defective subjects reflecting the smaller amount of endpoint adaptation they achieved during rotation. The labyrinthine-defective subjects' movements had significantly lower peak velocity, higher peak elevation, lower terminal velocity, and a more vertical touchdown than those of the control subjects. Thus the way their reaches terminated denied them the somatosensory contact cues necessary for full endpoint adaptation. These findings fully contradict equilibrium point theories of movement control. They emphasize the importance of contact cues in adaptive movement control and indicate that movement errors generated by Coriolis perturbations of limb movements reveal characteristics of motor planning and adaptation in both healthy and clinical populations.

Fingertip contact suppresses the destabilizing influence of leg muscle vibration

Touch of the hand with a stationary surface at nonmechanically supportive force levels (<1 N) greatly attenuates postural sway during quiet stance. We predicted such haptic contact would also suppress the postural destabilization caused by vibrating the right peroneus brevis and longus muscles of subjects standing heel-to-toe with eyes closed. In experiment 1, ten subjects were tested under four conditions: no-vibration, no-touch; no-vibration, touch; vibration, no-touch; and vibration, touch. A hand-held physiotherapy vibrator (120 Hz) was applied approximately 5 cm above the malleolous to stimulate the peroneus longus and brevis tendons. Touch conditions involved contact of the right index finger with a laterally positioned surface (<1 N of force) at waist height. Vibration in the absence of finger contact greatly increased the mean sway amplitude of the center of pressure and of the head relative to the no-vibration, no-touch control condition (P < 0.001). The touch, no-vibration and touch-vibration conditions were not significantly different (P > 0.05) from each other and both had significantly less mean sway amplitude of head and of center of pressure than the other conditions (P < 0.01). In experiment 2, eight subjects stood heel-to-toe under touch and no-touch conditions involving 40-s duration trials of peroneus tendon vibration at different duty cycles: 1-, 2-, 3-, and 4-s ON and OFF periods. The vibrator was attached to the subject's leg and remotely activated. In the no-touch conditions, subjects showed periodic postural disruptions contingent on the duty cycle and mirror image rebounds with the offset of vibration. In the touch conditions, subjects were much less disrupted and showed compensations occurring within 500 ms of vibration onset and mirror image rebounds with vibration offset. Subjects were able to suppress almost completely the destabilizing influence of the vibration in the 3- and 4-s duty cycle trials. These experiments show that haptic contact of the hand with a stable surface can suppress abnormal proprioceptive and motor signals in leg muscles.

Artificial gravity as a countermeasure in long-duration space flight

Long-duration exposure to weightlessness results in bone demineralization, muscle atrophy, cardiovascular deconditioning, altered sensory-motor control, and central nervous system reorganizations. Exercise countermeasures and body loading methods so far employed have failed to prevent these changes. A human mission to Mars might last 2 or 3 years and without effective countermeasures could result in dangerous levels of bone and muscle loss. Artificial gravity generated by rotation of an entire space vehicle or of an inner chamber could be used to prevent structural changes. Some of the physical characteristics of rotating environments are outlined along with their implications for human performance. Artificial gravity is the centripetal force generated in a rotating vehicle and is proportional to the product of the square of angular velocity and the radius of rotation. Thus, for a particular g-level, there is a tradeoff between velocity of rotation and radius. Increased radius is vastly more expensive to achieve than velocity, so it is important to know the highest rotation rates to which humans can adapt. Early studies suggested that 3 rpm might be the upper limit because movement control and orientation were disrupted at higher velocities and motion sickness and chronic fatigue were persistent problems. Recent studies, however, are showing that, if the terminal velocity is achieved over a series of gradual steps and many body movements are made at each dwell velocity, then full adaptation of head, arm, and leg movements is possible. Rotation rates as high as 7.5-10 rpm are likely feasible. An important feature of the new studies is that they provide compelling evidence that equilibrium point theories of movement control are inadequate. The central principles of equilibrium point theories lead to the equifinality prediction, which is violated by movements made in rotating reference frames.

Congenitally blind individuals rapidly adapt to coriolis force perturbations of their reaching movements

Reaching movements made to visual targets in a rotating room are initially deviated in path and endpoint in the direction of transient Coriolis forces generated by the motion of the arm relative to the rotating environment. With additional reaches, movements become progressively straighter and more accurate. Such adaptation can occur even in the absence of visual feedback about movement progression or terminus. Here we examined whether congenitally blind and sighted subjects without visual feedback would demonstrate adaptation to Coriolis forces when they pointed to a haptically specified target location. Subjects were tested pre-, per-, and postrotation at 10 rpm counterclockwise. Reaching to straight ahead targets prerotation, both groups exhibited slightly curved paths. Per-rotation, both groups showed large initial deviations of movement path and curvature but within 12 reaches on average had returned to prerotation curvature levels and endpoints. Postrotation, both groups showed mirror image patterns of curvature and endpoint to the per-rotation pattern. The groups did not differ significantly on any of the performance measures. These results provide compelling evidence that motor adaptation to Coriolis perturbations can be achieved on the basis of proprioceptive, somatosensory, and motor information in the complete absence of visual experience.

Reaching during virtual rotation: context specific compensations for expected coriolis forces

Subjects who are in an enclosed chamber rotating at constant velocity feel physically stationary but make errors when pointing to targets. Reaching paths and endpoints are deviated in the direction of the transient inertial Coriolis forces generated by their arm movements. By contrast, reaching movements made during natural, voluntary torso rotation seem to be accurate, and subjects are unaware of the Coriolis forces generated by their movements. This pattern suggests that the motor plan for reaching movements uses a representation of body motion to prepare compensations for impending self-generated accelerative loads on the arm. If so, stationary subjects who are experiencing illusory self-rotation should make reaching errors when pointing to a target. These errors should be in the direction opposite the Coriolis accelerations their arm movements would generate if they were actually rotating. To determine whether such compensations exist, we had subjects in four experiments make visually open-loop reaches to targets while they were experiencing compelling illusory self-rotation and displacement induced by rotation of a complex, natural visual scene. The paths and endpoints of their initial reaching movements were significantly displaced leftward during counterclockwise illusory rotary displacement and rightward during clockwise illusory self-displacement. Subjects reached in a curvilinear path to the wrong place. These reaching errors were opposite in direction to the Coriolis forces that would have been generated by their arm movements during actual torso rotation. The magnitude of path curvature and endpoint errors increased as the speed of illusory self-rotation increased. In successive reaches, movement paths became straighter and endpoints more accurate despite the absence of visual error feedback or tactile feedback about target location. When subjects were again presented a stationary scene, their initial reaches were indistinguishable from pre-exposure baseline, indicating a total absence of aftereffects. These experiments demonstrate that the nervous system automatically compensates in a context-specific fashion for the Coriolis forces associated with reaching movements.

Human orientation and movement control in weightless and artificial gravity environments

Our goal is to summarize what has been learned from studies of human movement and orientation control in weightless conditions. An understanding of the physics of weightlessness is essential to appreciate the dramatic consequences of the absence of continuous contact forces on orientation and posture. Eye, head, arm, leg, and whole body movements are discussed, but only experiments whose results seem relatively incontrovertible are included. Emphasis is placed on distinguishing between virtually immediate adaptive compensations to weightlessness and those with longer time courses. The limitations and difficulties of performing experiments in weightless conditions are highlighted. We stress that when astronauts and cosmonauts return from extended space flight they do so with both physical "plant" and neural "controller" structurally and functionally altered. Recent developments in adapting humans to artificial gravity conditions are discussed as a way of maintaining sensory-motor and structural integrity in extended missions involving transitions between different force environments.

Haptic stabilization of posture: changes in arm proprioception and cutaneous feedback for different arm orientations

Postural sway during quiet stance is attenuated by actively maintained contact of the index finger with a stationary surface, even if the level of applied force (<1 N) cannot provide mechanical stabilization. In this situation, changes in force level at the fingertip lead changes in center of foot pressure by approximately 250 ms. These and related findings indicate that stimulation of the fingertip combined with proprioceptive information about the hand and arm can serve as an active sensor of body position relative to the point of contact. A geometric analysis of the relationship between hand and torso displacement during body sway led to the prediction that arm and hand proprioceptive and finger somatosensory information about body sway would be maximized with finger contact in the plane of body sway. Therefore, the most postural stabilization should be possible with such contact. To test this analysis, subjects touched a laterally versus anteriorly placed surface while in each of two stances: the heel-to-toe tandem Romberg stance that reduces medial-lateral stability and the heel-to-heel, toes-outward, knees-bent, "duck stance" that reduces fore-aft stability. Postural sway was always least with finger contact in the unstable plane: for the tandem stance, lateral fingertip contact was significantly more effective than frontal contact, and, for the duck stance, frontal contact was more effective than lateral fingertip contact. Force changes at the fingertip led changes in center of pressure of the feet by approximately 250 ms for both fingertip contact locations for both test stances. These results support the geometric analysis, which showed that 1) arm joint angles change by the largest amount when fingertip contact is maintained in the plane of greatest sway, and 2) the somatosensory cues at the fingertip provide both direction and amplitude information about sway when the finger is contacting a surface in the unstable plane.

Precision contact of the fingertip reduces postural sway of individuals with bilateral vestibular loss

Contact of the hand with a stationary surface attenuates postural sway in normal individuals even when the level of force applied is mechanically inadequate to dampen body motion. We studied whether subjects without vestibular function would be able to substitute contact cues from the hand for their lost labyrinthine function and be able to balance as well as normal subjects in the dark without finger contact. We also studied the relative contribution of sight of the test chamber to the two groups. Subjects attempted to maintain a tandem Romberg stance for 25 s under three levels of fingertip contact: no contact; light-touch contact, up to 1 N (approximately 100 g) force; and unrestricted contact force. Both eyes open and eyes closed conditions were evaluated. Without contact, none of the vestibular loss subjects could stand for more than a few seconds in the dark without falling; all the normals could. The vestibular loss subjects were significantly more stable in the dark with light touch of the index finger than the normal subjects in the dark without touch. They also swayed less in the dark with light touch than when permitted sight of the test chamber without touch, and less with sight and touch than just sight. The normal subjects swayed less in the dark with touch than without, and less with sight and touch than sight alone. These findings show that during quiet stance light touch of the index finger with a stationary surface can be as effective or even more so than vestibular function for minimizing postural sway.

Adaptation in a rotating artificial gravity environment

The centripetal force generated by a rotating space vehicle is a potential source of artificial gravity. Minimizing the cost of such a vehicle dictates using the smallest radius and highest rotation rate possible, but head movements made at high rotation rates generate disorienting, nauseogenic cross-coupled semicircular canal stimulation. Early studies suggested 3 or 4 rpm as the highest rate at which humans could adapt to this vestibular stimulus. These studies neglected the concomitant Coriolis force actions on the head/neck system. We assessed non-vestibular Coriolis effects by measuring arm and leg movements made in the center of a rotating room turning at 10 rpm and found that movement endpoints and trajectories are initially deviated; however, subjects readily adapt with 10-20 additional movements, even without seeing their errors. Equilibrium point theories of motor control errantly predict that Coriolis forces will not cause movement endpoint errors so that subjects will not have to adapt their reaching movements during rotation. Adaptation of movement trajectory acquired during Coriolis force perturbations of one arm transfers to the unexposed arm but there is no intermanual transfer of endpoint adaptation indicating that neuromotor representations of movement endpoint and trajectory are separable and can adapt independently, also contradictory to equilibrium point theories. Touching a surface at the end of reaching movements is required for complete endpoint adaptation in darkness but trajectory adapts completely with or without terminal contact. We have also made the first kinematic measurements of unconstrained head movements during rotation, these movements show rapid adaptation to Coriolis force perturbations. Our results point to methods for achieving full compensation for rotation up to 10 rpm.

Auditory cues for orientation and postural control in sighted and congenitally blind people

This study assessed whether stationary auditory information could affect body and head sway (as does visual and haptic information) in sighted and congenitally blind people. Two speakers, one placed adjacent to each ear, significantly stabilized center-of-foot-pressure sway in a tandem Romberg stance, while neither a single speaker in front of subjects nor a head-mounted sonar device reduced center-of-pressure sway. Center-of-pressure sway was reduced to the same level in the two-speaker condition for sighted and blind subjects. Both groups also evidenced reduced head sway in the two-speaker condition, although blind subjects' head sway was significantly larger than that of sighted subjects. The advantage of the two-speaker condition was probably attributable to the nature of distance compared with directional auditory information. The results rule out a deficit model of spatial hearing in blind people and are consistent with one version of a compensation model. Analysis of maximum cross-correlations between center-of-pressure and head sway, and associated time lags suggest that blind and sighted people may use different sensorimotor strategies to achieve stability.

Aftereffects and sense of presence in virtual environments: formulation of a research and development agenda

This report represents a committee summary of the current state of knowledge regarding aftereffects and sense of presence in virtual environments (VEs). The work presented in this article, and the proposed research agenda, are the result of a special session that was set up in the framework of the Seventh International Conference on Human Computer Interaction. Recommendations were made by the committee regarding research needs in aftereffects and sense of presence, and, where possible, priorities were suggested. The research needs were structured in terms of the short, medium, and long term and, if followed, should lead toward the effective use of VE technology. The 2 most critical research issues identified were (a) standardization and use of measurement approaches for aftereffects and (b) identification and prioritization of sensorimotor discordances that drive aftereffects. Identification of aftereffects countermeasures (i.e., techniques to assist users in readily transitioning between the real and virtual worlds), reduction of system response latencies, and improvements in tracking technology were also thought to be of critical importance.

Combined influences of gravitoinertial force level and visual field pitch on visually perceived eye level

Psychophysical measurements of the level at which observers set a small visual target so as to appear at eye level (VPEL) were made on 13 subjects in 1.0 g and 1.5 g environments in the Graybiel Laboratory rotating room while they viewed a pitched visual field or while in total darkness. The gravitoinertial force was parallel to the z-axis of the head and body during the measurements. The visual field consisted of two 58 degrees high, luminous, pitched-from-vertical, bilaterally symmetric, parallel lines, viewed in otherwise total darkness. The lines were horizontally separated by 53 degrees and presented at each of 7 angles of pitch ranging from 30 degrees with the top of the visual field turned away from the subject (top backward) to 30 degrees with the top turned toward the subject (top forward). At 1.5 g, VPEL changed linearly with the pitch of the 2-line stimulus and was depressed with top backward pitch and elevated with top forward pitch as had been reported previously at 1.0 g (1,2); however, the slopes of the VPEL-vs-pitch functions at 1.0 g and 1.5 g were indistinguishable. As reported previously also (3,4), the VPEL in darkness was considerably lower at 1.5 g than at 1.0 g; however, although the y-intercept of the VPEL-vs-pitch function in the presence of the 2-line visual field (visual field erect) was also lower at 1.5 g than at 1.0 g as it was in darkness, the G-related difference was significantly attenuated by the presence of the visual field. The quantitative characteristics of the results are consistent with a model in which VPEL is treated as a consequence of an algebraic weighted average or a vector sum of visual and nonvisual influences although the two combining rules lead to fits that are equally good.

The role of reafference in recalibration of limb movement control and locomotion

The reafference model has frequently been used to explain spatial constancy during eye and head movements. We have found that its basic concepts also form part of the information processing necessary for the control and recalibration of reaching movements. Reaching was studied in a novel force environment--a rotating room that creates centripetal forces of the type that could someday substitute for gravity in space flight, and Coriolis forces which are side effects of rotation. We found that inertial, noncontacting Coriolis forces deviate the path and endpoint of reaching movements, a finding that shows the inadequacy of equilibrium position models of movement control. Repeated movements in the rotating room quickly lead to normal movement patterns and to a failure to perceive the perturbing forces. The first movements made after rotation stops, without Coriolis forces present, show mirror-image deviations and evoke perception of a perturbing force even though none is present. These patterns of sensorimotor control and adaptation can largely be explained on the basis of comparisons of efference copy, reafferent muscle spindle, and cutaneous mechanoreceptor signals. We also describe experiments on human locomotion using an apparatus similar to that which Mittelstaedt used to study the optomotor response of the Eristalis fly. These results show that the reafference principle relates as well to the perception of the forces acting on and exerted by the body during voluntary locomotion.

Motor function in microgravity: movement in weightlessness

Microgravity provides unique, though experimentally challenging, opportunities to study motor control. A traditional research focus has been the effects of linear acceleration on vestibular responses to angular acceleration. Evidence is accumulating that the high-frequency vestibulo-ocular reflex (VOR) is not affected by transitions from a 1 g linear force field to microgravity (<1 g); however, it appears that the three-dimensional organization of the VOR is dependent on gravitoinertial force levels. Some of the observed effects of microgravity on head and arm movement control appear to depend on the previously undetected inputs of cervical and brachial proprioception, which change almost immediately in response to alterations in background force levels. Recent studies of post-flight disturbances of posture and locomotion are revealing sensorimotor mechanisms that adjust over periods ranging from hours to weeks.

The role of brachial muscle spindle signals in assignment of visual direction

1. In the oculobrachial illusion, a target light attached to the unseen stationary hand is perceived as moving and changing spatial position when illusory motion of the forearm is elicited by brachial muscle vibration. Our goal was to see whether we could induce apparent motion and displacement of two retinally fixed targets in opposite directions by the use of oculobrachial illusions. 2. We vibrated both biceps brachii, generating illusory movements of the two forearms in opposite directions, and measured any associated changes in perceived distance between target lights on the unseen stationary hands. The stability of visual fixation of one of the targets was also measured. 3. The seen distance between the stationary targets increased significantly when vibration induced an illusory increase in felt distance between the hands, both with binocular and monocular viewing. 4. Subjects maintained fixation accuracy equally well during vibration-induced illusory increases in visual target separation and in a no-vibration control condition. Fixation errors were not correlated with the extent or direction of illusory visual separation. 5. These findings indicate that brachial muscle spindle signals can contribute to an independent representation of felt target location in head-centric coordinates that can be interrelated with a visual representation of target location generated by retinal and oculomotor signals. 6. A model of how these representations are interrelated is proposed, and its relation to other intersensory interactions is discussed.

Gravitoinertial force level influences arm movement control

1. The ability to move the forearm between remembered elbow joint angles immediately after rapid increases or decreases of the background gravitoinertial force (G) level was measured. The movements had been well-practiced in a normal 1G environment before the measurements in high-(1.8G) and low-force (0G) environments. The forearm and upper arm were always unsupported to maximize the influence of altered G-loading and to minimize extraneous cues about arm position. 2. Horizontal and vertical movement planes were studied to measure the effects of varying the G load in the movement plane within a given G background. Rapid and slow movements were studied to assess the role of proprioceptive feedback. 3. G level did not affect the amplitude of rapid movements, indicating that subjects were able to plan and to generate appropriate motor commands for the new G loading of the arm. The amplitude of slow movements was affected by G level, indicating that proprioceptive feedback is influenced by G level. 4. The effects of G level were similar for horizontal and vertical movements, indicating that proprioceptive information from supporting structures, such as the shoulder joint and muscles, had a role in allowing generation of the appropriate motor commands. 5. The incidence and size of dynamic overshoots were greater in 0G and for rapid movements. This G-related change in damping suggests a decrease in muscle spindle activity in 0G. A decrease in muscle spindle activity in 0G and an increase in 1.8G are consistent with the results of our prior studies on the tonic vibration reflex, locomotion, and perception of head movement trajectory in varying force backgrounds.

Gravitoinertial force level affects the appreciation of limb position during muscle vibration

Illusory motion and displacement of the restrained forearm can be elicited by vibrating the biceps brachii or triceps brachii muscle. We measured the influence of gravitoinertial force level on these perceptual responses to vibration during parabolic flight maneuvers where normal (1G) and high force (1.8G) background levels alternated with microgravity (0G). Subjects indicated the apparent forearm position of the vibrated arm with the other forearm and also made verbal reports. Biceps brachii vibration induced illusory extension of the forearm and triceps brachii, illusory flexion; these apparent motions and displacements were highly G force-dependent being enhanced at 1.8G and diminished at 0G relative to normal 1G force level. These alterations are discussed in terms of vestibulo-spinal and propriospinal influences on alpha-gamma motoneuronal control of muscle tone and the varying requirements for postural load support in different force backgrounds. Their implications for the control and appreciation of limb movements during exposure to different G force levels are also described.

Tonic vibration reflexes and background force level

On Earth, the functional stretch reflex is an important component in the maintenance of posture and muscle tone. In parabolic flight experiments, we evaluated whether the functional stretch reflex, as reflected in the tonic vibration reflex, adjusts appropriately for changes in background gravitoinertial force level. Virtually immediate alterations of appropriate sign occurred.

Space adaptation syndrome: multiple etiological factors and individual differences

Space motion sickness is a significant operational concern in the American and Soviet space programs. Nearly 70% of all astronauts and cosmonauts are affected to some degree during their first several days of flight. It is now beginning to appear that space motion sickness like terrestrial motion sickness is the consequence of multiple etiological factors. As we come to understand basic mechanisms of spatial orientation and sensory-motor adaptation we can begin to predict etiological factors in different motion environments. Individuals vary greatly in the extent to which they are susceptible to these different factors. However, individuals seem to be relatively self-consistent in terms of their rates of adaptation to provocative stimulation and their retention of adaptation. Attempts to relate susceptibility to motion sickness during the microgravity phases of parabolic flight maneuvers to vestibular function under 1G and 0G test conditions are described.

Motion sickness susceptibility in parabolic flight and velocity storage activity

In parabolic flight experiments, we have found post-rotary nystagmus to be differentially suppressed in free fall (OG) and in a high gravitoinertial force (1.8G) background relative to 1G. In addition, the influence of postrotary head movements on nystagmus suppression was found to be contingent on G level. The nature of this pattern indicated a G-dependency of the velocity storage and dumping mechanisms. Here, we have rank-correlated susceptibility to motion sickness during head movements in OG and 1.8G with the following: a) the decay time constant of the slow phase velocity of post-rotary nystagmus under 1G, no head movement, baseline conditions, b) the extent of time constant reduction elicited in OG and 1.8G; c) the extent of time constant reduction elicited by head tilts in 1G; and d) changes in the extent of time constant reduction in OG and 1.8G over repeated tests. Susceptibility was significantly correlated with the extent to which a head movement reduced the time constant in 1G, was weakly correlated with the baseline time constant, but was not correlated with the extent of reduction in OG or 1.8G. This pattern suggests a link between mechanisms evoking symptoms of space motion sickness and the mechanisms of velocity storage and dumping. Experimental means of evaluating this link are described.

Decreased susceptibility to motion sickness during exposure to visual inversion in microgravity

Head and body movements made in microgravity tend to bring on symptoms of motion sickness. Such head movements, relative to comparable ones made on Earth, are accompanied by unusual combinations of semicircular canal and otolith activity owing to the unloading of the otoliths in OG. Head movements also bring on symptoms of motion sickness during exposure to visual inversion (or reversal) on Earth because the vestibulo-ocular reflex is rendered anti-compensatory. Here, we present evidence that susceptibility to motion sickness during exposure to visual inversion is decreased in a 0G relative to a 1G force background. This difference in susceptibility appears related to the alteration in otolith function in 0G. Some implications of this finding for the etiology of space motion sickness are described.

Age differences in oculomotor responses to step changes in body velocity and visual surround velocity

We compared the slow-phase eye velocity elicited by sudden cessation of prolonged, constant velocity, vertical z-axis rotation of the body or the visual surround in 10 healthy college-age and over-65 individuals. The step gain of vestibular post-rotary nystagmus did not differ across age groups, but the time constant of slow-phase velocity decay was longer and more asymmetrical in the older group. The slow-phase velocity of optokinetic nystagmus attained the same initial levels for both age groups; it declined significantly during 60 s of stimulation for the older but not the younger group. The decay rate of optokinetic afternystagmus was quicker for the older subjects. This pattern of results may be related to already identified structural changes in the vestibular system and suggests the existence of yet unidentified changes in central vestibular and visual processing.

Sensory-motor factors triggering the suppression of post-rotary vestibular responses in different gravitoinertial force backgrounds

We studied suppression of oculomotor and perceptual after-responses by post-rotary head movements in high (1.8 G), low (0 G), and normal (1 G) gravitoinertial force backgrounds in parabolic flight. Our aim was to identify what sensory and motor signals are critical for triggering suppression. In a prior experiment (DiZio and Lackner 1988), we found suppression using 40 degrees post-rotary head tilts in 1 G and 1.8 G but not 0 G force backgrounds. However, in free fall even without head tilts there was a significant suppression of nystagmus relative to 1 G and 1.8 G force backgrounds, thus potentially masking an effect of head tilt on suppression in 0 G. We have retested four of the original subjects with 90 degrees head tilts to maximize the likelihood of detecting suppression in 0 G. Although nystagmus and illusory after-rotation were suppressed by post-rotary head tilts in normal and high gravitoinertial force environments, there was still no evidence of suppression in free fall. We present evidence that the lack of suppression in 0 G is not attributable to post-rotary responses already being at a "basement" level, but rather that suppression depends on the registration of a change in head position relative to a significant level of gravitoinertial force.

Perceived self-motion elicited by postrotary head tilts in a varying gravitoinertial force background

We measured the effects of postrotary head tilts on the perceived duration and the apparent axis of illusory self-rotation experienced following counterclockwise body rotation in high (1.8 G), normal (1 G), and low (0 G) gravitoinertial force environments. In the absence of head movements, the duration of illusory afterrotation was shorter in 0 G and 1.8 G than in 1 G, and it was further shortened by 40 degrees pitch-back head movements in 1 G and 1.8 G. Clockwise illusory afterrotation about the torso's vertical z-axis was always experienced in trials without postrotary head tilts. In trials with head movements, half the subjects experienced no change in this pattern; however, half experienced transient rightward roll of the torso's z-axis, which remained the rotation axis. The duration and extent of apparent roll were greater in 0 G and smaller in 1.8 G than in 1 G. We provide a functional explanation for the tendency for perceived self-rotation to be determined relative to the torso and to the gravitoinertial vertical rather than solely in relation to head position and head-fixed angular velocity sensors.

Altered sensory-motor control of the head as an etiological factor in space-motion sickness

Mechanical unloading during head movements in weightlessness may be an etiological factor in space-motion sickness. We simulated altered head loading on Earth without affecting vestibular stimulation by having subjects wear a weighted helmet. Eight subjects were exposed to constant velocity rotation about a vertical axis with direction reversals every 60 sec. for eight reversals with the head loaded and eight with the head unloaded. The severity of motion sickness elicited was significantly higher when the head was loaded. This suggests that altered sensory-motor control of the head is also an etiological factor in space-motion sickness.

Altered sensory-motor control of the head as an etiological factor in space-motion sickness

Mechanical unloading during head movements in weightlessness may be an etiological factor in space-motion sickness. We simulated altered head loading on Earth without affecting vestibular stimulation by having subjects wear a weighted helmet. Eight subjects were exposed to constant velocity rotation about a vertical axis with direction reversals every 60 sec. for eight reversals with the head loaded and eight with the head unloaded. The severity of motion sickness elicited was significantly higher when the head was loaded. This suggests that altered sensory-motor control of the head is also an etiological factor in space-motion sickness.

The effects of gravitoinertial force level and head movements on post-rotational nystagmus and illusory after-rotation

The effect of Coriolis, cross-coupled stimulation on the vestibuloocular reflex and the elicitation of motion sickness depends on background gravitoinertial force level (DiZio et al. 1986, 1987; Graybiel et al. 1977; Lackner and Graybiel 1984, 1986). We have explored whether this response dependency is related to the unusual patterns of sensorimotor activity present during exposure to non-terrestrial gravitoinertial force levels, to alterations in the encoding of head movements in different gravitoinertial force environments, or to some combination thereof. Blindfolded subjects were exposed to sudden stops after constant velocity, vertical z-axis rotation, sometimes with and sometimes without post-rotational head movements, in the 0 G, 1 G, and 1.8 G force phases of parabolic flight. After sudden stops without head movements, the time constant of decay of post-rotational nystagmus was significantly lower in 0 G than in 1 G and lower to a smaller extent in 1.8 G. Post-rotational head movements decreased the decay time constants in 1 G and in 1.8 G, but not in free fall. The same pattern emerged for the duration of illusory after-rotation. Systematic changes were not found in the peak slow phase velocity of nystagmus. These results suggest that tonic levels of otolithic and somatosensory activity in combination with canalicular, cervical, and motor activity regulate the velocity storage mechanism of the horizontal vestibuloocular reflex (Cohen et al. 1977; Raphan et al. 1979) and sensations of after-rotation. These same factors are likely to be important etiological elements in space motion sickness.

Visual stimulation affects the perception of voluntary leg movements during walking

When a limb is used for locomotion, patterns of afferent and efferent activity related to its own motion are present as well as visual, vestibular, and other proprioceptive information about motion of the whole body. A study is reported in which it was asked whether visual stimulation present during whole-body motion can influence the perception of the leg movements propelling the body. Subjects were tested in conditions in which the stepping movements they made were identical but the amount of body displacement relative to inertial space and to the visual surround varied. These test conditions were created by getting the subjects to walk on a rotatable platform centered inside a large, independently rotatable, optokinetic drum. In each test condition, subjects, without looking at their legs, compared, against a standard condition in which the floor and drum were both stationary, their speed of body motion, their stride length and stepping rate, the direction of their steps, and the perceived force they exerted during stepping. When visual surround motion was incompatible with the motion normally associated with the stepping movements being made, changes in apparent body motion and in the awareness of the frequency, extent, and direction of the voluntary stepping movements resulted.

The influence of gravitoinertial force level on oculomotor and perceptual responses to Coriolis, cross-coupling stimulation

Susceptibility to motion sickness during exposure to constant levels of Coriolis, cross-coupling stimulation is lower in zero G and higher in 1.8 G than in a 1-G force environment (10, 13). The goal of the present experiment was to determine whether gravitoinertial force magnitude also influences oculomotor and perceptual responses to Coriolis, cross-coupling stimulation. We had blind-folded subjects who were rotating at constant velocity make standardized head movements during the free-fall and high force phases of parabolic flight, and we measured both the characteristics of their horizontal nystagmus and the magnitude of their experienced self-motion. Both responses were less intense in the free-fall periods than in the high force periods. Although the slow phase velocity of nystagmus reached the same initial, peak level in both conditions, it decayed more quickly in zero G. These findings suggest that the response to semicircular canal stimulation depends on the background level of gravitoinertial force.

The influence of gravitoinertial force level on oculomotor and perceptual responses to sudden stop stimulation

Our goal was to determine whether the vestibular response to vertical, z-axis body rotation in the dark is influenced by the magnitude of gravitoinertial force. We measured the nystagmus and the duration of illusory self-motion elicited in blindfolded subjects by cessation of such rotation during the free-fall, high, and terrestrial force phases of parabolic flight maneuvers. Both measures were significantly lower in zero G than in 1 G, and lower to a smaller extent in 1.8 G. The decreased intensity of nystagmus was due specifically to a decrease in the time constant of slow phase velocity decay with no decrement in peak velocity. This pattern of findings is consistent with the responses we had observed earlier to constant levels of Coriolis, cross-coupled stimulation during parabolic flight maneuvers both in terms of the mode of nystagmus suppression and the effect of G-level. Attenuation of the vestibular response to rotary acceleration in free-fall causes sensory-motor mismatches during natural head movements in orbital flight that may be important factors in the evocation of space motion sickness.