Scott J. Wood

Orthostatic Intolerance and Motion Sickness After Parabolic Flight

Because it is not clear that the induction of orthostatic intolerance in returning astronauts always requires prolonged exposure to microgravity, we investigated orthostatic tolerance and autonomic cardiovascular function in 16 healthy subjects before and after the brief micro- and hypergravity of parabolic flight. Concomitantly, we investigated the effect of parabolic flight-induced vomiting on orthostatic tolerance, R-wave-R-wave interval and arterial pressure power spectra, and carotid-cardiac baroreflex and Valsalva responses. After parabolic flight 1) 8 of 16 subjects could not tolerate 30 min of upright tilt (compared to 2 of 16 before flight); 2) 6 of 16 subjects vomited; 3) new intolerance to upright tilt was associated with exaggerated falls in total peripheral resistance, whereas vomiting was associated with increased R-wave-R-wave interval variability and carotid-cardiac baroreflex responsiveness; and 4) the proximate mode of new orthostatic failure differed in subjects who did and did not vomit, with vomiters experiencing comparatively isolated upright hypocapnia and cerebral vasoconstriction and nonvomiters experiencing signs and symptoms reminiscent of the clinical postural tachycardia syndrome. Results suggest, first, that syndromes of orthostatic intolerance resembling those developing after space flight can develop after a brief (i.e., 2-h) parabolic flight and, second, that recent vomiting can influence the results of tests of autonomic cardiovascular function commonly utilized in returning astronauts.

Loss of otolith function with age is associated with increased postural sway measures.

Loss of balance and increased fall risk is a common problem associated with aging. Changes in vestibular function occur with aging but the contribution of reduced vestibular otolith function to fall risk remains unknown. We examined a population of 151 healthy individuals (aged 21-93) for both balance (sway measures) and ocular counter-rolling (OCR) function. We assessed balance function with eyes open and closed on a firm surface, eyes open and closed on a foam surface and OCR during +/-20 degree roll tilt at 0.005 Hz. Subjects demonstrated a significant age-related reduction in OCR and increase in postural sway. The effect of age on OCR was greater in females than males. The reduction in OCR was strongly correlated with the mediolateral measures of sway with eyes closed. This correlation was also present in the elderly group alone, suggesting that aging alone does not account for this effect. OCR decreased linearly with age and at a greater rate in females than males. This loss of vestibular otolith-ocular function is associated with increased mediolateral measures of sway which have been shown to be related to increased risk of falls. These data suggest a role for loss of otolith function in contributing to fall risk in the elderly. Further prospective, longitudinal studies are necessary to confirm these findings.

Neurovestibular and sensorimotor studies in space and Earth benefits.

This review summarizes what has been learned from studies of human neurovestibular system in weightless conditions, including balance and locomotion, gaze control, vestibular-autonomic function and spatial orientation, and gives some examples of the potential Earth benefits of this research. Results show that when astronauts and cosmonauts return from space flight both the peripheral and central neural processes are physiologically and functionally altered. There are clear distinctions between the virtually immediate adaptive compensations to weightlessness and those that require longer periods of time to adapt. However, little is known to date about the adaptation of sensory-motor functions to long-duration space missions in weightlessness and to the transitions between various reduced gravitational levels, such as on the Moon and Mars. Results from neurovestibular research in space have substantially enhanced our understanding of the mechanisms and characteristics of postural, gaze, and spatial orientation deficits, analogous to clinical cases of labyrinthine-defective function. Also, space neurosciences research has participated in the development and application of significant new technologies, such as video recording and processing of three-dimensional eye movements and posture, hardware for the unencumbered measurement of head and body movement, and procedures for investigating otolith function on Earth. In particular, devices such as centrifugation or off-vertical axis rotation could enhance clinical neurological testing because it provides linear acceleration which specifically stimulates the otolith organs in a frequency range close to natural head and body movement.

Spatial coding of eye movements relative to perceived earth and head orientations during static roll tilt

Abstract This purpose of this study was to examine the spatial coding of eye movements during static roll tilt (up to ±45°) relative to perceived earth and head orientations. Binocular videographic recordings obtained in darkness from eight subjects allowed us to quantify the mean deviations in gaze trajectories along both horizontal and vertical coordinates relative to the true earth and head orientations. We found that both variability and curvature of gaze trajectories increased with roll tilt. The trajectories of eye movements made along the perceived earth-horizontal (PEH) were more accurate than movements along the perceived head-horizontal (PHH). The trajectories of both PEH and PHH saccades tended to deviate in the same direction as the head tilt. The deviations in gaze trajectories along the perceived earth-vertical (PEV) and perceived head-vertical (PHV) were both similar to the PHH orientation, except that saccades along the PEV deviated in the opposite direction relative to the head tilt. The magnitude of deviations along the PEV, PHH, and PHV corresponded to perceptual overestimations of roll tilt obtained from verbal reports. Both PEV gaze trajectories and perceptual estimates of tilt orientation were different following clockwise rather than counterclockwise tilt rotation; however, the PEH gaze trajectories were less affected by the direction of tilt rotation. Our results suggest that errors in gaze trajectories along PEV and perceived head orientations increase during roll tilt in a similar way to perceptual errors of tilt orientation. Although PEH and PEV gaze trajectories became nonorthogonal during roll tilt, we conclude that the spatial coding of eye movements during roll tilt is overall more accurate for the perceived earth reference frame than for the perceived head reference frame.

Electrotactile Feedback of Sway Position Improves Postural Performance during Galvanic Vestibular Stimulation

The purpose of this study was to assess the influence of electrotactile feedback on postural control performance during binaural galvanic vestibular stimulation (GVS). Postural equilibrium was measured with a computerized hydraulic platform in 10 healthy adults (6M, 4F, 24–65 y). Feedback of anterior–posterior (AP) and mediallateral (ML) body sway was derived from a 2‐axis linear accelerometer mounted on a torso belt and displayed on a 144‐point electrotactile array held against the anterior dorsal tongue. Subjects were trained to use the tongue electrotactile feedback (TEF) by voluntarily swaying to draw figures on their tongue, both with and without GVS. Subjects performed 24 randomized trials (20‐s duration with eyes closed, 2 trials per condition), including 4 support surface conditions (fixed, rotational sway‐referenced, translating the support surface proportional to AP sway, and combined rotational–translational support‐platform sway referencing), and 3 feedback conditions (baseline, GVS, and GVS with TEF). Postural performance was assessed using deviations from upright (peak‐to‐peak and root‐mean‐square sway) and convergence toward stability limits (time and distance to limit of support boundaries). Postural stability was impaired (with respect to baseline) during GVS in all platform conditions, with larger decrements in performance during trials with rotation sway‐referencing. Electrotactile feedback improved performance with GVS toward non‐GVS levels, especially during trials with rotation sway‐referencing. These results demonstrate the effectiveness of TEF in providing sensory substitution to maintain postural stability during vestibular disturbances.

Inner ear dysfunction in caspase-3 deficient mice

BackgroundCaspase-3 is one of the most downstream enzymes activated in the apoptotic pathway. In caspase-3 deficient mice, loss of cochlear hair cells and spiral ganglion cells coincide closely with hearing loss. In contrast with the auditory system, details of the vestibular phenotype have not been characterized. Here we report the vestibular phenotype and inner ear anatomy in the caspase-3 deficient (Casp3-/-) mouse strain.ResultsAverage ABR thresholds of Casp3-/-mice were significantly elevated (P < 0.05) compared to Casp3+/-mice and Casp3+/+mice at 3 months of age. In DPOAE testing, distortion product 2F1-F2 was significantly decreased (P < 0.05) in Casp3-/-mice, whereas Casp3+/-and Casp3+/+mice showed normal and comparable values to each other. Casp3-/-mice were hyperactive and exhibited circling behavior when excited. In lateral canal VOR testing, Casp3-/-mice had minimal response to any of the stimuli tested, whereas Casp3+/-mice had an intermediate response compared to Casp3+/+mice. Inner ear anatomical and histological analysis revealed gross hypomorphism of the vestibular organs, in which the main site was the anterior semicircular canal. Hair cell numbers in the anterior- and lateral crista, and utricle were significantly smaller in Casp3-/-mice whereas the Casp3+/-and Casp3+/+mice had normal hair cell numbers.ConclusionsThese results indicate that caspase-3 is essential for correct functioning of the cochlea as well as normal development and function of the vestibule.

Diagnostic accuracy of dynamic posturography testing after short-duration spaceflight.

INTRODUCTION Astronauts face transient disruptions of sensorimotor functions after spaceflight. Computerized dynamic posturography (CDP) testing has been used to document functional recovery; however, its objective value in return-to-duty decision-making has not been established. Therefore, we studied the diagnostic accuracy of CDP to determine the most effective test components for probing post-spaceflight sensorimotor deficits. METHODS There were 11 first-time astronauts and 11 matched controls who were evaluated by CDP before and after spaceflight (controls did not fly). All CDP testing was conducted with eyes closed while standing on a computer-controlled force plate. Somatosensory influences were either unperturbed (stationary force plate) or altered (unstable force plate), and vestibular influences were either unperturbed (head erect) or altered by static (head pitched forward or back by 200) or dynamic (head pitched voluntarily in cadence with an auditory signal: +/- 20 degrees at 0.33 Hz) challenges. Using equilibrium (EQ) scores derived from peak A-P sway as the dependent measure, we determined the sensitivity and specificity of each test condition and then constructed receiver operator characteristic (ROC) curves to determine their diagnostic accuracies. RESULTS The greatest diagnostic accuracy was obtained from the test requiring the subject to make dynamic head movements while standing on an unstable force plate (94.9% sensitivity 96.6% specificity, area under ROC curve = 0.991). By contrast, the estimated ROC area for the standard clinical Romberg test (fixed support, head erect), which is often used to make postflight return-to-duty decisions, was 0.718. CONCLUSION We recommend that results from this test paradigm be considered during postflight return-to-duty decision-making.

Continuous equilibrium scores: Factoring in the time before a fall

The equilibrium (EQ) score commonly used in computerized dynamic posturography is normalized between 0 and 100, with falls assigned a score of 0. The resulting mixed discrete-continuous distribution limits certain statistical analyses and treats all trials with falls equally. We propose a simple modification of the formula in which peak-to-peak sway data from trials with falls is scaled according the percent of the trial completed to derive a continuous equilibrium (cEQ) score. The cEQ scores for trials without falls remain unchanged from the original methodology. The cEQ factors in the time before a fall and results in a continuous variable retaining the central tendencies of the original EQ distribution. A random set of 5315 Sensory Organization Test trials were pooled that included 81 falls. A comparison of the original and cEQ distributions and their rank ordering demonstrated that trials with falls continue to constitute the lower range of scores with the cEQ methodology. The area under the receiver operating characteristic curve (0.997) demonstrates that the cEQ retained near-perfect discrimination between trials with and without falls. We conclude that the cEQ score provides the ability to discriminate between ballistic falls from falls that occur later in the trial. This approach of incorporating time and sway magnitude can be easily extended to enhance other balance tests that include fall data or incomplete trials.

Eye movements and motion perception during off-vertical axis rotation after spaceflight

Constant velocity off-vertical axis rotation (OVAR) provides dynamic linear acceleration stimuli that can be used to assess otolith function. Eight astronauts were rotated in darkness about their longitudinal axis 20° off vertical at low (0.125 Hz) and high (0.5 Hz) frequencies and their responses were compared before and after spaceflight. Eye movements were recorded using infrared videography and perceived motion was evaluated using a joystick with four degrees of freedom - pitch and roll tilt, front-back and lateral translation. Low-frequency OVAR generates tilt otolith-induced responses - modulation of ocular counter-roll and counter-pitch with perceived conical motion path - whereas high-frequency OVAR generates translational otolith-induced responses - modulation of horizontal and vergence slow phase velocity with perceived cylindrical motion path. While there were transient changes in the amplitude of the translational ocular responses on landing day, there were no major changes in the tilt ocular reflexes after adaptation to weightlessness. However, there was an increase in sensitivity to motion perception after spaceflight. Direct comparisons of pre- and postflight stimuli suggested that OVAR on landing day was less provocative of motion sickness than before spaceflight. These results confirm that some otolith reflexes elicited during passive motion may not be altered by short-duration spaceflight - or may readapt very quickly - and that the resolution of sensory conflict associated with postflight recovery involves higher-order neural processes.

Phase-linking and the perceived motion during off-vertical axis rotation

Human off-vertical axis rotation (OVAR) in the dark typically produces perceived motion about a cone, the amplitude of which changes as a function of frequency. This perception is commonly attributed to the fact that both the OVAR and the conical motion have a gravity vector that rotates about the subject. Little-known, however, is that this rotating-gravity explanation for perceived conical motion is inconsistent with basic observations about self-motion perception: (a) that the perceived vertical moves toward alignment with the gravito-inertial acceleration (GIA) and (b) that perceived translation arises from perceived linear acceleration, as derived from the portion of the GIA not associated with gravity. Mathematically proved in this article is the fact that during OVAR these properties imply mismatched phase of perceived tilt and translation, in contrast to the common perception of matched phases which correspond to conical motion with pivot at the bottom. This result demonstrates that an additional perceptual rule is required to explain perception in OVAR. This study investigates, both analytically and computationally, the phase relationship between tilt and translation at different stimulus rates—slow (45°/s) and fast (180°/s), and the three-dimensional shape of predicted perceived motion, under different sets of hypotheses about self-motion perception. We propose that for human motion perception, there is a phase-linking of tilt and translation movements to construct a perception of one’s overall motion path. Alternative hypotheses to achieve the phase match were tested with three-dimensional computational models, comparing the output with published experimental reports. The best fit with experimental data was the hypothesis that the phase of perceived translation was linked to perceived tilt, while the perceived tilt was determined by the GIA. This hypothesis successfully predicted the bottom-pivot cone commonly reported and a reduced sense of tilt during fast OVAR. Similar considerations apply to the hilltop illusion often reported during horizontal linear oscillation. Known response properties of central neurons are consistent with this ability to phase-link translation with tilt. In addition, the competing “standard” model was mathematically proved to be unable to predict the bottom-pivot cone regardless of the values used for parameters in the model.