The Turbolift: Linear Sled Hybrid Artificial Gravity Concept

Future crewed space exploration missions into deep space will require enhanced countermeasure technologies to ensure astronaut health. One such hazard is extended exposure to reduced gravity levels (i.e., microgravity, lunar gravity, or Martian gravity). Reduced gravity negatively impacts many physiological systems, leading to hydrostatic intolerance, musculoskeletal atrophy, sensorimotor impairment, bone demineralization, cardiovascular deconditioning, and visual alterations. Various countermeasures have been employed for mitigating these effects, such as exercise, pharmaceuticals, diet, and fluid loading. However, these approaches treat individual symptoms, such that each physiological system is addressed with typically one countermeasure. An alternative to this approach is artificial gravity (AG), which promises to be a holistic, comprehensive countermeasure. The traditional approach to creating AG is through centrifugation. However, centrifugation is not a "pure" form of AG and typically includes the drawbacks of Coriolis forces, gravity gradients, and vestibular cross-coupled illusions.As an alternative, we have proposed a Linear Sled Hybrid (LSH) AG system to mitigate astronauts' physiological deconditioning. This system functions by applying pure linear acceleration to produce footward loading. There is a half rotation (180_) to reorient the rider between acceleration and deceleration phases, such that the loading remains footward, as when standing on Earth. The rotation also provides some footward acceleration to the lower body through centripetal acceleration; hence the "hybrid" aspect of the design. At the end of the deceleration, the rider than accelerates back in the opposite direction and the sequence repeats.This proposed system could be integrated with future crewed space vehicles in a variety of manners

HUMAN VESTIBULAR PERCEPTUAL THRESHOLDS FOR PITCH TILT ARE SLIGHTLY WORSE THAN FOR ROLL TILT ACROSS A RANGE OF FREQUENCIES

Vestibular perceptual thresholds measure vestibular sensory and perceptual noise by quantifying how small of a passive self-motion an individual is able to reliably perceive. Vestibular thresholds have clinical and operational relevance, as they are elevated in vestibular migraine patients, and even healthy individuals with higher (i.e., worse) thresholds have degraded balance. Vestibular thresholds have been quantified across a range of frequencies (motion durations) for rotations and translations, with differences identified for different motion directions (e.g., up/down thresholds are higher than those for left/right motions). While roll tilt thresholds have been well quantified, pitch tilt thresholds have not. Here we aim to quantify pitch tilt thresholds across a range of frequencies and test whether they are higher than in those for roll tilt. In ten normal subjects, we found pitch tilt thresholds at 0.15, 0.2, 0.5, and 1 Hz averaged 1.66, 1.61, 0.99, 0.51 degrees, respectively. Using a general linear model, we found subjects&rsquo; pitch tilt thresholds were slightly, but significantly, higher than their roll tilt thresholds across all frequencies tested. These differences were approximately 10% at 0.15, 0.2, and 1 Hz and 3% at 0.5 Hz. Pitch tilt thresholds exhibited a similar frequency response as in roll tilt (decreasing a higher frequencies). They also had substantial inter-individual variability, which correlated across pitch tilt frequencies and between pitch and roll tilt thresholds. We discuss why pitch tilt thresholds might be higher, including the pitched-up orientation of the utricular plane of the otoliths, compared to previous studies, and discuss functional implications. &nbsp;</p

Improved Feasibility of Astronaut Short-Radius Artificial Gravity through a 50-day Incremental, Personalized, Vestibular Acclimation Protocol

The &ldquo;Coriolis&rdquo; cross-coupled (CC) illusion has historically limited the tolerability of utilizing fast-spin rate, short-radius centrifugation for in-flight artificial gravity. Previous research confirms that humans acclimate to the CC illusion over 10 daily sessions, though the efficacy of additional training is unknown. We investigated human acclimation to the CC illusion over up to 50 daily sessions of personalized, incremental training. During each 25-min session, subjects spun in yaw and performed roll head tilts approximately every 30&thinsp;s, reporting the presence or absence of the illusion while rating motion sickness every 5&thinsp;min. Illusion intensity was modulated by altering spin rate based upon subject response, such that the administered stimulus remained near each individual&rsquo;s instantaneous illusion threshold. Every subject (n&thinsp;=&thinsp;11) continued to acclimate linearly to the CC illusion during the investigation. Subjects acclimated at an average rate of 1.17 RPM per session (95% CI: 0.63&ndash;1.71 RPM per session), with the average tolerable spin rate increasing from 1.4 to 26.2 RPM, corresponding to a reduction in required centrifuge radius from 456.6 to 1.3&thinsp;m (to produce loading of 1&thinsp;g at the feet). Subjects reported no more than slight motion sickness throughout their training (mean: 0.92/20, 95% CI: 0.35&ndash;1.49/20). We applied survival analysis to determine the probability of individuals reaching various spin rates over a number of training days, providing a tolerability trade parameter for centrifuge design. Results indicate that acclimation to a given, operationally relevant spin rate may be feasible for all subjects if given a sufficient training duration. &nbsp;</p

Sensorimotor Impairment from a New Analog of Spaceflight-altered Neurovestibular Cues

Exposure to microgravity during spaceflight causes central reinterpretations of orientation sensory cues in astronauts, leading to sensorimotor impairment upon return to Earth. Currently there is no ground-based analog for the neurovestibular system relevant to spaceflight. We propose such an analog, which we term the "wheelchair head-immobilization paradigm" (WHIP). Subjects lie on their side on a bed fixed to a modified electric wheelchair, with their head restrained by a custom facemask. WHIP prevents any head tilt relative to gravity, which normally produces coupled stimulation to the otoliths and semicircular canals, but does not occur in microgravity. Decoupled stimulation is produced through translation and rotation on the wheelchair by the subject using a joystick. Following 12 h of WHIP exposure, subjects systematically felt illusory sensations of self-motion when making head tilts and had significant decrements in balance and locomotion function using tasks similar to those assessed in astronauts postspaceflight. These effects were not observed in our control groups without head restraint, suggesting the altered neurovestibular stimulation patterns experienced in WHIP lead to relevant central reinterpretations. We conclude by discussing the findings in light of postspaceflight sensorimotor impairment, WHIP's uses beyond a spaceflight analog, limitations, and future work. We propose, implement, and demonstrate the feasibility of a new analog for spaceflight-altered neurovestibular stimulation. Following extended exposure to the analog, we found subjects reported illusory self-motion perception. Furthermore, they demonstrated decrements in balance and locomotion, using tasks similar to those used to assess astronaut sensorimotor performance postspaceflight.</p

The Effect of Noisy Galvanic Vestibular Stimulation on Learning of Functional Mobility and Manual Control Nulling Sensorimotor Tasks

Galvanic vestibular stimulation (GVS) is a non-invasive method of electrically stimulating the vestibular system. We investigated whether the application of GVS can alter the learning of new functional mobility and manual control tasks and whether learning can be retained following GVS application. In a between-subjects experiment design, 36 healthy subjects performed repeated trials, capturing the learning of either (a) a functional mobility task, navigating an obstacle course on a compliant surface with degraded visual cues or (b) a manual control task, using a joystick to null self-roll tilt against a pseudo-random disturbance while seated in the dark. In the &ldquo;learning&rdquo; phase of trials, bilateral, bipolar GVS was applied continuously. The GVS waveform also differed between subjects in each task group: (1) white noisy galvanic vestibular stimulation (nGVS) at 0.3 mA (2) high-level random GVS at 0.7 mA (selected from pilot testing as destabilizing, but not painful), or (3) with the absence of stimulation (i.e., sham). Following the &ldquo;learning&rdquo; trials, all subjects were blindly transitioned to sham GVS, upon which they immediately completed another series of trials to assess any aftereffects. In the functional mobility task, we found nGVS significantly improved task learning (p = 0.03, mean learning metric 171% more than the sham group). Further, improvements in learning the functional mobility task with nGVS were retained, even once the GVS application was stopped. The benefits in learning with nGVS were not observed in the manual control task. High level GVS tended to inhibit learning in both tasks, but not significantly so. Even once the high-level stimulation was stopped, the impaired performance remained. Improvements in learning with nGVS may be due to increased information throughput resulting from stochastic resonance. The benefit of nGVS for functional mobility, but not manual control nulling, may be due to the multisensory (e.g., visual and proprioceptive), strategic, motor coordination, or spatial awareness aspects of the former task. Learning improvements with nGVS have the potential to benefit individuals who perform functional mobility tasks, such as astronauts, firefighters, high performance athletes, and soldiers. &nbsp;</p

Objective Evaluation of the Somatogravic Illusion from Flight Data of an Airplane Accident

(1) Background: It is difficult for accident investigators to objectively determine whether spatial disorientation may have contributed to a fatal airplane accident. In this paper, we evaluate three methods to reconstruct the possible occurrence of the somatogravic illusion based on flight data recordings from an airplane accident. (2) Methods: The outputs of two vestibular models were compared with the “standard” method, which uses the unprocessed gravito-inertial acceleration (GIA). (3) Results: All three methods predicted that the changing orientation of the GIA would lead to a somatogravic illusion when no visual references were available. However, the methods were not able to explain the first pitch-down control input by the pilot flying, which may have been triggered by the inadvertent activation of the go-around mode and a corresponding pitch-up moment. Both vestibular models predicted a few seconds delay in the illusory tilt from GIA due to central processing and sensory integration. (4) Conclusions: While it is difficult to determine which method best predicted the somatogravic illusion perceived during the accident without data on the pilot’s pitch perception, both vestibular models go beyond the GIA analysis in taking into account validated vestibular dynamics, and they also account for other vestibular illusions. In that respect, accident investigators would benefit from a unified and validated vestibular model to better explain pilot actions in accidents related to spatial disorientation

Modeling orientation perception adaptation to altered gravity environments with memory of past sensorimotor states

Transitioning between gravitational environments results in a central reinterpretation of sensory information, producing an adapted sensorimotor state suitable for motor actions and perceptions in the new environment. Critically, this central adaptation is not instantaneous, and complete adaptation may require weeks of prolonged exposure to novel environments. To mitigate risks associated with the lagging time course of adaptation (e.g., spatial orientation misperceptions, alterations in locomotor and postural control, and motion sickness), it is critical that we better understand sensorimotor states during adaptation. Recently, efforts have emerged to model human perception of orientation and self-motion during sensorimotor adaptation to new gravity stimuli. While these nascent computational frameworks are well suited for modeling exposure to novel gravitational stimuli, they have yet to distinguish how the central nervous system (CNS) reinterprets sensory information from familiar environmental stimuli (i.e., readaptation). Here, we present a theoretical framework and resulting computational model of vestibular adaptation to gravity transitions which captures the role of implicit memory. This advancement enables faster readaptation to familiar gravitational stimuli, which has been observed in repeat flyers, by considering vestibular signals dependent on the new gravity environment, through Bayesian inference. The evolution and weighting of hypotheses considered by the CNS is modeled via a Rao-Blackwellized particle filter algorithm. Sensorimotor adaptation learning is facilitated by retaining a memory of past harmonious states, represented by a conditional state transition probability density function, which allows the model to consider previously experienced gravity levels (while also dynamically learning new states) when formulating new alternative hypotheses of gravity. In order to demonstrate our theoretical framework and motivate future experiments, we perform a variety of simulations. These simulations demonstrate the effectiveness of this model and its potential to advance our understanding of transitory states during which central reinterpretation occurs, ultimately mitigating the risks associated with the lagging time course of adaptation to gravitational environments

Galvanic vestibular stimulation produces cross-modal improvements in visual thresholds

Background: Stochastic resonance (SR) refers to a faint signal being enhanced with the addition of white noise. Previous studies have found that vestibular perceptual thresholds are lowered with noisy galvanic vestibular stimulation (i.e., "in-channel" SR). Auditory white noise has been shown to improve tactile and visual thresholds, suggesting "cross-modal" SR. Objective: We aimed to study the cross-modal impact of noisy galvanic vestibular stimulation (nGVS) (n=9 subjects) on visual and auditory thresholds. Methods: We measured auditory and visual perceptual thresholds of human subjects across a swath of different nGVS levels in order to determine if a subject-specific best nGVS level elicited a reduction in thresholds as compared the no noise condition (sham). Results: We found an 18% improvement in visual thresholds (p = 0.026). Among the 7 of 9 subjects with reduced thresholds, the average improvement was 26%. Subjects with higher (worse) visual thresholds with no stimulation (sham) improved more than those with lower thresholds (p = 0.005). Auditory thresholds were unchanged by vestibular stimulation. Conclusions: These results are the first demonstration of cross-modal improvement with nGVS, indicating galvanic vestibular white noise can produce cross-modal improvements in some sensory channels, but not all.Comment: 15 pages, 7 figure

COMPASS: Computations for Orientation and Motion Perception in Altered Sensorimotor States

Reliable perception of self-motion and orientation requires the central nervous system (CNS) to adapt to changing environments, stimuli, and sensory organ function. The proposed computations required of neural systems for this adaptation process remain conceptual, limiting our understanding and ability to quantitatively predict adaptation and mitigate any resulting impairment prior to completing adaptation. Here, we have implemented a computational model of the internal calculations involved in the orientation perception system&rsquo;s adaptation to changes in the magnitude of gravity. In summary, we propose that the CNS considers parallel, alternative hypotheses of the parameter of interest (in this case, the CNS&rsquo;s internal estimate of the magnitude of gravity) and uses the associated sensory conflict signals (i.e., difference between sensory measurements and the expectation of them) to sequentially update the posterior probability of each hypothesis using Bayes rule. Over time, an updated central estimate of the internal magnitude of gravity emerges from the posterior probability distribution, which is then used to process sensory information and produce perceptions of self-motion and orientation. We have implemented these hypotheses in a computational model and performed various simulations to demonstrate quantitative model predictions of adaptation of the orientation perception system to changes in the magnitude of gravity, similar to those experienced by astronauts during space exploration missions. These model predictions serve as quantitative hypotheses to inspire future experimental assessments. &nbsp;</p

Exhibition of Stochastic Resonance in Vestibular Tilt Motion Perception

Background: Stochastic Resonance (SR) is a phenomenon broadly described as &ldquo;noise benefit&rdquo;. The application of subsensory electrical Stochastic Vestibular Stimulation (SVS) via electrodes behind each ear has been used to improve human balance and gait, but its effect on motion perception thresholds has not been examined.Objective: This study investigated the capability of subsensory SVS to reduce vestibular motion perception thresholds in a manner consistent with a characteristic bell-shaped SR curve.Methods: We measured upright, head-centered, roll tilt Direction Recognition (DR) thresholds in the dark in 12 human subjects with the application of wideband 0-30 Hz SVS ranging from &plusmn;0-2700 &mu;A. To conservatively assess if SR was exhibited, we compared the proportions of both subjective and statistical SR exhibition in our experimental data to proportions of SR exhibition in multiple simulation cases with varying underlying SR behavior. Analysis included individual and group statistics.Results: As there is not an established mathematical definition, three humans subjectively judged that SR was exhibited in 78% of subjects. &ldquo;Statistically significant SR exhibition&rdquo;, which additionally required that a subject&rsquo;s DR threshold with SVS be significantly lower than baseline (no SVS), was present in 50% of subjects. Both percentages were higher than simulations suggested could occur simply by chance. For SR exhibitors, defined by subjective or statistically significant criteria, the mean DR threshold improved by -30% and -39%, respectively. The largest individual improvement was -47%.Conclusion: At least half of the subjects were better able to perceive passive body motion with the application of subsensory SVS. This study presents the first conclusive demonstration of SR in vestibular motion perception.</p