Human manual control precision depends on vestibular sensory precision and gravitational magnitude.

Precise motion control is critical to human survival on Earth and in space. Motion sensation is inherently imprecise, and the functional implications of this imprecision are not well understood. We studied a "vestibular" manual control task in which subjects attempted to keep themselves upright with a rotational hand controller (i.e., joystick) to null out pseudorandom, roll-tilt motion disturbances of their chair in the dark. Our first objective was to study the relationship between intersubject differences in manual control performance and sensory precision, determined by measuring vestibular perceptual thresholds. Our second objective was to examine the influence of altered gravity on manual control performance. Subjects performed the manual control task while supine during short-radius centrifugation, with roll tilts occurring relative to centripetal accelerations of 0.5, 1.0, and 1.33 GC (1 GC = 9.81 m/s2). Roll-tilt vestibular precision was quantified with roll-tilt vestibular direction-recognition perceptual thresholds, the minimum movement that one can reliably distinguish as leftward vs. rightward. A significant intersubject correlation was found between manual control performance (defined as the standard deviation of chair tilt) and thresholds, consistent with sensory imprecision negatively affecting functional precision. Furthermore, compared with 1.0 GC manual control was more precise in 1.33 GC (-18.3%, P = 0.005) and less precise in 0.5 GC (+39.6%, P < 0.001). The decrement in manual control performance observed in 0.5 GC and in subjects with high thresholds suggests potential risk factors for piloting and locomotion, both on Earth and during human exploration missions to the moon (0.16 G) and Mars (0.38 G). NEW & NOTEWORTHY The functional implications of imprecise motion sensation are not well understood. We found a significant correlation between subjects' vestibular perceptual thresholds and performance in a manual control task (using a joystick to keep their chair upright), consistent with sensory imprecision negatively affecting functional precision. Furthermore, using an altered-gravity centrifuge configuration, we found that manual control precision was improved in "hypergravity" and degraded in "hypogravity." These results have potential relevance for postural control, aviation, and spaceflight.

Human perception of whole body roll-tilt orientation in a hypogravity analog: underestimation and adaptation.

Overestimation of roll tilt in hypergravity ("G-excess" illusion) has been demonstrated, but corresponding sustained hypogravic conditions are impossible to create in ground laboratories. In this article we describe the first systematic experimental evidence that in a hypogravity analog, humans underestimate roll tilt. We studied perception of self-roll tilt in nine subjects, who were supine while spun on a centrifuge to create a hypogravity analog. By varying the centrifuge rotation rate, we modulated the centripetal acceleration (GC) at the subject's head location (0.5 or 1 GC) along the body axis. We measured orientation perception using a subjective visual vertical task in which subjects aligned an illuminated bar with their perceived centripetal acceleration direction during tilts (±11.5-28.5°). As hypothesized, based on the reduced utricular otolith shearing, subjects initially underestimated roll tilts in the 0.5 GC condition compared with the 1 GC condition (mean perceptual gain change = -0.27, P = 0.01). When visual feedback was given after each trial in 0.5 GC, subjects' perceptual gain increased in approximately exponential fashion over time (time constant = 16 tilts or 13 min), and after 45 min, the perceptual gain was not significantly different from the 1 GC baseline (mean gain difference between 1 GC initial and 0.5 GC final = 0.16, P = 0.3). Thus humans modified their interpretation of sensory cues to more correctly report orientation during this hypogravity analog. Quantifying the acute orientation perceptual learning in such an altered gravity environment may have implications for human space exploration on the moon or Mars. NEW & NOTEWORTHY Humans systematically overestimate roll tilt in hypergravity. However, human perception of orientation in hypogravity has not been quantified across a range of tilt angles. Using a centrifuge to create a hypogravity centripetal acceleration environment, we found initial underestimation of roll tilt. Providing static visual feedback, perceptual learning reduced underestimation during the hypogravity analog. These altered gravity orientation perceptual errors and adaptation may have implications for astronauts.

Multiple field tests on landing day: Early mobility may improve postural recovery following spaceflight.

Adaptation to microgravity causes astronauts to experience sensorimotor disturbances during return to Earth leading to functional difficulties. Recently, the Field Test (FT) study involving an incrementally demanding sensorimotor functional test battery has allowed for an unprecedented view into early decrements and recovery from multiple tests conducted on the landing day following 6-months International Space Station missions. Although the protocol was challenging and temporarily increased motion sickness symptoms, there were anecdotal reports that performing these tasks within the first few hours of landing accelerated their recovery. Therefore, results from computerized dynamic posturography (CDP) following return to Houston were used to compare recovery between crewmembers that participated in FT (n = 18) with those that did not (controls, n = 11). While there were significant decrements in postural performance for both groups, some FT participants tended to perform closer to their preflight baseline in the most challenging condition of the CDP sensitive to vestibular function-eyes closed, unstable support and head movements. However, the distribution of difference scores appeared bimodal with other FT participants in the lower range of performance. We attribute these observations to the manner in which the field tests were implemented-some benefitted by encouraging early movement to drive adaptation when performed in a constrained incremental fashion; however, movements above aversive thresholds may have impaired adaptation in others. Challenging the sensorimotor system with increasingly provocative movements performed as close to landing as possible, as long as within individual thresholds, could be a useful intervention to accelerate astronaut's sensorimotor readaptation that deserves further study.

Tandem Walk in Simulated Martian Gravity and Visual Environment.

Astronauts returning from long-duration spaceflights experience visual-vestibular conflicts that causes motion sickness, perceptions that the environment is moving when it is not, and problems with walking and other functional tasks. To evaluate whether astronauts will have similar decrements after they land on Mars following exposure to weightlessness, participants were held by a device that offloads their weight, first entirely (0 G), and then partially (0.38 G) or not at all (1 G). Tandem (heel-to-toe) walk on a medium-density foam surface was used to assess the subject's walking performance. Two visual conditions in virtual reality were investigated: normal vision and a visual-vestibular conflict generated by disorienting optokinetic stimulation (DOS). Tandem walking performance with DOS was better in 0.38 G compared to 1 G. Tandem walking performance in DOS in 1 G was not significantly different from tandem walking performance after spaceflight or bed rest. The increased tandem walking performance in 0.38 G compared to 1 G was presumably due to an increased cone of stability, allowing a larger amplitude of body sway without resulting in a fall. Tandem walking on a compliant foam surface with a visual-vestibular conflict is a potential analog for simulating postflight dynamic balance deficits in astronauts.

Efficacy of Gradient Compression Garments in the Hours After Long-Duration Spaceflight.

The incidence of presyncopal events is high soon after a long-duration spaceflight;>60% of returning astronauts could not complete a 10-min 80° head-up tilt test on landing day (R+0) after ~6 months of spaceflight. The objective of this study was to demonstrate the ability of a lower body gradient compression garment (GCG) to protect against an excessive increase in heart rate and a decrease in blood pressure during standing after long-duration spaceflight. Methods: Eleven astronauts (9 M, 2 F) volunteered to participate. The stand test protocol consisted of 2 min of prone rest followed by 3.5 min of standing. Subjects completed one familiarization session, two preflight data collection sessions in standard clothing, and three tests on landing day while wearing GCG. Postflight tests were conducted 1-4 h (R+0A), ~12 h (R+0B), and ~28 h after landing (R+0C). Results: All astronauts completed the stand test preflight. Three astronauts were unable to attempt the stand test at R+0A, and one of these was unable to start the test at R+0B. One astronaut was unable to complete 3.5 min of standing at R+0B (test ended at 3.3 min). Review of the individual's blood pressure data revealed no hypotension but the astronaut reported significant motion sickness. Of the astronauts who participated in testing on landing day, the heart rate and mean arterial pressure responses to standing (stand-prone) were not different than preflight at any of the postflight sessions. Conclusion: Wearing the GCG after spaceflight prevented the tachycardia that normally occurs while standing after spaceflight without compression garments and protected against a decrease in blood pressure during a short stand test.