BACKGROUND: The vestibulo-ocular reflex (VOR) is a basic function of the vestibular system that stabilizes gaze during head movement. Investigations on how spaceflight affects VOR gain and phase are few, and the magnitude of observed changes varies considerably and depends on the protocols used. OBJECTIVE: We investigated whether the gain and phase of the VOR in darkness and the visually assisted VOR were affected during and after spaceflight. METHODS: We measured the VOR gain and phase of 4 astronauts during and after a Space Shuttle spaceflight while the subjects voluntary oscillated their head around the yaw axis at 0.33 Hz or 1 Hz and fixed their gaze on a visual target (VVOR) or imagined this target when vision was occluded (DVOR). Eye position was recorded using electrooculography and angular velocity of the head was recorded with angular rate sensors. RESULTS: The VVOR gain at both oscillation frequencies remained near unity for all trials. DVOR gain was more variable inflight and postflight. Early inflight and immediately after the flight, DVOR gain was lower than before the flight. The phase between head and eye position was not altered by spaceflight. CONCLUSION: The decrease in DVOR gain early in the flight and after the flight reflects adaptive changes in central integration of vestibular and proprioceptive sensory inputs during active head movements.

Understanding the effects of microgravity on the vestibular system has been a primary focus of space research, driven by the need to counteract the often-debilitating impacts of altered gravity environments and maintain operational performance in space. Research using both space-based and ground-based models has identified structural and functional changes in the vestibular system, highlighting its significant capacity for sensorimotor adaptation. As human space exploration progresses towards missions beyond low Earth orbit for extended periods, additional stressors, such as space radiation, may impact the vestibular system. Early studies on space radiation using animal models and insights from radiotherapy have shown that the vestibular system is more vulnerable to radiation than previously understood. This paper provides a brief review of (1) dysfunctions in spatial orientation, gaze stabilization, posture, and locomotion observed in astronauts; (2) ground-based experiments on animals that likely explain these vestibular and sensorimotor dysfunctions; and (3) studies examining the effects of radiation on the vestibular system and its implications for vestibular function in space.

Effect of spaceflight on thresholds of perception of angular and linear motion. Psychophysical studies of vestibular function have been carried out in order to study adaptation within the vestibular sensory system to the weightless environment of orbital spaceflight. No significant change in the threshold of detection of whole-body angular acceleration was found, either during flight or post-flight. Experiments involving the perception of whole-body linear acceleration have yielded somewhat inconsistent results, although the weight of evidence points to an elevation and increased variability of threshold in the first few days following spaceflight. Although a change in the excitability of the saccular and macular otoliths in microgravity cannot be excluded, it is more probable that this decreased sensitivity is a manifestation of a central adaptive mechanism, in which the "weighting" of gravi-receptor information is reduced. Enhancement of the ability to detect linear acceleration stimuli, exhibited by some astronauts in microgravity, may be a manifestation of heightened utilization of cutaneous rather than otolithic cues. Published in Archives of oto-rhino-laryngology (1987)

Recovery of postural equilibrium control following spaceflight. Decreased postural stability is observed in most astronauts immediately following spaceflight. Because ataxia may present postflight operational hazards, it is important to determine the incidence of postural instability immediately following landing and the dynamics of recovery of normal postural equilibrium control. It is postulated that postflight postural instability results from in-flight adaptive changes in central nervous system (CNS) processing of sensory information from the visual, vestibular, and proprioceptive systems. The purpose of the present investigation was to determine the magnitude and time course of postflight recovery of postural equilibrium control and, hence, readaptation of CNS processing of sensory information. Thirteen crew members from six spaceflight missions were studied pre- and postflight using a modified commercial posturography system. Postural equilibrium control was found to be seriously disrupted immediately following spaceflight in all subjects.

European vestibular experiments on the Spacelab-1 mission: 7. Ocular counterrolling measurements pre- and post-flight. The static ocular counterrolling (OCR) of the four scientific crew members in the first Spacelab mission was measured during baseline-data-collection before and after the flight of SL-1. It was presumed that the modification of otolithic responses during spaceflight will be reflected in specific changes of the OCR-gain on the first days after recovery. The magnitude of OCR was determined analysing colour-transparencies of subjects right eyes that were produced in different positions of lateral body tilt. In general, one subject did not show any changes at all; three subjects exhibited a significant decrease of OCR-gain after exposure to weightlessness, whereby differences could be found between the responses for small and large angles of lateral body tilt. Moreover, asymmetrical effects of OCR-gain were found between body tilt to the left and tilt to the right side.

Space Motion Sickness (SMS) is a syndrome that affects around 70% of astronauts and includes symptoms of nausea, dizziness, fatigue, vertigo, headaches, vomiting, and cold sweating. Consequences range from discomfort to severe sensorimotor and cognitive incapacitation, which might cause potential problems for mission-critical tasks and astronauts and cosmonauts' well-being. Both pharmacological and non-pharmacological countermeasures have been proposed to mitigate SMS. However, their effectiveness has not been systematically evaluated. Here we present the first systematic review of published peer-reviewed research on the effectiveness of pharmacological and non-pharmacological countermeasures to SMS. We performed a double-blind title and abstract screening using the online Rayyan collaboration tool for systematic reviews, followed by a full-text screening. Eventually, only 23 peer-reviewed studies underwent data extraction. Both pharmacological and non-pharmacological countermeasures can help mitigate SMS symptoms. No definitive recommendation can be given regarding the superiority of any particular countermeasure approach. Importantly, there is considerable heterogeneity in the published research methods, lack of a standardized assessment approach, and small sample sizes. To allow for consistent comparisons between SMS countermeasures in the future, standardized testing protocols for spaceflight and ground-based analogs are needed. We believe that the data should be made openly available, given the uniqueness of the environment in which it is collected. https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021244131.