Gravity affects the perception of self-motion and of size on Earth. Any errors in these perceptions while in space could represent serious risks to astronauts, for example, for locating and moving to an escape hatch. When perception of self-motion is induced using only visual motion, vestibular cues indicate that the body remains stationary which may bias an observer’s perception. When lowering the reliability of the vestibular cue by lying down or adapting to microgravity, these biases may decrease, accompanied by a decrease in precision. Previous studies on the ISS using the perception of the shape of a cube have suggested that perceived size may be compressed also.

To assess these perceptions, we used (task 1) a move-to-target task and (task 2) a size comparison task in virtual reality. For task 1, astronauts (6 female, 6 male) and Earth-based controls (10 female, 10 male) were shown a simulated target. After the target disappeared, self-motion was induced by visual motion. Participants indicated when they had arrived at the target’s previously seen location. For task 2, they compared the height of a virtual square simulated at three distances with the length of a physical rod held in their hands. Astronauts completed these tasks on Earth (supine and upright) prior to space travel, twice onboard the International Space Station (ISS) (within 3-6 days of arrival, after ~85 days in space), and after landing (within 3-6 days of return and ~85 days later). Controls completed the experiment on Earth using a similar regime.

While variability was similar across all conditions, astronauts displayed higher gains (target distance/perceived travel distance) when supine than when upright in terrestrial sessions. No differences could be detected between astronauts’ performance on Earth and in space or between the controls’ sessions. We found no immediate effect of microgravity exposure on perceived object height. However, astronauts robustly underestimated the height of the target relative to the haptic reference and these estimates were significantly smaller after 60 days after return to Earth. No differences were observed in the precision of astronauts’ judgements.

We conclude that no countermeasures are required to mitigate acute effects of microgravity exposure on self-motion or object height perception. Despite adapting to a floating mode of travel in the ISS, astronauts’ performance in judging self-motion distance appears largely unaffected by exposure to microgravity.
However, space travelers should be warned about late-emerging and potentially long-lasting changes in these perceptual skills.

Introduction:
Space motion sickness (SMS) affects more than 50% of astronauts during the initial days of a mission (Davis et al., 1988b). The occurrence of SMS is hypothesized to be related to sensory mismatch within the vestibular system itself (Lackner and DiZio, 2006). In space, otolithic information, which provides cues about gravity direction on Earth, is absent, and the precise impact of gravity on vestibular capacities remains uncertain. SMS typically disappears after three to four days, following adaptation to microgravity. Evaluation of the gravitational influence on vestibular function primarily occurs in adaptated astronauts.

Aim:
In the experiments presented here, we focus on assessing vestibular capacities, with particular attention to the critical period preceding adaptation, during which SMS occurs. Additionally, we aim to investigate the impact of space motion sickness on visuo-vestibular integration.

Methods:
To examine how vestibular integration is affected, we conducted experiments during parabolic flights where participants were exposed to repeated short sessions of microgravity and hypergravity. Throughout these flights, we followed the evolution of motion sickness using both objective measurements and subjective feedbacks. For now, nine participants took part in the CNES parabolic flight campaign. Vestibular integration was evaluated through two investigations focusing on otoliths (Colebatch et al., 1994) and semicircular canals (Benson et al., 1986) respectively. The first assessment evaluated vestibular evoked myogenic potentials (VEMP) recordings, while the second measured the suppression of the vestibulo-ocular reflex (VOR). The impact of vestibular disturbance on visual integration was examined by assessing the optokinetic reflex (OKN) with virtual reality before and after the parabolic sessions. Motion sickness was continuously monitored through objective measurements such as skin conductance, temperature, heart rate, as well as through subjective information obtained from questionnaires and self-reports.

Discussion:
We will present the effects of short-term exposure to modified gravities on hard-wired vestibulo-spinal reflexes and concurrent integration. We wish to identify the most relevant physiological variables for motion sickness estimation and to discuss the relationship between physiological variables and vestibular responses over time. Furthermore, we aim to determine the differences between terrestrial motion sickness experienced in virtual environments and that encountered during parabolic flights, while evaluating their respective impacts on vestibular integration.

INTRODUCTION

Verticality is determined in relation to gravity, where a vertical orientation runs parallel to gravity, while a horizontal orientation is perpendicular to it. Verticality perception plays a crucial role in balance and spatial orientation. Exposure to microgravity during spaceflight is known to elicit verticality illusions. The brain constructs a representation of verticality by integrating vestibular and visual information. However, how mechanistically these sensory cues are integrated for the perception of verticality has been largely debated. Here we systematically investigated the dynamic integration of vestibular and visual cues, considering their respective reliability, in the perception of verticality.

MATERIAL AND METHODS

Twenty-four participants (10 male, 14 female), aged 18-50, engaged in a Verticality Detection Task (VDT). The task involved discriminating between vertical lines and those tilted 2.5 degrees clockwise or counterclockwise while seated upright with head support. To manipulate cue reliability, we introduced stimulations inducing artificial roll-tilt sensations using visual optokinetic stimulation cues (OKS), galvanic vestibular stimulation cues (GVS), or a combination of both (OKS+GVS). Sham controls were implemented to account for non-specific effects. Correct answers, perceptual sensitivity (d'), and post-perceptual bias (C) were measured and analyzed using signal detection approaches.

Sensitivity (d’) was then calculated as:
𝑑′ = 𝑍(𝑃𝑟𝑜𝑝𝐻𝑖𝑡) − 𝑍(𝑃𝑟𝑜𝑝𝐹𝐴)

Criterium was calculated as:
C = (-(𝑍(𝑃𝑟𝑜𝑝𝐻𝑖𝑡) + 𝑍(𝑃𝑟𝑜𝑝𝐹𝐴)))/(2)

Data were analyzed using JMP® (version Pro 16. SAS Institute Inc, Cary, NC, 1989-2001). We built three linear mixed models (LMM) for each variable (correct answer, sensitivity, and criterion) with condition as fixed effect and subject as random effect for each LMM.

RESULTS AND DISCUSSION

Linearly increasing errors in detecting verticality were observed from vestibular to visual and combined conditions. Sensitivity significantly differed between vestibular and combined visual-vestibular stimulations, with no discernible response bias variations. Results indicate that both visual and vestibular cues influence verticality perception, emphasizing the dominance of visual cues. Combining modalities produced summation effects, influencing how verticality is perceived.

Our findings not only contribute to understanding the mechanistic aspects of verticality but also provide insights for spaceflight countermeasures. Recommendations include providing astronauts with vertical visual cues, particularly during lunar surface motor tasks, to enhance orientation. Visual cues can prove advantageous in the recovery phase, aiding astronauts in regaining their upright perception and enhancing balance and spatial orientation upon return to Earth.

INTRODUCTION
Our research team has previously investigated the otolith-mediated ocular counter-roll (OCR) where we found a difference in eye torsion pre- to postflight [1], which was moreover governed by previous flight experience. Moreover, resting-state functional magnetic resonance imaging (rsfMRI) analysis in a largely overlapping cosmonaut cohort has revealed functional connectivity (FC) changes after spaceflight [2]. The current study aimed to investigate retrospectively if OCR changes from pre- to postflight correlate with functional connectivity changes in specific vestibular cortical regions after a long duration mission to the International Space Station (ISS).

METHODS
Fourteen cosmonauts were included in this analysis (mean age: 46.8 (SD=5.17); mission duration=186.92 (SD=51)). Brain MRI scans were acquired before the ISS mission (89 days (SD=199)) and after (9 days (SD=3)). The OCR was assessed through an independent study at 154 days (SD=109) before launch and 3 days (SD=1) after landing. Resting-state functional MRI (rsfMRI) data were acquired at each time point, from which FC was derived. We adopted a regions-of-interest (ROI) approach based on a cortical vestibular atlas from a previous fMRI study [3]. The vestibular function was measured using the OCR, which is an otolith-induced eye reflex generated by off-axis centrifugation. The pre- to post-flight difference in SBC was correlated with the pre- to post-flight difference of the OCR. The voxel-level threshold was set at p<0.001 uncorrected followed by a cluster-level threshold of p<0.05 corrected with the false discovery rate (FDR).

RESULTS
Significant alterations in FC were identified between the vestibular seed region, OP2_PIVC r, and clusters (+38 -72 +52) involving the right angular gyrus (AG) and (-52 -62 +48) encompassing the left angular gyrus (AG) (Figure 1). Location of both clusters was confirmed by using the AAL3 atlas toolbox of SPM12 as an additional atlas to the one of CONN. These changes corresponded with a simultaneous decrease in OCR (p(FDR)<0.021, p(FDR)<0.038), indicating a correlation with a higher decrease of the OCR and a lower connectivity post-flight. Highlighting the angular gyrus' role in spaceflight vestibular adaptation3–8, contributing to our understanding and facilitating accelerated adaptation in experienced cosmonauts during gravitational transitions. The more the otoliths are affected after spaceflight, the less the connectivity has changed between those regions.
[1] Catho Schoenmaekers. 10.1038/s41526-022-00208-5 [2] Jillings. 10.1038/s42003-022-04382-w
[3] Peter zu Eulenburg. 10.14293/S2199-1006.1.SOR-.PPCSDUO.v1.
[4] Hupfeld: 10.1093/cercor/bhab239.
[5] Linnea Banker. ID : NBK544218
[6] Joseph DiGuiseppi. ID: NBK549825
[7] Ruba R. Al-Ramadhani . 10.1684/epd.2021.1257 [8] Adnan A S Alahmadi: 10.1186/s13244-021-00993-9

Introduction and Background:
Astronauts experience spatial disorientation during and after gravity transitions. Such spatial disorientation poses a risk for mission critical tasks such as emergency egress and space craft manual control. With upcoming lunar missions, astronauts will be exposed to microgravity before transitioning to lunar gravity and may be required to take control of spacecraft landings (indeed all Apollo landings utilized manual control despite automated capabilities [1]). Thus, we need to better understand spatial disorientation during crewed lunar landing and develop countermeasures to mitigate the risk. A common analog used to study sensorimotor impairment for gravity transitions is Sickness Induced by Centrifugation (SIC), where participants are exposed to 2G through the chest (2Gx) for an hour. This analog has been used to study motion sickness, vestibular reflexes, and functional performance [2,3,4], but changes to spatial orientation perception following SIC have not yet been quantified. A detailed understanding of perceptual changes following SIC will allow for better predictions of spatial orientation, countermeasure development, and extensibility of knowledge to gravity transitions to and from hypo-gravity.
Experimental Methods:
This experiment utilized CU Boulder’s centrifuge to produce the SIC paradigm (2Gx for 1hr) and the Tilt Translation Sled (TTS) to provide roll tilts up to +/- 20 degrees. Participants completed two counterbalanced testing sessions at least a week apart. In one testing session they were exposed to the 2Gx SIC paradigm for an hour and in the other where they were exposed to 1Gx by lying supine for an hour. Following each gravitational exposure participants were seated in the TTS in the dark with their heads restrained and exposed to pseudorandom sum of sines roll tilt profiles. While being tilted, participants reported their orientation perception via a subject haptic horizontal (SHH) task. Here participants attempted to keep a bar connected to a potentiometer level with their perceived gravitational horizontal. SHH reports were compared following the 1Gx and 2Gx exposures.
Results:
Initial pilot testing suggests that there is substantial variability in perceptual reporting between participants regardless of the gravitational exposure, but that participants are less certain of their reports following the gravity transition. Through further testing we expect this to translate to decreased accuracy in perceptual reports.
Conclusion:
Gravity transitions, whether between hypo or hyper gravity, induce changes in orientation perception that can result in spatial disorientation. Perceptual changes measured during this experiment improve our understanding of spatial disorientation development following gravity transitions.

Introduction: Once more, plans are underway to send humans to the Moon or possibly even to Mars. It is therefore, important to know potential physiological effects of a prolonged stay in space and to minimize possible health risks to astronauts. It has been shown that spinal motor control strategies change during microgravity induced by parabolic flight. The way in which spinal motor control strategies change during partial microgravity, such as that encountered on the Moon and on Mars, is not known.
Methods: Spinal motor control measurements were performed during Earth, lunar, Mars, and micro-gravity conditions and two hypergravity conditions of a parabola. Three proxy measures of spinal motor control were recorded: spinal stiffness of lumbar L3 vertebra using the impulse response, muscle activity of lumbar flexors and extensors using surface electromyography, and lumbar curvature using two curvature distance sensors placed at the upper and lower lumbar spine. The participants were six females and six males, with a mean age of 33 years (standard deviation: 7 years).
Results: Gravity condition had a statistically significant (Friedmann tests) effect spinal stiffness (p < 0.001); on EMG measures (multifidus (p = 0.047), transversus abdominis (p < 0.001), and psoas (p < 0.001) muscles) and on upper lumbar curvature sensor (p < 0.001). No effect was found on the erector spinae muscle (p = 0.063) or lower curvature sensor (p = 0.170). Post hoc tests revealed a significant increase in stiffness under micro-, lunar-, and Martian gravity conditions (all p’s < 0.034). Spinal stiffness decreased under both hypergravity conditions (all p’s ≤ 0.012) and decreased during the second hypergravity compared to the first hypergravity condition (p = 0.012).
Discussion: Micro-, lunar-, and Martian gravity conditions resulted in similar increases in spinal stiffness, a decrease in transversus abdominis muscle activity, with no change in psoas muscle activity and thus modulation of spinal motor stabilization strategy compared to those observed under Earth’s gravity. These findings suggest that the spine is highly sensitive to gravity transitions but that Lunar and Martian gravity are below that required for normal modulation of spinal motor stabilization strategy and thus may be associated with LBP and/or IVD risk without the definition of countermeasures.