Since the beginning of time, all living organisms have evolved under a terrestrial gravitational acceleration of 9.81 m/s2. It’s hard to imagine a more fundamental and ubiquitous aspect of life on Earth than gravity. When the head moves with respect to gravity, the vestibular otoliths shift with the direction of gravitational acceleration, moving the hair cells receptors and signalling to the brain the magnitude and direction of gravity. Thus, the pull of gravity generates a constant sensory flow from early foetal life until death. But what can we say about the first human to be born in a non-terrestrial gravitational field? Little is known about how the information from the vestibular otoliths is coded by the brain to construct a model of Earth gravity, and whether this internalised model is innate and fixed to terrestrial gravity, or whether it can adjust to other gravitational environments, such as weightlessness or partial gravity on the Moon or Mars. Most recent neurocognitive models emphasize the predictability of sensory signals that make the organism feel anchored to the physical world. This construct invokes the idea of minimizing prediction error. A prediction error reflects the mismatch between a prior prediction or expectation and the actual sensory signals. On Earth gravity is always there and may play a primary role in minimizing prediction errors by providing a strong prior reference. Here I suggest that this “gravity hyper prior” is optimal in the terrestrial environment and permits regulation of multiple aspects of cognition by reducing unpredictability, and situating any behaviour within a general repertoire. I will share our research findings that endorse the gravity hyper prior in basic sensorimotor functions (Gallagher et al., 2021) and cognitive representations (Pavlidou et al., 2022). Across various models, from animals to humans, I will explore evidence supporting the idea that we possess innate predispositions towards a gravity hyper prior (Bliss et al., 2023). Furthermore, our research indicates that this gravity hyper prior can dynamically adjust when exposed to environments with non-terrestrial gravity.

INTRODUCTION: Crewmembers flying ~6-month missions to the International Space Station show signs of optic disc edema (ODE), chorioretinal folds, globe flattening, and/or hyperopic shifts in refractive error; these findings are known as spaceflight associated neuro-ocular syndrome (SANS). The chronic headward fluid shift is believed to be the primary cause, and use of the spaceflight analog strict 6˚ head-down tilt bed rest (HDTBR) has replicated key SANS findings. The purpose of this study was to test the ability for two candidate countermeasures to reverse the headward fluid shift during strict HDTBR.

METHODS: Using a cross-section study design, participants were exposed to 30 days of strict HDTBR and randomly assigned to 1 of 4 groups: Control (n=12, 6 female); Seated (n=11, 5 female); Lower Body Negative Pressure (LBNP, n=12, 6 female); Thigh Cuff (n=12, 4 female). Participants of the LBNP (50 mmHg) and Seated group were exposed to 6 hours of daily fluid shift reversal. Thigh cuffs were worn for 6 hours on 6 out of 7 days. Various ultrasound targets and intraocular pressure were used to assess the headward fluid shift before and during use of countermeasures throughout HDTBR. Optical coherence tomography images were analysed for the development of ODE.

RESULTS: All participants completed 6 hours of daily exposure to each countermeasure without incident. LBNP led to a small increase in HR, but this did not change throughout the 3-hour exposure, or throughout 30 days of HDTBR. Cardiac output was reduced during use of each countermeasure. Use of LBNP or thigh cuff led to ~50% reduction in internal jugular vein cross-sectional area, while the upright group demonstrated complete reduction. LBNP reduced IOP by ~1.5 mm Hg, while use of thigh cuff led to a modest ~0.5 mm Hg reduction. ODE developed in all 4 groups.

DISCUSSION: We rapidly tested 2 countermeasures by studying 47 volunteer participants exposed to 30 days of strict HDTBR. Both countermeasures were successfully implemented for 6 hours per day and resulted in an ~50% reversal of the headward fluid shift. However, SANS findings still emerged in all groups, suggesting that longer countermeasures exposures may be necessary.

On Earth, body weight is given by Newton’s laws as mass times gravitational acceleration. That is, an object’s weight is determined by the pull of gravity on it. Thus, the perceived body weight is – like actual weight – dependent on the strength of gravity. No single sensory signal informs the brain about the weight of the body. However, the vestibular organs constantly detect gravity. We therefore hypothesize that vestibular input is central to the perception of body weight.
We have recently demonstrated that experimental alterations of gravity produce rapid changes in the perceived weight of specific individual body parts (Ferrè et al., 2019). We have asked participants to estimate the weight of two body parts, their hand or their head, both in normal terrestrial gravity (1g) and during exposure to experimentally altered gravitational fields, 0g and +1.8g during parabolic flight and +1g using a short arm human centrifuge. For both body parts, there was an increase in perceived weight during the experience of hypergravity, and a decrease during the experience of microgravity.
However, how mechanistically vestibular signals shape weight perception is not clear. To address this question, we combined Galvanic Vestibular Stimulation (GVS) with
psychophysics measure of perceived hand weight (Ferrè et al., 2023). We used bilateral bipolar galvanic vestibular stimulation (GVS) to non-invasively stimulate the vestibular receptors. Brief left anodal and right cathodal GVS (L-GVS, which predominantly activates vestibular networks in the right cerebral hemisphere), or right anodal and left cathodal GVS (R-GVS), or sham stimulation were delivered at random, while participants estimated whether a weight placed on their wrist feels heavier or lighter than their left hand in two psychophysical staircases. We then used a planned comparisons approach to directly compare L-GVS and R-GVS conditions, in order to investigate how vestibular projections in each hemisphere might influence the neural processes responsible for weight perception. Preliminary results show a trend in which L-GVS, which enhances the gravity signal in the right hemisphere, make the hand feel heavier compared to R-GVS, reducing a baseline bias for underestimation. We also compared GVS stimulation of both hemispheres against sham stimulation to test a non-hemisphere-specific hypothesis regarding indirect effects of vestibular stimulation, such as those mediated by arousal. Results do not support this hypothesis, indicating that GVS inputs per se do not influence weight judgments.

Brain ventricular volume increases post long-duration International Space Station (ISS) missions, persisting up to a year after missions, suggesting potential carry-over effects [1-4]. This study explores ventricular volume changes across multiple ISS missions, aiming to elucidate long-term effects on cosmonauts.

METHODS

MRI scans from 24 cosmonauts over 10 years, part of ESA's BRAIN-DTI project, were analyzed. Six cosmonauts participated in consecutive missions, and nine control subjects underwent three measurements over 1.9 years. T1-weighted images were analyzed using cat12 toolbox, extracting CSF volumes from various ventricular regions. Linear mixed models in JMP Pro 16 and statistical analyses were employed.

RESULTS

Significant time effects on third (3V), right lateral (RLV), and left lateral ventricles (LLV) were observed (all p<0.0001). LLV exhibited increased volume 3 years post-mission compared to preflight. Linear increases in 3V, RLV, and LLV were noted when excluding immediate postflight data (p<0.0001). The fourth ventricle (4V) showed an impact of previous missions on pre- to postflight volume change. Control data exhibited time and age effects, with group-level increases influenced by two older subjects.

DISCUSSION

A trend towards accumulating ventricular volume increases in 3V, RLV, and LLV was observed. LLV maintained elevated volume three years post-mission. Although no significant difference in percentage volume change between missions was found, linear increases suggest cumulative effects. This study, analyzing the same cosmonauts across multiple missions, presents a unique framework. However, limited sample size and missing data constrain firm conclusions. Variability in ventricular volume dynamics and aging effects should be explored further. Despite limitations, these findings offer insights for future studies on how prior space experience impacts ventricular volume in space travelers.

Even though substantial research has revolved around developing food for Space to sustain astronauts in short- and mid-term missions, there are still concerns surrounding inadequate calorie consumption that leads to weight and bone loss due to menu fatigue and low palatability perception. Hence, it is important to investigate the perception of food senses in simulated space and microgravity environments, which will aid in developing palatable plants that may be produced in Space during long-term missions such as NASA-Artemis. Existing studies have focused on perception of orthonasal aromas and some basic tastes; however, these studies presented some flaws, and no or minimal research has been conducted to assess retronasal aromas and mouthfeel perception. This study focused on the assessment of retronasal aromas and trigeminal sensations (mouthfeel) intensity perception in two seating positions: (i) normal and (ii) simulated microgravity (reclined) and two simulated environments (i) neutral and (ii) immersive Space (rooms with 180° screens). Two sensory sessions for (i) retronasal aromas and (ii) mouthfeel were conducted with 12 trained panellists using the BioSensory© application (University of Melbourne, Parkville, VIC, Australia) to record participants self-reported (intensity) and subconscious (physiological and emotional) responses using non-invasive biometrics in different simulated environments and seating positions. Results were analysed using multivariate data analysis; therefore, four principal component analyses (PCA) were developed for each session/treatment PCAs 1 and 5 for neutral environment using samples in the two seating positions, PCAs 2 and 6 for simulated Space environment using samples from the two seating positions, PCAs 3 and 7 for normal seating position using samples tested in the two environments, and PCAs 4 and 8 for simulated microgravity seating position using samples tested in the two environments. Overall, results showed that retronasal aromas tested in simulated microgravity seating positions were associated with lower intensity in both environments (PCA 1 and 2); likewise, samples tested in both seating positions in the immersive Space environment were related to lower intensity (PCA 3 and 4). Mouthfeel samples were perceived with lower intensity in the simulated microgravity seating position in both environments (PCA 8) and the simulated space environment with the simulated microgravity position (PCA 6). However, mouthfeel samples tested in the neutral environment with normal seating position were associated with higher intensity (PCA 5). This evidence may be considered for developing plants suitable for Space environments, providing astronauts with tastier food options to address the challenges of altered taste perceptions in Space.

Introduction
The impact of spaceflight on the central nervous system, particularly the white matter (WM) of the brain, remains poorly understood. Previous studies using diffusion MRI and voxel-based analysis suggested an increase in WM in the cerebellum after spaceflight, potentially indicating neuroplasticity. However, questions persisted regarding whether these changes truly reflected alterations in connectivity or were a result of fluid and tissue displacements. To address these uncertainties, we conducted an advanced fixel-based analysis of diffusion MRI on 18 Roscosmos cosmonauts before and after extended space missions.

Material and Methods
During the study, cosmonauts were scanned using diffusion-weighted MRI before (average 81 days) and shortly after (9 days) their missions to the International Space Station (ISS), which typically lasted 195 days. The diffusion MRI data were preprocessed, and the white matter fiber orientation distribution function (fODF) was estimated in each voxel using multi-tissue constrained spherical deconvolution. Fixel-specific metrics, including Fiber Density (FD), Fiber Cross-section (FC), and Fiber Density modulated with Cross-section (FDC), were calculated to assess microscopic and macroscopic changes in WM fibers.

Results
Results from an omnibus F-test revealed widespread alterations in WM due to long-duration spaceflight, supporting previous findings. However, subsequent posthoc t-tests demonstrated that the majority of these changes were macroscopic (FC) rather than microscopic (FD). Further analysis, considering both FD and FC simultaneously (FDC), showed no net decreases in WM, indicating that most FD changes were likely due to macroscopic fluid shifts rather than true alterations in connectivity. Interestingly, FDC analysis identified a net increase in WM fibers in the left superior and left middle cerebellar peduncles, providing additional evidence for neuroplasticity induced by long-duration spaceflight.

Conclusion
In conclusion, this fixel-based analysis of diffusion MRI data in cosmonauts revealed widespread macroscopic changes in WM after spaceflight, with no significant net decreases when considering both microscopic and macroscopic effects. The study also uncovered a net increase in WM fibers in specific cerebellar regions, supporting the notion of neuroplasticity induced by prolonged space missions.

Abstract
This paper outlines a proposed theoretical study to explore the application of oxytocin in enhancing cognitive functions and team dynamics in simulated space environments. Utilising a neuroeconomic framework, it investigates how oxytocin inhalation could improve collaborative decision-making, reduce stress, and promote social cohesion—critical components for success and crew well-being during long-duration space missions.
The proposed multi-phased experiment, set in simulated microgravity conditions, will examine the hypothesis that oxytocin can modulate social behaviors, trust, and psychological resilience. Theoretical underpinnings from seminal works, including McEwen (1998) and Sterling and Eyer (1988), support oxytocin’s role in restoring homeostasis and adapting to environmental stressors, crucial for maintaining crew health under the rigors of space travel.

Objectives
1. Examine oxytocin's effects on neural mechanisms of stress and decision-making, drawing on empirical findings by Baumgartner et al. (2008) and Heinrichs et al. (2003).
2. Assess its impacts on economic behaviors and social cohesion using neuroeconomic frameworks from Fehr and Rangel (2011).
3. Evaluate the practicality and safety of oxytocin use in space missions, informed by recent research such as Quintana et al. (2021).

This research intends to extend the application of oxytocin from a neurobiological tool to a practical solution for enhancing interpersonal relations and decision-making capabilities in space missions. Although the findings will not be completed by the conference, the study aims to provide a foundational framework for developing targeted interventions to optimize team performance and mental health in space, further enriched by a wide body of literature including Carter (2014) and De Dreu et al. (2010).
 
Baumgartner, T., Heinrichs, M., Vonlanthen, A., Fischbacher, U. and Fehr, E., 2008. Oxytocin shapes the neural circuitry of trust and trust adaptation in humans. Neuron, 58(4), pp.639-650.

Carter, C.S., 2014. Oxytocin pathways and the evolution of human behavior. Annual review of psychology, 65, pp.17-39

De Dreu, C.K., Greer, L.L., Handgraaf, M.J., Shalvi, S., Van Kleef, G.A., Baas, M., Ten Velden, F.S., Van Dijk, E. and Feith, S.W., 2010. The neuropeptide oxytocin regulates parochial altruism in intergroup conflict among humans. Science, 328(5984), pp.1408-1411.

Fehr, E. and Rangel, A., 2011. Neuroeconomic foundations of economic choice—recent advances. Journal of Economic Perspectives, 25(4), pp.3-30.

Heinrichs, M., Baumgartner, T., Kirschbaum, C. and Ehlert, U., 2003. Social support and oxytocin interact to suppress cortisol and subjective responses to psychosocial stress. Biological psychiatry, 54(12), pp.1389-1398.

McEwen, B.S., 1998. Protective and damaging effects of stress mediators. New England journal of medicine, 338(3), pp.171-179.