Daily artificial gravity partially mitigates vestibular processing changes associated with head-down tilt bedrest

Long-duration spaceflight leads to transient impairments in balance and mobility upon return to Earth, largely attributed to altered vestibular signaling and multisensory reweighting in microgravity, especially down-weighting of otolith cues. Head-down tilt bedrest (HDT) models aspects of microgravity such as headward fluid shifts and unloading, and has been shown to alter vestibular processing and degrade balance and mobility. Prior neuroimaging studies show altered brain activation during vestibular stimulation after spaceflight and during HDT. Artificial gravity (AG) delivered via short-arm centrifugation has been proposed as an integrated countermeasure that could simultaneously target multiple physiological systems. The current study aimed to test whether 30 minutes of daily AG during 60 days of HDT would mitigate HDT-induced changes in vestibular processing measured with fMRI and whether individual brain changes would relate to changes in balance and mobility. The primary hypothesis was that controls would show greater pre- to post-HDT changes in brain activity during vestibular stimulation than the AG group, and that brain changes would correlate with decrements in balance and mobility.

Prior work demonstrates that spaceflight alters vestibular-related brain activity, including reduced in-flight EEG alpha power in vestibular, motor, and cerebellar regions and decreased vestibular network connectivity postflight. fMRI studies have shown reduced deactivation in sensorimotor, frontal, temporal, and occipital regions after long-duration flight during vestibular stimulation, consistent with sensory reweighting. HDT analog studies show increased activation in insular, frontal, and parietal cortices during vestibular stimulation and further effects when combined with elevated CO₂, with associations between greater deactivation and better preserved balance/mobility. AG has been explored as a multisystem countermeasure: intermittent AG (6×5 min) mitigated orthostatic intolerance after 5-day HDT; in the 60-day AGBRESA campaign, AG showed mixed effects, with some studies indicating partial mitigation of balance and sway deterioration and increased neural efficiency during sensorimotor adaptation, though other analyses found no AG mitigation for certain balance outcomes. Collectively, these findings motivate testing whether daily short-arm AG can preserve vestibular processing and balance during prolonged HDT.

Design and participants: Twenty-four healthy adults (8 female; mean age 33.3 ± 9.17 years; 174.6 ± 8.6 cm; 74.2 ± 10.0 kg) participated at DLR envihab (Cologne, Germany). All passed AG tolerability screening (AG2 protocol). Inclusion was aligned with astronaut-like age, sex, and education; exclusions included cardiovascular disease, relevant medications, and recent smoking. Participants provided informed consent; protocols were approved by University of Florida, NASA IRBs, and the regional medical association. HDT and AG intervention: All participants underwent 60 days of continuous 6° head-down tilt bedrest (HDT) with baseline and recovery testing before and after. Two groups received 30 minutes/day of short-arm centrifugation AG, either continuously (one 30-min bout) or intermittently (6×5-min bouts with 3-min rests). A third group received no AG (CTRL). There were n=8 per AG condition; as no differences were found between continuous and intermittent AG, these were pooled (AG n=16; CTRL n=8). Centrifugation rotational speed was individualized to produce ~1 g along the z-axis at the center of mass (~2 gz at the feet), with ramp up/down ≤ 5° s². Participants lay supine and were instructed to remain still (not restrained). Vestibular stimulation and fMRI task: Vestibular stimulation was applied to the right side using an MRI-compatible pneumatic tactile pulse system (PnTPS), delivering low-force taps (0.6 kg) to the lateral cheekbone, a method known to elicit VEMPs and vestibular cortical activation with concurrent deactivation of somatosensory and visual cortices. Each fMRI session included one run: five 24-s tap blocks (1 Hz; 24 taps per block) interleaved with 20-s rest blocks. Time points: 7 days pre-HDT (BDC-7), HDT day 29 (HDT29), HDT day 58 (HDT58), and 10 days post-HDT (R+10). Participants were in the HDT position in the scanner (on a foam wedge) with the head flat in the coil. Behavioral measures: Mobility was assessed with the Functional Mobility Test (FMT), an obstacle course sensitive to HDT and spaceflight effects; primary outcome was first-trial completion time. Balance was assessed using Sensory Organization Test conditions 5 and 5M (SOT-5: eyes closed, sway-referenced support; SOT-5M: same with ±20° head tilts at 0.33 Hz). Three 20-s trials per condition were performed; the median equilibrium quotient (EQ) score was used. MRI acquisition: 3T Siemens Biograph scanner. fMRI: T2*-weighted EPI, TR 2500 ms, TE 32 ms, FA 90°, FOV 192×192 mm, matrix 64×64, 37 slices, slice thickness 3.5 mm, voxel 3×3×3.5 mm. T1: TR 1.9 s, TE 2.4 ms, FA 9°, FOV 250×250 mm, matrix 512×512, 192 slices, voxel 0.49×0.49×1.0 mm. Preprocessing: Whole-brain cortex preprocessing used SPM12, ANTs, and FSL: fieldmap/topup for B0 correction; slice timing correction; realignment/reslicing; Artifact Detection Tool to flag outlier volumes (framewise displacement ≥ 2.0 mm, global signal Z ≥ 9) with outliers modeled as covariates; creation of participant-specific longitudinal T1 and fMRI templates with ANTs; coregistration and normalization to MNI space; smoothing with 8-mm FWHM Gaussian kernel. Cerebellar preprocessing used CERES for segmentation and SUIT for normalization; masked cerebellar fMRI data were normalized to SUIT space and smoothed with 2-mm FWHM kernel. First-level modeling: Vestibular stimulation versus rest contrasts were computed voxel-wise at each time point with first-level masking threshold set to -infinity. ROI analyses: Twelve a priori ROIs were selected based on prior HDT/spaceflight vestibular studies: left middle temporal gyrus, left insula, left postcentral, left superior frontal, left middle frontal, right angular gyrus, right rolandic operculum, right insula, right precentral gyrus, right cerebellar lobule VI, and bilateral parietal operculum 2 (OP2; putative human vestibular cortex). Longitudinal mixed models tested group (AG vs CTRL) by time effects, controlling for age and sex. Whole-brain analyses: Exploratory longitudinal analyses used the Sandwich Estimator (SwE) toolbox with nonparametric wild bootstrapping (999 permutations), modeling BDC-7, HDT29, and HDT58 to test HDT effects and R+10 to assess recovery within HDT-affected regions. Significance: p < 0.05 FWE-corrected (TFCE), gray-matter mask (CAT12, threshold 0.1). Cerebellum analyzed separately via SUIT pipeline. Behavioral statistics and brain–behavior correlations: Linear mixed-effects models (nlme in R 3.6.1) evaluated behavioral changes over time (HDT and recovery). Pearson correlations examined change–change relationships between ROI activation (BDC-7 to HDT58) and balance/mobility (BDC-1 to R+0), focusing on SOT-5/5M and FMT. Age and sex were covariates where applicable.

- Baseline vestibular stimulation (pre-HDT, all participants) elicited activation in bilateral rolandic operculum, right temporal supplementary region, right hippocampus/parahippocampus, and cerebellar lobule VIII, with deactivation in somatosensory and motor cortices, confirming expected task effects. - ROI analyses (12 ROIs): • Significant group × time interaction in right cerebellar lobule VI: CTRL showed decreased activation (reduced deactivation) during HDT with recovery post-HDT; AG maintained stable activation from pre- to late-HDT. • Group main effects (AG vs CTRL) observed in left superior frontal, right precentral gyrus, and left OP2, with AG showing less deactivation across time points, including at pre-HDT. - Brain–behavior correlations (AG group): Change in left OP2 activation (BDC-7 to HDT58) correlated with change in SOT-5 balance performance (pre- to post-HDT); participants with the least decrease in left OP2 activation exhibited the smallest decline in SOT-5 EQ scores (p = 0.019, t = 2.6911). - Exploratory whole-brain analysis (FWE-corrected): Two right-hemisphere clusters showed significant group × time effects: • Right inferior frontal gyrus (MNI: 48, 26, 10), PFWE-corr = 0.032, extent k = 235. • Right precentral gyrus (MNI: 54, 2, 30), PFWE-corr = 0.041, extent k = 50. In both clusters, CTRL increased activation after entering HDT (precentral gyrus continued to increase to HDT58; inferior frontal gyrus remained elevated during HDT and recovery), whereas AG maintained activation near pre-HDT levels with minimal change. - Overall, daily AG mitigated HDT-associated changes in vestibular processing-related brain activity and the maintenance of left OP2 activation was associated with better post-HDT balance (SOT-5).

Findings support the hypothesis that daily short-arm AG mitigates HDT-induced alterations in vestibular processing. In right cerebellar lobule VI, AG preserved the typical deactivation pattern during vestibular stimulation, whereas controls exhibited HDT-specific reductions in deactivation that normalized post-HDT. Given lobule VI’s role in complex sensorimotor control and vestibular-related balance, preservation of its response suggests AG helps maintain efficient vestibular-sensorimotor processing. The significant change–change correlation in the AG group between left OP2 (putative human vestibular cortex) activation and SOT-5 performance indicates that individuals who maintained vestibular cortical responsiveness also preserved vestibular-dependent balance. Whole-brain results revealed HDT-related increased activation in right precentral gyrus and right inferior frontal gyrus among controls—regions linked to motor execution and inhibitory control/attention—while AG prevented these increases, suggesting AG reduces compensatory or maladaptive recruitment during vestibular processing under unloading. The pattern parallels changes reported after spaceflight, implying translational relevance of AG as a countermeasure for in-flight neural adaptations. Collectively, AG appears to stabilize vestibular processing networks under HDT, with behavioral relevance for balance, although individual differences exist in responsiveness.

Daily 30-minute short-arm artificial gravity during 60 days of HDT mitigated HDT-specific changes in vestibular processing, maintaining cerebellar (lobule VI) and cortical (right precentral and right inferior frontal gyrus) activation profiles near baseline and linking preserved left OP2 activation to smaller declines in vestibular-dependent balance (SOT-5). These results suggest AG enhances sensory stimulation and helps preserve vestibular system function in unloading environments. Future work should optimize AG dosing (duration, intensity, and targeting to achieve ~1 g at the vestibular organs), increase sample sizes, examine longer exposures to better approximate spaceflight durations, and investigate individual variability and mechanisms to maximize countermeasure efficacy across sensorimotor and cognitive domains.

- Baseline group differences: Despite randomization, three of 12 ROIs showed group main effects at baseline, indicating potential pre-existing differences; analyses modeled changes from individual baselines to mitigate this. - Small sample size: n=24 total (AG n=16 pooled; CTRL n=8) limits power to detect subtle effects and generalizability; pooling AG protocols was used to increase power. - Analog duration: 60-day HDT may not fully capture adaptations from ~180-day ISS missions, potentially underestimating dysfunction. - AG dosing and targeting: Fixed 30 min/day may not be optimal; gz at the vestibular organs was less than 1 g despite 1 gz at center of mass; individualized dosing and improved centrifuge configurations may yield stronger effects. - Concurrent studies and activities: Participants engaged in other experiments and had free time in HDT, which could introduce variability, though all remained in 6° HDT.