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Abstract
Humans undergo extensive sensorimotor adaptation during spaceflight due to altered vestibular inputs and body unloading. No studies have yet evaluated the effects of spaceflight on human brain structure despite the fact that recently reported optic nerve structural changes are hypothesized to occur due to increased intracranial pressure occurring with microgravity. This is the first report on human brain structural changes with spaceflight. We evaluated retrospective longitudinal T2-weighted MRI scans and balance data from 27 astronauts (thirteen ~2-week shuttle crew members and fourteen ~6-month International Space Station crew members) to determine spaceflight effects on brain structure, and whether any pre to postflight brain changes are associated with balance changes. Data were obtained from the NASA Lifetime Surveillance of Astronaut Health. Brain scans were segmented into gray matter maps and normalized into MNI space using a stepwise approach through subject specific templates. Non-parametric permutation testing was used to analyze pre to postflight volumetric gray matter changes. We found extensive volumetric gray matter decreases, including large areas covering the temporal and frontal poles and around the orbits. This effect was larger in International Space Station versus shuttle crew members in some regions. There were bilateral focal gray matter increases within the medial primary somatosensory and motor cortex; i.e., the cerebral areas where the lower limbs are represented. These intriguing findings are observed in a retrospective data set; future prospective studies should probe the underlying mechanisms and behavioral consequences.
Highlights
Humans undergo extensive sensorimotor adaptation during spaceflight due to altered vestibular inputs and unloading of the body
These normalized images were broadly around: the temporal and frontal poles, in lateral inferior modulated by multiplying them with their Jacobian determinant temporal and frontal areas, around the orbits, and in bilateral image to preserve the amount of gray matter volume that was medial parts of Crus II of the cerebellum
To aid in the interpretation of the here reported focal gray matter changes from preflight to postflight we have made a random permutations and threshold-free cluster enhancement (TFCE)[35] implemented in FMRIB Software Library (FSL)’s randomize[36] were used to test (1) if qualitative comparison of the average focal gray matter changes in these astronauts with the average gray matter changes that we there were local gray matter increases or decreases as a function of spaceflight, (2) if there was a difference in GM changes between observed in a group of 18 subjects who participated in our microgravity-analog head down tilt bed rest study.[39,40]
Summary
Humans undergo extensive sensorimotor adaptation during spaceflight due to altered vestibular inputs and unloading of the body. The untoward effects of spaceflight on sensorimotor function have been well-studied, including postflight impairments in posture control[4,5] and locomotion,[4,6,7,8] as well as inflight spatial disorientation,[9] reduced mass discrimination[10] and increased manual tracking errors under cognitive load.[11,12] Astronauts gradually adapt their sensorimotor processing inflight in response to body unloading and altered vestibular inputs.
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