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ure, space travel makes bones and muscles atrophy and alters the distribution of blood and other bodily fluids, among other physiological consequences of microgravity, but what does it do to the brain? Since astronauts on a mission to Mars will need their wits about them, NASA and outside scientists have been keen to assess the effects of prolonged weightlessness on the 3 pounds of protoplasm inside the skull.

In a NASA-funded study published on Wednesday, Dr. Donna Roberts of the Medical University of South Carolina and her colleagues therefore compared before- and after-mission MRIs of 34 astronauts, 18 who spent months on the International Space Station (average voyage: 165 days) and 16 who had shorter jaunts (14 days, on average, on the space shuttle). Both groups were roughly the same age and had comparable flight experience. Among the findings:

Get back down here: Without gravity to pull the brain toward its owner’s feet, it shifts toward the top of the skull, Roberts and her team reported in the New England Journal of Medicine. In all 12 long-flight astronauts whose before-and-after images were lined up in a way that revealed movement, “you can see the brain shifting upward,” Roberts said. The brain remains up there days after the astronauts returned to Earth, but since none of the 28 men and six women had a second post-flight MRI, it’s not known if the brain eventually returned to its original location. (The single post-flight MRI was done two to 17 days after landing.) None of the short-trip astronauts seemed to experience upwardly mobile brains, but that might be misleading; if their brains did float upward, but perhaps less dramatically than during long flights, they might have returned to baseline before the MRI caught it.

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It’s getting crowded in here: A fold at the top of the brain, called the central sulcus, separates the cortex’s frontal lobe from the parietal lobe behind it, and in particular the motor cortex from the sensory cortex. In 17 of the 18 long-duration astronauts, it got narrower. That happened in only three of the 16 short-mission flyers. But since no astronauts have MRIs in space, it’s possible that the compression occurs during short flights, too, but doesn’t last long enough to be detected a few days after the return to Earth. The narrowing, due to the brain’s getting compressed against the top of the skull, “might lead to functional changes,” Roberts said, including the difficulty astronauts have adapting their movements to microgravity.

Where’d I leave my glasses? The brain’s upward shift can increase pressure on the optic disk, the point on the optic nerve where it leaves the retina and enters the brain, leading to swelling and, according to previous research on hundreds of U.S. astronauts, damaging vision, sometimes permanently. Three of the Space Station astronauts in Roberts’s study had swelling of the optic disk, and all three of them had narrowing of the central sulcus. But since many more astronauts had central sulcus narrowing, that’s apparently a necessary but not sufficient cause of optic disk compression.

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My cerebrospinal fluid runneth over: This clear, colorless liquid both cushions the brain and acts as its sewage system, removing metabolic and other waste products. Veins and other vessels carrying the cerebrospinal fluid (CSF) wend their way through the central sulcus, where they became narrower in 17 of the 18 long-haul voyagers and three of the 16 shuttle astronauts. With less CSF flowing through vessels at the top of the brain, “it has to go somewhere, since the skull is a fixed space,” Roberts said, “and what we found is that CSF-filled spaces in the center of the brain get larger.” Although disturbances in the flow of CSF have been linked to Alzheimer’s disease, multiple sclerosis, and other brain disorders, it’s not clear if that’s cause or effect. But since a mission to Mars would cause about two years worth of brain squashing, it would be useful to know.

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