When it comes to human health and spaceflight, there are a lot of concerns as NASA casts its eye toward deep space. Among the more familiar problems are muscle and bone mass loss due to the lack of gravity. But more recently, scientists have grown increasingly concerned about astronauts returning to Earth with blurred vision, flattened eyeballs, and inflamed optic nerves after long-duration missions.

The eyesight problems appear to be caused by changes in cerebrospinal fluid, a clear fluid that helps cushion the brain from pressure changes during bodily movements. Now scientists are probing what other kinds of health issues might be caused by a redistribution of this spinal fluid, and this has led to studying the changes in brain size during spaceflight.

A new study in Nature Microgravity provides some of the first data on the changes in brain structure during spaceflight. Led by University of Michigan researcher Rachael Seidler, the study reviewed MRI scans from 27 astronauts, 13 of whom flew space shuttle missions and 14 who had flown approximately six month increments on board the International Space Station. The data was obtained from the NASA Lifetime Surveillance of Astronaut Health.

In their analysis, the scientists found "extensive" decreases in some gray matter areas, including the covering of the temporal and frontal poles and around the orbits. The effect was more pronounced for astronauts who had spent more time in space, and it's likely these changes were caused partially by shifting cerebrospinal fluid. The study also found increases in gray matter volume for regions of the brain that control leg movement. It's possible that these areas of the brain grew as the brain "worked" to learn how to move in microgravity.

These findings are preliminary and from a relatively small sample size—as is often the case with astronaut studies. But there is enough data here for some basic conclusions. First of all, the brain clearly retains its plasticity in space, learning and adapting to new motor skills in microgravity. Second, the space-based brain reaction is similar to patients observed in a long-term bed rest study intended to mimic microgravity, in which volunteers spent up to three months in downward tilted position.