“The people who went for a year showed the largest changes,” Seidler said. “There were still some changes evident in people who went for two weeks, but duration seems to be the driving factor.”
She added that among astronauts who had stayed in space six months or longer, the upward movement was “pretty extensive,” particularly in structures at the top of the brain.
“It’s on the order of a couple of millimeters,” she said, “which doesn’t sound like a big number, but when you’re talking about brain movement, it really is. That kind of change is visible by eye.”
Seidler said the brain changes observed sometimes caused “sensory conflicts” for astronauts while in space, manifesting as temporary disorientation or motion sickness. Back on Earth, the shifts can also contribute to balance issues as the astronauts readjust to Earth’s gravity. But she said the study did not find serious symptoms, such as headaches or cognitive impairment, during or after spaceflight.
“That was surprising to me,” Seidler said.
For comparison, the research team also analyzed brain scans from 24 civilian participants on Earth who were put on bed rest for up to 60 days while tilted back, so that their heads were 6 degrees below their feet. The idea was to simulate a microgravity environment, and the scientists found similar changes in brain position and shape after the period of bed rest. The astronauts’ brains experienced greater upward shift, though.
Dr. Mark Rosenberg, an assistant professor of neurology at the Medical University of South Carolina and the director of its aerospace and performance neurology program, said scientists have known that spaceflight can affect the brain, but Seidler’s study is one of the first to document how those upward shifts influence how astronauts function in space and back on Earth.
“We knew the brain shifts upward, but does it actually have any kind of operational impact?” said Rosenberg, who was not involved with the study. “This study is able to make some of those associations.”
The findings raise new questions for potential follow-up, such as whether there are differences between brain changes among male and female astronauts, and whether a crew member’s age is a factor. Piecing together a complete picture, however, is limited by a sample size problem: Only up to a dozen or so astronauts launch to the International Space Station each year, and NASA’s astronaut corps has historically been dominated by men.
More research is also needed to understand if the observed brain changes have longer-term impacts.
So far, like most other changes to astronauts’ bodies after stints in space — including bone loss, deterioration of muscle strength and fluid redistribution — the shifts do not appear to be permanent. After the body readjusts to the familiar tug of Earth’s gravity, things more or less return to normal, Rosenberg said.
But it’s not yet known whether different gravity environments will introduce new complications.
“If you’ve been on Mars with one-third Earth’s gravity, or on the moon with one-sixth Earth’s gravity, will it take three or six times as long to get back to normal?” Rosenberg said.
He and Seidler both said the findings so far should not be viewed as an argument against humans spending extended periods in space. But it will be critical to figure out if there is any lasting damage and how to avoid it, if possible.
“Whether we care to admit it or not, we are eventually going to become a space-faring species,” Rosenberg said. “It’s only a matter of time. And these are just some of the unanswered questions that we need to sort out.”