The experience of weightlessness is confusing for human bodies. The eyes tell you you're gently bobbing up and down, while your inner ear screams that you're tumbling about, making you nauseous. The fluids in your body, freed from gravity, float upward, causing head congestion. Bones, suddenly useless in holding you up and moving you around, start thinning out. And something strange can happen to your eyeballs: They get squashed, blurring your vision.

About two-thirds of astronauts on the International Space Station report changes in their vision after they come back. Scans show that the backs of their eyeballs somehow get flattened, their retinas wrinkle, and their optic nerves swell after spending a prolonged period of time in microgravity, causing farsightedness. The leading explanation suggests that when bodily fluids rise and pool in astronauts' torsos and heads, they put pressure on the brain and the back of the eye, causing changes in its shape. Scientists don’t know for sure, so they keeping studying astronauts.

They don’t need the astronauts themselves to study vision changes, though, as a new study from researchers in Texas, published Thursday in the Journal of Physiology, shows. A team at UT Southwestern Medical Center instead tested cancer patients who had a device permanently placed in their heads as part of their treatments. The device, known as an Ommaya reservoir, allows doctors to inject medicine into patients’ cerebrospinal fluid or remove extra liquid. For the Texas researchers, the port allowed them to measure intracranial pressure—the force inside the skull that scientists suspect causes structural changes to the eye and optic nerve in microgravity.

Eight volunteers took turns flying on NASA’s parabolic aircraft, the kind used to train astronauts, to simulate conditions of microgravity. The aircraft created 20-second periods of weightlessness as it flew sharply up and down. Several researchers were also on board. They measured the patients’ intracranial pressure in those seconds, then compared it to measurements taken when the patients were sitting and lying down on their backs, face up.

The researchers found that intracranial pressure is higher in microgravity than when the patients were standing or sitting on the ground, but lower when they were sleeping. In orbit, astronauts never really sit or stand. Because they’re weightless, they remain in a perpetual state of lying down. And if they’re always lying down, that means their intracranial pressure doesn’t shift as they wake up, go to work, and go to sleep. That constant, the researchers say, is what leads to changes in the eye and worsening of vision.

Once again, free-floating bodily fluids are the prime suspect. The researchers say equipment that simulates the upright posture while astronauts sleep in space could help. They also say astronauts benefit from sometimes wearing negative-pressure boxes, devices that can regulate the flow and movement of blood and other fluids, on their lower bodies.

For now, vision problems are an accepted occupational hazard on the International Space Station, which is stocked with prescription glasses for residents over 40. Not all astronauts exhibit vision changes, so researchers are also studying potential risk factors that may make some more prone to them than others. NASA scientists take pictures of the back of the eye and conduct ultrasounds of the optic nerves before launch and after landing. Sometimes, the damage is permanent.

In 2005, John Phillips launched to the ISS with 20/20 vision and came back six months later seeing 20/100, he told The Washington Post last year. His vision improved a little after six months on Earth, but he has to wear glasses. Still, Phillips said, he wouldn’t trade his experience in space for better vision.