Astronauts’ brains begin to process visual information differently as they become acclimated to a weightless environment, a new study published in PLoS One reports.

Optical illusions made up of line drawings that can be seen in two different ways, for example a chair that can be seen either facing towards or away from you, are known as “reversible perspective figures.” Studying how people see these figures sheds light on how the human brain takes two-dimensional images from the eyes and puts them together to see in three dimensions.

When people look at these illusions, what they see tends to switch back and forth between the two possible visual interpretations, but one of these views is typically dominant and is seen about 70 percent of the time. This is thought to occur because of the way the brain interprets cues about visual depth. Because the ground is usually closer to us than the sky or ceiling, the brain is more prepared to see lower parts of the drawing as closer, making it easier to see the picture in one way than the other.

A new study, led by Gilles Clément of the Lyon Neuroscience Research Center, studied how astronauts saw these illusions to find out how prolonged weightlessness affects human depth perception. Six astronauts living on the International Space Station were tested on their vision of four reversible perspective figure optical illusions, viewing the figures on a screen and reporting each time their vision switched between the two possible interpretations. The astronauts were tested three times before leaving Earth, four times while in space for a period of up to six months, and another three times after returning.

Before going into space, the astronauts’ perception of the figures was the same as other people’s, with the dominant interpretation being seen about 70 percent of the time. Once they were in space, the astronauts’ perceptions began to change gradually over time. Instead of seeing one interpretation most of the time, they began to see both possibilities roughly equally. Their vision remained the same immediately after they returned to Earth, but after nine months they had returned to seeing one interpretation of the figure dominantly over the other.

According to the study authors, these results reveal that “depth is not so much the direct result of certain specifiable stimulus cues as it is a mental construction.” The brain pieces together information on visual depth based partly on experience with the environment. In an environment with gravity, this experience tells us that objects in the lower part of the field of vision are probably closer than those in the upper part. For someone who is weightless, the upper field of vision is just as likely to be close as the lower part, giving the brain fewer clues to depth.

In addition to advancing our understanding of how three dimensional vision is processed by the brain, these insights may help future astronauts combat “space fog,” the sense of confusion and disorientation reported after spending time in a weightless environment.