Your Perception of Gravity Is All Relative, Study Finds

girl doing headstand — Our perception of how an item will fall if it's tipping over is often biased toward the tilt of our body rather than gravity's true direction.

(Image credit: © Paul Simcock | Dreamstime.com)

In a discovery that could turn science on its head, researchers now find that you are a better judge of how objects fall when you are upright than when you lie on your side.

Our senses are known to play tricks on us. For instance, we can keep our balance when our eyes are closed, but are better at doing so when we open our eyes or touch a surface. This shows that our brain perceives gravity's direction through multiple senses -- our vision and the so-called vestibular system in our inner ear, among others.

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To measure how well people gauge how objects might fall, scientists equipped 15 volunteers with laptops that showed computer-rendered pictures of vaselike objects tipped at various angles over the edge of a table and asked them to indicate whether each item would fall or right itself. They were also asked to judge if a line on the screen was tilted clockwise or counterclockwise relative to which way was down. The participants were tested while they were sitting upright and lying on their sides.

The scientists found that volunteers were typically better judges of how objects fall when they were upright than when on their sides. Apparently, our perception of an item's stability is often biased toward the tilt of our bodies rather than visual cues of gravity's true direction. For instance, the Tower of Pisa "may appear more stable than it is if you lay in the same direction of its lean," said researcher Michael Barnett-Cowan, a postdoctoral neuroscientist and project leader for motion perception at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany. "Lay in the opposite direction, and it may appear even more likely to fall."

"Participants in our experiment fell along a spectrum from those that based their judgments mainly on gravity's true direction to those that were more dependent on vision and the sense of their body," Barnett-Cowan told LiveScience. "Why some people are more biased by their body orientation than others likely arises from the sense of their body being more reliable than the other senses, which may change with experience and damage to the senses or the way in which sensory information is integrated in the brain."

The researchers now plan to investigate the role of each sense involved in our perception of the stability of an object. "Administering our task before, during and after exposure to microgravity, for example, can help determine how objects are perceived as stable in the absence of gravity and whether the brain adapts to such environments by changing the relative role of sensory cues," Barnett-Cowan said. [Is There Gravity in Space?]

These findings might also yield insights into patients who have sensory problems. "In a previous experiment working with patients with Parkinson's disease, my research team in Canada and I were able to show that these patients rely less on their body and more on gravity when having to recognize objects in different orientations," Barnett-Cowan said. "If this is the case for perceived object stability, then we would predict that patients with Parkinson's disease may actually be better judges of object stability than nonpatients."

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.