Here's the catch. At zero gravity, the brain misjudges a falling ball's speed.

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Even if you can't name one of Newton's laws, your inner physicist is a whiz at applying them to the real world. New research conducted aboard the space shuttle shows that the brain is quite sophisticated about predicting when a falling object will hit the ground, taking into account not just its position and speed, but even the tug of gravity.

Accurately guessing when an approaching object will hit you is an important survival skill. But researchers are still in the dark about how the brain translates visual images--for instance, a car becoming larger as it gets nearer--into an accurate representation of reality. Since the 1940s, researchers have preferred to explain perception by external cues only, such as the object's apparent size and position. But many in the field are starting to favor a different idea: that the brain develops a model of the physical world, which we use to predict the actions of objects around us. Such a model could include, for instance, acceleration due to gravity.

To study whether the brain can take gravity into account, Neuroscientist Joseph McIntyre of the European Laboratory for the Neuroscience of Action in Paris and Rome took advantage of a rare opportunity to take it away entirely. McIntyre and his colleagues gave four astronauts aboard the Neurolab space shuttle mission three tries to catch a falling ball, released at a different speed each time. On Earth, gravity would speed up the ball's descent, but in space, the balls fell at a constant velocity. When McIntyre measured the upward rotation of the forearm and stiffening of the biceps just before the subjects expected to catch the ball, he discovered that the catchers reacted early. Their failure to compensate for the lack of acceleration suggests that an internal model of gravity, rather than pure observation, shaped their perception of the falling object's behavior, McIntyre reports in the July issue of Nature Neuroscience. But the model seems adaptable, McIntyre says: After 15 days in space, the subjects' motor reaction matched the ball's impact more closely.

Although the internal model of gravity has been proposed before, it's never been tested directly until now, says perceptual motor psychologist James Tresilian of the University of Queensland, Australia, who calls the Neurolab experiments "quite an interesting way of going about it." As a proponent of the internal model theory, Tresilian is not surprised by the outcome. "It's what I would have expected," he says.

Related sites

Abstract of the study from a recent meeting of the Society for the Neural Control of Movement
James Tresilian's home page
NASA Neurolab Web page