Playing catch looks easy, but there's more to it than meets the eye. A ball-catching experiment in space has revealed that human brains have a built-in model of gravity.

by Karen Miller

Playing catch is easy. Kids and even their parents can do it. Keep your eyes on the ball and - if you don't think too hard - your hand will grab it in mid-air. It's simple, really.

Or is it? In fact, playing catch is more complicated than it appears.

Just before the ball arrives, your hand twists slightly. The muscles tense, so your hand isn't knocked away by the force of the blow. The timing is surprisingly exact: the muscles tighten exactly one-tenth of a second before the ball's impact.

The brain prepares for the catch even earlier. The hand moves only because the brain tells it to - and it takes two tenths of a second for the brain's commands to travel down to the hand. Such delays require the brain to predict when the ball will arrive - a process made more difficult because, due to gravity, the speed of a soaring ball is always changing.

How does your brain do it?

According to neuroscientist Joe McIntyre of the College de France, the brain is so accurate because it contains an internal model of gravity. The brain, he says, seems able to anticipate, calculate and compensate for gravitational acceleration - naturally.

His conclusions, published recently in the journal Nature Neuroscience, are based on an innovative ball-catching experiment conducted in space: Astronauts on board space shuttle Columbia caught balls released from a spring-loaded canon. The balls moved with a constant speed as opposed to a constant acceleration as they would on Earth. While the astronauts were playing catch in this manner, infrared cameras tracked the motions of their hands and arms, and electrodes measured the electrical activity of their arm muscles.

McIntyre and colleagues designed the unusual experiment at the Centre National de la Recherche Scientifique (CNRS) in France and at the Santa Lucia Scientific Institute in Rome. It flew to space for a 17-day mission in 1998, one of 26 life sciences experiments in the Neurolab payload.

Credit: National Photo Co. The Charleston as an aid to the game. Created between 1920 and 1932. Prints and Photographs Division, Library of Congress. Basketball players learn the Charleston in the hope it will help their game. Does balancing on one leg offer conclusive proof as to gravity on the brain? (Perhaps Not)

In flight, says McIntyre, the astronauts were always able to catch the ball, but their timing was a little bit "off." They reacted as if they expected the ball to move faster than it did - in other words, as if gravity was "Earth-normal."

The astronauts' expectation of gravity was surprisingly persistent. They continued to mis-anticipate the ball's motion nearly fifteen days into the experiment - "although we did begin to see some evidence of adaptation at that point, " says McIntyre.

"The question is," he said, "if you do anticipate gravity, then why?"

Astronauts orbiting Earth clearly sense a change in acceleration: for example, the astronauts themselves float. And they do adapt to weightlessness in many ways. Motion sickness, for example, tends to disappear after two or three days in space. Yet for nearly fifteen days, astronaut brains continued to predict that balls would be accelerated as on Earth, even in the face of contrary evidence.

[more] An astronaut prepares for the ball-catching experiment inside the Neurolab trainer.

Such rigid, inflexible behaviour supports the notion that the brain contains a built-in model of gravity - like a specialized computer in our heads that calculates acceleration.

There's other evidence, too. For instance, says McIntyre, if you place an infant safely on a glass table where he or she can see the floor below, the baby will become fearful. He's not falling, yet he expects to fall - without any prior experience of falling. "It doesn't take much to elicit this response," he added. "It seems like a very robust, common effect that we expect a downward acceleration."

Eventually the amount of acceleration we anticipate can be changed from Earth-normal to other values. To wit: By the 15th day of the Neurolab mission astronauts on the shuttle were beginning to catch the ball better. The too-early movement of the arm remained, but its amplitude grew smaller. At the same time, the astronauts began to add in an additional arm movement - one timed to occur just before the ball's impact.

When the astronauts repeated the experiments on the ground, they all, said McIntyre, seemed surprised at how fast the ball dropped. But they adjusted far more quickly than they had in space. On Earth, if you're late with your response, you miss the ball. Perhaps that forces you to learn more quickly, he suggests.

It's possible that the astronauts did adapt to 0-g, and then readapted back to 1-g again. It's also possible that the brain is able to learn and retain multiple models of acceleration. In different situations, it might simply choose which one to apply. That, in fact, is what McIntyre and his colleagues believe is going on.

McIntyre's Neurolab experiment was surely fun, but there's more at stake than fun and games.

Researchers hope that understanding how astronauts adjust to the unexpected movements of objects in space will improve mission safety. There are also benefits to those of us on Earth: Such experiments, says McIntyre, offer a unique way to explore the nervous system. Some kinds of brain damage cause trouble with timing much like the astronauts experienced. Unraveling how the nervous system works is an important step in treating these kinds of problems.

Indeed, the human nervous system remains a puzzle in many respects. Experiments like this one show that at least some of the pieces may be found ... in space.