Researchers are learning new things about the human brain by studying how astronauts regain their balance.

by Karen Miller

Balancing is not as easy as it seems - just try to stand on one foot for a full minute, and you'll get a sense of the constant effort involved.

It's one of those complex skills like reading that becomes so automatic with practice, we simply forget how tricky they were to learn. And, like reading, you might suppose it would take something extraordinary to make you forget.

Indeed it does. Like travelling to space.

Researchers have found that astronauts who return from a space voyage can still balance, but they find it far more difficult. That's because, explains neuroscientist Bill Paloski, their brains are no longer sure how to interpret the information that comes from their senses.

When you balance, he says, you use information from as many as three sources: the proprioceptive sensors in your muscles, which tell you where your body parts are in relationship to each other, the vestibular system in your inner ear, which tracks the position of your head in space, and of course your eyes.

The brain deals with all that information by building "a model." Computer programmers might call it a mental subroutine, but it's more than an algorithm. Models provide context for interpreting and reacting to sensory data. The brain generates such models all the time - it's the way we learn and adapt. We do it on Earth, say, when we learn a new language, or even when we get accustomed to new prescription glasses.

Astronauts do it, too. On Earth, their brains have already constructed a model that tells them how to manage their bodies in 1-g (normal gravity). In space, they must build a 0-g (weightless) model. Then, back on Earth, they have to figure out that it's time to switch to the 1-g model again.

The transition isn't always easy.

Balancing in space requires new ways of thinking.

When you encounter a completely new context like space, your brain has some work to do. It has to decide whether this will be a persistent context or not - whether it's worth building a model. And if it is, then it has to develop one.

It takes time for the brain to learn how to interpret the new information, to form a new model, to figure out when to switch from one model to another. And during that transition, when the brain's confused about which model to use, it starts to interpret sensory data in odd ways. You get illusions, for example, that the world around you is moving, when all that's really moving is your head. Headaches and motion sickness are other symptoms of this disorienting transition. "The perceptual illusions that astronauts have are very interesting," he notes.

Paloski, who works with astronauts at the Johnson Space Centre, is trying to find out exactly what cues astronauts to switch models. He's doing this by sending their brains confusing sensory information, which, he believes, will force a shift from one state to another.

About ten years ago, he recalls, during a post-flight neurological test that involved a rotating chair, an astronaut who had already regained the ability to balance somehow lost that ability all over again. Retested, the astronaut kept falling over, "just like on landing day."

"Something happened in that person's brain that caused a switch, we think, from a terrestrial adaptation back to a 0-g adaptation. Probably the brain got confused by the funny signals it was receiving on the chair, and it chose to interpret those signals as saying, I must be back in space. And it flipped back to the model that was congruent with space flight."

Image credit: NASA Using this human-sized centrifuge, Bill Paloski plans to spin astronauts in order to learn more about how our brains manage mental models for balancing.

Now, Paloski is trying to recreate that effect.

"We know that astronauts are just on the verge of readapting to Earth in the 2 to 4 day time frame after short duration space flight. So we thought, why don't we go to day 3, when we think somebody is just about adapted, and see if we can cause the brain to switch states."

To do this, Paloski will put astronauts in a centrifuge. While they lie comfortably on their sides (the astronauts are tested one at a time), the device spins at varying rates of speed forward and back. After ten minutes of spinning, the astronauts are tested. They stand on a platform inside of a booth. All they have to do is stand as still as possible. But the platform and the booth are designed to isolate the different kinds of sensory information used in balancing - visual, vestibular and proprioceptive. For example, the most important proprioceptive sensors for balance control are the stretch receptors in your ankles, and the platform can prevent the body from receiving that sensory information. "If you begin to sway forward,� explains Paloski, "we move the platform to an angle that's identical to the angle you've moved through, so that your ankle angle never changes."

Image credit: NASA. After a spin in the centrifuge, this test subject steps into the "balance booth," also known as the "posturography system."

By spinning astronauts and then testing them in the "balance booth," Paloski hopes to learn how to facilitate the transition from one state to another. His subjects will be crew members of shuttle mission STS-107, which launched on January 16th 2003. "We plan to test these astronauts both before and after the mission," he says.

Paloski's research might help astronauts regain their sense of balance faster, but there's more to it than that. For instance, a side effect of transitioning between models is motion sickness. Paloski's work could help doctors understand such maladies. It might also be possible to train astronauts to develop models before they're needed. Mars explorers, for example, might be able to generate a 1/3-g model long before they reach the red planet.

And for us on Earth? Paloski's work may help here, too. Ultimately his research is about making it easier to learn - and that's something we do every day of our lives.