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 Nov.
22, 2002: 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 traveling 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 NASA neuroscientist Bill Paloski, their brains are no longer
sure how to interpret the information that comes from their senses.
Right:
Balancing is trickier than it looks. A royalty-free image from
corbis.com.
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.
 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.
Left:
Balancing in space requires new ways of thinking. [more]
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 Center, 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."
Below:
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. Image credit: NASA
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."
 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 crewmembers of shuttle
mission STS-107, which is slated for launch in January 2003. "We
plan to test these astronauts both before and after the mission,"
he says.
Right:
After a spin in the centrifuge, this test subject steps into the
"balance booth," also known as the "posturography
system." Image credit: NASA.
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.
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