The Kind of Noise That Keeps a Body on BalanceBy ANNE EISENBERG
EEPING your balance while standing upright can be tricky, particularly for older people.
That is because standing steady is partly a result of slight adjustments
to posture that are ordered by the brain in response to sensory information
from the feet. But as people age, they become less sensitive to touch and
send fewer signals.
Now a Boston University scientist and his colleagues have found a way to
use random signals to increase the sensory data coming from the feet.
In a series of experiments, healthy 75-year-olds stood on a platform that
transmitted randomly varying vibrations to the soles of their feet. With
these good vibrations, the subjects reflexively adjusted their balance until
they swayed about the same amount as 25-year-olds who did not receive the
random signals. Younger people who used the vibrating system also swayed
James J. Collins, a professor of biomedical engineering who
led the research group, attributed the improvement to stochastic resonance,
a well-known phenomenon in which random noise enhances the detection of weak
signals. In this case the noise made the nerves in the feet more sensitive
and better able to detect the kinds of pressure changes that occur when the
body goes slightly out of balance and puts more pressure on one part of the
"It's a foot massager with a twist," Dr. Collins said of the
research setup. The vibrations are not soothing because the motion is below
a detectable level, but they do make people more stable.
In the world
of signal detection, noise is traditionally viewed as a prime nuisance. Entire
courses at engineering schools are devoted to reducing it. Electromagnetic
noise creates snow on television sets; acoustic noise makes conversation
impossible in some restaurants.
But in Dr. Collins's experiments,
as in those of other researchers who have investigated stochastic resonance,
certain kinds of noise turn out to be helpful.
"For electrical signals,
the low levels of noise essentially tickle the membranes of the neurons,"
he said, making them more likely to fire when there is a physical stimulus
of some amplitude. For mechanical signals, noise serves to boost weak stimuli.
"The experiment is a good example of how noise lets a neuron fire in the
company of a signal that it is normally unable to detect,'' Dr. Collins said.
Although the principle of stochastic resonance has been investigated for
more than a decade, Dr. Collins said, these experiments were the first in
which it was shown to improve balance. The effect, described in a paper to
appear in the journal Physical Review Letters, may be sufficient to offset
age-related declines in balance control, he said.
The platform used
in the study has hundreds of small holes; a small plastic rod protrudes slightly
through each of them so that it contacts the bottom of the test subject's
foot. The rods are hooked up to motors that cause them to vibrate at random
frequencies generated by a computer while the test subject is standing quietly.
"They couldn't feel the random vibrations," Dr. Collins said. "We set the
noise up at too small an amplitude for them to detect it."
Priplata, a student of Dr. Collins and lead author of the paper, has designed
gel-based shoe insoles that contain small vibrating devices designed to produce
the same effect. When the researchers repeated the study with people using
the insoles, Dr. Collins said, they found even stronger effects.
It is important that the signals be random because neurons quickly get used to regular signals.
John Milton, a neuroscientist at the University of Chicago, said that Dr.
Collins's bionic inserts might one day prove to be an inexpensive remedy
for legions of aging baby boomers who have grown less steady on their feet.
"These noisy sneakers could save a lot of money if they were used for treatment,"
Dr. Milton said. "And there are no side effects I can imagine from wearing
Dr. Collins has written a series of research papers
on ways to use stochastic resonance to improve health. An earlier paper,
for instance, discussed the use of noise to improve the sensitivity of touch
in older adults who suffered diabetes or the effects of a stroke.
But he is well aware of more frivolous applications, particularly for sports
equipment. "I could imagine having noise introduced into the handles of golf
clubs or tennis rackets," he said. "Or into basketball shoes."
shoes might be something like an electronic version of flubber, the magic
substance that turned a so-so basketball player into a superstar in the old
Walt Disney movie and a recent remake. But Dr. Collins was quick to point
out the superiority of his discovery over flubber.
"The energy source
in flubber is the material itself," he said. "Here we are taking advantage
of the natural senses - the sensory neurons' shifting their detection thresholds
to a lower value."
Kurt Weisenfeld, a professor of physics at Georgia
Tech who did some of the early defining work in stochastic resonance, said
that Dr. Collins's experiments were a striking example of thinking creatively
about possible applications of the phenomenon.
"This is a practical
idea that could help people maintain their balance," he said. He said he
particularly admired Dr. Collins's solution because it is relatively simple.
"For someone with sensory problems, the high-tech answer might be a bionic
ankle," he said. "But maybe instead they'll just slip into a pair of bionic
socks. Those are a whole lot cheaper."