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The
Kind of Noise That Keeps a Body on Balance
By ANNE EISENBERG
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Keeping 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 less.
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 foot.
"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."
Attila 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 noisy sneakers."
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."
Vibrating 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."
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