Noise improves balance

Whether it is squelching the hiss from a stereo speaker or tuning out the static from a television speaker, engineers are usually trained to fight noise wherever they find it. But some engineers are adding noise to one complex control system and making it work better.

The control system in question is the neurological one that keeps us standing upright. The brain and spinal cord integrate data from eyes, inner ears, muscles, and skin to send the proper commands to the muscles that keep us standing. Using a phenomenon called stochastic resonance, researchers at Boston University and Afferent in Providence, RI, have found a way to boost the flow of data to the brain and improve balance.

Stochastic resonance is found in certain systems (sensory neurons are one) that have a built-in threshold of activation. Mechanical systems too weak to cross the threshold are not felt. But an imperceptible amount of noisy vibration can make the signals felt. "Just the right amount of noise provides the pedestal upon which signals can ride over the threshold" and be felt, write Boston University's James J. Collins and Attila A. Priplatta and Afferent's Jason D. Harry and James B. Niemi in the April issue of IEEE Spectrum.

Most people expect that their vision and hearing will degrade with age, but don't fully grasp that the same holds true for their mechanical senses. For the elderly, the decline of the sense of touch in the feet and of proprioception - the sense of what position their limbs are in--strongly contributes to the tendency to fall. In the US, roughly one-third of people older than 65 fall each year, and many of the falls result in serious and debilitating injuries such as broken hips.

In experiments with people in their twenties and people in their seventies, the engineers added a little noise to the sensory system by having research subjects stand on a pair of gel insoles containing electric motors. The motors sent an imperceptible vibration into the wearer's feet. Each group's balance, as measured by how much they swayed when standing still, improved. But the elderly group's balance improved so much that they swayed the same amount as the subjects in their twenties did without the benefit of noise.

Engineers at Afferent are working to develop a device based on the insoles used in the experiments, and they hope to release it within two years. They expect to market it first to people who have lost some nerve function in their feet from diabetes. "Lack of mechanical awareness, especially in the feet of people with diabetes, contributes to the occurence of open sores that can be extremely difficult to heal and that all too often lead to the amputation of affected areas," the engineers write.