To prove that someone's sense of touch could be improved, we had to first find that person's sensory limits. Clinicians routinely do this by poking patients with either a standard kit of filaments of different stiffnesses or a vibrating probe. Finding the level at which the patient feels the force of the filament or the vibration of the probe quantifies that patient's sensitivity. By recording a person's performance while randomly switching the stimulation on and off, we can show whether the noise boosts sensitivity.

In experiments we performed in collaboration with Aristidis Veves at the Joslin-Beth Israel Deaconess Foot Center, in Boston, we showed that we could improve sensitivity on foot soles of people with diabetes through the use of vibrating insoles. The vibrations were so small that the patients could not feel them, removing any chance that they would be biased in favor of the noise.

These findings complement earlier work by our group showing that imperceptible random vibration could enhance sensitivity on the fingers of the young, the elderly, and stroke patients, as well as those with diabetes. Future research will focus on the ability of diabetic subjects wearing vibrating insoles to improve their gait and walking patterns to minimize uneven pressure points.

We've gone a step further, so to speak, from just measuring sensitivity in our work with older people. Here we are looking at how a little noise can help them stay on their feet. We hypothesized that a heightened sense of touch on the soles of the feet can be integrated into the sensory-motor control system that allows people to maintain their balance while standing still.

We asked test subjects to stand on a pair of vibrating insoles, with their eyes closed [see illustration, "Still Standing"]. We set the amplitude of the vibrational noise to a low level and then slowly increased it until the research subject could feel it. (This level is different for each person.) Then we dialed down the amount of noise until it was just under that level, which effectively blinds the subject as to whether the stimulation is on or off during the trials. We put a reflective marker on the subject's shoulder and tracked its horizontal position with a special camera system. In an attempt to quantify the subject's balance from his or her posture, we plotted the shoulder marker's movements while the noise was turned on and when it was off.

The result, called a stabilogram, is an incomplete measure of how stable the subjects were when standing — there are too many joints between the shoulder and the feet to fully analyze posture from it. Nevertheless, the movement at the shoulder can tell us important things, such as how much a person sways before the internal control system makes a correction. In general, a tightening of the stabilogram indicates greater stability.

We performed the experiments with people in their 20s as well as people in their 70s, and we demonstrated a significant improvement in balance control in both groups when noisy vibration was applied to the soles of the feet. In particular, we found that both young and old subjects swayed considerably less when the vibrating insoles were turned on.

The improvement was more pronounced for the older group — so much so that when the septuagenarians wore the vibrating insoles, they swayed similarly to the twenty-somethings without vibrations.

Our results also fit well with a model of the body's balance control system we developed in the early 1990s. To bolster the model, we performed an analysis of our stabilograms that relied on the data's similarities to the random motions of microscopic particles. The analysis suggested that the body uses not one but two types of control systems while standing. If it drifts off center for less than a second or for a small distance, the nervous system appears to be using an open-loop control system — that is, one that does not use sensory feedback to keep it on target. But over longer time periods and greater distances, a closed-loop, feedback-driven system appears to kick in and help the body right itself.

In the vibrating sole experiments, we performed the same analysis, which showed that people's balance control systems seemed to be more tightly controlled when under the influence of noise. That is, when the insoles were vibrating, they swayed a shorter distance before feedback control helped correct their posture. Since we know that the vibrations enhance the sense of touch on the foot, the point at which the body switches to feedback-based control appears to be influenced by the sensitivity of mechanical sensors in the feet.

To make use of this discovery in a clinical device, you need to know two things: which body parts to stimulate to get the desired effect and which procedure will work best. To improve balance in the elderly or prevent foot problems in people with diabetes, the bottom of the foot has proven a good target. A noise-delivery device could be shoes with built-in stimulators or removable insoles.

Although both electrical and mechanical noise enhance the sense of touch, reliably delivering electric current to the skin on feet has proven difficult. So we have concentrated on mechanical stimulation, such as embedding small linear electromagnetic motors in gel-based insoles. The prototype we've used in recent experiments has two motors at the ball of the foot and one at the heel [see photo, "Vibrating Soles"]. The motors are externally controlled and powered.

But we must overcome several design challenges to go from this prototype to a commercial product. The device, ideally, will be a thin insole — consisting of motors, control electronics, and batteries — that can be inserted into most shoes. To that end, we are trying out a number of combinations of low-profile actuators with several types of gel. We are also looking into very thin batteries, but these may not have the energy density to deliver hours of vibration, so we might have to turn to external batteries.

It may be necessary to build a shoe instead of an insole, because there will be more room for the system, but then effort must be spent on considerations of fashion. In either case, comfort will be an important consideration. Because this technology is palliative — that is, it alleviates the symptom but doesn't cure the cause — people will most likely need to wear these devices a good portion of the day, perhaps 8 hours or more. So we will have to be careful that the motors are placed in such a way that they do not themselves become irritating. Whatever the final configuration, we hope to have it approved by the U.S. Food and Drug Administration and on the market within two years.

You have to learn to crawl before you can walk, as the saying goes. For our work, we learned how to improve standing, so that we could go on to more challenging and clinically relevant problems, such as walking and climbing stairs. We're also going to see if a little noise will help people recover from incidents such as tripping or being pushed. Some additional studies examining the effects of this stimulation on the balance of people with diabetes or those coping with the aftereffects of a stroke are under way, too.

Noise-based sensory enhancement technology was not originally intended to reverse or cure any underlying neurological dysfunction — but rather to help people live more safely and independently. However, there is a growing body of research that establishes the connection between sensory activity and the ability of the nervous system to repair itself following an injury.

The paralyzed actor Christopher Reeve, who died last year, became an example of this restoration, as his activity-based therapy was credited with the unprecedented return of his ability to feel a pin prick over most of his body. A new class of devices, now in limited testing, may be able to use the sensory-boosting attributes of our technology to help people relearn simple movements following a stroke and speed their recovery.

What's more, stochastic resonance technology may not be limited to those with medical problems. Perfectly healthy people might someday benefit, too. Just as in recovering from a stroke, there is a chance that learning a new activity, such as an effective golf swing, may be accelerated by the heightened sensory awareness that this technology might provide. Indeed, almost anywhere you need sensory input in order to interact with the environment, a little noise might make things better.