You’ve just had a stroke. Your left arm is mostly useless. The doctors tell you to go get some physical therapy, so every day you head off to the local physical therapy office for 3 hours of strength training and stretching designed to put you back on top.Realistic story—so far. But if the 8 employees of Providence, RI-based Afferent have their way, the first thing that will happen when you get there is that the PT will roll up your sleeve, slap on a long electrode that reaches from wrist to elbow, plug it into a cigarette-pack-sized battery, and crank up the power. Sound odd? Yeah. Particularly since even though you won’t feel the juice flowing through the electrode, it will (at least in theory) help you feel your own arm better.Founded in 2000 by Jason Harry, formerly VP of research engineering at Boston-based medical device maker NMT Medical, and James Collins, a Boston University biomedical engineer, Afferent is aiming to commercialize nerve-stimulating devices based on a quirky phenomenon called “stochastic resonance.” (The basic idea, if you want to get into the physics of it, is that under the right conditions adding noise to a system—in this case, the nervous system—can actually make a weak signal more detectable rather than less.)Collins (who’s also an Xconomist) began researching the phenomenon back in the 1990s. He found that it’s possible to increase the number of impulses that nerves in an arm, say, or foot send to the brain by applying a very small amount of electric or mechanical noise—not enough to be felt—to the receptors that form the business-end of the sense of touch. The end result is that the brain can do a better job of detecting sensations because more nerve impulses are reaching it. Harry, who stepped aside as President and CEO to become chief technical officer in 2006, calls it “turn[ing] up the volume.”Turning up the volume, it turns out, is a very good thing. According to Harry, when a person has a stroke, loses function in his arm, and subsequently recovers, it’s not because the “arm” part of his brain healed up, but rather that another part of the brain has learned to the job that the injured part used to do. Sensory signals from the arm are key to stimulating this process, and the louder the volume, the greater the effect, Harry says.Afferent’s stroke-rehab system is still undergoing human trials (30 patients are involved, and results are will be made public in April or so; until then, Harry’s saying mum lest he run afoul of the FDA), but the company already has data from a trial on rats. In that study, rats were trained to perform a task, given a stroke, and then retrained to do the same task. Some rats got stochastic resonance therapy through implanted electrodes along with their retraining, and some got retraining alone. The rats that got the highest levels of resonance therapy relearned the activities best.Current Afferent CEO and president David Hable, former worldwide president of Codman & Shurtleff, a neurosurgery device maker, thinks