The results are as reliable as flipping a light switch, but even after decades of testing, no one knows exactly why it works. Dr. Kaplitt, the surgeon who installed Rebecca Serdans' implant, explains it by likening the brain to a collection of electrical circuits. A disorder like dystonia is a failure of those circuits. When you install a brain stimulation device, "it's presumably blocking abnormal information from getting from one part of the brain to another, or normalizing that information." But Kaplitt is the first to acknowledge that this is just a theory. "The mechanism by which brain stimulation works is still somewhat unclear and controversial."

But the lingering questions haven't slowed down research. There are already patents that would use brain stimulation implants to enhance memory or prevent stuttering, to cure anorexia or bring a person to orgasm. Experimental studies use the device to treat Alzheimer's disease and drug addiction. Those circuits aren't as well understood as the circuits governing movement disorders, but the principle is no different. Once you've got a line into the circuitry of the brain, Parkinson's is just the beginning.