Deisseroth immediately put his lab to work figuring out what part of the brain needed to be stimulated to cure Parkinson's. Optogenetics was the ideal tool because it let researchers test various types of neurons to find which one would make legs move again, hands grasp again, faces smile again.But test after test failed. "This was a discouraging time," Deisseroth says. "The project was almost abandoned, because we had difficulty showing any therapeutic result."Many experts had thought the cure was to stimulate certain kinds of cells within the subthalamic nucleus, which coordinates motion. But when they tried that, it had no effect whatsoever. Then two of Deisseroth's grad students began experimenting with a dark-horse idea. They stimulated neurons near the surface of the brain that send signals into the subthalamic nucleus — a much harder approach because it meant working at one remove. It was as if, instead of using scissors yourself, you had to guide someone else's hands to make the cuts.Their idea worked. The mice walked. In their paper, published in April 2009, they wrote that the "effects were not subtle; indeed, in nearly every case these severely parkinsonian animals were restored to behavior indistinguishable from normal."Over at MIT, Boyden was asking the obvious question: Would this work on people? But imagine saying to a patient, "We're going to genetically alter your brain by injecting it with viruses that carry genes taken from pond scum, and then we're going to insert light sources into your skull." He was going to need some persuasive safety data first.