"Here's what happens when we turn on the light," Karl Deisseroth says. He points to a mouse, ordinary save for the thin optical fiber protruding through its skull. When a lab tech presses a lever, blue light shoots through the fiber, and the mouse -- which had been sauntering straight ahead -- starts to run in circles. "He's doing that because the blue light turns the neural circuit on," Deisseroth explains. "As soon as we stop the stimulation, he'll walk straight again."