Deisseroth has no interest in a 21st-century mind-control campaign, however. He's trying to get a handle on the fundamentals of brain function. When he turns his neural circuits on and off, he can observe exactly what behaviors those circuits control. "We know how the heart works. It's a pump," he says. "But with the brain, we don't have that kind of model. We can't even imagine one." Deisseroth's fibers will let researchers explore what the brain does on a neuron-by-neuron level. "Karl's big breakthrough of using light to map the function of neural circuits will allow us, for the first time, to really know how the brain works," says Duke University neurobiologist George Augustine. "This has been a big problem for more than a century. He's poised to blow the field wide open."