When neuroscientist Sébastien Tremblay set out to manipulate monkeys’ brains with light, colleagues had sobering advice: “It’s more difficult than it sounds.” Tremblay, who works in neuroscientist Michael Platt’s lab at the University of Pennsylvania, uses light to activate or silence precise groups of neurons and probe their role in brain function. The method, called optogenetics, works well in rodents, but studies in nonhuman primates are critical if it’s ever going to become a therapy for humans—to suppress seizures, for example, disrupt tremors in Parkinson’s disease, or even project images into the brain of a blind person.