Optogenetics has revolutionized brain research in recent years. Using light, Wallenberg Academy Fellow Marie Carlén and her colleagues can turn selected neurons on and off and see the direct effect on behavior. This enables the team to piece together the brain’s networks with its activities, knowledge shedding light on the cause of mental illness, among other things. In future it is hoped that this will lead to more effective drugs to combat schizophrenia, for example.