In our work, we use optical and electrical methods to stimulate and record from the brain, in order to study rodent behavior, memory, and disease models on the level of neural populations. Light offers powerful ways to study the living brain. We use new optogenetic methods to selectively stimulate selected neural subpopulations, and for example, use this to study the role of monoaminergic activity during behavioral tasks. We also use optical imaging methods in combination with state-of-the-art in vivo electrophysiology, to study the neural correlates of learning and memory. In close collaboration with other groups, we also conduct rodent functional imaging (fMRI, SPECT).