The last century has seen great progress in our understanding of those aspects of neural computation that can be studied through experimentation on one or a few cells--for example, how synapses enable a neuron to talk to one of its neighbors. But the phenomena that got many neuroscientists interested in the brain in the first place--learning, emotion, consciousness, and mysterious disorders such as depression and schizophrenia--remain difficult to explain through experiments on just one or even a few cells. Thousands or millions of cells, computing as an ensemble, are responsible for practically all of our behaviors, as well as the derangements thereof. ...