Despite a century of sustained research, brain scientists remain ignorant of the workings of the three-pound organ that is the seat of all conscious human activity. Many have tried to attack this problem by examining the nervous systems of simpler organisms. In fact, almost 30 years have passed since investigators mapped the connections among each of the 302 nerve cells in the roundworm Caenorhabditis elegans. Yet the worm-wiring diagram did not yield an understanding of how these connections give rise to even rudimentary behaviors such as feeding and sex. What was missing were data relating the activity of neurons to specific behaviors.