Work in our laboratory attempts to understand the flow of information through the hippocampal formation and the computations performed by the various subfields of the hippocampus and its inputs from the entorhinal cortex. To address these issues, we use multi-electrode arrays to record the extracellular action potentials from scores of well-isolated hippocampal neurons in freely moving rats. These neurons have the fascinating property of being selectively active when the rat occupies restricted locations in its environment. They are termed "place cells," and it has been suggested that these cells form a cognitive map of the environment (O'Keefe and Nadel, The hippocampus as a cognitive map). The animal uses this map to navigate efficiently in its environment and to learn and remember important locations. It is also hypothesized that these cells play a major role in the formation of episodic (autobiographical) memories. Place cells thus constitute a tremendous opportunity to investigate the mechanisms by which the brain transforms sensory input into an internal, cognitive representation of the world "out there" and then uses this representation as the framework that organizes and stores memories of past events.