A prominent theory holds that the hippocampus is the neural locus of a “cognitive map”—a mental representation of one’s surrounding environment and one’s location within it.  This map is thought to be the scaffold upon which the brain organizes and relates the individual components of an experience (the sights, sounds, thoughts, emotions) and stores it in such a way that it can be later retrieved as a conscious recollection.  To understand how these memories are represented, encoded, and stored in neural circuits, we record the activity of populations of neurons in the hippocampal system as rats perform various behavioral tasks.  We investigate the properties of hippocampal input structures (e.g., grid cells of the medial entorhinal cortex and object representations of the lateral entorhinal cortex), output structures (e.g., place cells of the CA1 region), and the intervening computational stages (e.g., granule cells and mossy cells of the dentate gyrus and the place cells of the CA3 region).  We collaborate with theorists and engineers to bridge the gap between experimental data and computational theory in order to understand the neural basis of memory and how it is affected during healthy and abnormal aging.