Many studies have sought to clarify the nature of hippocampal information processing, using neuropsychological and electrophysiological approaches in animals. The predominant view from research on humans is that the hippocampus mediates declarative memory, the memory for everyday facts and events that can be brought to conscious recollection. There have been many efforts to understand the role of the hippocampus in animals as well. Among several proposals generated by these studies, one that has captured considerable attention is the view, first espoused by O'Keefe & Nadel, that the hippocampus mediates a neural representation of physical space, that is, a cognitive map. This theory was based on a thorough and systematic analysis of the expansive literature on diverse behavioral abnormalities following hippocampal damage. In addition, O'Keefe and Nadel's proposal incorporated a central observation about the behavioral physiology of hippocampal neurons, specifically that some cells increased firing rate when a rat was at a particular location in its environment. The discovery of these "place cells" appeared to perfectly complement the findings on the behavioral deficits, showing that spatial information was encoded within the cellular activity of the very hippocampal structures that are necessary for spatial learning and memory.
Despite considerable effort to elaborate the details of how place cells encode space, several recent findings call into question the spatial mapping view. These studies have revealed aspects of hippocampal firing patterns that are inconsistent with a straightforward allocentric spatial mapping, and have identified a broader scope of information processing. I will discuss this evidence and argue that hippocampal location-related firing patterns are incidental to the fundamental representation of regularities in the nonspatial and spatial stimuli as well as behavioral actions that occur during the course of learning experiences. These codings incorporate the temporal structure of episodic memories and, through repeated and related experiences, are linked to serve the continuous reorganization of a "memory space". Furthermore, like memory performance, the stability and organization of the memory space embodied in hippocampal neural activity appears to be dependent on NMDA receptor mediated plasticity. Taken together, recent evidence on properties of the hippocampal memory space are consistent with the full scope of the literature on critical hippocampal function in humans and animals, and help us understand how hippocampal neural populations mediate declarative memory processing.