Place, space, and taste: combining context and spatial information in a hippocampal navigation system

The hippocampus appears to play an important role in episodic-like, or "what, where, when" memory that may be used for goal finding. We have previously presented a model of the hippocampus describing how navigation to a distal goal location could be achieved through gradient ascent processes based on place field density. Here we extend that model to show that information about both where a goal is, and the attributes of that goal, can be incorporated with relatively simple modifications. In this model both the spatial and attribute information would be available to the animal at any location within the environment although differential recall requires two different firing modes. "Where" information is available when cells in the model are firing in "place field" firing mode. The recall of "what" information requires, however, near simultaneous firing of a large number of place cells. We discuss how this "what" firing mode could potentially depend upon sharp-wave ripple (SPW-R) events. This conception has implications for how we interpret SPW-R related firing. In particular, we suggest that the near simultaneous firing that occurs in large groups of cells during SPW-Rs may be sufficient for "what" recall and that the forward and reverse "replay" of place cell sequences that are often observed during SPW-Rs may be an epiphenomenon of this process.