Place cells can flexibly terminate and develop their spatial firing. A new theory for their function

In this study, hippocampal place cells were recorded in a behavioral paradigm previously not employed in place-cell research. Rats were exposed to the same fixed environment for as long as 8-24 h without interruption, while the firing of CA1 and CA3 place cells was monitored continuously. The first finding was that all place cells that were detected at the beginning of the recording sessions ceased to produce location-specific firing in their original firing fields within 2-12 h. This was observed despite the fact that the animals kept visiting the original firing fields, the hippocampal EEG was virtually unchanged, and the discriminated action potentials of the cells could be clearly recorded. The second finding was that some complex-spike cells that produced no spatially selective firing pattern at the beginning of the recording sessions developed location-specific discharges within 3-12 h. Thus, place cells can flexibly terminate and develop their spatial firing. even in a fixed environment and during similar behaviors, if that environment is explored continuously for a prolonged period. To explain this phenomenon, a new place-cell theory is outlined. Accordingly, the high-frequency discharges of these neurons may serve to create, under multiple extrahippocampal control and within limited periods, stable engrams for specific spatial sites in the association cortex where the cognitive map probably resides. After the creation of a stable engram, or in the absence of favorable extrahippocampal inputs, place cells may suspend their location-specific firing in the original field, and initiate the processing of another spatial site.