The presence of a second rat has only subtle effects on the location-specific firing of hippocampal place cells.

We compared the spatial firing properties of hippocampal place cells as a hungry rat foraged for randomly scattered food pellets in a familiar environment while it was by itself and while it shared the arena with a second rat that also was trained in the same task. Our goal was to determine if the hippocampal mapping system remained functional in the presence of the second rat, despite a strong initial tendency of the two animals to stay close together and despite the increased complexity of the sensory surroundings. We found that almost all place cell firing fields were only marginally changed by introducing the second rat. In particular, there was no evidence of the remapping characteristic of place cells in a sufficiently different novel environment. Instead, firing fields became somewhat less well organized and slightly weaker in the presence of the second rat. These second order changes were found to be distance dependent; the degradation of firing properties was maximal when the two rats were near each other. We conclude that signals in the hippocampal mapping system are affected to a small enough extent that accurate navigational is still possible when the environment is enriched in this realistic fashion.