Local remapping of place cell firing in the Tolman detour task.

The existence of place cells, whose discharge is strongly related to a rat's location in its environment, has led to the proposal that they form part of an integrated neural system dedicated to spatial navigation. It has been suggested that this system could represent space as a cognitive map, which is flexibly used by animals to plan new shortcuts or efficient detours. To further understand the relationships between hippocampal place cell firing and cognitive maps, we examined the discharge of place cells as rats were exposed to a Tolman-type detour problem. In specific sessions, a transparent barrier was placed onto the maze so as to block the shortest central path between the two rewarded end locations of a familiar three-way maze. We found that rats rapidly and consistently chose the shortest alternative detour. Furthermore, both CA1 and CA3 place cells that had a field in the vicinity of the barrier displayed local remapping. In contrast, neither CA1 nor CA3 cells that had a field away from the barrier were affected. This finding, at odds with our previous report of altered CA3 discharge for distant fields in a shortcut task, suggests that the availability of a novel path and the blocking of a familiar path are not equivalent and could lead to different responses of the CA3 place cell population. Together, the two studies point to a specific role of CA3 in the representation of spatial connectivity and sequences.