Distinct patterns of hippocampal formation activity associated with different spatial tasks: a Fos imaging study in rats.

Fos levels were measured in rats trained in one of two qualitatively different spatial memory tasks in a water maze. In one task, ( landmark condition) rats found a submerged platform that was always 25 cm south of a visible landmark, the absolute position of the platform and landmark changing after every trial. In the other task ( place condition), rats swam to a platform that remained in the same absolute position on every session, but changed session to session, with this task relying on the memory of allocentric cues. Despite matched swim times, the landmark condition resulted in higher levels of Fos in a wide range of cortical and subcortical sites, including the hippocampus and its connections. Structural equation modelling revealed two different patterns of hippocampal function. In the allocentric place task there was a significant association between Fos activity in the entorhinal cortices and the hippocampus proper, while in the non-allocentric landmark task this relationship was not present, but was replaced by a connection from the entorhinal cortices to the subiculum. Thus, the two different tasks engage two different modes of hippocampal activity as demonstrated by Fos expression.