Dissociation of spatial representations within hippocampal region CA3.

Classic models of the hippocampus uniformly ascribe pattern completion to CA3, but recent data suggest CA3c (enclosed by the dentate gyrus) may act in a manner more consistent with the dentate and aid in pattern separation. The ideal test for functional distinction within CA3, however, is to compare the responses in these regions in the same animal in multiple contexts. To accomplish this, animals visited two contexts with varying degrees of similarity and the pattern of repeated Arc expression was examined across the pyramidal cell layer. Under conditions of partial cue change, responses in CA3c are far more distinct than CA3a/b, consistent with evidence for functional diversity along the transverse axis of CA3. These data add to the mounting evidence that "classic" roles ascribed to CA3 in learning and memory require re-evaluation.