"Keeping on track": firing of hippocampal neurons during delayed-nonmatch-to-sample performance.

Hippocampal neurons that encode critical events during a delayed-nonmatch-to-sample (DNMS) task were proposed to have functional topography as demonstrated by Hampson et al. (1999b). Functional cell types (FCTs) that encode similar task features were located within alternating transverse segments along the hippocampal longitudinal axis. On this basis, Redish et al. (2001) suggested that firing of populations of CA1 neurons recorded from the same hippocampal locations in animals running on linear or curvilinear tracks should be spatially and temporally correlated; however, they failed to find such correlations. The current study addresses the issues raised by Redish et al. (2001). Initially we found that modeling of simulated place fields revealed absences in temporal correlations in the study by Redish et al. (2001) that should have been present given the reported spatial correlations. In addition, the correlation methods used by those investigators failed to detect robust but transient event-related cross-correlations between FCTs in the DNMS task. Furthermore, demonstration of such transient, short-latency correlated firing between similar CA3 and CA1 FCTs corroborated the anatomic scheme proposed by Hampson et al. (1999b) and reaffirmed the potential existence of a functional topography within hippocampus.