The Effects of Repetition and Sequence Length on Hippocampal Memory Trace Reactivation

Abstract

Patterns of hippocampal ensemble activity that occur during a spatial experience are reactivated during subsequent rest periods and slow wave sleep. Connections between active cells are thought to be strengthened, via long term potentiation (LTP), by repeated co-activation during experience, which suggests that the level of memory trace reactivation would increase proportionately with repetition. Alternatively, plasticity associated with memory formation, such as LTP-dependent place field expansion and the induction of activity-dependent immediate early gene, ARC, saturates after only a few laps, indicating that reactivation would plateau after a few repetitions. The length of the repeated sequence may also affect reactivation, since activation of a very short sequence can be repeated more frequently than a long sequence in a given time period. We studied how memory trace reactivation was affected by repetition and the length of the repeated sequence by observing the reactivated patterns of cell-pair correlations after a rat ran laps around a long circular track versus running more laps around a short track. On the shorter track, fewer cells had place fields, but they covered more of the track, resulting in generally stronger correlations among active cells. In addition, neuronal activity was recorded from dorsal and mid-ventral CA1. In mid-ventral CA1, there were fewer place fields in the environment but they were larger, with generally stronger correlations among active cells. The comparison between dorsal and mid-ventral regions is thus analogous to the comparison between the sequence of place fields on a long versus short track, respectively. Although there were more cells active in the dorsal region, but more potent correlations in the middle region, no differences in memory trace reactivation were found with respect to repetitions, track length or hippocampal region. This suggests that although spatial scaling increased along the dorsoventral axis of the hippocampus, reactivation is balanced, and possibly coherent across the hippocampal axis and it is relatively independent of sequence length or number of repetitions, at least when that number exceeds about 20.