Sequential firing of hippocampal neurons during both running and rest is believed to contribute to episodic-memory encoding. In particular, sharp-wave (SPW) sequences, which normally occur during rest, have been suggested to serve as a physiological substrate of memory. This hypothesis arose because (i) elimination of SPW sequences impairs learning and memory and (ii) the similarity of some SPW sequences to average running sequences (such as place-cell and episode-cell sequences) creates the impression that SPWs “replay” an animal’s experience of running. Using these average running sequences as templates, it has been shown that SPWs can replay the running sequences in both forward and backward directions relative to the templates. This led to the conjecture that SPW sequences are the sequences that are activated bidirectionally (i.e., forward and backward).
We used a novel method to test this bidirectionality conjecture by directly comparing pairs of SPW sequences without the use of average running sequences. Assuming that SPW sequences can be activated in both forward and backward directions, correlations among SPW sequences should be both positive and negative. Surprisingly, our analysis of correlations among SPW sequences revealed a very significant number of positive correlations but only a chance-level number of negative correlations. This lack of negative correlations among SPW sequences suggests that SPW sequences are activated unidirectionally, not bidirectionally as previously conjectured. This same method was also robust enough to reproduce the seemingly contradictory findings that SPW sequences are positively and negatively correlated with running sequences. More than suggesting that backward SPW replay does not exist, this analysis questions the entire “replay” framework since SPW sequences are statistically correlated positively with each other regardless of their similarity to running sequences.