The anisotropic structure of hippocampal SPW sequences
Sharp wave sequences correlate only positively with each other despite replaying running sequences forward and backward.
Evidence against bidirectional sharp-wave replay.
Hippocampal sharp-wave (SPW) sequences are believed to contribute to the encoding of episodic memories by “replaying” experience on a faster timescale. This belief stems from the observation that some SPW sequences correlate significantly with templates generated from an animal’s running experience. As these correlations are both positive and negative, it is believed that SPW sequences replay experience bidirectionally. We compared SPW sequences directly without the use of running templates. Surprisingly, correlations between SPW sequences were significantly positive, with negative correlations occurring at chance level. This suggests that SPW sequences are not activated bidirectionally. Further, this observation held regardless of how SPW sequences correlated with running sequences, suggesting that SPW sequences do not replay experience as previously believed.