The content of hippocampal “replay”

One of the most striking features of the hippocampal network is its ability to self‐generate neuronal sequences representing temporally compressed, spatially coherent paths. These brief events, often termed “replay” in the scientific literature, are largely confined to non‐exploratory states such as sleep or quiet rest. Early studies examining the content of replay noted a strong correlation between the encoded spatial information and the animal's prior behavior; thus, replay was initially hypothesized to play a role in memory formation and/or systems‐level consolidation via “off‐line” reactivation of previous experiences. However, recent findings indicate that replay may also serve as a memory retrieval mechanism to guide future behavior or may be an incidental reflection of pre‐existing network assemblies. Here, I will review what is known regarding the content of replay events and their correlation with past and future actions, and I will discuss how this knowledge might inform or constrain models which seek to explain the circuit‐level mechanisms underlying these events and their role in mnemonic processes.