Time cells in the hippocampus: a new dimension for mapping memories

Key Points Hippocampal time cells fire at successive moments in temporally structured experiences. Temporal coding in the hippocampus is observed across a broad range of behavioural tasks and in different animal species and humans. Time cells cannot be explained by variations in location or movement through space. Time cells also encode spatial variables and other dimensions of specific events. Time cells provide a mechanism for the temporal organization of episodic memories. The recently discovered hippocampal 'time cells' are thought to represent the flow of time in specific memories. In this Review, Howard Eichenbaum discusses the evidence for the existence of time cells, describes their characteristics and relationship with place cells, and considers their role in memory. Recent studies have revealed the existence of hippocampal neurons that fire at successive moments in temporally structured experiences. Several studies have shown that such temporal coding is not attributable to external events, specific behaviours or spatial dimensions of an experience. Instead, these cells represent the flow of time in specific memories and have therefore been dubbed 'time cells'. The firing properties of time cells parallel those of hippocampal place cells; time cells thus provide an additional dimension that is integrated with spatial mapping. The robust representation of both time and space in the hippocampus suggests a fundamental mechanism for organizing the elements of experience into coherent memories.