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

Subjects

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.

Abstract

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.

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Figure 1: Key features of time-cell firing sequences.
Figure 2: Early discoveries on temporal coding by hippocampal neurons.
Figure 3: Time coding and spatial coding.
Figure 4: Time cells have a role in memory.
Figure 5: The influence of temporal context on spatial coding.
Figure 6: Temporal context versus chaining models.

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Acknowledgements

The author would like to acknowledge funding support from the US National Institutes of Mental Health (grants MH095297 and MH094263).

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Correspondence to Howard Eichenbaum.

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41583_2014_BFnrn3827_MOESM486_ESM.mp4

Time cells in rats running on the treadmill. The firing patterns of three CA1 neurons are shown. For each, the time and location of the rat when individual spikes occur are plotted as dots (in a different colour for each neuron) on the rat's head. Note that even though the rat's head is approximately in the same location during the run, the neurons fire in sequence (pink then green then blue). Also note that each neuron additionally fires at a location on the maze outside the treadmill. The treadmill is on when a red triangle appears in bottom left corner. (MP4 5332 kb)

Supplementary information S1 (movie)

Time cells in rats running on the treadmill. The firing patterns of three CA1 neurons are shown. For each, the time and location of the rat when individual spikes occur are plotted as dots (in a different colour for each neuron) on the rat's head. Note that even though the rat's head is approximately in the same location during the run, the neurons fire in sequence (pink then green then blue). Also note that each neuron additionally fires at a location on the maze outside the treadmill. The treadmill is on when a red triangle appears in bottom left corner. (MP4 5332 kb)

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Eichenbaum, H. Time cells in the hippocampus: a new dimension for mapping memories. Nat Rev Neurosci 15, 732–744 (2014). https://doi.org/10.1038/nrn3827

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