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  • Review Article
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Rhythms of the hippocampal network

Nature Reviews Neuroscience volume 17, pages 239–249 (2016)Cite this article

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Key Points

  • The hippocampus shows three main classes of rhythms: theta (∼4–12 Hz), sharp wave–ripples (∼150–200 Hz ripples superimposed on ∼0.01–3 Hz sharp waves) and gamma (∼25–100 Hz).

  • Theta rhythm generation involves a variety of mechanisms, including theta rhythmic firing in septal and hippocampal interneurons, excitatory inputs to hippocampus and intrinsic properties of hippocampal neurons.

  • Theta rhythms are likely to be important for the formation of memories of sequences of events.

  • Sharp wave–ripple complexes are composed of two distinct network patterns: sharp waves (excitatory events that propagate from CA3 to CA1) and ripples (which reflect high frequency firing in hippocampal interneurons).

  • Accumulating evidence suggests that sharp wave–ripples are important for intrinsic hippocampal operations, including offline memory processing, retrieval of previously stored memories and planning of future behaviours.

  • The class of brain rhythms traditionally defined as gamma probably contains at least two different variants of oscillatory activity.

  • Recent findings suggest that slow (∼25–55 Hz) and fast (∼60–100 Hz) variants of gamma have different origins and may have different functions.

Abstract

The hippocampal local field potential (LFP) shows three major types of rhythms: theta, sharp wave–ripples and gamma. These rhythms are defined by their frequencies, they have behavioural correlates in several species including rats and humans, and they have been proposed to carry out distinct functions in hippocampal memory processing. However, recent findings have challenged traditional views on these behavioural functions. In this Review, I discuss our current understanding of the origins and the mnemonic functions of hippocampal theta, sharp wave–ripples and gamma rhythms on the basis of findings from rodent studies. In addition, I present an updated synthesis of their roles and interactions within the hippocampal network.

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Figure 1: Theta sequences in the hippocampus during spatial memory operations.
Figure 2: Replay during sharp wave–ripples.
Figure 3: Slow and fast gamma rhythms.

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Acknowledgements

The author thanks K. Bieri, E. Hwaun, and C. Zheng for assistance with data collection and/or figure preparation. The work of L.L.C. is supported by the Klingenstein Fund, the Whitehall Foundation, Alzheimer's Association grant NIRP-14- 305205, National Institute of Mental Health (NIMH) grant 1R01MH102450-01A1, the Office of Naval Research Young Investigator Program award N00014-14-1-0322 and National Science Foundation CAREER award #1453756.

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  1. Department of Neuroscience, Center for Learning and Memory, The University of Texas at Austin, Austin, 78712, Texas, USA

    Laura Lee Colgin

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PowerPoint slides

Glossary

Sleep spindles

Thalamocortical oscillations of 7–14 Hz that occur in bouts lasting for a few seconds and recurring approximately once every 10 seconds during slow-wave sleep, particularly around the onset of sleep.

Slow oscillations

Cortically generated oscillations of ∼0.5–1 Hz, consisting of alternating depolarizing 'up' states and hyperpolarizing 'down' states that regulate the occurrence of other oscillations, including sleep spindles, during slow-wave sleep.

Neuronal ensembles

Co-active neurons that work together to carry out neuronal computations and operations such as stimuli coding or memory storage.

Theta sequences

Ordered series of place cell spikes that occur within theta cycles and that represent the succession of locations traversed during active behaviours.

Head direction cells

Neurons that fire when an animal's head is pointing in a particular direction and are found in several brain areas including the postsubiculum, the thalamus and the medial entorhinal cortex.

Replay

The phenomenon by which ordered place cell spike trains that occur during exploratory theta-related behaviours later reactivate in a temporally compressed manner during sharp wave–ripples while the animal is at rest and during slow-wave sleep.

Stratum radiatum

Apical dendritic layer in CA3 and CA1 in which axons from CA3 pyramidal neurons terminate.

Stratum lacunosum-moleculare

Distal apical dendritic layer in CA3 and CA1 in which perforant pathway fibres from the entorhinal cortex terminate.

Delayed nonmatching-to-place task

A behavioural task that assesses memory by first allowing animals to visit one of two goal locations and then, after a delay period, requiring animals to navigate to the other goal location in order to receive a reward.

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Colgin, L. Rhythms of the hippocampal network. Nat Rev Neurosci 17, 239–249 (2016). https://doi.org/10.1038/nrn.2016.21

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