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Rhythms of the hippocampal network
Laura Lee Colgin
Abstract | The hippocampal local field potential (LFP) shows three major types of rhythms: theta, sharp waveripples 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 waveripples and gamma rhythms on the basis of findings from rodent studies. In addition, Ipresent an updated synthesis of their roles and interactions within the hippocampal network.
Brain rhythms are periodically fluctuating waves of neuronal activity that are readily observed using local field potential (LFP) recordings. Such rhythms reflect the synchronized activity of large numbers of neurons because synchronous currents sum together to generate large- amplitude fluctuations in LFP, whereas non-synchronized currents do not sum together and thus remain too small to be detected. It is known that individual neurons are not capable of carrying out complex cognitive operations in isolation and instead must form functional networks with other neurons1 and so synchronous neuronal activity is thought to be relevant to cognition.
One such cognitive operation that requires coor dination across multiple neurons is memory. Memories are thought to be represented by distributed ensembles of neurons that are concurrently activated, a concept that can be traced back to Donald Hebbs cell assembly hypothesis2. Brain rhythms are thought to have a key role in memory formation by synchronizing, and thereby coordinating, the activity of distributed neurons during memory operations.
The hippocampus is essential for spatial and episodic memory3 and thus is an ideal region in which to investigate how brain rhythms affect memory operations. In addition, several features of the hippocampus facilitate the study of brain rhythms. The hippocampus contains densely packed neurons that generate large LFPs and, in turn, generate large rhythms. This is especially true for area CA1 because the pyramidal cell dendrites are aligned in parallel. As a result of this, synaptic currents flow in the same direction and sum together to produce large amplitude signals that...