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The role of phase synchronization in memory processes
Juergen Fell and Nikolai Axmacher

Abstract | In recent years, studies ranging from single-unit recordings in animals to electroencephalography and magnetoencephalography studies in humans have demonstrated the pivotal role of phase synchronization in memory processes. Phase synchronization — here referring to the synchronizationof oscillatory phases between different brain regions — supports both working memory and long-term memory and acts by facilitating neural communication and by promoting neural plasticity. There is evidence that processes underlying working and long-term memory might interact in the medial temporal lobe. We propose that this is accomplished by neural operations involving phase–phase andphase–amplitude synchronization. A deeper understanding of how phase synchronization supports the flexibility of and interaction between memory systems may yield new insights into the functions of phase synchronization in general.
Local field potential
A neural voltage fluctuation recorded from the extracellular space, which mainly originates from postsynaptic potentials.

Neural oscillation
A periodicand continuous (wave-like) variation of a neural signal.

Oscillatory phase
The angle that corresponds to the momentary deflection of an oscillation (referring to the cosine function; for example, 0° at the peak and 180° at the trough of an oscillation).

Gamma frequency range
The frequency range between 30–100 Hz.

Department of Epileptology, University of Bonn, SigmundFreud-Strasse 25,D-53105 Bonn, Germany Correspondence to J.F. e-mail: doi:10.1038/nrn2979

Neurons do not function in isolation. They are embedded in assemblies and networks, in which they influence each other through excitatory and inhibitory synaptic connections. As a result, the neurons in a network are rhythmically activated and inhibited1. This rhythmicity is reflected inoscillations of the extracellular field potential that can be measured through recordings of local field potentials and through electroencephalography (EEG). The frequency of neural oscillations depends on various time constants and network properties and may range from slow activity, with oscillation periods of several seconds, to fast activity in which one cycle lasts a few milliseconds. Moreover,oscillations of different frequencies can occur at the same time in the same brain regions2–4. In networks of synchronized neurons, the oscillatory phase determines the degree of excitability of the neurons and influences the precise discharge times of the cells in the network5,6 (although not each neuron discharges action potentials during every oscillatory cycle). Consequently, phase relationshipsbetween brain regions affect the relative timing of action potentials in those regions. One speaks of phase synchronization between oscillations in two regions when oscillatory phases in these regions are correlated (for example, if the peaks of oscillatory activity in region A invariantly occur at the same time as the peaks of oscillatory activity in region B; BOX 1 and FIG. 1). Phasesynchronization is a fundamental neural mechanism. It supports neural communication and

neural plasticity and is probably relevant for many cognitive processes7–11. Despite a growing amount of empirical data, however, a fundamental understanding of these functions and their interplay is lacking. In this Review, we focus on the role of phase synchronization in memory processes, which is currently a matter ofintense investigation. We hypothesize that understanding this role is particularly well suited to illuminating how the basic functions of phase synchronization might interact in other contexts. We first describe the main basic functions of phase synchronization of high- and low-frequency oscillations: neural communication and plasticity. We then discuss the relevance of phase synchronization...