Elsevier

Brain Research

Volume 1235, 15 October 2008, Pages 31-44
Brain Research

Review
A short review of slow phase synchronization and memory: Evidence for control processes in different memory systems?

https://doi.org/10.1016/j.brainres.2008.06.049Get rights and content

Abstract

An integrative theoretical approach about memory related oscillations is presented. The basic assumptions are that memory related oscillations are probably confined to theta and upper alpha and that other frequencies particularly in the gamma range are important for memory primarily because they become coupled to lower frequencies and/or because they play a specific role for a high precision timing of neural events (including phenomena such as LTP or LTD). In contrast to previous studies, where we related theta and upper alpha to a variety of different memory processes, we suggest here that these oscillations are associated with top-down control processes in two large storage systems, working memory (WM) and long-term memory (LTM). These systems may have their own types of top-down processes that control access to and/or manipulation of stored information. The hypothesis, suggested here is that theta and upper alpha reflect these processes which can be best studied when analyzing phase.

Introduction

The functional meaning of different EEG/MEG oscillations is still a controversial issue. Even a single frequency domain (such as theta, alpha and gamma) has been associated with a variety of different functions. Because most of the research has been based on power measurements, the question arises, whether more sophisticated analysing methods will allow new insights into the mechanisms of human information processing. We will argue that findings about phase are particularly helpful to advance our understanding of the functional meaning of oscillations. We suggest that slow oscillations – particularly in the theta and alpha frequency range – are related to control processes in two large memory systems, a working memory (WM) and long-term memory (LTM) system.

Section snippets

The issue of memory related brain oscillations

Findings based on power measurements Most of the studies investigating the functional meaning of EEG/MEG oscillations have focused on different types of power measures such as event-related synchronization and desynchronization (ERD/ERS; cf. Pfurtscheller and Lopes da Silva, 1999). Here, we use these terms in their general meaning denoting an event-related increase (ERS) or decrease (ERD) in power. Findings from this type of research revealed frequency specific responses that differ largely

The importance of phase: timing of neuronal activity and dynamic interconnectivity

Power estimates reflect – besides unspecific factors (such as skull thickness or recoding method) – the number of neural units that are synchronously active. But in contrast to power, phase is a measure that is directly related to the timing of neural activity. EEG oscillations are associated with rhythmic fluctuations in the excitability of the dendritic tree of a large number of neurons (cf. Logothetis et al., 2001). During the phase of high excitability a neuron is very likely to fire,

Theta and alpha phase synchronization: evidence for memory related operating systems?

Although theta and alpha have been associated with different memory related functions, we suggest here that the reported findings allow for a consistent interpretation. The basic idea is that synchronized theta and synchronized alpha cannot be functionally related to a single and specific process such as episodic or semantic encoding, but they can be related to different classes of functions within two large memory systems, a WM and LTM system. But what is the nature of these different

Conclusions and critical questions

The reviewed findings demonstrate that different measures of phase synchronization processes within and between different frequency domains offer a promising new way to investigate not only memory related processing modes but also specific memory processes such as semantic encoding or memory matching. Nonetheless, there are at least two critical aspects that limit the interpretation of the reported findings.

First, the functional and methodological relationship between phase and power on one

Acknowledgments

This research was supported by the Austrian Science Fund (FWF), P – 16849-B02.

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