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