Memory related EEG oscillations:
The functional meaning of phase and amplitude

W. Klimesch
Department of Physiological Psychology
University of Salzburg
Salzburg, Austria

    Event-related changes in EEG oscillations are traditionally quantified in terms of power changes as is best exemplified in research on event-related de/synchronization (ERD/ERS). Based on this method, convergent findings indicate that an event-related increase in theta power (i.e. theta ERS) reflects working memory (WM) processes, whereas an event-related decrease in upper alpha power (i.e. upper alpha ERD) reflects sensory-semantic long-term memory (LTM) processes. The aim of the talk is to show that in addition to an event-related change in amplitude, theta and alpha exhibit an event-related change in phase that leads to an alignment in the absolute phase angle at about 160 ms during the time window of the N1 component. Thus, the question is addressed whether event-related potentials (ERPs) are generated by a superposition of evoked oscillations in the theta and alpha frequency range. The question is investigated whether evoked theta and alpha have a similar functional meaning for memory as is known from event-related de/synchronization (ERD/ERS).
    Recent findings indicate that (i) theta and alpha show a significant increase in phase locking during the time window of early ERP components as compared to a prestimulus reference, (ii) the dynamics of event-related changes in evoked theta and alpha power obey the same principles as are known from event-related de-/synchronization research and (iii) latency measures of the P1-N1 complex are negatively correlated with individual alpha frequency. In addition, we have found that theta phase locking is larger during encoding than recognition and that good memory performers show a larger increase in theta and alpha phase locking during recognition in the time window of the N1.
The reported findings suggest that cognitive performance may be based at least in part by an interplay between the synchronous activation of two distributed neuronal network systems, a WM and sensory-semantic LTM system, each operating with a different frequency, the first in the theta (about 6 Hz), and the second in the upper alpha (about 12 Hz) frequency range.

 

 

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