EEG-alpha rhythms and memory processes

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Abstract

The results of several experiments indicate that alpha frequency varies as a function of memory performance. It was found that in samples of age matched subjects alpha frequency of good memory performers is about 1 Hz higher than those of bad performers. The difference in alpha frequency between good and bad performers reaches a maximum during the retrieval of information, is much smaller during encoding and is minimal — but still significant- during a resting period. These results suggest that alpha frequency may be a permanent and not only a functional parameter that determines the speed with which information can be retrieved from memory. The calculation of changes in band power indicate further that the upper alpha band is particularly sensitive to sematic memory demands. The lower alpha band, on the other hand, seems to reflect attentional processes. These findings are discussed on the basis of a hypothesis which assumes that EEG frequencies within the alpha band stem at least in part from the thalamus and that the activity of thalamo-cortical networks reflects processes that are related to searching, accessing and retrieving information from (somatic) long-term memory. © 1997 Elsevier Science B.V.

Section snippets

Introduction: memory search and the possible meaning of thalamo–cortical feedback loops

One of the crucial issues in memory research is based on the question of how a search process finds the relevant information in memory. Whereas most researchers would agree that (parts of) the cortical neural network can be considered a storage network for memory, little is known about the way a search process is initiated and how information is accessed and retrieved. Memory theories assume that a search process can be described as a spreading activation process in the cortical storage network

Interindividual differences in alpha frequency and memory performance

In a first step, before testing this prediction in detail, it is important to show whether alpha frequency does vary interindividually and what the factors are that affect alpha frequency. It is well known that alpha frequency decreases with increasing age. Köpruner et al. (1984) have found a linear relationship (alpha peak frequency=11.95–0.053× age) within the age range of adult subjects that is depicted in Fig. 2.

According to this relationship, a young adult of, e.g. 20 years has an expected

Intraindividual differences in alpha frequency and memory performance

Although Experiments 1 and 2 support the hypothesis of a relationship between alpha frequency and memory performance, the results, reported so far, could theoretically also be explained in terms of interindividual differences in attention. It could be objected that the higher IAF of good memory performers reflects a state of increased attention which is responsible for an increase in memory performance. There is ample evidence that alpha desynchronization may reflect attention (Mulholland, 1969

Shifts in alpha power and memory performance

Amplitude analyses in an individually defined lower and upper alpha band will be an important additional measure for studying cognitive processes and memory in particular. Topographical as well as interindividual differences in EEG power are very large. Thus, an adequate way of measuring event-related shifts in EEG power should not be based on absolute but instead on relative differences as was first proposed by Pfurtscheller and Aranibar (1977) who have coined the term `event-related

Discussion

The most general conclusion is that alpha frequency is a sensitive measure for cognitive performance or cognitive processing capacity. Alpha frequency varies inter- and intraindividually as a function of processing capacity. The decrease in alpha frequency with increasing age and the lowered alpha frequency as a result of a variety of neurological disorders are well-known examples underlining the validity of a general relationship between alpha activity and cognitive performance.

Memory and

Acknowledgements

This research was supported by the Austrian `Fonds zur Förderung der wissenschaftlichen Forschung', Project P-10235.

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