The relation between alpha and theta oscillations in memory tasks

Klimesch, W.1, and Schack, B.2
1Institute of Psychology, University of Salzburg, Austria; 2Institute of Medical Statistics, Computer Sciences and Documentation, University Jena, Germany

Converging evidence from the human EEG as well as animal research indicates that theta oscillations are associated with working memory processes. We discuss arguments and present data suggesting that alpha oscillations also play an important role during short-term memory retention and retrieval. Three different findings are of importance. First, for the retention period of memory scanning tasks an interesting similarity between the reactivity of theta and upper alpha oscillations with respect to memory load was found. Second, increased evoked upper alpha activity was found over parietal regions during retrieval from short-term memory storage. Third, during attempts to retrieve information from long-term memory, evoked theta oscillations spread from anterior to posterior recording sites and when information actually is retrieved, the direction reverses and theta spreads to frontal sites. Most interestingly, this time point - when direction reverses - varies between subjects to a large extent but is significantly correlated with memory performance and the onset of upper alpha desynchronization. Forth, in a recent experiment we found significant theta : upper alpha phase synchronization during retention and retrieval. These findings suggest that both frequencies play an important role for working memory an that a specific 'co-activation' between theta and upper alpha can be observed when information stored in short- or long-term memory is retrieved or processed in working memory. Thereby, upper alpha oscillations may reflect the reactivation of long-term memory codes in working memory.