Phase synchronization between theta and upper alpha oscillations in a working memory task.

Motivated by findings that theta and upper alpha oscillations respond selectively to different types of memory demands, we investigated the role of phase synchronization in a memory scanning task. During retention, we found a load dependent increase in upper alpha power at O2 and P4 and a significant upper alpha:theta phase synchronization between right posterior, central and left anterior sites. During retrieval, a load dependent increase in upper alpha phase locking was observed at O2 and an increase in upper alpha:theta phase synchronization between right posterior and left anterior sites. We suggest that theta reflects central executive functions whereas upper alpha may be important for the reactivation of long-term memory codes in short-term memory. The interplay between theta and upper alpha may be reflected by phase synchronization between these frequencies.