EEG alpha distinguishes between cuneal and precuneal activation in working memory

In the literature on EEG during working memory (WM), the role of alpha power (8-13 Hz) during WM retention has remained unclear. We recorded EEG while 18 subjects retained sets of consonants in memory for 3 s; setsize (ss4, ss6, ss8) determines memory workload. Theta power (4-8 Hz) increased with workload in all subjects in middle frontal electrodes. Using ICA, the increase in theta could be attributed to one component whose generators were localized by sLORETA in the medial frontal gyrus. Alpha power in parietal electrode Pz showed a mean increase during retention as compared to prestimulus fixation (event-related synchronization, ERS). On an individual basis, alpha power increased with workload in 9 subjects (WL+ group) and decreased in 9 subjects (WL- group). The alpha increased in upper alpha for the WL+ group (mean: 10.4 Hz) and decreased in lower alpha for the WL- group (mean: 8.9 Hz). Time-frequency representations show high alpha power early during retention for the WL+ group and high alpha power late during retention for the WL- group. sLORETA revealed maximal contrast for the WL+ group in the cuneus and for the WL- group in the precuneus. In subjects with WL+, alpha increase in the cuneus may reflect WM maintenance or active inhibition of task-irrelevant areas. In subjects with WL-, alpha decrease in the precuneus may reflect release of inhibition associated with attentional demands. Thus, alpha EEG characterizes two aspects of processing in the same WM task.