EEG alpha synchronization and functional coupling during top-down processing in a working memory task

Electroencephalogram (EEG) alpha (similar to 10 Hz) is the dominant rhythm in the human brain during conditions of mental inactivity. High amplitudes as observed during rest usually diminish during cognitive effort. During retention of information in working memory, however, power increase of a oscillations can be observed. This a synchronization has been interpreted as cortical idling or active inhibition. The present study provides evidence that during top-down processing in a working memory task, a power increases, at prefrontal but decreases at occipital electrode sites, thereby reaching a state in which a power and frequency become very similar over large distances. Two experimental conditions were compared. In the first, visuospatial information only had to be retained in memory whereas the second condition additionally demanded manipulation of the information. During the second condition, stronger a synchronization at prefrontal sites and larger occipital a suppression was observed as compared to that for pure retention. This effect was accompanied by assimilation of prefrontal and occipital a frequency, stronger functional coupling between prefrontal and occipital brain areas, and a latency shifts from prefrontal cortex to primary visual areas, possibly indicating the control of posterior cortical activation by anterior brain areas. An increase of prefrontal EEG a amplitudes, which is accompanied by a decrease at posterior sites, thus may not be interpreted in terms of idling or "global" inhibition but may enable a tight functional coupling between prefrontal cortical areas, and thereby allows the control of the execution of processes in primary visual brain regions.