Parieto-occipital sources account for the increase in alpha activity with working memory load

The role of oscillatory alpha activity (8-13 Hz) in cognitive processing remains an open question. It has been debated whether alpha activity plays a direct role in the neuronal processing required for a given task or whether it reflects idling and/or functional inhibition. Recent electroencephalography (EEG) studies have demonstrated that alpha activity increases parametrically with load during retention in working memory paradigms. While it is known that the parieto-occipital cortex is involved in the generation of the spontaneous alpha oscillations, it remains unknown where the sources of the memory-dependent alpha activity are located. We recorded brain activity using magnetoencephalography (MEG) from human subjects performing a Sternberg memory task where faces were used as stimuli. Spectral analysis revealed a parametric increase in alpha activity with memory load over posterior brain areas. We then applied a source reconstruction technique that allowed us to map the parametric increase in alpha activity to the anatomical magnetic resonance (MR) images of the subject. The primary sources of the memory-dependent alpha activity were in the vicinity of the parieto-occipital sulcus. This region is not directly involved in working memory maintenance of faces. Our findings are consistent with the notion that alpha activity reflects disengagement or inhibition of the visual dorsal stream. We propose that the disengagement reflected in alpha power serves to suppress visual input in order to devote resources to structures responsible for working memory maintenance.