Reactivity of magnetic parieto-occipital alpha rhythm during visual imagery.

Spontaneous MEG signals were recorded during visual imagery from 13 healthy adults with a whole-scalp neuromagnetometer. The parieto-occipital 7-14 Hz alpha activity was suppressed strongly while subjects visualized and evaluated letters. The act of forming a visual image caused a smaller suppression than did inspection of the imaged pattern for a named property. The maximum suppression depended on the baseline alpha level and, for the majority of the subjects, occurred close to the area with the strongest alpha, showing no systematic hemispheric asymmetry. Sources for the alpha activity, modeled with equivalent current dipoles, clustered in the parietal and occipital lobes. The strongest suppression of the activity occurred near the parieto-occipital sulcus.