Event-related desynchronization (ERD) of alpha components was studied in 10 subjects in a visuoverbal judgment task. EEG was recorded from 29 electrodes: ERD. measured by percentage of alpha power decrease, was calculated in 125-ms intervals and displayed in a time course over 7-s intervals and in the form of topographical ERD maps. ERD. interpreted as a measure of activated cortical areas, was studied in the upper and lower alpha bands. It was found that upper alpha band ERD was short-lasting (<1 s). localized to the posterior region and not found before stimulation; in contrast to this, lower alpha band ERD was longer-lasting (> 1 s). more widespread, with a left hemisphere dominance. and was already present before stimulation. ANOVA revealed significant interactions among alpha frequency bands, scalp location, and pre-/poststimulus intervals. The self-organizing topological Kohonen network was used to analyze the stability of consecutive ERD maps. There is evidence that topographical ERD patterns can be stable for about 300–400 ms.