Functional topography during sensorimotor activation studied with event-related desynchronization mapping.

Event-related desynchronization (ERD) of alpha components was studied in four subjects during self-paced voluntary movements and somatosensory stimulation. EEG was recorded from 29 electrodes with a common right ear reference; local average reference derivations were calculated and used for computing ERD maps at 250-ms intervals. It was found that upper alpha components were desynchronized close to electrodes C3 and C4 overlying the primary sensorimotor cortex during voluntary movements and somatosensory stimulation. ERD started contralaterally more than 2 s before the movement and some hundreds of milliseconds before the stimulation. During the performance of unilateral movement and unilateral stimulation, ERD was bilaterally symmetric. Desynchronization of lower alpha components was found over the superior parietal cortex, clearly starting some hundreds of milliseconds after the desynchronization of upper frequency components. Therefore, the desynchronization of lower and upper alpha components reflects activation of different neuronal structures.