Magnetic source localization agrees with electric source localization of the auditory P300 [34]. Recently, spatially filtered MEG has made it possible to detect and localize sources of task-related changes in the on-going oscillatory activity involved in auditory attention and memory [35]. The physiological mechanisms underlying alpha synchronization and desynchronization reflect a complex pattern of microscale synchronization during information processing in the brain [36]. A phasic, i.e. short, local desynchronization is present during sensory stimulation and also preceding the imperative signal and motor response including motor preactivation in the contingent negative variation paradigm. ERD is not seen in comatose patients but is a phenomenon of the conscious state of the brain, related to preactivation or priming of the cortical areas needed for a specific task, sensory and cognitive processing, and motor behaviour [36-40]. ERD has an electrophysiological correlation with cortical activation related to stimulus processing or motor output, and is characteristic of cortical areas or neural structures preparing to process sensory information or ready and prepared to execute a motor command [40, 41].
