Fast modulation of alpha activity during visual processing and motor control

Whereas some studies suggest that alpha rhythm promotes information processing in the human cortex (processing hypothesis), other studies suggest its involvement in an active cortical idling which prevents the interfering action of irrelevant information (idling hypothesis). In this study, this apparent contradiction was analyzed using a computing procedure which distinguishes phase-locked and non-phase-locked alpha response during the execution of a complex event-related visual-motor task. The electroencephalographies (EEGs) of 12 male volunteers were digitized (128 electrodes), band-pass filtered for isolating α wave, and event-related averaged during the execution of the visual-motor tasks. This procedure showed a phase-locked α response to stimuli and suppressed the non-phase-locked response. When EEG envelope of the α-wave was computed (Hilbert transformation) before the event-related average, the response of the alpha amplitude to stimuli was observed while the phase-locked α response vanished. Visual stimuli induced a short-lasting increase of phase-locked alpha activity and a long-lasting decrease of non-phase-locked alpha activity whose latency and amplitude changed with the cortical region (visual vs. parietal vs. frontal cortex), with the sensory-semantic information of visual stimuli, and with the tasks associated to them (comparing the alpha response to stimuli which were used for a visual-motor tasks with those passively observed). Alpha sub-bands around the individual alpha frequency peak showed a different phase-locked response. Finally, two early evoked potentials (C1-P1) showed a time latency similar to that computed for the phase-locked alpha response, suggesting that early evoked potentials are modified by the superposition of this alpha activity. Present data suggest that alpha activity promotes the cortical processing of information by increasing the phase-locked alpha activity and by decreasing the non-phase-locked alpha activity, and disturbs cortical processing under basal conditions when it is not phase-locked to any particular task.