[Alpha ERD and human visual selective attention]

We compared the alpha band EEG depression (event-related desynchnization, ERD) level in two tasks, involving activation of different attentional processes: visual search for a deviant relevant stimulus among many similar ones and visual oddball. Control data for the visual search task consisted of simple viewing of several stimuli being of the same shape as the relevant stimulus in the search trials. Gaze position was verified by eye tracking method. We interpreted alpha band ERD as a correlate of activation of attentional processes. Fixating the target in visual search task caused a significantly larger ERD than fixating the same stimuli in control trials over all leads. We suppose this to be related with task and visual environment complexities. The frontal ERD domination may indicate attentional control over voluntary movements execution (top-down attention). The caudal ERD may be related with updating of visual information as a result of search process (bottom-up attention). Both relevant and irrelevant stimuli in the oddball task also induced alpha band ERD, but it was larger in response to relevant one and reached maximum level over occipital leads. Domination of caudal ERD in oddball task is supposed to indicate bottom-up attention processes.