EEG activity of 8 male volunteers was monopolarly recorded at P3 and P4 during four resting periods and during the performance of three series of cognitive tasks: one verbal, one spatial and one demanding verbal and spatial processing or "mixed" task. Beta, alpha and theta relative power were compared between successful and unsuccessful trials, between start and end of performance interval and among resting periods and tasks. There were no significant differences between successful and unsuccessful trials, nor between start and ending of performance period. The effect of tasks and hemispheres on relative power showed different results for each band: beta was responsive to hemispheres while alpha and theta were sensitive to tasks; beta relative power was significantly higher in the left parietal and the same pattern of asymmetry was maintained during the three series of tasks; alpha relative power decreased and theta increased significantly during the three series of tasks regardless of their cognitive nature as compared to baseline.