Experimental design. Top: After mounting EEG and tDCS electrodes, the experiment started with a 15-minute Sham-tDCS. For the next 10 minutes, participants performed a letter two-back working memory (WM) task while EEG was recorded. Subsequently, a 15 minute active-tDCS (anodal or cathodal) was applied, followed by a further 10-minute WM task. EEG recording was stopped during the stimulation periods. Each participant performed one anodal and one cathodal tDCS session separated by at least one day. The session order was counterbalanced across participants. An additional control group underwent one experimental session performing the similar WM task twice but without receiving a treatment during the 15 minute break in between. Bottom: Schematic description of the letter two-back working memory task.