As shown in Figure 2, the working memory was assessed using a verbal N-back task [5, 31]. A pseudorandom set of 4 digit numbers was displayed on a monitor, and the subjects were instructed to determine whether specific digit one appeared on the screen (0-back task); or the currently displayed number at any given time had been already displayed in the previous presentation (1-back task). Stimuli consisted in a 0.5 sec. Inter-stimulus interval (ISI) was 2.5 sec in all conditions. Subjects had to distinguish between targets and nontargets by pressing a keyboard. Continuous ERP signals were acquired using an EEG/ERP amplifier system (NATION Inc.). For all ERP recordings, 18 electrodes were placed according to the 10–20 international system. The chosen electrode positions were EOG1, EOG2, Fp1, Fp2, F3, F4, F7, F8, Fz, C3, Cz, C4, P3, Pz, P4, O1, Oz, and O2 (see Figure 2). The data were sampled at 256 Hz. Signals were recorded for 120 s during each task. Each task was repeated for three sessions. Each session contained 40 trials with a 1 : 1 target/nontarget relation.
