Figure 1.

Experimental procedure. (A) Time course of the current experiment. First 90 s of eyes-closed EEG were acquired to determine participants’ individual alpha frequency (IAF) which was used as stimulation frequency in the subsequent steps. Next, stimulation intensity was adjusted to the individual sensation threshold. During the following 113 min participants performed a visual vigilance task (indicated in blue) while 3 min of baseline EEG was measured followed by 20 min of tACS or sham stimulation and 90 min post-stimulation EEG. (B) Electrode setup. Stimulation electrodes were placed above Cz (5 × 7 cm) and Oz (4 × 4 cm) following the international 10–20 system. Additionally 10 EEG electrodes were positioned over five frontal and five parietal sides. (C) Visual vigilance task. Participants fixated a white cross at the center of a computer screen. Every 30–40 s the cross was rotated by 45° for a duration of 500 ms. Participants were given 2 s to manually respond to the rotation and received 0.05 € for each detected target. A total of 191 targets were presented during the experiment.