a) Montage of 22-channel EEG (bandpass filtered: 0.3 – 120 Hz, sampling rate: 512 Hz) and sites of tACS electrode placement (a, b, c, d). tACS electrodes a and b, as well as c and d were connected pair-wise. The AC current flowed mainly sagitally between a and c, respectively b and d. The combined impedances of the paired tACS electrodes were kept below 5 kΩ. EEG electrodes were referenced to Cz instead of mastoids because of the close proximity of the latter to the tACS electrodes c and d. b) Measured scalp surface potentials applied during one arbitrarily chosen maximum of a 40 Hz sinusoidal tACS stimulation in a single subject, with all polarities reverting every 12.5 milliseconds (voltage scale given in relative units). See Online Methods for details. c) Mathematically derived dura potential (CSD estimate). The dura potential indicates the effect of the stimulation current entering the skull, spreading from fronto-temporal to parieto-occipital regions, as well.