tACS electrodes (two conductive rubber 4 × 4 cm) were attached over standardized Cz and Oz channel locations (Jasper, 1958 (link); Herwig et al., 2003 (link)) underneath the EEG recording cap and the sinusoidal alternating current was applied at IAF (calculated according to the procedure described above) using a battery-driven stimulator (DC-Stimulator Plus, NeuroConn, Ilmenau, Germany). Stimulation electrode positions were selected based on previous studies, in which modulations of visual alpha oscillations by tACS were reported (Neuling et al., 2013 (link); Helfrich et al., 2014 (link); Ruhnau et al., 2016 (link)). Impedances were kept below 10 kΩ with Ten20 conductive paste (Weaver and Company, Aurora, CO, USA). tACS at IAF was applied with an intensity of 1 mA (peak-to-peak) for all subjects. The stimulation signal for every trial was initially determined in a closed-loop application (custom made, C++), then generated through NI DAQ card (USB 6343, National Instruments, TX, USA) and transmitted to the DC-Stimulator Plus “remote input” port.
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