The tACS was applied via two surface conductive-rubber electrodes (5 × 7 cm) enclosed in saline-soaked sponges (Neuroconn, Ilmenau, Germany) centered at Cz and Oz underneath the EEG recording cap (see Figure 2A ). Stimulation electrode positions were chosen in order to affect the occipital cortex (Figure 1 ). The impedance was kept below 10 kΩ. An alternating, sinusoidal current at the IAF of each participant was applied using a battery-operated stimulator system (Eldith, Neuroconn, Ilmenau, Germany). The intensity of the sinusoidal current was adjusted individually to the highest intensity at which the stimulation was not noticed by the participants. To obtain this threshold, we started with an intensity level of 1500 μA (peak-to-peak). If the subject indicated no skin sensation or phosphene perception, we increased the intensity in steps of 100 μA. As soon as the participant either indicated skin sensation or phosphene perception, we decreased the intensity in steps of 100 μA. Each intensity step was applied for approximately 20 s, without fade-in/out. The obtained threshold level was used as stimulation intensity. The experimental group received 20 min of stimulation. In the beginning, and at the end, the stimulation was faded-in and faded-out for 10 s. In the control group, sham stimulation was applied. While all other stimulation parameters were the same as in the experimental group, the control group received only 30 s of stimulation, a procedure that has been used in previous studies (e.g., Polania et al., 2012 (link)).
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