FIGURE 1.

Experimental procedure and paradigm. (A) The experiment started with a short training period. Next, participants conducted the task (pre, see B), followed by a 10 min break. Next, participants conducted the task again, now with additional tACS (peri), followed by a 25 min break. Finally, participants conducted the task for the third time, now again without tACS (post), followed by the questionnaire. Participants repeated the entire procedure on three different days. Each day differed only in stimulation frequency of tACS (10, 5 Hz, or sham) during the peri section. Quest., questionnaire. (B) The task used in the pre, peri, and post session (see A) started with a fixation point, which decreased in luminance after 500 ms. This darker fixation point was shown for a jittered period of 500–700 ms. The jittered period was followed by electric stimulation of the left index finger with varying SOAs (0, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 130 ms). After another jittered period of 300–800 ms showing the fixation point, participants were asked to respond whether they perceived one stimulus or two stimuli. Then the next trial started with the brighter fixation point. (C) Electrode placement. S1 was determined by neuronavigation. M1 was determined by the strongest FDI response when TMS was applied. Starting from M1, we applied TMS in steps of 0.5 cm moving to posterior (dashed line), until FDI response was no longer visible (“no FDI response”). At this spot we placed the most anterior border of the stimulation electrode (red). The reference electrode (blue) was placed on the contralateral forehead.