Stimulation parameters
skin sensations
PHOSPHENES
Theta tACS stimulation during the arithmetic training was delivered for 25 min using a battery-driven NeuroConn (NeuroConn, Germany) DC-stimulator Plus. The target electrode was a 3 × 3 cm rubber electrode placed at the F3 location (based on the 10–20 system) targeting the L-DLPFC. The return electrode was a 7 × 5 cm rubber electrode placed on the left shoulder. We also adjusted the stimulation intensity and the simulation frequency for each subject. Stimulation intensity was adjusted with a stepwise increase of 0.25 mA starting at 1 mA until either the participants reported skin sensations or phosphenes or the current reached 1.5 mA. If skin sensations or phosphenes were reported at or below 1 mA, the current intensity was reduced by 0.1 mA until no sensations were reported. We used a similar procedure for the sham group, but the stimulation lasted only 30 s. Theta frequency was determined based on resting-state IAF assessment (IAF minus 5 Hz) before the arithmetic learning session. This was based on the calculation of individual frequency bands proposed by (Klimesch The mean and standard deviations of the stimulation intensities and frequencies assessed for the different groups on the three stimulation days are shown