During tACS, an alternating electrical current with a 1 milliamp peak-to-peak amplitude will be delivered, featuring a 15-second ramp-up at the start and a 15-second ramp-down at the end. The stimulation duration is 20 minutes and four channels are used for delivering sinusoidal alternating currents to the head. The alternating current was delivered to the prefrontal cortex bilaterally with the electrodes at F3 and F4 and the return electrodes were placed at the ipsilateral mastoids at TP9 and TP10 (international 10-20 EEG electrode placement system).

The intermittent theta burst stimulation (iTBS) is regarded as safe for high-frequency neuromodulation. The pulse bursts are applied for two seconds in 5Hz intervals followed by an eight-second break before the next bursts are applied. This pattern is repeated for 190 seconds resulting in 600 pulses per protocol. 80% of the individual subjects' active motor threshold (AMT) are used for stimulation intensity and will be applied four times within a 20-minute time frame starting at 0 minutes, 5 minutes, 10 minutes, and 15 minutes from the beginning of the intervention resulting in 2400 delivered pulses for the whole intervention

To combine tACS and the iTBS protocol, a customized circuit is developed in-house to phase-lock the TMS pulses to the waveform generated by tACS. TMS pulses were delivered using two figure-of-eight shaped coils (double 70 mm, PMD coil type; Mag and More GmbH, Munich, Germany). For the stimulation, the center of the TMS coils are placed over the two prefrontal tACS electrodes (F3 and F4 electrode positions) with an orientation of 45 degrees to the midline and the handles pointing backward. Using the customized circuit, TMS pulses will be delivered at the peak of the tACS wave.

Transcranial alternating current was delivered utilizing a Starstim 8-channel constant current, battery-powered electric stimulator (Neuroelectrics, Barcelona, Spain). Circular carbon rubber electrodes (2cm radius, 12,57 cm2) were used throughout the stimulation. Before placing the electrodes on the head, the subject's hair was moved to the side

Participants view a continuous sequence of alphabet letters, presented as stimuli in Arial font, size 120pt, on a tube screen positioned at 60cm eye distance. The stimuli are shown with a time interval of two seconds between them. The task consisted of six blocks with 70 stimuli, including 12 possible hit trials. Participants were instructed to press the left key on a response box if the presented stimulus matched either the one presented one trial prior (1-Back) or three trials prior (3-Back). In all other cases, participants press the right key. The left button is pressed with the index finger, while the right button is pressed with the middle finger of the right hand, with buttons aligned horizontally. The response condition varied by block, with three blocks for 3-Back and three for 1-Back, and the order was counterbalanced across participants and sessions.

Change in the EEG power in alpha, delta, theta and beta bands

Change in EEG functional connectivity, specifically global efficiency, coherence, and phase-locking value.