Participants
Participants were between 18 to 45 years old without any current or past mental or neuropsychiatric disorder. A psychologist screened all potential participants using the Diagnostic and Statistical Manual of Mental Disorders - version 5 (DSM-5) and the Beck Depression Inventory (BDI) (Beck et al., 1961). Exclusion criteria for the study were as follows: (1) contraindications for the applied techniques, including tDCS and Magnetic Resonance Imaging (MRI), (2) habitual smoking (more than 10 cigarettes per day) or abuse/dependence on other drugs, (3) pregnancy, (4) use of psychoactive drugs, including antidepressant drugs, benzodiazepines, and Z-drugs, and (5) serious clinical conditions. The participants included in the study were contacted from a pool of participants who have previously undergone other studies conducted by our laboratory. Prior to participation, all participants provided written informed consent.
The required sample size for this study was determined by employing a small to moderate effect size of f = 0.25, alpha of 0.05, and a power of 0.80, based on three repeated measurements (G*Power 3.1 software), which generated a required sample size of 28 participants. To account for potential dropouts or data loss, a total of 40 participants were recruited.

Experiment Procedure

The experiment consisted of four separate visits. Firstly, an anatomical 3-Tesla MRI acquisition was performed (T1- and T2-weighted sequences; repetition time (TR) = 1900 milliseconds, echo time (TE) = 2.2 milliseconds, flip angle = 9°, 176 slices/volume, slice thickness = 0.8 mm). These images were posteriorly used for neuronavigation and for electric field modeling analyses. Afterwards, participants underwent three tDCS experiment sessions (Figure 1).
All experiment sessions took place in a well-controlled laboratory environment. Participants were asked to sleep sufficiently, and abstain from intense physical activity and alcohol/caffeine consumption 2 hours prior to the session. Throughout all three sessions, participants were seated in a comfortable chair and positioned their knees at a 90-degree angle. In the first experiment session, a neuronavigation procedure (Brainsight, Rogue Resolutions, Inc) based upon the individual subject’s MRI image was performed to individually determine the targeted DLPFCs. These targets were marked in a cap that was used in the following sessions.
At the beginning of each session, self-report baseline measurements were collected and the physiological equipment to record cardiac activity was set up. Cardiac activity was then collected for 5 minutes at rest (i.e. baseline period). The neuromodulation session started after the end of the baseline period. A 0-back task was performed together with the tDCS to control and standardize ongoing neural activity during tDCS. For the 0-back, random words appeared on a computer screen and the participant had to press a specific letter on the keyboard of the occurrence of a specific word on the screen. Cardiac activity was collected during the entire tDCS session (20 minutes) (Figure 1).
Figure 1. Study Design