Working memory task
The N-back task is a frequently used working memory task in which participants have to determine if the current trial has been presented. The ‘N’ refers to the position of the previous trial that participants have to remember and actively update with every trial. In our study, we used a 2-back task with letters i.e., the target letter was two trials before the current trial. A total of 120 trials were presented, each lasting for 2 s, and the whole task took 4 min in total. In each trial, a letter appeared on the screen for 0.5 s, followed by a black screen appeared for the remaining 1.5 s. Participants had to decide, whether the letter just presented was the same as 2 trials before. If they thought it was the same letter, they were asked to press a corresponding button, and if not, no action was required. They could react during the 2 s of trial duration, in which participants had to make a response. Out of 120 trials, 40 were matching target trials. The N-back task was performed on Day 1 and after the 3 days of stimulation training on Day 5 of the experiment, each time before the arithmetic task and MRI assessment took place.