Fig. 1: Schematic structure of the experimental design and an experimental trial.

a Experimental structure. The first day started with a preacquisition phase, where CS+ and CS− were presented twice without any US. Next, the US-intensity was adjusted to be perceived as unpleasant, but not painful by the participant (shock workup). During the following acquisition, first (extinction 1) and second half of extinction training (extinction 2), CS+ and CS− were presented 10 times, each. During acquisition, 6 CS+ trials were paired with the US, while no US was presented during the CS− or during extinction. The second experimental session followed 24 h later and began with an extinction recall phase, where CS+ and CS− were each presented 10 times without any US. A following return of fear test started with a reinstatement procedure, during which the background color of the monitor changed to white and three non-signaled USs were administered. Then, CS+ and CS− were each presented 10 times without the US. TDCS electrodes were always attached aside from preacquisition and shock workup, but electrical currents were only applied during extinction 1 (sham stimulation) and extinction recall (tDCS vs. sham) to minimize contextual effects. b Trial structure. A prompting slide required participants to rate the expectancy (in percent range 0–100) to receive a shock during an upcoming CS presentation (English translation: “Next, this picture will follow. How likely do you think is it, to receive an electrical shock during the upcoming presentation of this picture?”). After a three-second post-rating interval, the CS was presented full-size, followed by an inter-trial interval (ITI). Startle probes were administered during CSs and ITIs.