Study Design and Procedure
After signing the informed consent and completing the questionnaires, participants were seated in a reclining chair in a separate soundproof EEG room, where the EEG and SCR electrodes were applied. The seat was positioned 1.5 meters away from a 22-inch iiyama HM204DT-A computer screen with 120 Hz refresh rate. Participants were instructed to remain as motionless as possible during the experiment and were informed that they will see two faces but only one will sometimes be followed by a loud scream. The experiment included three phases: Habituation, Acquisition, Generalization. In the habituation phase, participants saw the CS+ and CS- for 10 times each, without the US (see Figure 1). In the Acquisition phase, each CS was presented 15 times and only the CS+ was followed by the US at the offset of the CS+. The reinforcement rate differed between groups (LU: 80%, MU: 60%, HU: 40%). The following Generalization phase was identical for all the groups, in which all six faces were presented (15 times each). That means the four GSs and the CSs were presented 15 times each. The CS+ was reinforced 20% of the time to minimize extinction (Lissek et al., 2008). All faces flickered at 15 Hz for 5 s, to evoke ssVEPs. All stimuli were presented in a pseudorandomized order so that the same faces were not presented more than twice in a row and that the Acquisition phase always started with a CS+ presentation for all groups. Each stimulus was presented for 5 s and the US appeared at the CS+ offset . The Inter-Trial Interval (ITI) ranged from 9 to 10 s.
Valence and arousal ratings were measured at the end of each phase, US-expectancy was measured at three time points: half-way through Acquisition, at the end of Acquisition, and at the end of Generalization. Discrimination of stimuli was tested only at the end of the Generalization phase to avoid priming the participants about the number of different faces presented. The whole task lasted approximately 45 mins.