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.