Task and stimuli
pain
Fear acquisition training comprised 40 trials (20 CS+, 20 CS−), and the fear recall test 30 trials (15 CS+, 15 CS−). Each CS was followed by a 15-s trace interval. During acquisition training, US was presented after the trace interval (15 s after CS offset) in all CS+ trials (100% reinforcement; CS were two differently colored [yellow (RGB: 225, 224, 177) and purple (RGB: 238, 194, 244)] isoluminant triangles, presented for 2 s at the center of an isoluminant gray (RGB: 175, 175, 175) computer screen at a visual angle of ∼4.1°. Association of CS+/CS− to CS color was counterbalanced across participants. As an identification task, participants were asked to indicate the color of the CS by pressing the left/right cursor keys during CS presentation on a standard computer keyboard. If participants gave the wrong or no response, the words “wrong key” and “no response,” respectively, were presented immediately after CS offset. During the 15-s trace interval, a white (RGB: 255, 255, 255) fixation cross was presented at the center of the gray background screen at a visual angle of ∼0.8°.US consisted of a sequence of 83 square electric pulses of 0.2-ms duration with a duty cycle of 1.67%, summing up to a total duration of 1000-ms. US were delivered to the participants’ dominant forearm via a pin-cathode/ring anode configuration. Electric pulses were generated by a constant current stimulator (Digitimer DS7A, Digitimer). Intensity of the US was set to a perceived intensity between 80–90% of the lowest clearly painful stimulus. US intensity was estimated in three phases. In the first phase, US intensity was increased until a painful level was firmly reached, marking the upper limit for the second phase, during which 14 US with random intensities were delivered. Participants were asked to rate their subjective pain perception for each of them from 0 to 100. These ratings were then linearly interpolated to estimate a US intensity corresponding to 90% of a clearly painful stimulus. Stimuli with this intensity were once more presented to the participants and adjusted if necessary.During the ITI, a simple visual detection task was presented to keep participants attentive, because previous studies showed that participants become drowsy even with shorter ITIs (15 s; In the recall session, white noise startle probes of 20 ms duration, instantaneous rise time, and 102-dB loudness, were delivered binaurally via headphones (HD 202, Sennheiser). The experiment was conducted in a dark, soundproof chamber. The experimental task was presented on a Dell P2014h 20-inch screen, set to an aspect ratio of 4:3 at 60 Hz, with a resolution of 1152 × 864 pixels. Participants’ heads were positioned with a chin rest at 70 cm distance from the monitor and 47 cm from the eye tracker.