Figure 1.

Study design

All participants underwent three phases of the experiment: habituation, acquisition, and extinction. During the habituation and extinction phases, both discriminative stimuli (DS_pain and DS_money, blue and yellow arrows on black background, respectively, duration: 6 s) were presented eight times each and followed by an inter-trial interval of 8–10 s (random jitter). During acquisition, twenty DS of each type were presented for 6 s and followed by an anticipation phase of 4 s. In the monetary reward condition, a monetary reward (1 €) or no reward (fixation cross) was subsequently presented for 3 s. In the pain-relief condition, the anticipation phase was always followed by a painful stimulus lasting 1.5 s. During the feedback phase of the pain-relief condition, participants received either pain relief (no pain) or pain (pain for 1.5 s). Participants were instructed to press a button with their index finger as soon as they saw any of the two discriminative stimuli, with the goal of winning a monetary reward or pain relief if they pressed the button “fast enough.” In reality, both rewards were delivered in 50% of the trials. Participants never received rewards if they did not press the button. The feedback phase was followed by an inter-trial interval of 8–10 s (random jitter). Ratings of the contingency between each DS and the rewards (numerical rating scale from 1 to 9) and the arousal and valence associated with each DS (self-assessment manikins converted to a scale from 1 to 9) were presented after the habituation, acquisition, and extinction phases.