Figure 1. Schematic of the experimental procedures.

The three tasks were based on a discrimination fear conditioning paradigm with partial reinforcement. The aversive outcome was a mild electric shock to the wrist (US, unconditioned stimulus). The conditioned stimuli were colored squares (in extinction and regulation) or faces (in reversal). For discrimination, one specific stimulus (e.g., a yellow square) was designated as the conditioned stimulus (CS+) and was paired with the shock on about 30% of the trials, whereas the other stimulus (e.g., a blue square) was never paired with the shock (CS−). In extinction, the conditioning session was followed by two extinction sessions (one immediately after and the other 24 hours later) consisted of repeated non-reinforced presentations of the CS+ and CS−. In reversal, the conditioning session was immediately followed by an identical conditioning session only with reversed reinforcement contingencies, such that the stimuli designated as CS+ and CS− flipped roles. In regulation, the conditioning trials were interleaved with the regulation trials. Before each trial, subjects were instructed to either attend (“Try to focus on your natural feelings”) or to regulate (“Try to think of something calming in nature”). The index of fear was skin conductance responses (SCR) detected by two electrodes attached to the first and second fingers. In all tasks, the stimuli were presented for 4 sec and the inter-trial-interval was 12 sec. The US lasted 200 msec co-terminating with the conditioned stimulus. Each trial type was typically presented 12-16 times.