Figure 1. Study design.

Study design illustrating the conditioning protocol consisting of an acquisition phase (A) during which painful rectal distensions were paired with a visual CS⁺ while a second visual stimulus (CS⁻) was presented without US (differential conditioning using geometric symbols as CSs). A total of 32 CSs were presented (16 CS⁺; 16 CS⁻) in pseudo-randomized order. Twelve out of the 16 CS⁺ were followed by a US (i.e., 75% reinforcement schedule). The onset of the US presentation varied randomly between 7.2 and 12s after CS⁺ onset, and both stimuli were co-terminated (i.e., delay conditioning). In the extinction phase (B), CSs (12 CS⁺; 12 CS⁻) were presented without US. In the reinstatement phase (C), one single unpaired US was delivered during the initial off-phase. Subsequently, only CSs (6 CS⁺; 6CS⁻), were presented. At the end of each phase, online ratings assessing CS unpleasantness were accomplished. In addition, at the end of the acquisition and reinstatement phases, perceived CS-US contingency ratings together with distension-induced pain and current tension ratings were accomplished.