The main experimental task consisted of 76 trials (see Fig. 1). On each trial, a 1.5-s predictive cue was presented and followed by an interstimulus interval (ISI) with a fixation cross at the screen center. In experiment 1, this ISI had a fixed duration of 2 s, whereas in experiment 2, the ISI duration was jittered, ranging from 1.75 to 6.25 s (average 4 s) with an incremental step of 0.25 s, to accommodate the standard requirements of MRI research. Next, the instruction “breathe out” was presented together with a numerical 3-s countdown. When the countdown reached 0, participants had to breathe in evenly while the “breathe in” instruction was presented and the olfactory/thermal stimulus delivered. These breathing instructions ensure that the stimuli were synchronized with the inspiration cycle and stabilized intra- and interparticipant breathing pattern variability (18, 23). Olfactory stimuli lasted 2 s (experiment 1) or 3 s (experiment 2; the duration of the olfactory stimulus was longer in experiment 2 following pilot testing in the MRI setting for the olfactometer). Thermal stimuli always lasted 2 s, although additional 3 s was necessary for the thermal stimulator to reach the target temperature. After stimulation, participants rated its unpleasantness on a VAS with their right hands using the appropriate response keys. The VAS remained onscreen until a response was delivered for a maximum of 6 s. The scale was followed by an intertrial interval (ITI) with a fixation cross at the center of the screen [experiment 1: duration, 4 s; experiment 2: duration, ranging from 1.75 to 6.25 s (average 4 s) with an incremental step of 0.25 s].

In this paradigm, the four stimulus conditions (HP, LP, HD, and LD) were presented following their corresponding cues, which were schematic representations of either a smelly sock or a flame (see Fig. 1). Thus, each cue was always correctly predictive of the upcoming stimulation. Furthermore, in 32 of the 76 trials (50%), a dilemma was presented between the cue and the stimulus. For each participant, the 32 dilemmas (16 moral versus 16 nonmoral, selected from the pilot results) were randomly associated with each of the four cue/stimulus conditions, to minimize putative idiosyncratic confounds of the vignettes. This yielded eight balanced conditions of interest in which moral and nonmoral dilemmas were preceded by each of the four possible cues. For experiment 2 only, a graphical representation of the previously presented cue was also displayed on the top-left corner of the screen during the presentation of the dilemmas (this was performed to enhance any expectancy effect and minimize distraction due to a noisy/stressful environment such as the MRI). The dilemma remained on screen until participants pressed a key, for a maximum duration of 60 s. Subsequently, participants rated how much a course of action associated with the story was appropriate on a VAS ranging from −50 (extremely inappropriate) to +50 (extremely appropriate). The VAS remained on screen until a response was delivered for a maximum of 10 s.

The experimental structure is fully described in Fig. 1. Participants received 32 reference trials in which cues were followed directly by the predicted stimuli (eight trials HP, eight trials LP, eight trials HD, and eight trials LD) and 32 trials in which dilemmas were presented between the cues and the predicted stimuli according to the eight conditions described above. These 32 postdilemma trials were the main objective of the experiment. Last, these 64 trials (32 reference trials + 32 postdilemma trials) were intermingled with 12 trials in which the positive odor was administered following a corresponding cue (schematic flower).

Experiment 1 was organized in one unique block of about 40 min, in which all 76 trials were presented in random order. Experiment 2 was instead split into four independent blocks, each lasting about 12 min and comprising one-fourth (19) of the overall trials, to minimize potential movement artifacts and signal drop in MRI data. The experiments were all run using Cogent 2000 (Wellcome Department, London, UK), as implemented in MATLAB R2015b (MathWorks, Natick, MA).

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