We identified for each participant two odorants (expected to elicit HD and LD) and two thermal stimulations (expected to elicit HP and LP). Odorants arose from vials containing isovaleric acid or sclarymol at different concentrations and were delivered to the subjects’ nostrils by means of rubber cannulas connected to a computer-controlled, multichannel, custom-built olfactometer. Thermal stimuli were delivered through a computer controlled thermal stimulator with an MRI-compatible 25 × 50 mm fluid-cooled Peltier probe (MSA, Thermotest), attached to participants’ legs. Odorants and temperatures were selected on a participant-by-participant basis from preexperimental sessions. In particular, participants first underwent two random presentations of 13 olfactory stimulations, comprehending isovaleric acid and sclarymol (embedded in a solution of odorless dipropylene glycol at five different concentrations: 0.1, 0.5, 1, 5, and 10%), two positive odorants (shampoo and lavender, at 10%) and an odorless control (dipropylene glycol alone). Participants provided unpleasantness ratings on a visual analog scale (VAS) ranging from 50 (extremely unpleasant) to −50 (extremely pleasant). This allowed the selection of two main odorants of interest, one unpleasant (~40 of unpleasantness) and one neutral (~5), plus a positive stimulation of no interest (<0), used only to give participants relief from the aversive stimuli and to reduce the occurrence of putative habituation or sensitization effects. Subsequently, thermal stimulations were selected through a modified double random staircase session aimed at identifying a temperature whose unpleasantness was the closest possible to that of the previously identified negative odorant. This approach led to a highly unpleasant temperature, which averaged across participants at 48.87°C (SD = 1.50) for experiment 1 and 47.15°C (SD = 1.51) for experiment 2. On the basis of this temperature, an additional neutral stimulus was selected, corresponding to an average value of 46.81°C (SD = 1.61) for experiment 1 and 44.68°C (SD = 2.01) for experiment 2. See previous studies for detailed description of the selection procedure (18, 23).

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