Particular care was taken to ensure that participants understood all aspects of the experiment. To this end, participants were instructed to carefully read detailed instructions and were required to fill out an extensive questionnaire probing their understanding of the experimental procedures. The accuracy of each participant’s answers was confirmed by the experimenters and discussed in a brief interview that lasted for ca. 10 min. Participants were then placed inside the scanner for a brief practice session consisting of 12 trials to ensure that they could view all stimuli, perform the task, make decisions in the allotted 5.5 s per trial, understood the experimental setup, and were subsequently given the opportunity to ask further questions.

After completion of practice, participants were taken out of the scanner and washed their hands before placement of SCR and stimulation electrodes. Participants were then placed inside the scanner, and two ring electrodes were attached to the dorsum of the left hand: (i) The electrode providing relatively higher intensity stimulation was placed between 1 and 2 cm below the second carpometacarpal joint, and (ii) the electrode providing relatively lower intensity stimulation was placed 1 to 2 cm below the fifth carpometacarpal joint. To determine individual thresholds for high- and low-intensity stimulation, we followed a standard procedure (50) and used a visual analog rating scale with end points defined as 0 = “cannot feel anything” and 10 = “maximum tolerable pain.” Tactile stimulation was delivered via two Digitimer DS5 isolated bipolar constant current stimulators (bipolar constant current, 5 V, 50 mA; Digitimer Ltd., Welwyn Garden City, UK) and a custom-made fMRI compatible 5-mm ring electrode, which delivered a maximally focused and centered tactile stimulus. By varying current amplitude between 1 and 99% of the maximum amperage, stimuli with varying intensity levels were repeatedly delivered to each participant until stable ratings were achieved at least three times according to the following criteria: between 1 and 2 for the low-intensity stimulus and between 8 and 9 for the high-intensity stimulus. Visual and tactile stimulus presentation, as well as recording of responses, was controlled by Cogent 2000 (www.vislab.ucl.ac.uk/cogent.php).

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