224 an effect size of η2= 0.17, alpha error probability set at α = 0.05, and desired power set at 0.9, the sample size indicated 22 participants per pharmacological group. Given the potential for dropout and artefacts in imaging data, we recruited 25 participants per group in this study. Inclusion criteria were: age between 18 and 35 years, a good command of the English language, and (corrected to) normal vision and hearing. Exclusion criteria were any history of chronic pain, serious medical or psychiatric conditions, experiencing pain on the day of the study or use of analgesic medication in the 24 hours prior to testing, use of psychotropic drugs in the month prior to testing, and being pregnant or breastfeeding. A physician performed a brief health screening based on our exclusion criteria and to assess vital signs. Participants also needed to be eligible to undergo MRI and were screened for standard MRIcompatibility exclusion criteria. Participants were recruited via the recruitment website Sona (Sona Systems, Tallinn, Estonia). All participants signed written informed consent and were reimbursed with a 90-euro payment. Thermal pain stimulation Thermal pain stimuli were delivered to participants’ right volar forearm and pain intensities were rated on a numeric rating scale (NRS) ranging from 0 (no pain) to 10 (worst pain imaginable on the arm). In the screening part, pain stimuli were delivered via a Thermal Sensory Analyzer with a 3×3 cm thermode probe (TSA-II; Medoc Advanced Medical Systems, Ramat Yishai, Israel). In the MRI part, pain was delivered with an MR-compatible ATS 3x3 thermode attached to a Pathway device (Medoc Advanced Medical Systems, Israel).
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