Chapter 6 – EEG imaging 203 Results Participants and pain reports Thirty-nine participants were enrolled in this study and underwent calibration, conditioning, and evocation of nocebo hyperalgesia (Fig. 1a, b). Testing of three participants was discontinued: one due to technical difficulties, one for experiencing discomfort and headache during testing, and one for not reporting differences in experienced pain between acquisition control and nocebo trials. A total of 36 participants (25 female) were included in final analyses. Mean warmth detection threshold across participants was 33.7°C (standard deviation; SD = 0.7) and mean pain threshold was 41.9°C (SD = 3.2). Mean temperatures used to induce moderate and high pain were 46.6°C (SD = 0.8) and 48.1°C (SD = 0.5), respectively. At baseline, mean NRS pain rating for control trials was 4.4 (SD = 1.7), while mean pain rating for nocebo trials was 7.4 (SD = 1.2). During nocebo acquisition, mean pain rating for control trials was 3.9 (SD = 1.8) and mean pain rating for nocebo trials was 7.3 (SD = 1.4). Conditioning of pain during the acquisition phase successfully induced negative associations with the gel and evoked nocebo responses during evocation (Fig. 1c). A repeated measures ANOVA was conducted with trial type as within-subjects factor with two levels (first evocation nocebo trial, first evocation control trial), to establish whether significant nocebo hyperalgesia was induced. There was a significant difference between NRS reports for the first nocebo and first control trial of the evocation phase (F (1,35) = 27.44, p = 0.000008, ηp 2 = 0.44) indicating the presence of nocebo hyperalgesia.
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