Chapter 7 – Pharmacological fMRI 231 Behavioral data were analyzed and visualized by use of R programming software (version 4.1.2; R Core Team, 2019), including the MASS 48, stargazer 49, and ggplot2 50 packages. The magnitude of reported nocebo hyperalgesia was measured withinsubjects and was defined as the difference in pain ratings for the first nocebo trial compared to the first control trial of the extinction phase. The first evocation trials were selected to check whether significant nocebo hyperalgesia was induced, as previous studies indicate the effect to be clearest in those trials 14,51. We also compared the average of pain ratings of the first 5 pairs of extinction trials for a significant nocebo response, as these were used for brain imaging analysis where more trials are required. Repeated measures ANOVAs were conducted with trial type as within-subjects factor with two levels (nocebo trial, control trial), as a separate model, to test whether significant nocebo hyperalgesia was induced. The first and last pairs of trials of the extinction phase were used to calculate the magnitude of extinction of nocebo responses. The reduction of nocebo responses was measured as the change in magnitude of nocebo responses (nocebo minus control trial difference score) between the start and the end of the extinction phase. A repeatedmeasures ANOVA was performed with time of measurement (pre to post) as within-subjects factor with two levels (nocebo magnitude before extinction, nocebo magnitude after extinction). Pearson correlation analyses were performed between all questionnaire data (psychological questionnaires, exit questions) and the magnitude of nocebo responses (nocebo minus control trial difference score), to establish whether these factors impacted nocebo responding. We also conducted post-hoc exploratory mediation analyses, to explore potential between-groups mediating effects of the questionnaire scores on nocebo magnitudes.
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