Figure 5. Mediation of psychological pain modulation (Studies 5–6).

(a) Study 5's experimental design and the behavioral findings: In Study 5, the expectancy level was manipulated by two different cues associated with two levels of heat intensity (high and low). After participants learned the cue-heat intensity association, the low-pain cue was followed by a low (LL trial type) or medium pain (LM) with 50% chance and the high-pain cue was followed by a medium (HM) or high pain (HH) with 50% chance. Violin plots show participants' averaged pain ratings for two medium pain conditions (LM and HM) and grey lines connect the same individuals' pain ratings. (b) The significant expectancy effects on pain were mediated by the SIIPS1, not by the NPS. The path coefficients and s.e.m. (in parenthesis) for the mediation effects (Path a × b) are reported here. (c) Study 6's experimental design and the behavioural findings: in Study 6, we manipulated the levels of perceived control and expectancy with a 2-by-2 design (for more details, see Methods). The bar plots show participants' averaged pain ratings for four different experimental conditions and the right ones show the multilevel general linear model results (beta coefficients). Error bars represent s.e.m. (d) The perceived control effects were mediated only by the SIIPS1 (that is, not by the NPS) and the expectancy effects were mediated by both the SIIPS1 and the NPS. The path coefficients and s.e.m. (in parenthesis) for the mediation effects (Path a × b) are reported here. For more detailed methods and statistics of path coefficients, see Methods and Supplementary Table 5. ⁺P<0.05, one-tailed; *P<0.05, **P<0.01 and ***P<0.001, two-tailed. Significance tests in this figure include paired t-test, multi-level mediation analyses (bootstrap test) and multi-level general linear model (bootstrap test).