Associations Between Interindividual Differences, Expectations and Placebo and Nocebo Effects in Itch

Area of Science:

• Psychoneuroimmunology
• Behavioral Medicine
• Experimental Psychology

Background:

• Placebo and nocebo effects are health outcomes influenced by psychosocial context and expectations.
• Individual differences in personality and affective states may impact placebo/nocebo responses, but findings are inconsistent.
• Understanding these modulators is crucial for harnessing positive health outcomes and mitigating negative ones.

Purpose of the Study:

• To investigate expectation as a mediator between verbal suggestions (VSs) and histamine-induced itch.
• To examine if individual differences (optimism, neuroticism, BAS, body ignorance) moderate VSs effects on itch.
• To explore direct moderation and moderated mediation of VSs effects by individual traits.

Main Methods:

• Three experimental studies utilized verbal placebo and nocebo suggestions (VSs) in open-label and closed-label contexts.
• Histamine-induced itch was measured as the primary outcome.
• Mediation analyses tested expectation's role, while conditional process analyses explored moderation by individual traits (e.g., BAS, body ignorance).

Main Results:

• VSs effects on itch were significantly mediated by expectation in Study 1 and open-label conditions of Studies 2 & 3.
• Body ignorance marginally moderated direct VS effects in closed-label conditions; lower body ignorance amplified VS effects.
• Moderated mediation was observed in open-label conditions: lower BAS drive strengthened expectation-mediated VS effects on itch.

Conclusions:

• Expectation mediates the effects of verbal suggestions on itch, particularly in open-label contexts.
• Placebo and nocebo effects are modulated by ignorance of bodily signals and the behavioral activation system (BAS).
• Limited evidence suggests other interindividual differences play a significant role in modulating itch responses to VSs.