Nocebo effects are adverse effects induced by patients' expectations. Nocebo effects on pain may underlie several clinical conditions, such as chronic pain. These effects can be learned via classical conditioning mechanisms. In the lab, nocebo effects are commonly studied via conditioning with continuous reinforcement (CRF) during which 100 percent of unconditioned pain stimuli are paired to conditioned stimuli (i.e., the activation of a mock medical device). Partial reinforcement (PRF) provides a more uncertain pairing during conditioning, where less than 100 percent of unconditioned pain stimuli are paired to conditioned stimuli. This method provides a potentially more clinically relevant learning platform to study how nocebo effects on pain are induced. In this study, the efficacy of conditioning with PRF, CRF, and sham-conditioning in inducing nocebo effects on pain will be compared. Furthermore, a counterconditioning method will be compared to an extinction method for the attenuation of nocebo effects on pain. Given the relevance of nocebo effects for patients, it is important to ascertain effective & clinically relevant methods to understand how nocebo effects may be formed and attenuated. This study is conducted by Leiden University.

Main planned analyses:

Primary Outcome; Comparing the effectivity of conditioning with PRF, conditioning with CRF, and sham conditioning, in inducing nocebo effects on pain:

The magnitude of the induced nocebo effect during the first evocation phase (dependent variable), will be compared between induction groups (independent variable). The pain ratings for the first control trial will be measured against the pain ratings for the first reinforced trial, yielding the magnitude of the nocebo effect. A 3x2 mixed model Analysis of Variance (ANOVA) will be used, treating group as the between-subjects factor with three levels (PRF, CRF, sham) and magnitude of the nocebo effect on pain as a within-subjects measure with two levels (first control and first reinforced trial). Planned contrasts will be conducted.

Secondary Outcome; Comparing the effectivity of counterconditioning and extinction in attenuating nocebo effects on pain:

The nocebo effect still present in the second evocation phase (dependent variable) will be compared between attenuation groups (independent variable). This effect will be measured as the change in pain ratings reported in the first reinforced trial of the first evocation phase (after induction) and pain ratings reported in the first reinforced trial of the second evocation phase (after attenuation). This effect will be further analysed by measuring any change in pain reports for the control trials (second dependent variable) in the same way. The effect of induction group will also be further explored. A 2x2 mixed model ANOVA will be conducted, with attenuation group as the between-subjects factor (counterconditioning and extinction) and the degree to which the nocebo effect has been attenuated as the within-subjects measure with two levels (first and second evocation phase for the first reinforced trials). Further secondary analyses will be conducted.

Inclusion Criteria:

• Aged 18 - 35 years
• Good understanding of the Dutch language

Exclusion Criteria:

• Severe or chronic physical morbidity (e.g., heart and lung diseases, diabetes)
• Psychiatric disorders (DSM diagnosis; e.g., depression, autism)
• Chronic pain complaints (≥ 6 months) at present or in the past
• Current pain
• Current use of analgesic medication
• Pregnancy
• After inclusion, participants who do not reach a sensation of high pain with the highest temperature administered or participants who appear unable to distinguish between moderate and high pain stimuli will also be excluded

Participants whose gender identity, gender expression, and lived gender is male, will be included as male. Participants whose gender identity, gender expression, and lived gender is female, will be included as female.

All data are collected pseudonymised thus no personal data are stored or shared. Consent forms are the only sources containing personal data and will not be shared, but are monitored by the department's Data Monitor.

Data will become available immediately after publication of the study and will be retained for 15 years.

Data can be shared with scientists in relevant fields for the purpose of future studies such as replication or meta-analysis (or with designated persons for monitoring purposes).

A double-blind randomization list (stratified for gender) was created by an independent statistician. Complete blinding of the researchers during the experiment is not possible due to the nature of conditioning paradigms. However, in this study blinding is optimized: the researchers are informed of the (conditioning) group to which participants are allocated after their final inclusion to the study, and after all questionnaires and pain calibration procedures have been completed on the testing day.