Nocebo hyperalgesia can be induced by classical conditioning without involvement of expectancy.

Influential theoretical accounts take the position that classical conditioning can induce placebo effects through conscious expectancies. In the current study two different conditioning procedures (hidden and open) were used to separate expectancy from conditioning in order to reveal the role of expectancy in the formation of nocebo hyperalgesia. Eighty-seven healthy females were randomly assigned to three groups (hidden conditioning, open conditioning, and control). Participants were selected according to the Fear of Pain Questionnaire scores and assigned to two subgroups: high and low level of fear of pain (trait). They received electrocutaneous pain stimuli preceded by either an orange or blue color. During the conditioning phase, one color was paired with pain stimuli of moderate intensity (control stimuli) and the other color was paired with pain stimuli of high intensity (nocebo stimuli) in both hidden and open conditioning groups. Only participants in the open conditioning group were informed about this association, however just before the testing phase the expectancy of hyperalgesia induced in this way was withdrawn. In the control group, both colors were followed by control pain stimuli. During the testing phase all participants received a series of stimuli of the same intensity, regardless of the preceding color. Participants rated pain intensity, expectancy of pain intensity and fear (state). We found that nocebo hyperalgesia was induced by hidden rather than open conditioning. The hidden conditioning procedure did not produce conscious expectancies related to pain. Nocebo hyperalgesia was induced in participants with low and high fear of pain and there was no difference in the magnitude of the nocebo effect between both groups. Nocebo hyperalgesia was not predicted by the fear of upcoming painful stimuli.