Mechanisms of Placebo Analgesia (2011)

Undergraduates: Quinn Tracy, Daniel Harper

Faculty Advisor: Mark Hollins
Department: Psychology & Neuroscience

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The placebo effect can play a critical role in the effectiveness of clinical pain treatments. Previous research has linked the placebo effect to pain signaled by one type of pain fiber (C fibers), but not to the other (A? fibers). However, any suspected differences in the effects of a placebo on the two pain fiber types has not been well investigated. A total of 42 undergraduate students participated in a series pain tasks involving thermal pain. Participants were led to believe that the placebo, a skin moisturizer, was an effective pain killer. To contrast placebo effects between pain fiber types, a skin location with a mix of A? and C fibers (forearm) was compared to a location with only C fibers (palm). To further contrast effects, temporal summation was induced, a phenomenon only occurring in C fibers where pain increases in response to the same stimulus applied multiple times in quick succession. With or without the placebo, initial pain ratings tended to be lower for the palm than forearm, while summation tended to be greater for the palm than forearm. There was no consistent placebo effect, though a trend towards a nocebo effect (i.e., an increase in pain) was found for summation on the palm at the area the placebo was applied. Although not expected, this nocebo effect suggests that the mechanisms underlying placebos are more involved in summation on the palm than on the forearm.