Placebo mechanisms and reward circuitry: clues from Parkinson's disease.

Recent evidence indicates that the placebo effect in Parkinson's disease is mediated by the release of dopamine in the dorsal striatum. Interestingly, there is also placebo-induced dopamine release in the ventral striatum, which establishes a connection between the placebo effect and reward mechanisms. Specifically, we propose that placebo responses are related to the activation of the reward circuitry. Here, the clinical benefit induced by placebos represents the reward. The magnitude of the placebo effect likely depends on the a priori probability of clinical benefit. This notion has profound implications in the design of clinical trials and placebo investigations.