Great expectations: the placebo effect in Parkinson's disease.

Our understanding of the neural mechanisms underlying the placebo effect has increased exponentially in parallel with the advances in brain imaging. This is of particular importance in the field of Parkinson's disease, where clinicians have described placebo effects in their patients for decades. Significant placebo effects have been observed in clinical trials for medications as well as more invasive surgical trials including deep-brain stimulation and stem-cell implantation. In addition to placebo effects occurring as a byproduct of randomized controlled trials, investigation of the placebo effect itself in the laboratory setting has further shown the capacity for strong placebo effects within this patient population. Neuroimaging studies have demonstrated that placebos stimulate the release of dopamine in the striatum of patients with Parkinson's disease and can alter the activity of dopamine neurons using single-cell recording. When taken together with the findings from other medical conditions discussed elsewhere in this publication, a unified mechanism for the placebo effect in Parkinson's disease is emerging that blends expectation-induced neurochemical changes and disease-specific nigrostriatal dopamine release.