ABSTRACT
Since the first medical practices and healing rituals were performed in ancient civilizations, the ability of the mind to influence the healing of the body has been recognized across many cultures. Modern medical research has termed this the placebo effect, which is essentially the patient’s ability to demonstrate improvement in condition in response to some type of “inert” treatment-whether it be a pill, an injection, or even sham surgery-but not from any properties that the treatment itself possesses. In 1811, Hooper’s Medical Dictionary defined a placebo as “an epithet given to any medicine adapted more to please than to benefit the patient.”1 Ironically, scientific investigation, in the realization of this phenomenon, needed to account for the placebo effect in the interpretation of experimental results, and thus the placebo effect was largely considered to be a nuisance obscuring the true effects of the active treatment. However, with the growing amount of research available from clinical drug trials, the ability of placebos to produce therapeutic benefit in patients who suffer from various neurological disorders has proven to be real and effective. It is now accepted that a prominent placebo effect may be present in pain disorders, depression, and Parkinson’s disease.2-4 In the case of the latter, several randomized, placebo-controlled trials aimed at testing new pharmaceutical therapies have shown objective improvements in motor performance following placebo administration.5 However, the precise neuropsychological and biochemical mechanisms underlying the placebo effect are only beginning to be unraveled. The original observation by Levine and colleagues in the late 1970s that placebo analgesia can be blocked by naloxone suggested that the placebo effect in pain disorders involves the release of endogenous opioids.6 Following recent studies revealing direct biochemical evidence that a patient’s expectation is central to the placebo response in Parkinson’s disease,7 research is currently directed at characterizing the psychological and biochemical links between the expectation of benefit and the improvement of motor function in patients. This “expectation theory” of the placebo effect is thought to depend on reward circuitry in the brain, and more specifically, as recent evidence suggests, to dopamine release in the ventral striatum.
