Citation: Atlas, L.Y., Wager. T. D., Dahl, K., and Smith, E. E. (2009). Placebo effects. In: Handbook of Neuroscience for the Behavioral Sciences
A placebo is a treatment that is expected to have no inherent pharmacological or physical benefit ? for instance, a starch capsule given for anxiety or pain, or sham surgery in which the critical surgical procedure is not performed. Placebos are often used for comparison in clinical studies, as a baseline against which to evaluate the efficacy of investigational clinical treatments. However, placebo treatments often elicit observable improvements in signs or symptoms on their own ? these are placebo effects . For this reason, placebos have been used as healing agents for a variety of ailments; they have had a place in the healer's repertoire for thousands of years, and they are still used as a viable treatment option by physicians in industrialized countries with surprising frequency. Psychologists and neuroscientists today are most inter- ested in the placebo response , the brain and body response to the psychosocial (and perhaps neurobiological) context surrounding treatment. The study of the placebo response reveals active processes that provide a powerful window into brain - body interactions and the brain substrates of human behavior. Studies of drug treatments for various disorders have investigated the effects of exogenous regulation of neural and psychological end - points, such as reported emotion, behavioral responses, and disease - specific brain activity. The brain however, comprises interlocking feedback mechanisms that provide powerful endogenous control of neural and psychological processes. These endogenous processes regulate perceptual, affective, and cognitive processes based on the evaluation of situational context. Contextual information leading to placebo responses arises from either conscious expectancies about anticipated effects of treatment, or from prior learning in the form of conditioning with active treatments. In some cases, these two sources of placebo responses can be complementary, while in other cases, they may be mutually exclusive in their influence on observed placebo effects. The context surrounding placebo administration may lead individuals to expect improvement, and positive outcomes would compose the placebo effect. Alternatively, contextual information can lead individuals to expect worsening of symptoms; changes in the negative direction are observed as part of the nocebo effect. There is some evidence that the two involve separate mechanisms, although the placebo response has been much more thoroughly studied. A literature on experimental manipulations of placebo treatments has produced substantial evidence that placebo effects result, in many cases, from active brain responses to context, rather than statistical artifacts and reporting biases. Neuroimaging and related techniques have allowed us to begin to understand the brain mechanisms by which placebos exert their effects.