The neurobiology of placebo effects

The placebo effect is a psychobiological phenomenon whereby treatment cues trigger improvement. While traditionally viewed as a nuisance variable to be controlled for, the past three decades have seen a surge in interest in the placebo effect in light of some remarkable clinical and laboratory discoveries that have demonstrated its potential power to improve patient outcomes. Many attempts have been made to conceptualize the placebo effect and most of the research on placebos has focused on expectations and learning as the two main factors involved in placebo responsiveness. Expectation, reward and conditioning have also been shown to modulate the reinforcing effects of drugs of abuse, with reward being of crucial importance. In fact, placebo effects appear to be related to reward mechanisms in different types of addiction. The aim of this review is to outline what is currently known about the neurobiology of placebo effects, with particular emphasis on reward mechanisms and motivated behaviors, and to focus on some of the most significant studies that uncovered both psychological and biological mechanisms underlying placebo and placebo-like effects in addiction.