Expectation plays an essential role in placebo analgesia. Modulation of pain perception by placebos depends on expectation, as shown in many studies (Kirsch, 1999; Price et al., 1999, 2008). Montgomery and Kirsch conducted one of the first studies in which expected pain levels were manipulated and directly measured (Montgomery and Kirsch, 1997).
They used a design in which subjects were given cutaneous pain via iontophoretic stimuli. Once baseline stimuli were applied, subjects were surreptitiously given stimuli with reduced intensities in the presence of a placebo cream (conditioning procedure). Then the stimulus strength was restored to its original baseline level and several stimuli were then used in placebo test trials to test the effect of conditioning.
Subjects rated their expected pain levels just before placebo test trials and were divided into two groups. The first did not know about the stimulus manipulation and their pain ratings during placebo trials were markedly diminished by prior conditioning. However, by performing regression analyses, it was clear this effect was mediated by expected pain levels.
Expectancy accounted for 49% of the variance in post-manipulation pain ratings. The second group was informed about the experimental design and learned that the cream was inert. They showed no placebo analgesic effect. Regression analysis and the difference in results across the groups showed that conscious expectation is necessary for placebo analgesia and clarified some previous studies which concluded that conditioning was important in placebo analgesia (Voudouris et al., 1989, 1990).
-Researchers discovered that placebo effects can be increased or decreased through embedded medical rituals and expectation-
Benedetti, Fabrizio. Placebo Effects (p. 118). OUP Oxford.
Kirsch I (1999). How expectancies shape experience. American Psychological Association, Washington, DC.
Montgomery GH and Kirsch I (1996). Mechanisms of placebo pain reduction: an empirical investigation. Psychological Science, 7, 174-6.
Price DD (1999). Psychological Mechanisms of Pain and Analgesia. IASP Press, Seattle, WA.
Voudouris NJ, Peck CL and Coleman G (1989). Conditioned response models of placebo phenomena: further support. Pain, 38, 109-16.