CLASSICAL-CONDITIONING AND THE PLACEBO-EFFECT

Stimulus substitution models posit that placebo responses are due to p airings of conditional and unconditional stimuli. Expectancy theory ma intains that conditioning trials produce placebo response expectancies , rather than placebo responses, and that the expectancies elicit the responses. We tested these opposing models by providing some participa nts with information intended to impede the formation of placebo expec tancies during conditioning trials and by assessing placebo expectanci es. Although conditioning trials significantly enhanced placebo respon ding, this effect was eliminated by adding expectancies to the regress ion equation, indicating that the effect of pairing trials on placebo response was mediated completely by expectancy. Verbal information rev ersed the effect of conditioning trials on both placebo expectancies a nd placebo responses, and the magnitude of the placebo effect increase d significantly over 10 extinction trials. These data disconfirm a sti mulus substitution explanation and provide strong support for an expec tancy interpretation of the conditioned placebo enhancement produced b y these methods. (C) 1997 International Association for the Study of P ain. Published by Elsevier Science B.V.