An analysis of factors that contribute to the magnitude of placebo analgesia in an experimental paradigm

Placebo analgesia was produced by conditioning trials wherein heat induced experimental pain was surreptitiously reduced in order to test psychologica l factors of expectancy and desire for pain reduction as possible mediators of placebo analgesia. The magnitudes of placebo effects were assessed afte r these conditioning trials and during trials wherein stimulus intensities were reestablished to original baseline levels. In addition, analyses were made of the influence of these psychological factors on concurrently assess ed pain and remembered pain intensities. Statistically reliable placebo eff ects on sensory and affective measures of pain were graded according to the extent of surreptitious lowering of stimulus strength during the manipulat ion trials, consistent with conditioning. However, all of these effects wer e strongly associated with expectancy but not desire for relief. These resu lts show that although conditioning may be sufficient for placebo analgesia , it is likely to be mediated by expectancy. The results further demonstrat ed that placebo effects based on remembered pain were 3 to 4 times greater than those based on concurrently assessed placebo effects, primarily becaus e baseline pain was remembered as being much more intense than it actually was. However, similar to concurrent placebo effects, remembered placebo eff ects were strongly associated with expected pain levels that occurred just after conditioning. Taken together, these results suggest that magnitudes o f placebo effect are dependent on multiple factors, including conditioning, expectancy, and whether analgesia is assessed concurrently or Introspectiv ely. (C) 1999 international Association for the Study of Pain. Published by Elsevier Science B.V.