As I’ve written before, the placebo effect is a rather messy phenomenon. It usually refers to the difference in outcomes in a study that are not due to the intervention but to multiple other variables associated with being in a study. More colloquially, “placebo” often means a positive effect seen from the administration of a biologically inert substance. There’s a bit of a buzz brewing about a recent brief communication in Science. The report used fMRI to look for physiologic correlates to pain responses that were attenuated by an inert substance. (For the purposes of this discussion, I’ll assume that the fMRI technique used is valid.) According to the authors:
Placebo analgesia is a prime example of how psychological factors can influence pain perception (1). It refers to a situation where the administration of an inactive treatment has a pain-relieving effect, presumably because of the participant’s belief in the analgesic effectiveness of the treatment.
This study did not involve an “inactive treatment”. If you administer a presumably inert substance with the intent of causing an effect, you are no longer studying placebo, but a specific set of behaviors to manipulate the patient’s pain response. In other words, when you set up the theater to encourage a patient to feel better, you can no longer separate out the “placebo” response from the, er, placebo response. ”Placebo” is what is left to explain an effect when your intervention is expected to be inert, and setting strong expectations in patients that a cream will reduce pain is hardly an “inactive treatment”, as the fMRI findings showed.
Measurements with real lidocaine were never taken. Subjects were subjected only to an inert cream, one presented as lidocaine, and one presented as control, so we don’t know what would have happened had the patients been exposed to a “real” analgesic.
What these researchers measured was not “how placebo works” but how a certain type of medical theater affects pain behaviors. Medical theater is probably a significant component of placebo effects measured in clinical studies, but it is not the only component, and should not be confounded with “placebo”. So while this study is interesting in showing a possible physiologic and anatomic correlate to pain modulation, I’m not convinced it illuminates our understanding of placebo.
Eippert, F., Finsterbusch, J., Bingel, U., & Buchel, C. (2009). Direct Evidence for Spinal Cord Involvement in Placebo Analgesia Science, 326 (5951), 404-404 DOI: 10.1126/science.1180142 (Methods available here.)