The role of the prefrontal cortex in hypnotic and placebo analgesia.

Abstract

Background: Prominent theoreticians have referred to hypnosis as a mega placebo (Kirsch, 1997) or “hypnobo” (Raz, 2007) suggesting that hypnosis and placebo are effectively the same phenomenon. However, there is evidence that their mechanisms of action are different (Parris, 2016a) and that whilst placebo effects rely on the prefrontal cortex (PFC; Benedetti, 2010), the role of the PFC in hypnosis and hypnotic suggestibility is inconclusive. Methods: Fifty-four participants (27 male) performed tasks known to tap either left (the Stroop Task; Stroop, 1938), right (the Hayling Task; Burgess & Shallice, 1996) or global PFC functions (Fluid IQ; Wechsler, 1939; 2008). Participants then completed the Cold Pressor Test (CPT), a commonly used test for pain-threshold (the time at which pain is first felt) and pain-tolerance (from pain threshold until hand removal), under three counterbalanced conditions: 1) After receiving a hypnotic suggestion for analgesia; 2) After receiving a placebo analgesia (hand cream); 3) After receiving “cleansing” hand cream. Results: Placebo analgesia was not effective on pain-tolerance, but did affect pain-threshold and was related to right and global Fluid IQ, which appeared to positively modify the relationship between the Hayling Task predictor, the right prefrontal cortex and placebo analgesic. Hypnotic analgesia was effective for both pain-threshold and pain-tolerance but was not related to performance on any PFC task. Conclusions: The findings showed that hypnotic analgesia was greater in magnitude than placebo analgesia. Additionally, placebo analgesia but not hypnotic analgesia was related to PFC function, indicating that participants with frontal atrophy through stroke or dementia would benefit from hypnotic analgesia but not from placebo analgesia. The results are discussed in terms of both cognitive and neural theories of hypnosis and placebo.