Implicit versus explicit associative learning and experimentally induced placebo hypoalgesia

Abstract

The present study examined whether 1) placebo hypoalgesia can be generated through implicit associative learning (ie, conditioning in the absence of conscious awareness) and 2) the magnitude of placebo hypoalgesia changes when expectations about pain are made explicit. The temperature of heat pain stimuli was surreptitiously lowered during conditioning trials for the placebo cream and the magnitude of the placebo effect was assessed during a subsequent set of trials when the temperature was the same for both placebo and control conditions. To assess whether placebo hypoalgesia could be generated from an implicit tactile stimulus, a 2 × 2 design was used with direction of cream application as one factor and verbal information about which cream was being applied as the second factor. A significant placebo effect was observed when participants received verbal information about which cream was being applied but not following implicit conditioning alone. However, 87.5% of those who showed a placebo response as the result of implicit conditioning were able to accurately guess the order of cream application during the final trial, despite a lack of awareness about the sensory manipulation and low confidence in their ratings, suggesting implicit learning in some participants. In summary, implicit associative learning was evident in some participants but it was not sufficient to produce a placebo effect suggesting some level of explicit expectation or cognitive mediation may be necessary. Notably, the placebo response was abolished when expectations were made explicit, suggesting a delicate interplay between attention and expectation.

Keywords:

• placebo hypoalgesia
• associative learning
• expectancy
• implicit learning

Acknowledgements

We thank Eileen Halket for help with data collection and entry. Andrea Martin-Pichora was supported by a Canadian Institutes of Health Research (CIHR) Doctoral Award and a CIHR Strategic Training Fellowship in Pain: Molecules to Community. Joel Katz is supported by a CIHR Canada Research Chair (CRC) in Health Psychology at York University. The study was funded by a CIHR CRC in Health Psychology and by infrastructure grants from the Canadian Foundation for Innovation and the Ontario Innovation Trust. This article is derived, in part, from Andrea Martin-Pichora’s Doctoral Dissertation.

Disclosure

The authors report no conflicts of interest in this work.

Notes

a After removing the one participant from the Direction group who reported being aware of the direction of cream application, the proportion of placebo responders who accurately identified the order of cream application (6/7) remained significantly greater than that of nonresponders (1/7), Fisher exact test, P = 0.03.