Classical conditioning of analgesic and hyperalgesic pain responses without conscious awareness

Karin Jensen; Irving Kirsch; Sara Odmalm; Ted J. Kaptchuk; Martin Ingvar

doi:10.1073/pnas.1504567112

New Research In

Edited by Howard L. Fields, University of California, San Francisco, CA, and accepted by the Editorial Board April 22, 2015 (received for review March 5, 2015)

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Learning of analgesia and hyperalgesia
- Jun 08, 2015

Significance

It is unclear to what extent new learning can take place outside of conscious awareness. In the present study, we used psychophysical measures and classical conditioning to establish whether psychologically mediated analgesic and hyperalgesic responses could be acquired by unseen (subliminally presented) stimuli. Our study demonstrates that analgesia and hyperalgesia can be learned without conscious awareness, suggesting that higher-order cognitive processes may be affected by implicit learning mechanisms.

Abstract

Pain reduction and enhancement can be produced by means of conditioning procedures, yet the role of awareness during the acquisition stage of classical conditioning is unknown. We used psychophysical measures to establish whether conditioned analgesic and hyperalgesic responses could be acquired by unseen (subliminally presented) stimuli. A 2 × 2 factorial design, including subliminal/supraliminal exposures of conditioning stimuli (CS) during acquisition/extinction, was used. Results showed significant analgesic and hyperalgesic responses (P < 0.001), and responses were independent of CS awareness, as subliminal/supraliminal cues during acquisition/extinction led to comparable outcomes. The effect was significantly larger for hyperalgesic than analgesic responses (P < 0.001). Results demonstrate that conscious awareness of the CS is not required during either acquisition or extinction of conditioned analgesia or hyperalgesia. Our results support the notion that nonconscious stimuli have a pervasive effect on human brain function and behavior and may affect learning of complex cognitive processes such as psychologically mediated analgesic and hyperalgesic responses.

Footnotes

↵¹To whom correspondence should be addressed. Email: karin.jensen{at}ki.se.

Author contributions: K.J., I.K., T.J.K., and M.I. designed research; K.J. and S.O. performed research; K.J., I.K., and M.I. analyzed data; and K.J., I.K., S.O., T.J.K., and M.I. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. H.L.F. is a guest editor invited by the Editorial Board.
See Commentary on page 7624.

Freely available online through the PNAS open access option.

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