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Coolness both underlies and protects against the painfulness of the thermal grill illusion

Harper, Daniel E.*; Hollins, Mark

doi: 10.1016/j.pain.2014.01.017
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Summary The thermal grill illusion is affected by warm and cool preadaptation in ways that shed light on its mechanism.

ABSTRACT We investigated the contributions of warm and cool signals in generating the thermal grill illusion (TGI), a phenomenon in which interlaced warm and cool bars generate an experience of burning, and under some conditions painful, heat. Each subject underwent 3 runs, 2 of which tested the effects of preadapting subjects to the grill's warm or cool bars (while the interlaced bars were thermally neutral) on the subsequent intensity of the illusion. In a control run, all bars were neutral during the adaptation phase. Thermal visual analogue scale ratings during the warm and cool adaptation periods revealed significant and equivalent adaptation to the 2 temperatures. Adaptation to the grill's cool bars significantly reduced pain and perceived thermal intensity of the TGI, compared to the control condition, while adaptation to the grill's warm bars had little effect. These results suggest that the cool stimulus triggers the pain signals that produce the illusion. The inability of warm adaptation to attenuate the TGI is at odds with theories suggesting that the illusion depends upon a simple addition of warm and cool signals. While the grill's cool bars are necessary for the TGI's painfulness, we also observed that the more often a participant reported feeling coolness or coldness, the less pain he or she experienced from the TGI. These results are consistent with research showing that cool temperatures generate activity in both thermoreceptive-specific, pain-inhibitory neurons and nociceptive dorsal horn neurons.

Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA

* Corresponding author. Address: Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. Tel.: +1 704 996 0773; fax: +1 919 962 2537.

E-mail address:harperd@email.unc.edu

Article history:

Received 5 August 2013

Received in revised form 15 January 2014

Accepted 16 January 2014

Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.

© 2014 International Association for the Study of Pain
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