Conditioned pain modulation dampens the thermal grill illusion

• Published in: European journal of pain 2017-10, Vol.21 (9), p.1591-1601
• Main Authors: Harper, D.E., Hollins, M.
• Format: Article
• Language: English
• Subjects: Adolescent; Cold Temperature; Female; Hot Temperature; Humans; Illusions - physiology; Male; Pain - physiopathology; Pain Measurement - methods; Pain Perception - physiology; Pain Threshold - physiology; Thermosensing - physiology; Young Adult
• ISSN: 1090-3801; 1532-2149
• DOI: 10.1002/ejp.1060

Background The thermal grill illusion (TGI) refers to the perception of burning heat and often pain that arises from simultaneous cutaneous application of innocuous warm and cool stimuli. This study utilized conditioned pain modulation (CPM) to help elucidate the TGI's underlying neural mechanisms, including the debated role of ascending nociceptive signals in generating the illusion. Methods To trigger CPM, subjects placed the left hand in noxious cold (6 °C) water before placing the right volar forearm onto a thermal grill. Lower pain and unpleasantness ratings of the grill in this CPM run compared to those in a control run (i.e. 33 °C water) were taken as evidence of CPM. To determine whether CPM reduces noxious heat pain and illusory heat pain equally, an experimental group of subjects rated pain and unpleasantness of a grill consisting of innocuous alternating warm (42 °C) and cool (18 °C) bars, while a control group rated a grill with all bars controlled to a noxious temperature (45 °C). Results CPM produced significant and comparable reductions in pain, unpleasantness and perceived heat of both noxious heat and the TGI. Conclusions This result suggests that the TGI results from signals in nociceptive dorsal horn convergent neurons, since CPM involves descending inhibition with high selectivity for this neuronal population. More broadly, CPM's ability to produce a shift in perceived thermal sensation of both noxious heat and the TGI from ‘hot’ to ‘warm’ implies that nociceptive signals generated by a cutaneous stimulus can contribute to its perceived thermal intensity. Significance Conditioned pain modulation reduces the perceived painfulness, unpleasantness and heat of the thermal grill illusion and noxious heat similarly. The results have important theoretical implications for both types of pain.