The spatial characteristics of the painful thermal grill illusion

Interlaced cold and warm stimuli may induce a paradoxical burning sensation termed the "thermal grill illusion". Studies on the grill illusion have yielded contradictory results regarding its quality and intensity, which in turn led to controversies concerning the underlying mechanism. Some controversies may result from testing the illusion with absolute temperatures thereby disregarding inter-subjects' variation in temperature sensitivity. Therefore, our aim was to measure the individual threshold of the painful grill illusion (PGI). Another aim was to measure, here for the first time, the spatial boundaries of the PGI by spatially separating between the cooling and warming stimuli. Subjects (10 males, 15 females) underwent measurements of heat-pain (HPT) and cold-pain thresholds (CPT) with 9 and 18 cm² stimulating probes, on the forearm. Subjects also underwent measurement of pain threshold (PT), with one cooling and one heating probe (9 cm² each) activated simultaneously, and separated by 0-30 cm (distances encompassing one or two dermatomes). Simultaneous cold and warm stimuli produced burning pain at all separation distances. PT (∼26 and 38 °C) was significantly lower than CPT and HPT, respectively, and was relatively fixed across all distances except for 30 cm at which PT increased towards CPT and HPT values. Gender did not affect the PGI. In conclusion, innocuous cold and warm stimuli can spatially summate, both within and between dermatomes and evoke a PGI. Possibly, non-nociceptive channels integrate onto 2nd or 3rd order nociceptive neurons which in turn induce a unique painful burning resulting from the blend of cold and warm sensations.