Investigation of the paradoxical painful sensation (‘illusion of pain’) produced by a thermal grill

Abstract

A paradoxical painful sensation can be elicited by the simultaneous application of innocuous warm and cold stimuli to the skin. In the present study, we analyzed the conditions of production of this unique experimental illusion of pain in 52 healthy volunteers (27 men, 25 women). The stimuli were produced by a thermode composed of six bars whose temperature was controlled by Peltier elements. The temperature of alternate (even- and odd-numbered) bars could be controlled independently to produce various patterns of the 'thermal grill'. After measuring the cold and heat pain thresholds, a series of combinations of warm and cold stimuli, whose distance to the thermal pain threshold was at least 4 degrees C, were applied on the palmar surface of the right hand during 30s. After each stimulus, the subjects had to describe and rate their sensations on visual analog scales. Paradoxical painful sensations, mostly described as burning, were reported by all the subjects but three. However, the phenomenon was less frequent in approximately one third of ('low responder') volunteers. The frequency and intensity of such painful sensations were directly related to the magnitude (i.e. 5-25 degrees C) of the difference of the temperature between the warm and cold bars of the grill. The combination of increasingly colder temperature to a given warm temperature induces similar effects as combining increasingly warmer temperature to a given cold temperature. These results suggest that pain can be the result of a simple addition of non-noxious warm and cold signals.

Organization of the Thermal Grill Illusion by Spinal Segments

“…Previous work indicated several alternative neurophysiological mechanisms and pathways, including the difference in activity between cold‐specific and polymodal nociceptive neurons at the thalamocortical level,7, 11 summation by wide dynamic range neurons in the deep dorsal horn4, 6 and type 2 C‐afferents 36. More specifically, animal work suggests reciprocal cross‐inhibition between cold and warm afferents in the spinal cord 37, 38.…”

Pharmacological dissection of the paradoxical pain induced by a thermal grill

“…As described previously (Bouhassira et al, 2005) the temperature of the warm and cold bars was set prior to each stimulus. Then, the volunteers were asked to apply their hand to the grill, orthogonally to the long axis of the bars and each stimulus was applied for 30 seconds.…”

“…A series of stimuli combining cold (CPT + 2°C, +4°C or +6°C ) and warm (HPT-2°C, -4°C or -6°C) non noxious stimuli were applied to define the optimal pattern of the thermal grill producing a paradoxical pain in each subject. These parameters were based on our previous results showing that the intensity of paradoxical pain was directly related to the difference in the intensity of the warm and cool bars of the grill (Bouhassira et al, 2005). Then, two measures of paradoxical pain separated by a 3 minute-interval were taken as baseline control values with these parameters.…”

“…As in our previous study (Bouhassira et al 2005), thermal stimuli were produced with a thermode designed and built by SEICER (Mouy, France). The thermode was composed of six bars (1.2 by 16 cm) covered with a copper plate, spaced 2 mm apart for thermal isolation, whose temperature was controlled by thermoelectric Peltier elements (three per bar).…”

“…This phenomenon was reinvestigated recently using modern techniques, to identify causative conditions and to characterize further its psychophysical properties. Non painful "synthetic heat" has been reported (Green, 2002;Fruhstorfer et al, 2003), but it has been repeatedly demonstrated that, with appropriate stimuli parameters, stimulation with a thermal grill composed of alternate warm and cold bars also induces a painful burning sensation (Craig and Bushnell, 1994;Bouhassira et al, 2005;Leung et al, 2005). In particular, we showed in a large group of volunteers that the occurrence and intensity of this paradoxical pain were directly related to the magnitude of the difference in the temperatures between the warm and cold bars of the grill; this suggests that pain can be the result of the simple addition of two non-noxious, normally non-painful, thermal stimuli (Bouhassira et al, 2005).…”