The thermal grill illusion and what is painful about it

The 'thermal grill illusion' refers to paradoxical sensations of heat and pain, resulting from simultaneous application of interlaced warm and cold stimuli to the skin. It provides an interesting model of integrative mechanisms in the nervous system, supposed to be relevant in explaining the hypersensitivity found in chronic pain of unclear etiology. The aim of this study is to investigate the perceptual qualities elicited by a reconstruction of the original grill stimulator and to compare these qualities with those elicited by a single temperature thermode of identical dimensions. Healthy participants performed these comparisons by choosing adjectives describing the perceived sensory qualities. We hypothesized that the thermal grill would be perceived as different from a single temperature hot stimulus near pain threshold because of varying sensory qualities. Moreover, the qualities elicited by the grill were expected to be different from the qualities elicited by its single component temperatures. The thermal grill elicited a complex percept, which was distinguished almost perfect from a hot stimulus. The pattern of perceived qualities of the thermal grill differed from single temperature warm and hot stimuli. Pain-related sensations were less present in the grill percept than in a single hot stimulus near pain threshold. The spectrum of qualities of the grill stimulus changed marginally with stimulus duration by decrease of a cold component and concurrent increase of a heat component. In conclusion, the percept of the thermal grill is not simply pain--it can be understood as a metaesthetic percept at the transition from heat to pain.