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In Thunberg's thermal grill illusion, first demonstrated in 1896, a sensation of strong, often painful heat is elicited by touching interlaced warm and cool bars to the skin. Neurophysiological recordings from two classes of ascending spinothalamic tract neurons that are sensitive to innocuous or noxious cold showed differential responses to the grill. On the basis of these results, a simple model of central disinhibition, or unmasking, predicted a quantitative correspondence between grill-evoked pain and cold-evoked pain, which was verified psychophysically. This integration of pain and temperature can explain the thermal grill illusion and the burning sensation of cold pain and may also provide a basis for the cold-evoked, burning pain of the classic thalamic pain syndrome.
The sensations of pain and temperature stem from parallel ascending sensory channels that are regarded as physiologically separate (1). However, these sensations can be shown to interact. In 1896, Thunberg reported that innocuous warm and cool stimuli applied simultaneously to the skin by means of interlocking spiral tubes elicited a sensation of strong heat, which he compared to the burning sensation that commonly accompanies cold pain (2). We investigated the cause of this illusion with neurophysiological and psychophysical methods.
The prevailing explanation of the thermal grill illusion is based on Alrutz's proposal that the perception of "heat" (evoked at temperatures between 45deg and 50degC) is not a specific sensation but rather a fusion resulting from the simultaneous activation of specific warm and cold spots (3). (Cold spots can be activated by cooling and also, paradoxically, by high temperatures.) The grill was thought to evoke this fusion by the simultaneous activation of sensory channels for warmth and cold by warm and cool, rather than hot, temperatures. In the 1920s, several experimental psychologists concurred with this proposal, whereas others concluded that the sensation evoked by the grill was more tactual, like a pricking sensation. One group questioned whether the illusion was due to suggestion and to the confusion resulting from an unnatural stimulus.
Modern physiological findings have confirmed the existence of specific cutaneous receptors for warm and for cold. However, many warm receptors cease their discharge at temperatures above 45degC and are thus not active at high temperatures (1). Instead, specific heat nociceptors have thresholds around 45degC, which is now the...