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How does the thermal grill illusion create a burning sensation from warm and cool bars?

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thermoreception in polar bears — Thermoreception Warm-blooded animals, such as polar bears, maintain stable body temperatures and adapt to substantial geographic and seasonal temperature fluctuations. Thermal adaptation is supported by the function of sensory structures called thermoreceptors.

The thermal grill illusion creates a burning sensation through a central neural mechanism involving the interaction between thermoreception and pain pathways. When spatially interlaced warm (40 °C) and cool (20 °C) bars are touched simultaneously, the brain integrates these inputs in a way that reduces normal thermoreceptive activity.1

This reduction in activity effectively "unmasks" or disinhibits polymodal C-fibre nociceptors, which are responsible for the sensation of cold pain. Under normal conditions, this nociceptive activity is inhibited centrally by thermoreceptive input; however, the specific combination of warm and cool stimuli disrupts this inhibition. Consequently, the brain perceives a burning sensation that mimics the pain typically associated with extreme cold (temperatures below 15 °C). This phenomenon demonstrates that thermal stimuli interact with pain sensation through central neural mechanisms rather than solely through peripheral processes.1