Thermal-grill illusion
Thunberg’s (1896) demonstration, given a modern interoceptive interpretation in Craig (2002) (Box 1). Interlaced innocuous warm and cool bars — neither painful alone — together produce a burning, ice-like pain. See ad-craig.
What it shows
The illusion reveals two things about the lamina-i-spinothalamocortical-pathway:
- “Cold inhibits pain.” Below ~24°C, a polymodal C-nociceptive channel is increasingly active but is normally masked centrally by activity in the specific A-fibre cooling channel. The grill (or an A-fibre block) reduces cooling-channel activity, disinhibiting the nociceptive channel → burning pain at merely cool temperatures.
- Thermal pain is a central comparison. The perception depends on the relative balance of two thermosensory/nociceptive labelled lines, computed in the forebrain — not on a dedicated “pain receptor.”
Why it matters for the wiki
The grill is Craig’s evidence that pain and temperature are homeostatic interoceptive channels (see homeostasis) that signal thermoregulatory distress, and that the ACC carries their affect: functional imaging finds the grill selectively unmasks ACC activation, matching the perception of thermoregulatory urgency. It anchors Craig’s broader argument that pain “is not a binary modality” but the extreme of a homeostatic afferent continuum.