Beyond labeled lines: A population coding account of the thermal grill illusion

Heat and pain illusions (synthetic heat and the thermal grill illusion) can be generated by simultaneous cold and warm stimulation on the skin at temperatures that would normally be perceived as innocuous in isolation. Historically, two key questions have dominated the literature: which specific pathway conveys the illusory perceptions of heat and pain, and where, specifically, does the illusory pain originate in the central nervous system? Two major theories - the addition and disinhibition theories - have suggested distinct pathways, as well as specific spinal or supraspinal mechanisms. However, both theories fail to fully explain experimental findings on illusory heat and pain phenomena. We suggest that the disagreement between previous theories and experimental evidence can be solved by abandoning the assumption of one-to-one relations between pathways and perceived qualities. We argue that a population coding framework, based on distributed activity across non-nociceptive and nociceptive pathways, offers a more powerful explanation of illusory heat and pain. This framework offers new hypotheses regarding the neural mechanisms underlying temperature and pain perception.