There's an interesting overview in Science today of a recent Swedish study that aimed to determinine how the thermal grill illusion tricks the mind.
The thermal grill illusion, for those not familiar with the term, describes the pain sensation of feeling burning heat when placing your hand on a surface with alternating warm and cold bars. In the study, researchers used functional magnetic resonance imaging (fMRI) machines and a grill-like device comprised of silver bars that every 20 seconds were either warmed to 41*C, cooled to 18*C, or warmed and cooled in an alternating pattern. The magazine reports:
[Neuroscientist Fredrik Lindstedt and colleagues at the Karolinska Institute] tested every pattern on 20 people, and as expected the volunteers found the illusion more unpleasant or painful than normal hot and cold. The fMRI showed that those experiencing the illusion had a particularly busy thalamus, a relay station in the brain through which sensory impulses pass, and part of the pain matrix, a collection of brain regions that manage pain. The thalamus is also active during pain caused by cold allodynia, a neurological disorder in which even normal levels of cold hurt.
Building off these findings, the research team, whose long-term goal is to ease pain disorders, plans to repeat the experiment with allodynia patients. Researchers say the results of future studies could yield new insights into where and how the brain transforms feelings of temperature into pain.