The ‘Thermal Grill’ Illusion

How the ‘Thermal Grill’ Illusion Tricks the Mind
Science. 12 December 2011

The trick is called the thermal grill illusion, and it’s the topic of a paper published last month in PLoS ONE.
If you feel harmless levels of cold and warm all at once and in a grill-like pattern, it can hurt. “It feels a bit like the burning of cold pain, when you put your hand in snow,” says Lindstedt, who works at the Karolinska Institute in Sweden. “It’s a very, in want of a better word, weird stimulus.”

… silver, which isn’t magnetic. … strapped the box to her leg … they warmed all the bars to 41°C, cooled all the bars to 18°C

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.

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