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Chilly heat. A device built to create the thermal grill illusion, which uses an alternating pattern of warm and cool to cause a strange sensation like burning pain, is shown in infrared.

Adapted from F. Lindstedt et al., PLoS ONE (2011)

How the 'Thermal Grill' Illusion Tricks the Mind

Neuroscientist Fredrik Lindstedt tried a homemade pilot experiment one day with a pack of frankfurters. He briefly put half in the microwave and the rest in the fridge. Lindstedt then laid them out in a warm-cool-warm-cool pattern on his countertop and placed his hand on them. The result: An unpleasant burning sensation. "My girlfriend wondered what the heck I was doing," he says.

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."

Though scientists have known about the illusion for over a century, it remains unclear why it causes pain. So Lindstedt and his colleagues did something that had never been tried: they tested the trick on people in an fMRI scanner. It wasn't easy. fMRI scanners use powerful magnets to generate pictures that show where the brain is active, so that ruled out using any magnetic metal parts, which could turn into dangerous projectiles near the scanner. And hot dogs were out of the question because the scientists had to be able to precisely control the grill's exact temperature while their subjects were being tested.

So the researchers created a plastic box and covered one surface with bars made of silver, which isn't magnetic. With a subject inside the scanner, Lindstedt's team strapped the box to his or her leg. To heat or cool the silver bars, the scientists pumped hot and cold water into channels inside the box, changing the silver surface's temperature. Every 20 seconds they warmed all the bars to 41°C, cooled all the bars to 18°C, or warmed and cooled alternating bars to create the illusion.

Lindstedt's team 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.

Easing such pain disorders is a long-term goal of Lindstedt's research. So he plans to repeat his experiment, only this time with allodynia patients, to see if their thalami react similarly to pain from both the illusion and the disorder. Lindstedt thinks allodynia pain, which can result from a stroke, will probably be the stronger of the two sensations. But if the same subjects describe both pain from the illusion and allodynia in the same way—"burning" for example—that could reveal where and how the brain transforms feelings of temperature into pain.

The study has "opened the door for further progress" in this field, says neuroanatomist A. D. "Bud" Craig of the Atkinson Pain Research Institute in Phoenix, Arizona. Craig conducted similar research in 1996, except his team used a PET scanner, which detects brain activity by measuring the brain's consumption of radioactive glucose injected into volunteers. That work showed no activity in the thalamus, but plenty in the anterior cingulate cortex, which is part of the pain matrix and also plays a role in decision-making. Still, he says Lindstedt's results make sense because the anterior cingulate cortex and thalamus are heavily connected. "They are highly integrated structures."