[1] The illusion is created by a grill of bars which are alternately warm, e.g. 40 °C (104 °F; 313 K), and cool, e.g. 20 °C (68 °F; 293 K).
When someone presses a hand against the whole grill, they experience the illusion of burning heat.
Due to shortcomings in the body map—the multisensory representation of the body—and this particular sensory input configuration, for some people the brain is tricked into thinking the middle finger is in the warm water and the index and ring fingers in cold water.
In an fMRI experiment of the illusion, researchers in 2011 observed an activation of the thalamus not seen for control stimuli.
Also, activity in a portion of the right mid/anterior insula correlated with the perceived unpleasantness of the illusion.