If you’ve ever accidentally brushed your hand against a hot stove, you’ll probably remember immediately clutching the burn with your other hand—an instinct that seems to help relieve the pain. In contrast, we often pull back in fear if someone else tries to touch a wound. Although psychologists have long recognized this distinction, no one properly understood the cognitive mechanisms that allow reflexive self-touch—rather than a stranger’s touch—to soothe pain. A new study published online September in Current Biology suggests that touching an injured area on one’s own body reduces pain by enhancing the brain’s map of the body in a way that touch from another cannot mimic.
In the study cognitive neuroscientist Marjolein Kammers of University College London and her colleagues asked blindfolded participants to place their index and ring fingers in tubs of warm water while their middle fingers rested in cool water, a common experimental trick that creates the illusion that the middle fingers are burning hot. When the participants withdrew from the water and touched only their middle fingers of both hands together or joined only their outer fingers, they found little relief. Touching all three fingers to an experimenter’s hand also failed to reduce pain. Only when the participants entwined their three affected fingers on both hands did they soothe themselves, diminishing perceived pain by 64 percent.
Uniting two parts of the same body, Kammers explains, sends diverse signals to the brain about temperature, spatial position and identity that can come only from self-contact. In this case, bringing all three fingers together probably provided the brain with enough comparative information to readjust its interpretation of skin temperature on each finger. “When you get input from many different signals, the brain increases the coherence of its body map, which reduces acute pain,” Kammers says. The new findings parallel previous work demonstrating that adding more sensory input can relieve chronic phantom limb pain experienced by some amputees: when a mirror tricks the brain into thinking the body is whole again, the pain subsides.