Grabbing hold of an injured area of the body can ameliorate acute pain. Researchers have known that touch can reduce pain, but why then are we often reluctant to let someone else put their hands on the injury?
To answer that question, Marjolein Kammers at University College London and her colleagues subjected volunteers to a modified version of a classic sensory trick called a “thermal grill illusion” (TGI). Normally, this illusion is achieved when you touch a grill made up of interlaced warm and cool bars – touching both warm and cool simultaneously elicits a feeling of burning pain.
For this study, explains Kammers, because her team was interested in changes in body representation, they applied the stimulus to the fingers directly: Submerging a subject’s index and ring fingers in warm water and the middle finger in cool water created the sensation of painful heat in the middle finger.
When both hands experienced this apparent heat pain, pressing the fingers of each hand together after removing them from the water cut the pain levels by 64%. Partially overlapping the fingers on the two hands, or having their “burning” fingers pressed on the experimenter’s, didn’t have this effect. The study was published online yesterday in Current Biology.
Kammers believes that such complete self-contact does the job by increasing the coherence of the brain’s map of the body. This map update, she speculates, is what curbs the pain. “We don’t fully understand yet all the brain mechanisms that underlie the creation of the body representation, or how the brain maintains the current state of the body,” she says. “We think the reason why full touch works is that you need lots of sensory channels to update” the representation.
The role of such body map updating has been suggested before for chronic pain. One example is a phenomenon known as phantom limb pain, in which amputees experience pain in regions of their body that are no longer there, often as if the missing limb is stuck in a clenched position. In a now-classic demonstration of the mind’s control over the body, neuroscientist Ramachandran, now at the University of California, San Diego, and his colleagues asked amputees who were experiencing phantom limb pain to place their good limb and their stump into two sides of a mirror box they had constructed. As the patients moved their good limb, the reflection in the mirror created the illusion that they were also moving their amputated limb, and they could trick their brain into representing it in an uncurled – and pain-free – position.
Image: Wikipedia — Sensory homunculus in the human cortex, as mapped by neurologist Wilder Penfield