If you get an injury, you might clench your hand.  It turns out there is a brain-related reason for that,  namely a somatosensory body one.   Perception and multisensory interactions at the spinal level.

A new report in Current Biology says 'self-touch', like hand clenching, offers significant relief for acute pain under experimental conditions and they used the Thermal Grill Illusion to show it.

Unlike magic tricks, the Thermal Grill Illusion is a sensory one involving touch instead of eyesight and was first introduced by T. Thunberg in 1896 (1) and showed that heat and cold are similar to the brain but that they were also relative so if a warmer signal reached the brain before a colder one, the sensory experience would be warmth even if the object (in his case a metal bar) was not warm.  The Thermal Grill Illusion is ideal for pain studies because it allows researchers to study the experience of pain without actually causing any injury to those who participate in the studies.

Patrick Haggard and Marjolein Kammersof University College London used the Thermal Grill Illusion to study the effects of self-touch in people regarding pain sensations.   

"The TGI is one of the best-established laboratory methods for studying pain perception," Haggard explained. "In our version, the index and ring fingers are placed in warm water and the middle finger in cold water. This generates a paradoxical feeling that the middle finger is painfully hot."

When the Thermal Grill Illusion was induced in an individual's two hands and then the three fingers of one hand were touched to the same fingers on the other hand immediately afterwards, the painful heat experienced by the middle finger dropped by 64 percent compared to a condition without self-touch. That relief didn't come when only one hand was placed under Thermal Grill Illusion conditions.

Partial self-touch in which only one or two fingers were pressed against each other didn't work either. Nor did it work to press the affected hand against an experimenter's hand that had also been warmed and cooled in the same way.

"In sum," the researchers wrote, "TGI was reduced only when thermosensory and tactile information from all three fingers was fully integrated. That is, TGI reduction required a highly coherent somatosensory pattern, including coherence between tactile and thermal patterns and coherence of stimuli between the two hands."

Haggard said that earlier studies of chronic pain had suggested the importance of body representation in the experience of pain. For example, the phantom pain that is often felt following amputation of a limb appears to lessen with time as the brain converges on an updated representation of the body. Haggard said the new findings extend the important role of body representation to acute pain and may lead to a better understanding of the brain mechanisms involved in chronic pain as well.

The findings might be put to practical use, the researchers say. "Our work suggests that therapies aimed at strengthening the multisensory representation of the body may be effective in reducing pain," Haggard said.

Citation: Marjolein P.M. Kammers, Frédérique de Vignemont, Patrick Haggard, 'Cooling the Thermal Grill Illusion through Self-Touch', Current Biology, 23 September 2010 DOI: 10.1016/j.cub.2010.08.038


(1) Thunberg T., 'Förnimmelserne vid till samma ställe lokaliserad, samtidigt pågående köld-och värmeretning', Uppsala Läkfören Förh, 1896, 1: 489–95.