In a breakthrough that might also help to explain why amputees suffer phantom limb pain, scientists have hit the nail on the head in explaining why we clasp our two hands together after we injure one of them.
UCL scientist Patrick Haggard and his colleagues, writing in Current Biology, used a trick called the thermal grill illusion (TGI) to fool participants into experiencing a painful sensation in one of their fingers. This clever trick involves placing the longest finger into a cup of cool water (at 14 C) and the two adjacent fingers into warm water (at 43 C). Paradoxically, subjects report that the middle finger feels painfully hot.
This is probably because the warm sensations from the index and ring fingers partially shut off the cold sensations coming from the central finger. The reduction in the cold sensations then in turn makes the brain think that a different population of nerves, which signal painful heat, have become more active, so a burning sensation is experienced, despite the subject remaining totally uninjured.
Intriguingly, however, if the illusion is triggered in both hands simultaneously, and then the equivalent three fingers on both hands are brought into contact, finger pad to finger pad, the pain perception abruptly drops by over 60%. But this only works if both hands are subjected to the illusion at the same time, and does not work unless all three involved fingers are brought into contact, or if one of the hands is substituted by another individual. The researchers suspect that a specialised population of sensory brain cells, which integrate information from both sides of the body to produce a coherent body map, are responsible. "Correlated multisensory information is a major source of the sense of one's own body as a coherent ''self''. Our results show that coherence between hands, as well as across modalities, may contribute to pain modulation. The present study suggests that multisensory, multieffector information may rapidly boost coherence of cortical activity and thus reduce (thermal) pain," say the scientists.
Apart from its functional significance, this discovery could also have practical applications, such as in the treatment of phantom limb syndrome, a frequent complaint of amputees who experience painful sensations arising from their missing body part. The pain tends to reduce with time, perhaps as the amputee's brain re-creates a new coherent map. However, developing techniques to manipulate the map and achieve this new coherence sooner could help to reduce the duration of discomfort experienced by sufferers.