Amputees who experience phantom limb pain could find relief in a surprisingly simple way - by paying more attention to the people around them.
Phantom limbs occur when an amputee feels the often painful sensation of touch arising from a limb that is no longer present. Working with combat veterans, Vilayanur Ramachandran, of the Center for Brain and Cognition at the University of California, San Diego, has now discovered a potential cure.
His treatment makes use of the newly discovered properties of mirror neurons. Mirror neurons fire when a person performs an intentional action - such as waving - and also when they observe someone else performing the same action. They are thought to help us predict the intentions of others by creating a "virtual reality" simulation of the action in our minds.
"You also find cells like this for touch," says Ramachandran. "They fire when you touch yourself and when you watch someone else being touched in the same location."
This begs the question: if the same touch neurons fire when you rub your hand as when you watch somebody else rubbing their hand, why is it that we don't constantly go around "feeling" what we are watching?
Ramachandran proposed that messages from sensory cells in the hand would partially inhibit the output of mirror neurons, preventing the message from going to higher centres of the brain.
"They're telling the brain: 'I feel your touch in some abstract way but not in a literal sense'," he says. "This mechanism allows you to simultaneously empathise and recognise that someone else is being touched but not think you are being touched yourself. "
To test this theory, Ramachandran and his colleague and wife Diane Rogers-Ramachandran used a "mirror box" - a tool that creates the visual illusion of two hands for people who actually only have one. By placing an amputee's arms either side of a mirror - with the missing limb on the non-reflective side, the amputee sees the reflection of their normal hand superimposed on the location of their missing hand.
Two amputees watched their normal hand being prodded, and both felt the remarkable sensation of "being prodded" in their missing hand. In another experiment, when the amputees watched a volunteer's hand being stroked, they too began to experience a stroking sensation arising from their missing limb.
The amputees "felt" the actions of others because their missing limb provided no feedback to partially inhibit their mirror neurons, no longer telling them that they were not "literally" being touched, says Ramachandran.
One subject also reported that watching a volunteer rubbing her hand caused the cramping sensation within the phantom limb to cease for 10 to 15 minutes. "If you do it often enough perhaps this pain will go away for good," suggests Ramachandran.
"If an amputee experiences pain in their missing limb, they could watch a friend or partner rub their hand to get rid of it."
Massaging the skin helps relieve a painful sensation by restoring blood flow and activating sensory fibres, which inhibit pain messages to the brain. By watching another person rubbing their hand, these amputees are apparently tapping into this latter mechanism, says Ramachandran.
The number of amputations as a result of conflict are increasing globally. In Iraq, for example, amputations are performed on 6% of wounded US soldiers, twice the amount as has been seen in other conflicts. But Ramachandran says there could be broader applications to the work than helping amputees.
"If performed early enough, this type of therapy may also be used to help stroke patients regain movements by watching others perform their lost actions," Ramachandran suggests.
The research has implications that go beyond the case of amputees, agrees Valeria Gazzola, at the School of Behavioural and Cognitive Neurosciences' Neuro Imaging Center at the University of Groningen, the Netherlands.
"Ramachandran has provided a very plausible answer to other problems such as why echopraxics imitate most of the actions they observe, although it will be important to see the full-length papers on the topic before relating it to other disorders."
Journal ref: Medical Hypotheses, DOI: 10.1016/j.mehy.2008.01.008