A team of Japanese and Australian researchers have conducted new studies on how phantom limbs form that show there is no default position that the phantom limb moves into after it forms as often thought, but instead coalesces into positions that are dependent on experiences the limb undergoes while sensation is lost. The results were published in the December issue of The Journal of Physiology.
“Our research suggests that the state of nerves in the limb at the time the phantom is forming is very important in determining how the phantom develops,” said Simon Gandevia, MD, PhD, DSc, FAA, FRACP, deputy director and foundation scientist from Neuroscience Research Australia (NeuRA), Randwick, and corresponding author.
Because distortions of body image, such as phantom limbs, are difficult to treat, a better understanding of the mechanisms behind their formation will help in developing more effective treatments.
Gandevia and colleague Lee Walsh, PhD, a NeuRA research officer, conducted studies in which they temporarily anaesthetized participants’ hands to induce a phantom limb. The team found that the state of the nerves-i.e. how much they were being stimulated-over the period when participants were losing sensation in their hand was key in determining the final perceived position of the phantom hand.
“This might also be true for phantom pain,” Gandevia said. “In other words, the amount and type of nerve stimulation around the time of amputation or injury could also be important in determining the type and degree of pain perceived in the phantom limb.”
Walsh told the Australian-based The Conversation, that phantom limbs are believed to form because the brain keeps maps, or representations, of the body. “Even though we cut off sensory information below the elbow, the brain still has a map that says below the elbow is an arm and a hand, four fingers, and a thumb,” Walsh said. “What we think is happening is that the brain holds onto that map but certain things can still manipulate that map-certain things that don’t depend on sensory information. Or in this case, because the sensory information is removed slowly over time, we think the brain is actually interpreting that slow removal as a change in the position of the hand.”
While the current study was not clinically oriented, it did have relevance to clinical inquiries, Walsh said. “An amputee’s phantom limb can hurt quite a lot-but pain is just another sensory signal so from a clinical point of view there would be interest in whether you could manipulate sensation to help alleviate some of the symptoms, and there are other conditions that upset these body maps and representations that are completely unrelated to amputations: strokes, for example, can upset how you perceive your body. It could help give us an understanding of how those maps develop.”
