Inside the human brain there is a map of the body drawn in neural tissue. When a person loses a limb, the neural representation of that body part still exists in the map, and more often than not, it continues to give rise to “phantom” sensations. Sometimes neurons in adjacent areas of the body map invade the tissue that represents the missing limb. This can lead to the curious situation where stimulation of a person’s face (or other areas) provokes feelings in their phantom limb, as documented by the great neuroscientist V.S. Ramachandran. Cases like this are often cited as evidence for the brain’s plasticity.
Now Ramachandran and his colleague Paul McGeoch have reported a phantom limb case that illustrates how aspects of the body map are apparently hard-wired. The case is a 57-year-old woman (known as R.N.) who was born with a deformed right hand consisting of only three fingers and a rudimentary thumb. After a car crash at age 18, R.N.’s deformed hand was amputated, which gave rise to feelings of a phantom hand. Curiously, R.N. experienced her phantom hand as having a full complement of five fingers, albeit that some of the digits were foreshortened. In other words, she was experiencing the sensation of having fingers that she’d never physically possessed.
R.N. was referred to the researchers more than 35 years after her accident, after her phantom hand had become unbearably painful and uncomfortable, including two of the fingers feeling as if they’d become twisted and bent until their tips touched. McGeoch and Ramachandran trained R.N. in using “mirror visual feedback”, in which the reflection of her healthy left-hand was seen as superimposed onto where she felt her phantom right hand to be. After two weeks of 30-minutes daily feedback, R.N. was able to move her phantom fingers and was relieved of pain. Crucially, she also experienced that all five of her phantom fingers were now normal length.
McGeoch and Ramachandran said this case provides evidence that the brain has an innate template of a fully-formed hand. Freed from the visual, proprioceptive and tactile sensations of her deformed hand, and aided by the mirror training, R.N.s brain re-instated its innate map of a normal hand. “There appears to be a ‘hard-wired’ innately specified scaffold for body image,” the researchers said. This account also helps explain the occurrence of phantom limbs in people born with missing limbs.
The researchers conceded that they were taking R.N.s account of her feelings on trust. It’s possible she was confabulating – although they think this unlikely. If she were, McGeoch and Ramachandran think it more likely that R.N. would have claimed to have had normal length fingers prior to the mirror training.
McGeoch, P., and Ramachandran, V. (2012). The appearance of new phantom fingers post-amputation in a phocomelus. Neurocase, 18 (2), 95-97 DOI: 10.1080/13554794.2011.556128