Describing Phantom Limb Experience
Of people who have had body parts amputated, about 80 percent experience some sort of phantom limb sensation. This experience, which can range from severe shooting pain to merely feeling the presence of the absent limb, most often occurs in amputees but sometimes manifests itself in individuals whose limbs have been missing since birth. The sensations patients experience are not necessarily of the same strength, location, or duration from occurrence to occurrence, and the frequency of episodes often fluctuates over time. Especially in the case of amputees, who have lived a significant portion of their lives with the limb in question, it would make sense that there be a psychological element to phantom limb sensation. This notion is corroborated by the fact that phantom limb sensation is rare in children under the age of four; it is thought that these children are young enough to not see the loss of a body part as so significant a trauma who has.lived with full use of the limb for so long. Yet this phantom limb sensation also has a physiological component. Both pain and light-touch sensations (both of which phantom limbs may feel) are the result of impulses traveling through the thalamus, which relays the information the cerebral cortex, where sensations are mapped.
This mapping is believed to be done on what the Macalester College Psychology Department calls a (somatosensory) "homunculus." Neurologists think that in the cerebral cortex is a map of the human body, where certain impulse locations correlate with specific locations on the body. That is, cortical regions represent individual parts of the body. The amputation, or even the congenital lack, of a body part, would be problematic to this homunculus, eliminating the physical location to which the cerebral cortex mapped the sensation. This "wiring" between the cerebral cortex and the rest of the body has been the focus of research on phantom limb pain. From this research, several theories have emerged.
Merzenich and Kaas amputated monkeys' fingers and expected to find no nervous response in the part of the homunculus whose bodily analog no longer existed. Instead, this region of the cerebral cortex fired when the fingers next to the amputation site were stimulated. Their conclusion was that preexisting axon branches that had innervated the region were "unmasked," which allows for nervous response without any new neuronal growth. This idea upheld Hubel and Wiesel's concept of the "hardwired" brain, in which the cerebral cortex's map is set at birth. Ramachandran also had a similar idea of "hidden circuitry," arguing that phantom limb sensation could not be the result of new neuronal growth as the sensation's onset was often as soon as a month or two after amputation, which be enough time for the brain to remap but not for new cells to develop.
Pons' research, however, came to conclusions that challenged this hardwired model. His work with...