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 monkeys indicated cortical remapping significant enough that sensation for an entire amputated arm could be caused by stimulation of the face; this part of the cortex had taken over responding for the nonexistent arm. Such change in the nervous system would have to incorporate new neuronal growth.

Case studies described on Macalester's web site seem to support the possibility of this sort of neuroplasticity. Phantom limb sensation triggered by postoperative trauma has been documented; it is thought that this is a result of changes in the pattern of innervation in the limb, which results in reception by a different region of the cerebral cortex. Sensation in amputated arms as a result of facial stimulation, as described by Merzenich and Kaas, is relatively common, most likely because of the proximity of the facial and hand regions in the motor cortex.

Melzack's theory situates itself halfway between those of Pons and Merzenich and Kaas. He proposes the neuromatrix - a massive body of interconnected neurons found in all humans. Part of this concept is the neurosignature, based in the parietal lobe, which tells you when what you are feeling is a part of your body. This suggests, to a degree, that sensation is genetically predetermined. However, some who agree with the idea of the neuromatrix, but not with hardwiring, say that the neuromatrix is wired by experience and learning rather than by genetics. Hardwiring of the neuromatrix might explain phantom limb sensation limbs missing since birth, though.

So far, the only theory about phantom limb sensation that has been disproved is that the sensation is due to the neuromas (the ends of nerve severed during amputations) firing. Researchers, doctors, and neurologists have all tried various techniques to treat phantom limb sensation, particularly pain: ultrasound, physical therapy, medication, surgery, etc. To date, the most (though far from universal) success has been found with drugs. Beta-blockers, serotonin agonists, tricyclic antidepressants, and calcitonin have all worked for some patients. Yet, the only information doctors have is that elevated serotonin excites pathways that inhibit pain. Phantom limb pain is as much of a mystery to them as it is to the patients who can feel but not see their missing body parts.

WWW Sources

Pain of the Phantom Limb
Neurological Theories
Plasticity
Monkey Manipulation--Merzenich's research
Ramachandran's Theory on Cortical Remapping
Can't Get Enough Of Those Monkeys--Pons' research
What does it feel like to have a phantom limb? a casestudy.
How about a phantom arm? another case...
A case of delayed phantom limb presentation...
Touch the face and feel the amputated hand...
A Remarkable Case of Phantom Limbs