Phantom Limb - Neurological Basis

Neurological Basis

Until recently, the dominant theory for cause of phantom limbs was irritation in the severed nerve endings (called "neuromas"). When a limb is amputated, many severed nerve endings are terminated at the residual limb. These nerve endings can become inflamed, and were thought to send anomalous signals to the brain. These signals, being functionally nonsense, were thought to be interpreted by the brain as pain. Treatments based on this theory were generally failures. In extreme cases, surgeons would perform a second amputation, shortening the stump, with the hope of removing the inflamed nerve endings and causing temporary relief from the phantom pain. But instead, the patients' phantom pains increased, and many were left with the sensation of both the original phantom limb, as well as a new phantom stump, with a pain all its own. In some cases, surgeons even cut the sensory nerves leading into the spinal cord or in extreme cases, even removed the part of the thalamus that receives sensory signals from the body.

By the late 1980s, Ronald Melzack had recognized that the peripheral neuroma account could not be correct. In his 1989 paper, "Phantom Limbs, The Self And The Brain" Melzack proposed the theory of the "neuromatrix." According to Melzack the experience of the body is created by a wide network of interconnecting neural structures. In 1991, Tim Pons and colleagues at the National Institutes of Health (NIH) showed that the primary somatosensory cortex undergoes substantial reorganization after the loss of sensory input. Hearing about these results, Vilayanur S. Ramachandran theorized that phantom limb sensations could be due to this reorganization in the somatosensory cortex, which is located in the postcentral gyrus, and which receives input from the limbs and body. Ramachandran and colleagues illustrated this theory by showing that stroking different parts of the face led to perceptions of being touched on different parts of the missing limb.

Ramachandran argued that the perception of being touched in different parts of the phantom limb was the perceptual correlate of cortical reorganization in the brain. However, research published in 1995 by Flor et al. demonstrated that pain (rather than referred sensations) was the perceptual correlate of cortical reorganization. In 1996 Knecht et al. published an analysis of Ramachandran's theory that concluded that there was no topographic relationship between referred sensations and cortical reorganization in the primary cortical areas Recent research by Flor et al. suggests that non-painful referred sensations are correlated with a wide neural network outside the primary cortical areas.