As mentioned above, amputation is not essential for the occurrence of a phantom. Brachial plexus avulsion is a condition most often resulting from motorcycle accidents, where all nerves from the arm are ripped from the spinal cord. In such a case the phantom, usually extremely painful, occupies the immobile arm and seems coordinated with it unless the arm is moved unbeknownst the owner. Similarly, paraplegics, individuals who have experienced a complete break of the spinal cord and thus have no feeling or control over their body below the break-line, also often experience having phantom limbs. A similar experience also occurs with patients whose spinal cords have been anesthetized, such as by spinal block during labor (3).
Several theories have been proposed regarding the cause of phantom limbs, although there is still much to be learned and understood. The oldest explanation for phantom limbs is based upon neuromas, the nodules that result at the end of severed nerves in the remaining limb stump. These neuromas were said to keep on firing signals to the spinal cord and parts of the thalamus to the somatosensory cortex (7). This theory was later disproved when neuromas, nerve roots, pathways within the spinal cord, and the areas of the thalamus and cortex that ultimately receive sensory information from the limb, were removed or severed as attempts to cease impulse transmissions at every level of the somatosensory projection system. These procedures not only failed in permanently relieving phantom pains, but did nothing to effect the phantom limb itself at all (3).
A related hypothesis moves on to the spinal cord as a possible initiator of the phantom sensations, resting on the possibility that spinal cord neurons, which have lost their normal sensory input from the body, tend to generate high-level spontaneous impulses. This output is transmitted to the cortex as if the spinal neurons had received an external stimulation. However, this does not explain how paraplegics experiencing phantom sensations below the spinal cord break. The spinal neurons which carry messages from those lower areas to the brain originate well below the level of the break, and therefore any nerve impulses arising from those neurons cannot possibly traverse the break to the brain (3).
Recent work has led researchers to look into the thalamus and the somatosensory cortex within the brain itself, as the root cause of phantom sensations. In 1992, Ronald Melzack introduced a theory attesting that phantom pain could be explained through genetics and the neuromatrix (8). Melzack proposed that a neuromatrix, a network many interconnected neurons, exists in every person (7). This neuromatrix, in addition to responding to sensory stimulation, continuously generates a characteristic pattern of impulses indicating that the body is intact, and identifies the body as its own (1). Melzack calls this pattern of self-identification a neurosignature (8), which is imprinted on the output signal carrying information about the sensory input (3).
Melzack’s theory is based on the supposition that the neuromatrix and its image of the human body is pre-wired by genetics. This blueprint assumes that the human body is complete with all limbs intact, whether or not in actuality the person does possess all the limbs to constitute a complete body. Therefore if the brain expects a limb is there, it might send an output signal to the limb through certain neural pathways in the neuromatrix. Because there is no limb, the brain receives no sensory feedback. The brain will then subsequently increase the intensity of its signals, thus causing phantom pain (7). This theory successfully explains cases of individuals who were born without limbs and still experience phantom pain (8).
The homunculus, located in the sensory cortex of the brain, is what contains the actual "blueprint" representation of the entire body surface, and identifies the locations of sensations felt on the skin. Thus a pinch on the left index finger stimulates a location on the homunculus which represents the left index finger . If the finger is amputated and an input signal is started at any point along the remaining nerve pathway between the stump and homunculus, the resulting sensation would seem to emanate from the phantom finger (1). Therefore the long-accepted theory of the hard-wired adult brain is rapidly changing. Michael Merzenich, Ph.D. and Vilayanur Ramachadran were the among the firsts to challenge this idea through their research.
In his experiments, Merzenich amputated the finger of a monkey and showed that neurons within the relevant region of the homunculus fired anytime the two fingers adjacent to the amputated one were touched. Where we would expect no neural activity reaching the section of the monkey's cortical map for the amputated finger, Merzenich concluded that there are existing axon branches that become unmasked when normal input ceases (11). It has also been found that patients with phantom limbs will often mislocalize sensations from the face to the phantom (4).
Ramachadran's theory on Cortical re-mapping poses the idea that, in response to peripheral injury, neurons grow and regenerate in the homunculus (9). To show this, Ramachadran performed an experiment using Q-Tips and discovered that regions of an amputated hand are mapped out on different areas of the body. For example, by brushing the amputee's chin with a Q-tip, the subject would feel the sensations in specific areas of the phantom limb. Because the amount of time after the amputation was too short to allow new neurons to generate and re-map, Ramachadran believes that the neural circuitry associated with cortical re-mapping is already present and waiting to activate. An amputation causes activity in the specific area of the homunculus to cease, allowing "hidden" neural circuits to activate and link other areas of the body through that supposedly inert section of the homunculus (9).
Further studies have been conducted at the University of Toronto and the Toronto University Hospital in 1998, investigating the re-mapping component in the brain. The study involved amputees who experienced phantom limbs to undergo surgery to map the sensory areas in the brain. During the mapping process the patients were able to report sensations they felt when certain areas of the thalamus were stimulated. Patients reported phantom sensations when the areas of the thalamus which were formally innervated by neurons from the missing arm, were stimulated. Similar sensations were reported when areas on the stump were stimulated which then activated the reorganized regions in the brain. This study enabled researchers to learn about sensory output from the thalamus and the ability of neurons to adapt to change.
Whereas it was once thought that the brain retained its plasticity (the ability of neurons to adapt to change (6)) only during neonatal or juvenile periods, we have now discovered through recent studies that the brain is more malleable than previously thought. Over the next few decades, this research will pave the way for the possible recovery of those with lost bodily functions due to brain damage, according to Tim Pons, Ph.D. (2). In the meantime however, this reorganization of the sensory cortex is what is currently thought to be responsible for phantom limb pain (10).
2) Damaged Area of Brain can Reorganize Itself
3) Phantom Limbs Melzack, Ronald Ph.D.
5) Macalester College Department of Psychology Phantom Limb Homepage
6) New Piece to Puzzle in Phantom Pain Mystery
8) Bach, Beethoven and the Neuromatrix
10) Cortical Reorganization and the "Phantom Limb"