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2001 First Web Report
The so-called "I-function" which describes the brain's sense of self takes on interesting connotations when discussing phantom limbs and associated phantom pain. The loss of an arm or leg through amputation is not an easy experience to endure, and is even more difficult when the patient begins to feel sensations in their now missing limb. These feelings, sometimes referred to as "stump hallucination", is the subjective sensation, not arising from an external stimulus, that an amputated limb is still present (1). Although they no longer exist, patients perceive these limbs as still being essential components of their body-image, and continues to move in sync with their torso and other limbs.
For some amputees, these phantom sensations may be no more than painless distractions of pressure, warmth, and cold that do not interfere with their everyday lives. But for the majority of amputees, about 50% to 80% (2), they experience phantom pains that vary in classification from cramping, burning, tingling, shocking, shooting and stabbing pains. These episodes are severe enough to interfere with work, sleep and normal function and do require some kind of treatment. Phantom pain can occur anytime, from immediately after an amputation to several years later.
The powerful impression of a stable, embodied self is taken for granted. But it's an perception that's possible only because of the body image created by the brain. A significant element of that image is a mental map of the body surface, generated by the cortex using the sensory signals it receives from the skin. Other regions of the cortex control other components, such as the position of muscles and joints (proprioception), the intention to move, and also the viewing of the body's movements (3). But the brain's idea of itself can be distorted by the amputation of a limb. Since there is no visual feedback, initiating motor intention does not activate proprioreceptors (4). Over time, phantom limbs are deemed by the amputee to be overflexed, which causes a cramping pain. The question that arises is, if the inconsistency between the intention of the brain and the perception of the body's actions was to be resolved, could the phantom pain also be eliminated? Several theories have been developed over the years that have attempted to answer this question, most notably by Ronald Melzack and Vilayanur Ramachandran.
The earliest hypothesis regarding the cause of phantom limbs and pain was that of neuromas. These were thought to be nodules comprised of remaining nerves located at the end of the stump. These neuromas presumably continued to generate impulses that traveled up the spinal cord to portions of the thalamus and somatosensory domains of the cortex. As a result, treatment involved cutting the nerves just above the neuroma in an attempt to interrupt signaling at each somatosensory level (5). This and other related theories were deemed unsatisfactory because of the fact the phantom pain always returned, indicating that there was a more complex reason.
Psychologist Ronald Melzack developed the concept of the neuromatrix and the neurosignature. This idea held that the brain contained a neuromatrix or a network of neurons that analyzed the sensory information and allowed the perception of feeling (1). Then the neurosignature, which consisted of the three primary neural pathways (from the thalamus to the somatosensory cortex, from the reticular formation of brain stem to limbic system, and the parietal lobes) was activated and informed the brain that the detection of sensation were from itself. He also maintained that the neuromatrix, which was essentially a brain map of the body, was pre-wired by genetics. Melzack pointed to his research that showed that people born without a limb could experience phantom pain as well (6). He postulated that the brain was predisposed to believe that all its limbs existed and so sent out an output signal to it through the neural pathways in the neuromatrix. But because there was no limb, the brain acquired no sensory feedback, and in an attempt to compensate increased the intensity of its signals, which induced the phantom pain. These findings led Melzack to believe that "the body we perceive is in large part built into our brain-it's not entirely learned. In fact, you do not need the body to feel the body (6)."
Other researchers, such as Vilayanur Ramachandran had other answers to the question of phantom pain etiology. He was inspired by previous experiments by Michael Merzenich that had studied the homunculus (blueprint representation of the entire body surface, which identifies the locations of sensations felt on the skin) of monkeys. In these experiments, the sensory nerves in the arms of a group of monkeys were severed. It was found that that despite the lack of sensory input from the arm, the arm region of the body map in the cortex hadn't gone silent. Instead, signals from the face (next door on the map) had taken over for the phantom arm (3). They concluded that there were existing axon branches that become unmasked when normal input had disappeared (1) .
Ramachandran wondered if amputees who complained of phantom pain could be suffering from rearranged body maps, and formulated his cortical remapping theory. He examined the reorganized homunculus of patients with removed limbs. By using q-tips to brush the face of a patient, he was able to produce sensations in their phantom limb. So each time a patient smiled or scratched their face, they stimulated the arm region of the body map causing a sensation in their phantom limb (3). Paralysis occurred because even after the limb was severed, the brain continued to send signals commanding the missing limb to move. This created an illusion of movement as the brain still monitored the intention. As a result of the absence of visual and proprioceptive activation, the different signals feeding into the body image contradicted each other. Ultimately the brain learned to interpret the lack of response as paralysis of the non-existent limb (3).
Ramachandran reasoned that in order to treat the paralysis, it was necessary to eliminate the discrepancy by allowing the patient to see the movement they wanted to make. He developed an ingenious method using mirrors that provided the brain with this visual stimulation. A midvertical sagittal mirror was put in front of the patient, and they placed their remaining limb in an exact mirror-symmetric location opposite to their phantom limb (7). The reflection the intact limb was optically superimposed on the perceived location of the phantom limb (7). Ramachandran has been successful with helping patients ease the pain for their phantom limbs using this therapy. Six of ten patients instantly felt their paralyzed phantom limbs moving, and a few were able to shift their phantom limbs out of painfully awkward positions (7). One patient even managed to correct his body image, and his phantom limb eventually shrank away to nothing (7).
The occurrence of phantom limbs raises several questions about the I-Function. What happens to a person's sense of self when their homunculus is disrupted by the loss of a limb? Both Melzack's and Ramachandran's theories indicate that the visual and proprioceptive systems are essential in maintaining body image. The I-Function depends on accurate information from the body in order to maintain an correct concept of self. Melzack's neuromatrix/neurosignature theory proves that the assumption that sensations are produced only by external stimuli is incorrect. The brain does more than merely detect and analyze inputs. The phenomenon of phantom limbs and pain reveal that it can also create perceptual experience even when there are no external catalysts. Ramachandran's theory of cortical remapping demonstrates how brain maintains its self-image, and despite its appearance of durability and permanence, it is at the core nothing more than an ephemeral internal construct.
1)Phantom Homepage, Macalester Psychology page
2)Phantom Limb Pain, Electromyography: Applications in Physical Therapy Webpage
3)THEY DO IT WITH MIRRORS , From New Scientist
4) Harris, J. A. "Cortical Origin of Pathological Pain." in Lancet, vol. 354 (pg. 1464-1466) 1999
5)Scientific American, Ronald Melzack article
6)Discover Phantom limbs , Brief Article
7) Ramachandran, V.S. & Rogers-Ramachandran, D. "Phantom Limbs and Neural Plasticity." in Archives of Neurology. Vol. 57 (pg. 317-320), 2000, Ramachandran article