Biology 202, Spring 2005
First Web Papers
On Serendip

In a quest to separate ourselves from other species, we have attempted to distinguish humans as somehow special and superior. Before Nicholas Copernicus (1473-1543), it was assumed that we lived at the center of the universe. It is still widely accepted that humans have unique, more advanced capabilities that other species, even other higher primates, must lack. Philosophers and scientists have for years have attempted to tackle the problems of consciousness—perhaps one of the most important concepts in what it means to be human. Most animals (and to an extent, other organisms including plants) have some level of "awareness" in that they can sense and respond to environmental stimuli, although it is not commonplace to describe their behaviors in terms of awareness. Humans possess another type of awareness: awareness of themselves, both in terms of their body interacting with their environment and of their body as being whole and belonging to them. To explore this sense of self, cases where the body is an altered state from how the brain perceives it are particularly useful. Using the phantom limbs phenomenon as a model, can we put the self in the brain?

The occurrence of phantom limbs has been described throughout history. Silas Weir Mitchell first coined the name 'phantom limb' in 1871, after soldiers in the Civil War described vivid sensations in limbs that had been amputated after injury (5). Experiencing a phantom limb can, quite understandingly, cause a great deal of distress. The feeling is precise in its location and qualitatively very real. The most commonly reported sensations are pressure, warmth or cold, dampness, itchiness, and many different kinds of pain. As many as 70% of amputees experience pain in their missing limb (3). Phantom limbs often move in coordination with the rest of the body, behaving essentially as the normal limb would. For example, a phantom leg will bend when the amputee is in a sitting position. In some cases, though, the limb may feel stuck in an awkward position. In one case, a man turned sideways whenever moving through a doorway to avoid hitting the wall with his phantom limb, which he believed was stuck at a right angle from his body (3). These varying sensations persist throughout the course of the amputee's life. Children and adolescents also experience phantom limbs, especially in the case of traumatic amputations. One study found that 33% of individuals who were born without a limb or who had an amputation in the first 6 years of life still experienced phantoms (7). This finding is perhaps one of the most controversial, as it casts into doubt theories which explain phantom limb phenomenon in terms of nerve memory.

René Descartes (1596-1650) used phantoms as proof of his argument that all sensation is located in the brain. He reasoned that the "the whole mind seem[ed] to be united to the whole body", but noted that "if a foot or arm...is cut off, nothing has thereby been taken away from the mind" (1). Descartes believed the mind is unified, while the body is more fragmentary in nature. More recently, scientists in the field of cognitive informatics have described the mind's relationship to the brain as that of a program to a computer. Wang believed phantom limbs shows that the mind is "partially programmed and partially wired" and that phantom sensations occur at the lower (programmed) level of the brain which cannot be eliminated, even though higher layers may be overridden or modified (6). Descartes and Wang are essentially saying the same thing, although their terms (and potentially, their underlying beliefs about the mind/brain distinction) differ: there is something in the brain which causes sensation in the missing limbs and perhaps 'believes' this limb is still present.

One of the most fascinating characteristics of phantom limbs is that they are experienced as part of the self, not as foreign entities by any means. The missing arm or leg feels very much like part of the amputee's body—it belongs to them. This is not unique to amputees; phantoms occur in numerous other conditions, including paraplegia, brachial plexus avulsion (where the nerves from the arm are disconnected from the spinal cord), and in patients whose spinal cords have been anesthetized. In these cases, the spinal cord is not operating normally by sending signals to the brain from the body. Yet they experience sensations in the affected extremities and identify those parts of the body as their own. This suggests that the sensations and the affected person's experience of their limbs in a certain position is generated in the brain.

A series of observations of brain lesions have highlighted the role of the right hemisphere in the sense of self. In a comparison left hemisphere (LH) and right hemisphere (RH) lesions, acute lesions on the RH caused more severe disruptions in self-image and self-awareness in relating the self, visually and emotionally, to the environment (2). In left-sided hemispatial motor neglect, lesions on the right side of the brain impair the movements of extremities on the left side of the body. The muscles are still functional and can be activated on commands from an observer. When the same lesion occurs on the LH, however, the RH is able to accommodate and activate the right-sided muscles. This suggests that the defect ultimately stems from impairment of the sense of self, which occurs with right-side damage. Unilateral neglect is more extreme; it can affect visual, tactile, and auditory stimuli in any combination. Again, the motor and sensory functions are still in tact, yet cannot be initiated or activated on the patient's own. In the case of Anosognosia, the affect patient can no longer identify their left arm as belonging to them and experience hemiparesis (weakness or slight paralysis) in the left-side. What is most remarkable in such cases is that the LH is oblivious to the damage and patients continue to hold false beliefs about their abilities even in the face of information (from doctors and from their own visual input). Patients with phantom limbs are able to 'correct' their feelings through visual and tactile input. Some have even found tactile input beneficial in coping with painful phantom sensations; massaging the stump and applying heat decrease the pain (2). Their sense of self is intact; they experience their sensations as part of them. Anosognosia, however, is essentially a delusional disorder and their sense of self is severely disturbed.

Another interesting case is that of the phantom supernumerary limb, sometimes called the 'alien hand syndrome'. As the name implies, people experiencing a phantom supernumerary limb experience an additional limb. Both the real and imagined limbare perceived simultaneously and, similarly to other phantoms, these sensations persist over time. The sense of self under normal conditions results in one, unified concept of the self and the body. With supernumerary phantoms, there are multiple conscious representations of a particular body part. McGoinigle, et. al studied one particular case, that of subject E.P., a stroke patient with a right frontomesial lesion. She experienced a 'ghost' left arm which she described as "having a will of its own," and less often, a ghost left leg. This began immediately after her operation. Similarly to phantom limb sufferers, E.P.'s perception of the ghost is "cancelled by vision of the normal left arm or leg" and prevented by "continual tactile stimulation" (4). Unlike phantom limbs, the supernumerary limb is the result of brain damage. E.P.'s phantom does not feel like an ordinary limb, but rather interferes with her body and feels out of her control. Her phantom conflicts with her sense of self and this distortion occurred after a RH lesion.

There are several theories which aim to explain phantom limbs and similar phenomenon in terms of the nervous system. Nerve memory is an older theory which essentially states that the stump of amputees continues to generate impulses. These impulses act as normal sensory signals, sending a message through the spinal cord and into the thalamus and cortex in the form of action potentials and resulting in sensations at the specific site. Yet how can there be a 'memory' in the case of people born without a limb? Another blow to this theory is the unsuccessful treatments for pain that cut the nerves above the neuroma, or at the roots where the sensory nerves divide into smaller branches before entering the spinal cord (3). Amazingly, the pain returns and the presence of the phantom itself never disappears. This follows from the phantoms that occur in paraplegics and other such patients, making it very unlikely that nerve endings alone can account for phantoms. Melzack has recently proposed that the brain contains a "neuromatrix" which responds to sensory inputs and "continuously generates a characteristic pattern of impulses indicating that the body is intact and unequivocally one's own" (3). He calls this pattern a "neurosignature" and suggests that that its connections are largely prewired and determined by genes. He does acknowledge a role for experience in modifying the synapses of the neuromatrix. In other words, connections may be added, deleted, strengthened, or weakened, through experience. His model implies that the neuromatrix can function on its own—the brain is generating its own inputs in the absence of external stimuli. His proposal is a good fit with our current observations; it will be interesting to see how it holds up over time and with new information.

The right hemisphere has been implicated in various conditions affecting the sense of self and of the body in relation to space, yet it cannot be fully understood without considering the role of the left hemisphere. The LH has been identified as the site of language and communication processes. Although beyond the scope of this paper, language may play an important role in consciousness as a whole. The specialization of language in the LH may by responsible for the RH's dominance in self-awareness via a "crowing out" mechanism (2). At some point in our evolutionary history, both hemispheres may have been responsible for our perception of self and of spatial relations. The RH may have become increasingly responsible for these skills, as development of language took place in the LH. This could have several implications, including the possibility that animals lacking language and communication in the traditional sense may still have a self-awareness, although without the lateralization seen in humans. The concept of animals possessing awareness is a difficult pill for many to swallow. And what would this mean for our use of animals in experiments and the notion of humane treatment? Perhaps sense of self does not make humans special at all; but if the search for it will likely lead us to the brain.

References

1)Descartes, R. The Philosophical Writings of Descartes, Volume 2. Trans. J Cottingham, D Murdoch, and R Stoohoff. Cambridge: Cambridge P, 1984.

2)Devinsky, O. "Right cerebral hemisphere dominance for a sense of corporeal and emotional self." Epilepsy & Behavior 1. (2000): 60-73.

3)Melzack, R. "Phantom limbs". Scientific American: Mysteries of the Mind.(1997): 85-91.

4)McGoinigle, D.J., et al. "Whose arm is it anyway? An fMRI case study of supernumerary phantom limb." Brain 125. (2002): 1265-1274.

5)Wade, N.J. "The legacy of phantom limbs". Perception 32. (2003): 517-524

6)Wang, Y. "On cognitive informatics." Brain and Mind 4. (2003): 151-167.

7)Wilkins, K.L., et al. "Phantom limb sensations and phantom limb pain in child and adolescent amputees." Pain 78. (1998): 7-12.


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