Read Online Phantoms in the Brain: Probing the Mysteries of the Human Mind by V. S. Ramachandran, Sandra Blakeslee - Free Book Online Page B
Authors: V. S. Ramachandran, Sandra Blakeslee
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is still there: hence the phantom limb and the notion that the accompanying pain arises because the neuromas are painful.
On the basis of this tenuous reasoning, surgeons have devised various treatments for phantom limb pain in which they cut and remove neuromas. Some patients experience temporary relief, but surprisingly, both the phantom and the associated pain usually return with a vengeance. To alleviate this problem, sometimes surgeons perform a second or even a third amputation (making the stump shorter and shorter), but when you think about this, it's logically absurd. Why would a second ampu−
tation help? You'd simply expect a second phantom, and indeed that's usually what happens; it's an endless regress problem.
Surgeons even perform dorsal rhizotomies to treat phantom limb pain, cutting the sensory nerves going into the spinal cord. Sometimes it works; sometimes it doesn't. Others try the even more drastic procedure of cutting the back of the spinal cord itself—a cordotomy—to prevent impulses from reaching the brain, but that, too, is often ineffective. Or they will go all the way into the thalamus, a brain relay station that processes signals before they are sent to the cortex, and again find that they have not helped the patient. They can chase the phantom farther and farther into the brain, but of course they'll never find it.
Why? One reason, surely, is that the phantom doesn't exist in any one of these areas; it exists in more central parts of the brain, where the remapping has occurred. To put it crudely, the phantom emerges not from the stump but from the face and jaw, because every time Tom smiles or moves his face and lips, the impulses activate the "hand" area of his cortex, creating the illusion that his hand is still there. Stimulated by all these spurious signals, Tom's brain literally hallucinates his arm, and perhaps this is the essence of a phantom limb.
29
If so, the only way to get rid of the phantom would be to remove his jaw. (And if you think about it, that wouldn't help either. He'd probably end up with a phantom jaw. It's that endless regress problem again.) But remapping can't be the whole story. For one thing, it doesn't explain why Tom or other patients experience the feeling of being able to move their phantoms voluntarily or why the phantom can change its posture. Where do these movement sensations originate? Second, remapping doesn't account for what both doctor and patient are most seriously concerned about—the genesis of phantom pain. We'll explore these two subjects in the next chapter.
When we think of sensations arising from skin we usually only think of touch. But, in fact, distinct neural pathways that mediate sensations of warmth, cold and pain also originate on the skin surface. These sensations have their own target areas or maps in the brain, but the paths used by them may be interlaced with each other in complicated ways. If so, could such remapping also occur in these evolutionarily older pathways quite independently of the remapping that occurs for touch? In other words, is the remapping seen in Tom and in Pons's monkeys peculiar to touch, or does it point to a very general principle—would it occur for sensations like warmth, cold, pain or vibration? And if such remapping were to occur would there be instances of accidental "cross−
34 / Phantoms in the Brain
wiring" so that a touch sensation would evoke warmth or pain? Or would they remain segregated? The question of how millions of neural connections in the brain are hooked up so precisely during development—and the extent to which this precision is preserved when they are reorganized after injury—is of great interest to scientists who are trying to understand the development of pathways in the brain.
To investigate this, I placed a drop of warm water on Tom's face. He felt it there immediately but also said that his phantom hand felt distinctly warm. Once, when the water accidentally trickled
On the basis of this tenuous reasoning, surgeons have devised various treatments for phantom limb pain in which they cut and remove neuromas. Some patients experience temporary relief, but surprisingly, both the phantom and the associated pain usually return with a vengeance. To alleviate this problem, sometimes surgeons perform a second or even a third amputation (making the stump shorter and shorter), but when you think about this, it's logically absurd. Why would a second ampu−
tation help? You'd simply expect a second phantom, and indeed that's usually what happens; it's an endless regress problem.
Surgeons even perform dorsal rhizotomies to treat phantom limb pain, cutting the sensory nerves going into the spinal cord. Sometimes it works; sometimes it doesn't. Others try the even more drastic procedure of cutting the back of the spinal cord itself—a cordotomy—to prevent impulses from reaching the brain, but that, too, is often ineffective. Or they will go all the way into the thalamus, a brain relay station that processes signals before they are sent to the cortex, and again find that they have not helped the patient. They can chase the phantom farther and farther into the brain, but of course they'll never find it.
Why? One reason, surely, is that the phantom doesn't exist in any one of these areas; it exists in more central parts of the brain, where the remapping has occurred. To put it crudely, the phantom emerges not from the stump but from the face and jaw, because every time Tom smiles or moves his face and lips, the impulses activate the "hand" area of his cortex, creating the illusion that his hand is still there. Stimulated by all these spurious signals, Tom's brain literally hallucinates his arm, and perhaps this is the essence of a phantom limb.
29
If so, the only way to get rid of the phantom would be to remove his jaw. (And if you think about it, that wouldn't help either. He'd probably end up with a phantom jaw. It's that endless regress problem again.) But remapping can't be the whole story. For one thing, it doesn't explain why Tom or other patients experience the feeling of being able to move their phantoms voluntarily or why the phantom can change its posture. Where do these movement sensations originate? Second, remapping doesn't account for what both doctor and patient are most seriously concerned about—the genesis of phantom pain. We'll explore these two subjects in the next chapter.
When we think of sensations arising from skin we usually only think of touch. But, in fact, distinct neural pathways that mediate sensations of warmth, cold and pain also originate on the skin surface. These sensations have their own target areas or maps in the brain, but the paths used by them may be interlaced with each other in complicated ways. If so, could such remapping also occur in these evolutionarily older pathways quite independently of the remapping that occurs for touch? In other words, is the remapping seen in Tom and in Pons's monkeys peculiar to touch, or does it point to a very general principle—would it occur for sensations like warmth, cold, pain or vibration? And if such remapping were to occur would there be instances of accidental "cross−
34 / Phantoms in the Brain
wiring" so that a touch sensation would evoke warmth or pain? Or would they remain segregated? The question of how millions of neural connections in the brain are hooked up so precisely during development—and the extent to which this precision is preserved when they are reorganized after injury—is of great interest to scientists who are trying to understand the development of pathways in the brain.
To investigate this, I placed a drop of warm water on Tom's face. He felt it there immediately but also said that his phantom hand felt distinctly warm. Once, when the water accidentally trickled
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