Phantoms in the Brain: Probing the Mysteries of the Human Mind (8 page)

Figure 2.3
Magnetoencephalography (MEG) image superimposed on a magnetic resonance (MR) image of the
brain in a patient whose right arm was amputated below the elbow. The brain is viewed from the top. The
right hemisphere shows normal activation of the hand (hatched), face (black) and upper arm (white) areas of
the cortex corresponding to the Penfield map. In the left hemisphere there is no activation corresponding to
the missing right hand, but the activity from the face and upper arm has now "spread" to this area.

these frayed nerve endings form little clumps of scar tissue called neuromas, which can be very painful. When neuromas are irritated, the theory goes, they send impulses back to the original hand area in the brain so that the brain is "fooled" into thinking the hand 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.

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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−

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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 down his face, he exclaimed with considerable surprise that he could actually feel the warm water trickling down the length of his phantom arm. He demonstrated this to me by using his normal hand to trace out the path of the water down his phantom. In all my years in neurology clinics, I had never seen anything quite so remarkable—a patient systematically mislocalizing a complex sensation such as a "trickle" from his face to his phantom hand.

These experiments imply that highly precise and organized new connections can be formed in the adult brain in a few days. But they don't tell us how these new pathways actually emerge, what the underlying mechanisms are at the cellular level.

I can think of two possibilities. First, the reorganization could involve sprouting—the actual growth of new branches from nerve fibers that normally innervate the face area toward cells in the hand area in the cortex. If this hypothesis were true, this would be quite remarkable since it is difficult to see how highly organized sprouting could take place over relatively long distances (in the brain several millimeters might as well be a mile) and in such a short period. Moreover, even if sprouting occurs, how would the new fibers "know" where to go? One can imagine a higgledy−piggledy jumble of connections, but not precisely organized pathways.

The second possibility is that there is in fact a tremendous redundancy of connections in the normal adult brain but that most of them are nonfunctional or have no obvious function. Like reserve troops, they may be called into action only when needed. Thus even in healthy normal adult brains there might be sensory inputs from the face to the brain's face map
and
to the hand map area as well. If so, we must assume that this occult or hidden input is ordinarily inhibited by the sensory fibers arriving from the real hand. But when the hand is removed, this silent input originating from the skin on the face is unmasked and allowed to express itself so that touching the face now activates the hand area and leads to sensations in the phantom hand. Thus every 30

time Tom whistles, he might feel a tingling in his phantom arm.

We have no way at present of easily distinguishing between these two theories, although my hunch is that both mechanisms are at work. After all, we had seen the effect in Tom in less than four weeks and this seems too short a time for sprouting to take place. My colleague at the Massachusetts General Hospital Dr. David Borsook9 has seen similar effects in a patient just twenty−four hours after amputation, and there is no question of sprouting's occurring in such a short period. The final answer to this will come from simultaneously tracking perceptual changes and brain changes (using imaging) in a patient over a period of several days. If Borsook and I are right, the completely static picture of these maps that you get from looking at textbook diagrams is highly misleading and we need to rethink the meaning of brain maps completely. Far from signaling a specific location on the skin, each neuron in the map is in a state of dynamic equilibrium with other adjacent neurons; its significance depends strongly on what other neurons in the vicinity are doing (or not doing).

These findings raise an obvious question: What if some body part is lost other than the hand? Will the same kind of remapping occur? When my studies on Tom were first published, I got many letters and phone calls from amputees wanting to know more. Some of them had been told that phantom sensations are imaginary and were relieved to learn that that isn't true. (Patients always find it comforting to know that there is a logical explanation for their otherwise inexplicable symptoms; nothing is more insulting to a patient than to be told that his pain is "all in the mind.")

One day I got a call from a young woman in Boston. "Dr. Ramachandran," she said, "I'm a graduate student at Beth Israel Hospital and for several years I've been studying Parkinson's disease. But recently I decided to switch to the study of phantom limbs."

"Wonderful," I said. "The subject has been ignored far too long. Tell me what you are studying."

"Last year I had a terrible accident on my uncle's farm. I lost my left leg below the knee and I've had a phantom limb ever since. But I'm calling to thank you because your article made me understand what is going on." She cleared her throat. "Something really strange happened to me after the amputation that didn't make sense. Every time I have sex I experience these strange sensations in my phantom foot. I didn't dare tell anybody because it's so weird. But when I saw your diagrams, that in the brain the foot is next to the genitals, it became instantly clear to me."

She had experienced and understood, as few of us ever will, the remapping phenomenon. Recall that in the Penfield map the foot is beside the genitals. Therefore, if a person loses a leg and is then stimulated in the genitals, she will experience sensations in the phantom leg. This is what you'd expect if input from the genital area were to invade the territory vacated by the foot.

The next day the phone rang again. This time it was an engineer from Arkansas.

"Is this Dr. Ramachandran?"

"Yes."

"You know, I read about your work in the newspaper, and it's really exciting. I lost my leg below the knee about two months ago but there's still something I don't understand. I'd like your advice."

"What's that?"

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"Well, I feel a little embarrassed to tell you this."

I knew what he was going to say, but unlike the graduate student, he didn't know about the Penfield map.

"Doctor, every time I have sexual intercourse, I experience sensations in my phantom foot. How do you explain that? My doctor said it doesn't make sense."

"Look," I said. "One possibility is that the genitals are right next to the foot in the body's brain maps. Don't worry about it."

He laughed nervously. "All that's fine, doctor. But you still don't understand. You see, I actually experience my orgasm in my foot. And therefore it's much bigger than it used to be because it's no longer just confined to my genitals."

Patients don't make up such stories. Ninety−nine percent of the time they're telling the truth, and if it seems incomprehensible, it's usually because we are not smart enough to figure out what's going on in their brains.

This gentleman was telling me that he sometimes enjoyed sex
more
after his amputation. The curious implication is that it's not just the tactile sensation that transferred to his phantom but the erotic sensations of sexual pleasure as well. (A colleague suggested I title this book "The Man Who Mistook His Foot for a Penis.")

This makes me wonder about the basis of foot fetishes in normal people, a subject that—although not exactly central to our mental life— everyone is curious about. (Madonna's book,
Sex,
has a whole chapter devoted to the foot.) The traditional explanation for foot fetishes comes, not surprisingly, from Freud. The penis resembles the foot, he argues, hence the fetish. But if that's the case, why not some other elongated body part?

Why not a hand fetish or a nose fetish? I suggest that the reason is quite simply that in the brain the foot lies right next to the genitalia. Maybe even many of us so−called normal people have a bit of cross−wiring, which would explain why we like to have our toes sucked. The journeys of science are often tortuous with many unexpected twists and turns, but I never suspected that I would begin seeking an explanation for phantom limbs and end up explaining foot fetishes as well.

Given these assumptions, other predictions follow.10 What happens when the penis is amputated? Carcinoma of the penis is sometimes treated with amputation, and many of these patients experience a phantom penis—sometimes even phantom erections! In such cases you would expect that stimulation of the feet would be felt in the phantom penis. Would such a patient find tap dancing especially enjoyable?

What about mastectomy? An Italian neurologist, Dr. Salvatore Aglioti, recently found that a certain proportion of women with radical mastectomies experience vivid phantom breasts. So, he asked himself, what body parts are mapped next to the breast? By stimulating adjacent regions on the chest he found that parts of the sternum and clavicle, when touched, produce sensations in the phantom nipple. Moreover, this remapping occurred just two days after surgery.

Aglioti also found to his surprise that one third of the women with radical mastectomies tested reported tingling, erotic sensations in their phantom nipples when their earlobes were stimulated. But this happened only in the phantom breast, not in the real one on the other side. He speculated that in one of the body maps (there are others besides the Penfield map) the nipple and ear are next to each other. This makes you wonder why many women report feeling erotic sensations when their ears are nibbled during sexual foreplay. Is it a coincidence, or does it have something to do with brain anatomy? (Even in the original Penfield map, the genital area of women is mapped right next to the nipples.)

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A less titillating example of remapping also involving the ear came from Dr. A. T. Caccace, a neurologist who told me about an extraordinary phenomenon called gaze tinnitus.

People with this condition have a weird problem. When they look to the left (or right), they hear a ringing sound. When they look straight ahead, nothing happens. Physicians have known about this syndrome for a long time but were stymied by it. Why does it happen when the eyes deviate? Why does it happen at all?