Phantoms in the Brain: Probing the Mysteries of the Human Mind (9 page)

After reading about Tom, Dr. Caccace was struck by the similarity between phantom limbs and gaze tinnitus, for he knew that his patients had suffered damage to the auditory nerve—the major conduit connecting the inner ear to the brain stem. Once in the brain stem the auditory nerve hooks up with the auditory nucleus, which is right next to another

structure called the oculomotor nerve nucleus. This second, adjacent structure sends commands to the eyes, instructing them to move. Eureka! The mystery is solved.11 Because of the patient's damage, the auditory nucleus no longer gets input from one ear. Axons from the eye movement center in the cortex invade the auditory nucleus so that every time the person's brain sends a command to move the eyes, that command is sent inadvertently to the auditory nerve nucleus and translated into a ringing sound.

The study of phantom limbs offers fascinating glimpses of the architecture of the brain, its astonishing capacity for growth and renewal12 and may even explain why playing footsie is so enjoyable. But about half the people with phantom limbs also experience the most unpleasant manifestation of the phenomenon—phantom limb pain. Real pain, such as the pain of cancer, is hard enough to treat; imagine the challenge of treating pain in a limb that isn't there! There is very little that can be done, at the moment, to alleviate such pain, but perhaps the remapping that we observed with Tom may help explain why it happens.

We know, for instance, that intractable phantom pain may develop weeks or months after the limb is amputated. Perhaps as the brain adjusts and cells slowly make new connections, there is a slight error in the remapping so that some of the sensory input from touch receptors is accidentally connected to the pain areas of the brain. If this were to happen, then every time the patient smiled or accidentally brushed his cheek, the touch sensations would be experienced as excruciating pain. This is almost certainly not the whole explanation for phantom pain (as we shall see in the next chapter), but it's a good place to start.

As Tom left my office one day, I couldn't resist asking him an obvious question. During the last four weeks, had he ever noticed any of these peculiar referred sensations in his phantom hand when his face had been touched—when he shaved every morning, for example?

"No, I haven't," he replied, "but you know, my phantom hand sometimes itches like crazy and I never know what to do about it. But now," he said, tapping his cheek and winking at me, "I know exactly where to scratch!"

You never identify yourself with the shadow cast by your body, or with its reflection, or with the body you see
in a dream or in your imagination. Therefore you should not identify yourself with this living body, either.

—
Shankara (a.D. 788−820),
Viveka Chudamani
(Vedic scriptures)
When a reporter asked the famous biologist J.B.S. Haldane what his biological studies had taught him about God, Haldane replied, "The creator, if he exists, must have an inordinate fondness for beetles," since there are more species of beetles than any other group of living creatures. By the same token, a neurologist might 33

conclude that God is a cartographer. He must have an inordinate fondness for maps, for everywhere you look in the brain maps abound. For example, there are over thirty different maps concerned with vision alone.

Likewise for tactile or somatic sensations—touch, joint and muscle sense—there are several maps, including, as we saw in the previous chapter, the famous Penfield homunculus, a map draped across a vertical strip of cortex on the sides of the brain. These maps are largely stable throughout life, thus helping ensure that perception is usually accurate and reliable. But, as we have seen, they are also being constantly updated and refined in response to

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vagaries of sensory input. Recall that when Tom's arm was amputated, the large patch of cortex corresponding to his missing hand was "taken over" by sensory input from his face. If I touch Tom's face, the sensory message now goes to two areas—the original face area (as it should) but also the original "hand area." Such brain map alterations may help explain the appearance of Tom's phantom limb soon after amputation. Every time he smiles or experiences some spontaneous activity of facial nerves, the activity stimulates his "hand area," thereby fooling him into thinking that his hand is still there.

But this cannot be the whole story. First, it doesn't explain why so many people with phantoms claim that they can move their "imaginary" limbs voluntarily. What is the source of this illusion of movement? Second, it doesn't explain the fact that these patients sometimes experience intense agony in the missing limb, the phenomenon called phantom pain. Third, what about a person who is born without an arm? Does remapping also occur in his brain, or does the hand area of the cortex simply never develop because he never had an arm?

Would he experience a phantom? Can someone be
born
with phantom limbs?

The idea seems preposterous, but if there's one thing I've learned over the years it's that neurology is full of surprises. A few months after our first report on phantoms had been published, I met Mirabelle Kumar, a twenty−five−year−old Indian graduate student, referred to me by Dr. Sath−yajit Sen, who knew about my interest in phantoms. Mirabelle was born without arms. All she had were two short stumps dangling from her shoulders. X rays revealed that these stumps contained the head of the humerus or upper arm bone, but that there were no signs of a radius or ulna. Even the tiny bones of her hands were missing, although she did have a hint of rudimentary fingernails in the stump.

Mirabelle walked into my office on a hot summer day, her face flushed from walking up three flights of stairs.

An attractive, cheerful young lady, she was also extremely direct with a "don't pity me" attitude writ large on her face.

As soon as Mirabelle was seated, I began asking simple questions: where she was from, where she went to school, what she was interested in and so forth. She quickly lost patience and said, "Look, what do you really want to know? You want to know if I have phantom limbs, right? Let's cut the crap."

I said, "Well, yes, as a matter of fact, we do experiments on phantom limbs. We're interested in . . ."

She interrupted. "Yes. Absolutely. I've never had arms. All I've ever had are these." Deftly, using her chin to help her in a practiced move, she took off her prosthetic arms, clattered them onto my desk and held up her stumps. "And yet I've always experienced the most vivid phantom limbs, from as far back in my childhood as I can remember."

I was skeptical. Could it be that Mirabelle was just engaging in wishful thinking? Maybe she had a deep−seated desire to conform, to be normal. I was beginning to sound like Freud. How could I be sure she was not making it up?

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I asked her, "How do you know that you have phantom limbs?"

"Well, because as I'm talking to you, they are gesticulating. They point to objects when I point to things, just like your arms and hands."

I leaned forward, captivated.

"Another interesting thing about them, doctor, is that they're not as long as they should be. They're about six to eight inches too short."

"How do you know that?"

"Because when I put on my artificial arms, my phantoms are much shorter than they should be," said Mirabelle, looking me squarely in the eye. "My phantom fingers should fit into the artificial fingers, like a glove, but my arm is about six inches too short. I find this incredibly frustrating because it doesn't feel natural.

I usually end up asking the prosthetist to reduce the length of my artificial arms, but he says that would look short and funny. So we compromise. He gives me limbs that are shorter than most but not so absurdly short that they look strange." She pointed to one of her prosthetic arms lying on the desk, so I could see. "They're a little bit shorter than normal arms, but most people don't notice it."

To me this was proof that Mirabelle's phantoms were not wishful thinking. If she wanted to be like other people, why would she want shorter−than−normal arms? There must be something going on inside her brain that was giving rise to the vivid phantom experience.

Mirabelle had another point. "When I walk, doctor, my phantom arms don't swing like normal arms, like your arms. They stay frozen on the side, like this." She stood up, letting her stumps drop straight down on both sides. "But when I talk," she said, "my phantoms gesticulate. In fact, they're moving now as I speak."

This is not as mysterious as it sounds. The brain region responsible for smooth, coordinated swinging of the arms when we walk is quite different from the one that controls gesturing. Perhaps the neural circuitry for arm swinging cannot survive very long without continuous nurturing feedback from the limbs. It simply drops out or fails to develop

when the arms are missing. But the neural circuitry for gesticulation— activated during spoken language—might be specified by genes during development. (The relevant circuitry probably antedates spoken language.) Remarkably, the neural circuitry that generates these commands in Mirabelle's brain seems to have survived intact, despite the fact that she has received no visual or kinesthetic feedback from those

"arms" at any point in her life. Her body keeps telling her, "There are no arms, there are no arms," yet she continues to experience gesticulation.

This suggests that the neural circuitry for Mirabelle's body image must have been laid down at least partly by genes and is not strictly dependent on motor and tactile experience. Some early medical reports claim that patients with limbs missing from birth do not experience phantoms. What I saw in Mirabelle, however, implies that each of us has an internally hard−wired image of the body and limbs at birth—an image that can survive indefinitely, even in the face of contradictory information from the senses.1

In addition to these spontaneous gesticulations, Mirabelle can also generate voluntary movements in her phantom arms, and this is also true of patients who lose arms in adulthood. Like Mirabelle, most of these patients can "reach out" and "grab" objects, point, wave good−bye, shake hands, or perform elaborate skilled maneuvers with the phantom. They know it sounds crazy since they realize that the arm is gone, but to them these sensory experiences are very real.

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I didn't realize how compelling these felt movements could be until I met John McGrath, an arm amputee who telephoned me after he had seen a television news story on phantom limbs. An accomplished amateur athlete, John had lost his left arm just below the elbow three years earlier. "When I play tennis," he said, "my phantom will do what it's supposed to do. It'll want to throw the ball up when I serve or it will try to give me balance in a hard shot. It's always trying to grab the phone. It even waves for the check in restaurants," he said with a laugh.

John had what is known as a telescoped phantom hand. It felt as if it were attached directly to his stump with no arm in between. However, if an object such as a teacup were placed a foot or two away from the stump, he could try to reach for it. When he did this, his phantom no longer remained attached to the stump but felt as if it were zooming out to grab the cup.

On a whim I started thinking, What if I ask John to reach out and grab this cup but pull it away from him before he "touches" it with his phantom? Will the phantom stretch out, like a cartoon character's rub−

bery arm, or will it stop at a natural arm's length? How far can I move the cup away before John will say he can't reach it? Could he grab the moon? Or will the physical limitations that apply to a real arm also apply to the phantom?

I placed a coffee cup in front of John and asked him to grab it. Just as he said he was reaching out, I yanked away the cup.

"Ow!" he yelled. "Don't do that!"

"What's the matter? "

"Don't do that," he repeated. "I had just got my fingers around the cup handle when you pulled it. That really hurts!"

Hold on a minute. I wrench a real cup from phantom fingers and the person yells, ouch! The fingers were illusory, of course, but the pain was real—indeed, so intense that I dared not repeat the experiment.

My experience with John started me wondering about the role of vision in sustaining the phantom limb experience. Why would merely "seeing" the cup be pulled away result in pain? But before we answer this question, we need to consider why anyone would experience movements in a phantom limb. If you close your eyes and move your arm, you can of course feel its position and movement quite vividly pardy because of joint and muscle receptors. But neither John nor Mirabelle has such receptors. Indeed, they have no arm. So where do these sensations originate?

Ironically, I got the first clue to this mystery when I realized that many phantom limb patients—perhaps one third of them—are
not
able to move their phantoms. When asked, they say, "My arm is cast in cement, doctor" or "It's immobilized in a block of ice." "I try to move my phantom, but I can't," said Irene, one of our patients. "It won't obey my mind. It won't obey my command." Using her intact arm, Irene mimicked the position of her phantom arm, showing me how it was frozen in an odd, twisted position. It had been that way for a whole year. She always worried that she would "bump" it when entering doorways, and that it would hurt even more.

How can a phantom—a nonexistent limb—be paralyzed? It sounds like an oxymoron.

I looked up the case sheets and found that many of these patients had had preexisting pathology in the nerves entering the arm from the spinal cord. Their arms really had been paralyzed, held in a sling or cast for a few 36

months and later been amputated simply because they were constantly getting in the way. Some patients were advised to get rid of it, perhaps in a misguided attempt to eliminate the pain in the arm or to correct postural abnormalities caused by the paralyzed arm or leg. Not surprisingly, after the operations these patients often experience a vivid phantom limb, but to their dismay the phantom remains locked in the same position as before the amputation, as though a memory of the paralysis is carried over into the phantom limb.