Phantoms in the Brain: Probing the Mysteries of the Human Mind (12 page)

But the student now did something different. Instead of placing his left hand into the other side of the box, in an exact mirror image of the phantom, he inserted his right hand, palm up. Since the hand was gloved, it looked exactly like her "palm−down" phantom right hand. Then the student flexed his index finger to touch his palm. To Mary, peering into the box, it appeared as if her phantom index finger were bending backward to touch the back of her wrist—in the wrong direction!5 What would her reaction be?

When Mary saw her finger twisted backward, she said, "One would have thought it should feel peculiar, doctor, but it doesn't. It feels exactly like the finger is bending backward, like it isn't supposed to. But it doesn't feel peculiar or painful or anything like that."

Another subject, Karen, winced and said that the twisted phantom finger hurt. "It felt like somebody was grabbing and pulling my finger. I felt a twinge of pain," she said.

These experiments are important because they flatly contradict the theory that the brain consists of a number of autonomous modules acting

as a bucket brigade. Popularized by artificial intelligence researchers, the idea that the brain behaves like a computer, with each module performing a highly specialized job and sending its output to the next module, is widely believed. In this view, sensory processing involves a one−way cascade of information sensory 44

receptors on the skin and other sense organs to higher brain centers.

But my experiments with these patients have taught me that this is not how the brain works. Its connections are extraordinarily labile and dynamic. Perceptions emerge as a result of reverberations of signals between different levels of the sensory hierarchy, indeed even across different senses. The fact that visual input can eliminate the spasm of a nonexistent arm and then erase the associated memory of pain vividly illustrates how extensive and profound these interactions can be.

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Studying patients with phantom limbs has given me insights into the inner working of the brain that go far beyond the simple questions I started with four years ago when Tom first walked into my office. We've actually witnessed (directly and indirecdy) how new connections emerge in the adult brain, how information from different senses interacts, how the activity of sensory maps is related to sensory experience and more generally how the brain is continuously updating its model of reality in response to novel sensory inputs.

This last observation sheds new light on the so−called nature versus nurture debate by allowing us to ask the question, Do phantom limbs arise mainly from nongenetic factors such as remapping or stump neuromas, or do they represent the ghostly persistence of an inborn, genetically specified "body image"? The answer seems to be that the phantom emerges from a complex interaction between the two. I'll give you five examples to illustrate this.

In the case of below−the−elbow amputees, surgeons will sometimes cleave the stump into a lobster claw−like appendage, as an alternative to a standard metal hook. After the surgery, people learn to use their pincers at the stump to grasp objects, turn them around and otherwise manipulate the material world. Intriguingly, their phantom hand (some inches away from real flesh) also feels split in two—with one or more phantom fingers occupying each pincer, vividly mimicking the movements of the appendage. I know of one instance in which a patient underwent amputation of his pincers only to be left with a permanently cleaved phantom—striking evidence that a surgeon's scalpel can dissect a phantom.

After the original surgery in which the stump was split, this patient's brain must have reshaped his body image to include the two pincers— for why else would he experience phantom pincers?

The other two stories both entertain and inform. A girl who was born without forearms and who experienced phantom hands six inches below her stumps frequently used her
phantom
fingers to calculate and solve arithmetic problems. A sixteen−year−old girl who was born with her right leg two inches shorter than her left leg and who received a below−knee amputation at age six had the odd sensation of possessing four feet! In addition to one good foot and the expected phantom foot, she developed two supernumerary phantom feet, one at the exact level of amputation and a second one, complete with calf, extending all the way down to the floor, where it should be had the limb not been congenitally shorter.6 Although researchers have used this example to illustrate the role of genetic factors in determining body image, one could equally use it to emphasize nongenetic influences, for why would your genes specify three separate images of one leg?

A fourth example that illustrates the complex interplay between genes and environment harks back to our observation that many amputees experience vivid phantom movements, both voluntary and involuntary, but in most the movements disappear eventually. Such movements are experienced at first because the brain continues sending motor commands to the missing limb (and monitors them) after amputation. But sooner or later, the lack of visual confirmation (Gee, there is no arm) causes the patient's brain to reject these signals and the movements are no longer experienced. But if this explanation is correct, how can we understand the continued presence of vivid limb movements in people like Mirabelle, who was born without arms? I can only guess that a normal adult has had a lifetime of visual and kinesthetic feedback, a process that leads the brain to 45

expect such feedback even after amputation. The brain is "disappointed" if the expectation is not fulfilled—leading eventually to a loss of voluntary movements or even a complete loss of the phantom itself.

The sensory areas of Mirabelle's brain, however, have never received such feedback. Consequently, there is no learned dependence on sensory feedback, and that lack might explain why the sensation of movements had persisted, unchanged, for twenty−five years.

The final example comes from my own country, India, which I visit every year. The dreaded disease leprosy is still quite common there and often leads to progressive mutilation and loss of limbs. At the leprosarium at Vellore, I was told that these patients who lose their arms do not experience phantoms, and I personally saw several cases and verified these claims. The standard explanation is that the patient gradually "learns" to assimilate the stump into his body image by using visual feedback, but if this is true, how does it account for the continued presence of phantoms in amputees? Perhaps the
gradual
loss of the limb or the simultaneous presence of progressive nerve damage caused by the leprosy bacterium is somehow critical. This might allow their brains more time to readjust their body image to match reality.

Odder still, when such a patient develops gangrene in his stump and the diseased tissue is amputated, he
docs
develop a phantom. But it's not a phantom of the old stump; it's a phantom of the entire hand! It's as though the brain has a dual representation, one of the original body image laid down genetically and one ongoing, up−to−date image that can incorporate subsequent changes. For some weird reason, the amputation disturbs the equilibrium and resurrects the original body image, which has always been competing for attention.7

I mention these bizarre examples because they imply that phantom limbs emerge from a complex interplay of both genetic and experiential variables whose relative contributions can be disentangled only by systematic empirical investigations. As with most nature/nurture debates, asking which is the more important variable is meaningless—despite extravagant claims to the contrary in the IQ literature. (Indeed, the question is no more meaningful than asking whether the wetness of water results mainly from the hydrogen molecules or from the oxygen molecules that constitute H20!) But the good news is that by doing the right kinds of experiments, you can begin to tease them apart, investigate how they interact and eventually help develop new treatments for phantom pain. It seems extraordinary even to contemplate the possibility that you could use a visual illusion to eliminate pain, but bear in mind that pain itself is an illusion—constructed entirely in your brain like any other sensory experience. Using one illusion to erase another doesn't seem very surprising after all.

The experiments I've discussed so far have helped us understand what is going on in the brains of patients with phantoms and given us hints as to how we might help alleviate their pain. But there is a deeper message here:
Tour own body
is a phantom, one that your brain has temporarily constructed purely for convenience. I know this sounds astonishing so I will demonstrate to you the malleability of your own body image and how you can alter it profoundly in just a few seconds. Two of these experiments you can do on yourself right now, but the third requires a visit to a Halloween supply shop.

To experience the first illusion, you'll need two helpers. (I will call them Julie and Mina.) Sit in a chair, blindfolded, and ask Julie to sit on another chair in front of you, facing the same direction as you are. Have Mina stand on your right side and give her the following instructions: "Take my right hand and guide my index finger to Julia's nose. Move my hand in a rhythmic manner so that my index finger repeatedly strokes and taps her nose in a random sequence like a Morse code. At the same time, use your left hand to stroke my nose with the same rhythm and timing. The stroking and tapping of my nose and Julia's nose should be in perfect synchrony."

After thirty or forty seconds, if you're lucky, you will develop the uncanny illusion that you are touching your nose out there or that your nose has been dislocated and stretched out about three feet in front of your face.

The more random and unpredictable the stroking sequence, the more striking the illusion will be. This is an 46

extraordinary illusion; why does it happen? I suggest that your brain "notices" that the tapping and stroking sensations from your right index finger are perfectly synchronized with the strokes and taps felt on your nose.

It then says, "The tapping on my nose is identical to the sensations on my right index finger; why are the two sequences identical? The likelihood that this is a coincidence is zero, and therefore the most probable explanation is that my finger must be tapping my nose. But I also know that my hand is two feet away from my face. So it follows that my nose must also be out there, two feet away."8

I have tried this experiment on twenty people and it works on about half of them (I hope it will work on you).

But to me, the astonishing thing is that it works at all—that your certain knowledge that you have a normal nose, your image of your body and face constructed over a lifetime should be negated by just a few seconds of the right kind of sensory stimulation. This simple experiment not only shows how malleable your body image is but also illustrates the single most important principle underlying all of perception—that the mechanisms of perception are mainly involved in extracting statistical correlations from the world to create a model that is temporarily useful.

The second illusion requires one helper and is even spookier.9 You'll need to go to a novelty or Halloween store to buy a dummy rubber hand. Then construct a two−foot by two−foot cardboard "wall" and place it on a table in front of you. Put your right hand behind the cardboard so that you cannot see it and put the dummy hand in front of the cardboard so you can see it clearly. Next have your friend stroke identical locations on both your hand and the dummy hand synchronously while you look at the dummy. Within seconds you will experience the stroking sensation as arising from the dummy hand. The experience is uncanny, for you know perfectly well that you're looking at a disembodied rubber hand, but this doesn't prevent your brain from assigning sensation to it. The illusion illustrates, once again, how ephemeral your body image is and how easily it can be manipulated.

Projecting your sensations on to a dummy hand is surprising enough, but, more remarkably, my student Rick Stoddard and I discovered that you can even experience touch sensations as arising from tables and chairs that bear no physical resemblance to human body parts. This experiment is especially easy to do since all you need is a single friend to assist you. Sit at your writing desk and hide your left hand under the table. Ask your friend to tap and stroke the surface of the table with his right hand (as you watch) and then use his hand simultaneously to stroke and tap your left hand, which is hidden from view. It is absolutely critical that you not see the movements of his left hand as this will ruin the effect (use a cardboard partition or a curtain if necessary). After a minute or so, you will start experiencing taps and strokes as emerging from the table surface even though your conscious mind knows perfectly well that this is logically absurd. Again, the sheer statistical improbability of the two sequences of taps and strokes—one seen on the table surface and one felt on your hand—lead the brain to conclude that the table is now part of your body. The illusion is so compelling that on the few occasions when I accidentally made a much longer stroke on the table surface than on the subject's hidden hand, the person exclaimed that his hand felt "lengthened" or "stretched" to absurd proportions.

Both these illusions are much more than amusing party tricks to try on your friends. The idea that you can actually
project
your sensations to external objects is radical and reminds me of phenomena such as out−of−body experiences or even voodoo (prick the doll and "feel" the pain). But how can we be sure the student volunteer isn't just being metaphorical when she says "I feel my nose out there" or "The table feels like my own hand." After all, I often have the experience of "feeling" that my car is part of my extended body image, so much so that I become infuriated if someone makes a small dent on it. But would I want to argue from this that the car had become part of my body?

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These are not easy questions to tackle, but to find out whether the students really identified with the table surface, we devised a simple experiment that takes advantage of what is called the galvanic skin response or GSR. If I hit you with a hammer or hold a heavy rock over your foot and threaten to drop it, your brain's visual areas will dispatch messages to your limbic system (the emotional center) to prepare your body to take emergency measures (basically telling you to run from danger). Your heart starts pumping more blood and you begin sweating to dissipate heat. This alarm response can be monitored by measuring the changes in skin resistance—the so−called GSR—caused by the sweat. If you look at a pig, a newspaper or a pen there is no GSR, but if you look at something evocative—a Mapplethorpe photo, a
Playboy
centerfold or a heavy rock teetering above your foot—you will register a huge GSR.