by Oliver Sacks
There was general astonishment, therefore, when Matthew Botvinick and Jonathan Cohen showed in 1998 that a rubber hand, under the right circumstances, could be mistaken for one’s own. If a subject’s real hand is hidden under a table while the rubber hand is visible before him, and both are stroked in synchrony, then the subject has the convincing illusion, even though he knows better, that the rubber hand is his—and that the sensation of being stroked is located in this inanimate though lifelike object. As I found when I looked through the “eyes” of a robot, knowledge in such a situation does nothing to dispel the illusion. The brain does its best to correlate all the senses, but the visual input here trumps the tactile.
Henrik Ehrsson, in Sweden, has developed a great range of such illusions, using the simplest equipment—video goggles, mannequins, and rubber arms. By disrupting the usual unity of touch, vision, and proprioception, he has induced uncanny experiences in some people, convincing them that their bodies have shrunk or grown enormous, even that they have swapped bodies with someone else. I experienced this for myself when I visited his laboratory in Stockholm for a number of experiments. In one, I was convinced that I possessed a third arm; in another, I felt embodied in a two-foot-high doll, and as I looked through “its” eyes via video goggles, normal objects in the room appeared enormous.
It is evident, from all of this work, that the brain’s representation of the body can often be fooled simply by scrambling the inputs from different senses. If sight and touch say one thing, however absurd, even a lifetime of proprioception and a stable body image cannot always resist this. (Individuals may be more or less susceptible to such illusions, and one might imagine that dancers or athletes, who have an exceptionally vivid sense of where their bodies are in space, may be harder to fool in this way.)
The body illusions Ehrsson is exploring are very much more than party tricks; they point to the ways in which our body ego, our sense of self, is formed from the coordination of senses—not just touch and vision but proprioception and perhaps vestibular sensation, too. Ehrsson and others favor the idea that there are “multisensory” neurons, perhaps at a number of places in the brain, which serve to coordinate the complex (and usually consistent) sensory information coming into the brain. But if this is interfered with—by nature or experiment—our seemingly unassailable certainties about the body and the self can vanish in an instant.
1. The term “out-of-body experience” was introduced in the 1960s by Celia Green, an Oxford psychologist. While there had been stories of out-of-body experiences for centuries, Green was the first to systematically examine a large number of firsthand accounts, from more than four hundred people whom she located by launching a public appeal through the newspapers and the BBC. In her 1968 book, Out-of-the-Body Experiences, she analyzed these in detail.
2. Several of Celia Green’s subjects described similar feelings. “My mind was clearer and more active than before,” one wrote; another spoke of being “all-knowing and understanding.” Green wrote that such subjects felt they “could obtain an answer to any question they chose to formulate.”
3. August Strindberg noted, in his autobiographical novel Inferno, an odd body double, an “other” who mirrored his every movement.
This unknown man never uttered a word; he seemed to be occupied in writing something behind the wooden partition that separated us. All the same, it was odd that he should push back his chair every time I moved mine. He repeated my every movement in a way that suggested that he wanted to annoy me by imitating me.… When I went to bed the man in the room next to my desk went to bed too.… I could hear him lying there, stretched out parallel to me. I could hear him turning the pages of a book, putting out the lamp, breathing deeply, turning over and falling asleep.
Strindberg’s “unknown man” is identical with Strindberg in one sense: a projection of him, at least of his movements, his actions, his body image. Yet, at the same, he is someone else, an Other who occasionally “annoys” Strindberg, but perhaps, at other times, seeks to be companionable. He is, in the literal sense of the term, Strindberg’s “Other,” his “alter ego.”
15
Phantoms, Shadows, and Sensory Ghosts
While hallucinations of sight and sound—“visions” and “voices”—are described in the Bible, in The Iliad and The Odyssey, in all the great epics of the world, none of these so much as mentions the existence of phantom limbs, the hallucinatory feeling that one still has a limb even though it has been amputated. Indeed, there was no term for these before Silas Weir Mitchell gave them their name in the 1870s. And yet they are common—more than a hundred thousand people in the United States have amputations every year, and the vast majority of them experience phantoms after their amputations. The experience of phantom limbs must be as old as amputation itself, and amputations are not new—they were performed thousands of years ago: the Rig Veda tells the story of the warrior queen Vishpla, who went to battle with an iron prosthesis after she lost a leg.
In the sixteenth century, Ambroise Paré, a French military surgeon who was called upon to amputate dozens of injured limbs, wrote, “Long after the amputation is made, patients say that they still feel pain in the amputated part … which seems almost incredible to people who have not experienced this.”
Descartes, in his Meditations on First Philosophy, observed that, just as the sense of vision was not always reliable, so “errors in judgment” could occur in the “internal senses” as well. “I have sometimes been informed,” he wrote, “by parties whose arm or leg had been amputated, that they still occasionally seemed to feel pain in that part of the body which they had lost—a circumstance that led me to think that I could not be quite certain even that any one of my members was affected when I felt pain in it.”
But by and large, as the neurologist George Riddoch brought out (in 1941), a curious atmosphere of silence and secrecy seems to surround the subject. “Spontaneous description of phantoms is rarely offered,” he wrote. “Dread of the unusual, of disbelief, or even of the accusation of insanity may be behind this reticence.”
Weir Mitchell himself hesitated for years before writing professionally on the subject; he introduced it first in the form of fiction (he was a writer as well as a physician), in “The Case of George Dedlow,” published anonymously in the Atlantic Monthly in 1866. As a neurologist working at a military hospital in Philadelphia during the Civil War (the place was informally known as the “Stump Hospital”), Mitchell saw dozens of amputees and, driven by his own curiosity and compassion, he encouraged them to describe their experiences. It would take him several years to fully digest what he had seen and heard from his patients, but in 1872, in his classic Injuries of Nerves, he was able to provide a detailed description and discussion of phantom limbs—the first such in the medical literature.1
Mitchell devoted the final chapter of his book to phantom limbs, introducing the subject as follows:
No history of the physiology of stumps would be complete without some account of the sensorial delusions to which persons are subject in connection with their lost limbs. These hallucinations are so vivid, so strange, and so little dwelt upon by authors, as to be well worthy of study, while some of them seem to me especially valuable, owing to the light which they cast upon the subject of the long-disputed muscular sense.
Nearly every man who loses a limb carries about with him a constant or inconstant phantom of the missing member, a sensory ghost of that much of himself.
After Mitchell had brought attention to the subject, other neurologists and psychologists were drawn to study phantom limbs. Among them was William James, who sent a questionnaire to eight hundred amputees (he was able to contact them with the help of prosthetic manufacturers), and of these, nearly two hundred answered the questionnaire; a few he was able to interview personally.2
Where Mitchell’s observations, working with Civil War amputees, were of fresh, just developed phantom limbs, James was able to study a much more varied population (one m
an, in his seventies, had had a thigh amputation sixty years earlier), and so he was in a better position to describe the changes in phantom limbs over years or decades, changes which he described in detail in his 1887 paper on “The Consciousness of Lost Limbs.”
James was especially interested in the way that initially vivid and mobile phantoms often tended to shorten or disappear with time. This surprised him more than the presence of phantoms, which he felt was only to be expected with continuing activity in the areas of the brain that represented sensation and movement in the lost limb. “The popular mind wonders how the lost feet can still be felt,” James wrote. “For me, the cause for wonder are those in which the lost feet are not felt.” Hand phantoms, he observed, unlike leg or arm phantoms, rarely disappeared. (We now know that this is because the fingers and hands have a particularly massive representation in the brain.) He did, however, note that the intervening arm might disappear, so that the preserved phantom hand now seemed to sprout from the shoulder.3
He was also struck by the way in which an initially mobile phantom could become immobile or even paralyzed, so that “no effort of will can make it change [its position].” (In rare cases, he said, “the very attempt to will the change has grown impossible.”) James saw that fundamental questions were raised here about the neurophysiology of “will” and “effort,” though he could not answer them. And they were not to be answered for more than a century, until V. S. Ramachandran clarified the nature of “learned” paralysis in phantom limbs in the 1990s.
Phantom limbs are hallucinations insofar as they are perceptions of something that has no existence in the outside world, but they are not quite comparable to hallucinations of sight and sound. While losing one’s eyesight or hearing may lead to corresponding hallucinations in 10 or 20 percent of those affected, phantom limbs occur in virtually all who have had a limb amputated. And while it may be months or years before hallucinations follow blindness or deafness, phantom limbs appear immediately or within days after an amputation—and they are felt as an integral part of one’s own body, unlike any other sort of hallucination. Finally, while visual hallucinations such as those of Charles Bonnet syndrome are varied and full of invention, a phantom closely resembles the physical limb that was amputated in size and shape. A phantom foot may have a bunion, if the real one did; a phantom arm may wear a wristwatch, if the real arm did. In this sense, a phantom is more like a memory than an invention.
The near universality of phantom limbs after amputation, the immediacy of their appearance, and their identity with the corporeal limbs in whose stead they appear suggest that, in some sense, they are already in place—revealed, so to speak, by the act of amputation. Complex visual hallucinations get their material from the visual experiences of a lifetime—one has to have seen people, faces, animals, landscapes to hallucinate them; one has to have heard pieces of music to hallucinate them. But the feeling of a limb as a sensory and motor part of oneself seems to be innate, built-in, hardwired—and this supposition is supported by the fact that people born without limbs may nonetheless have vivid phantoms in their place.4
The most fundamental difference between phantom limbs and other hallucinations is that they can be moved voluntarily, whereas visual and auditory hallucinations proceed autonomously, outside one’s control. This was also emphasized by Weir Mitchell:
[The majority of amputees] are able to will a movement, and apparently they themselves execute it more or less effectively.… The certainty with which these patients describe their [phantom motions], and their confidence as to the place assumed by the parts moved, are truly remarkable … the effect is apt to excite twitching in the stump.… In some cases the muscles which act on the hand are absent altogether; yet in these cases there is fully as clear and definite a consciousness of the movement of the fingers and of their change of positions as in cases [where the muscles of the hand are partially preserved].
Other hallucinations are only sensations or perceptions, albeit of a very special sort, whereas a phantom limb is capable of phantom action. Given a suitable prosthesis, the phantom limb will slip into the prosthesis (“like a hand into a glove,” as many patients say)—slip into it and animate it, so that the artificial limb can be used like a real one. Indeed, this must happen if one is to use a prosthesis effectively. The artificial limb becomes part of one’s body, of one’s body image, as a cane in a blind man’s hand becomes an extension of himself. One may say that an artificial leg, for instance, “clothes” the phantom, allows it to be effective, gives it an objective sensory and motor existence, so that it can often “feel” and respond to minute irregularities in the ground almost as well as the original leg.5 (Thus the great climber Geoffrey Winthrop Young, who lost a leg during World War I, was able to climb the Matterhorn using a prosthetic limb of his own design.)6
One might go further and say that a phantom is a portion of body image which is lost or dissociated from its natural, embodying home (the body)—and, as such, as something extraneous, it may be intrusive or deceptive (thus the danger of walking off a curb with a phantom leg). The lost phantom (if one can speak figuratively) longs for a new home, and it will find this in a suitable prosthesis. I have had many patients tell me how they may be disturbed by their phantom at night but relieved in the morning, for the phantom disappears the moment they put on their prosthesis—disappears, that is, into the prosthesis, merging so seamlessly with it that phantom and prosthesis become one.
Knowledge of what one is doing with one’s phantom—even without a prosthesis—can be exquisitely refined. As a young student, Erna Otten, a distinguished pianist, was a pupil of the great Paul Wittgenstein, who lost his right arm in the First World War but continued to play with his left hand (and commissioned a number of composers to write music for the left hand). Yet he continued to teach, in a sense, with both hands. In a letter to the New York Review of Books, responding to an article I had written, Otten wrote:
I had many occasions to see how involved his right stump was whenever we went over the fingering for a new composition. He told me many times that I should trust his choice of fingering because he felt every finger of his right hand. At times I had to sit very quietly while he would close his eyes and his stump would move constantly in an agitated manner. This was many years after the loss of his arm.
Unfortunately, not all phantoms are as well formed, as painless, or as mobile as Wittgenstein’s. Many show a tendency to shrink or “telescope” with time—a phantom arm may be reduced to a hand seemingly sprouting from the shoulder. This tendency to shrink is minimized by embedding the phantom in a prosthesis and using it as much as possible. A phantom may also become paralyzed or contorted in painful positions, with its “muscles” in spasm. Thus Admiral Lord Nelson, after losing his right arm in battle, developed a phantom limb with the hand permanently clenched, the fingers digging excruciatingly into the palm.7
Such disorders of body image have long seemed inexplicable and untreatable. But over the last few decades, it has become clear that the body image is not as fixed as we once thought; indeed, it is remarkably plastic, and extensive reorganization or remapping can occur with phantom limbs.
If there is interruption of nerve function from injury or disease in the spinal cord or peripheral nerves, cutting off or reducing normal sensory input to the brain, this may cause major disturbance of body image, with strange phantom images superimposed on the real but insentient body parts. This was very striking with a colleague of mine, Jeannette W., who broke her neck in a car accident and became quadriplegic, with a complete absence of sensation below the level of the fracture. She had, in a sense, been “amputated” from the neck down and had little sense of her body below this. But in its place, she had a phantom body, which was unstable and prone to distortions and deformations. She could reverse these, for a while, by seeing that her body still had a normal shape and conformation, and she arranged for mirrors to be set up in her office and in the hospital corridors, so that she could glance
up and (in her words) take “visual sips” from them as she bowled past in her wheelchair.
As normal sensation is blocked, body image disturbances can occur very quickly. Most of us have had strange phantom experiences with dental anesthesia, of a grotesquely swollen, deformed, or misplaced cheek or tongue. Looking in a mirror will do little to dispel these illusions, which disappear only with the return of normal sensation. One patient of mine, with the removal of a large brain tumor, had to sacrifice the roots of the sensory nerves on one side of her face. For years following this, she had a persistent sense that the whole right side of her face was “slipping,” “caved in,” or “missing”; that her tongue and cheek on this side were tremendously swollen and grotesque-looking. She later came to have a leg amputated, and soon after surgery became aware of a phantom leg. Now, she said, “I know what’s wrong with my face. It’s exactly the same feeling—I have a phantom face.”
There can also be extra limbs—supernumerary phantoms—if certain areas of the body are denervated. A striking example of this was described by Richard Mayeux and Frank Benson. Their patient was a young man with multiple sclerosis who developed a numbness on his right side and then experienced, as they wrote,
a tactile illusion that a second right arm was lying across his lower chest and upper abdomen. The extra arm seemed to be attached to the chest wall.… There was only a vague sensation of the duplicate illusory lower forearm, wrist, and palm, but a vivid impression of the fingers lying on the abdominal wall.… The illusion persisted for period of 5 to 30 minutes and was accompanied by a “gripping” sensation of the illusory hand.… The phantom limb sensation was always coincident with feelings of increased stiffness, numbness, and burning [sensations] of the actual right arm.
Nelson’s clenched hand exemplifies an unpleasant evolution which phantom limbs may undergo—phantoms which are initially loose, mobile, and obedient to the will may subsequently become paralyzed, contorted, and often intensely painful. Before the 1990s, there was no plausible explanation as to why phantom limbs might get frozen in this way, nor any notion of how to unfreeze them. But in 1993, V. S. Ramachandran suggested a physiological scenario which might explain the progressive loss of voluntary movement so common in phantom limbs. The vivid sense that one could move a phantom limb freely, he thought, went with the brain being able to monitor its own motor commands to the phantom. But with the continuing absence of visual or proprioceptive confirmation of movement, the brain, in effect, might “abandon” the limb. Thus, Ramachandran thought, paralysis was “learned,” and he wondered whether it could be unlearned.
