Penfield's Homunculus and the mystery of phantom limbs

Phantom limbs are not a modern phenomenon. There are records of people "haunted" by amputated appendages dating all the way back to the sixteenth century. Consequently, we have more than 500 years worth of theories about what causes phantom limbs--some quite ingenious. After losing his right arm in the Napoleonic Wars, British naval hero Lord Nelson believed that his phantom arm was proof positive of the existence of a soul. After all, if his arm could outlive its corporeal existence, why not the rest of him? This was a soothing hypothesis. Most were not.

For the uninitiated, phantom limbs are ghost appendages. After having an arm or leg removed, some patients continue to experience feeling in them. This is not some vague sense that an arm or leg is "revisiting" them. It is a visceral experience of continued "life" in the lost appendage. In his compulsively readable book Phantoms in the Brain, V.S. Ramachandran recounts the story of Tom Sorenson's phantom left arm. Sorenson lost his arm in a car accident.

[After his crash], even though he knew that his arm was gone, Tom could still feel its ghostly presence below the elbow. He could wiggle each "finger," "reach out" and "grab" objects that were within arm's reach. Indeed, his phantom arm seemed to be able to do anythcoming that the real arm would have done . . . Since Tom had been left-handed, his phantom would reach for the receiver whenever the telephone rang.

(Phantoms in the Brain, 21)

This, in and of it self, is unsettling. But to make matters worse, many people's phantom limbs cause them excruciating pain. If someone had arthritis in the appendage that was removed, they often continue to experience joint pain. But given the limb's ghostly nature, pain medications do nothing to alleviate their suffering. Some phantom hands torture their owners by balling up into fists. Phantom nails dig into phantom palms and the patient is powerless to stop it.

Until fairly recently, many psychologists maintained that phantom limbs were caused by pathological denial. "As recently as fifteen years ago, a paper in The Canadian Journal of Psychiatry stated that phantom limbs are merely the result of wishful thinking." The agony of living with a phantom was compounded by patients' belief that they were somehow bringing it on themselves.

Over the years, a variety of physiological explanations for phantom limbs have been bandied about, but none were terribly convincing. The most credible theory was that phantoms were caused by frayed nerve endings misfiring in the stump. Working off this hypothesis, some people with phantoms underwent repeated surgeries to shorten their stumps only to have the pain return again.

Stymied, medical professionals threw up their hands. For years, phantom limbs were relegated to the category of "unsolvable medical mysteries." Then neurologist V.S. Ramachandran read an article by Dr. Tim Pons, of the National Institutes of Health, which sparked his interest.

Pons and his colleagues were researching the Penfield Homunculus:

To further their research, they did some abominable things to a monkey. Pons and his team severed the nerve connections to a monkey's arm, leaving it paralyzed. Then, they opened up its brain and rooted around in an area called the somatosensory cortex.

The somatosensory cortex contains a neural map of all of our body parts. A neurologist called Penfield charted the points corresponding to each body region. His homunculus was designed to illustrate these connections. By and large, the somatosensory cortex follows a logical progression, i.e., the lips are next to the jaw, which is next to the tongue, etc. But if you look at it closely, you'll see that the map contains a couple of inconsistencies. The face, for some odd reason, is next to the hand. And the feet are next the genitals.

Why does this matter to us? Well, what Pons found was that when he stimulated the part of the somatosensory cortex associated with the face, the cells associated with the hand also fired. One of the great drawbacks of research monkeys is that they can't talk. But Ramachandran guessed that if this one could, he'd be saying, 'Wow. My dead arm's moving.'

To test his theory, Ramachandran developed a study so simple it almost defies belief. He recruited some volunteers, bought a box of Q-tips, and went to work in his basement laboratory. He sat Tom in a chair, moistened his swab, and began gently stoking different areas of his face. I'll let Ramachandran tell the rest:

I swabbed his cheek. "What do you feel?"

"You are touching my cheek."

"Anything else?"

"Hey, you know it's funny," said Tom. "You're touching my missing thumb--my phantom thumb."

I moved the Q-tip to his upper lip. "How about here?"

"You're touching my index finger and my upper lip."

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.

(Phantoms in the Brain, 29-34)

How is this possible? According to Ramachandran, when the somatosensory cortex learned that Tom's arm no longer worked, it decided to delegate "arm" space for another purpose. The neurons correlating with his face started invading the neighboring area associated with his arm. The face neurons and the arm neurons began to overlap. So, when Tom's face was stimulated, it inadvertently triggered feelings in his missing appendage.

Using materials purchased at a corner drugstore, Ramachandran solved the mystery of phantom limbs. But he didn't stop there. Having figured out roughly what caused phantom appendages, he was eager to help alleviate patients' pain.

For reasons that are too complicated to go into here, he surmised that patients couldn't control their phantom limbs because of cross wiring in their optic circuitry. So, he designed a simple contraption to prove his hypothesis. He took a cardboard box, cut two armholes in the side, and placed a mirror inside. When subjects put their working arms and phantoms in the box, it created the illusion that they were seeing both their limbs.

One of Ramachandran's patients regularly struggled with the feeling that his phantom hand was making a tight fist that he couldn't release. When he placed his real and ghost arm in the box, Ramachandran instructed him to make a fist with his working hand. Because of the mirror, the patient saw two fists. He was then asked to loosen his real hand. Magically, both fists opened. By tricking the brain into "seeing" the phantom, the patient was able to control its movements.

But that's not all. One of Ramachandran's patients used the box repeatedly for a week and called him with some exciting news.

"Doctor," he exclaimed, "it's gone!"

"What's gone?" (I thought maybe he'd lost the mirror box.)

"My phantom is gone . . . my phantom arm, which I had for 10 years. It doesn't exist anymore."

(Phantoms in the Brain, 49)

How could a rudimentary illusion result in the first ever "phantom amputation?" Ramachandran believes that, thanks to the mirror, the parietal lobe was overwhelmed with conflicting signals. Eventually, it solved the sensory conundrum by saying, "To hell with it, there's no arm here."

This neurological magic hasn't worked on all of Ramachandran's patients, but his research promises to revolutionize the treatment of phantom pain. In the mean time, at least amputees can be assured that it's not "all in their mind."

Virtual Footnote

In studying phantoms, Ramachandran solved another, less pressing mystery: what causes foot fetishes.

After his first paper on phantom limbs was published, he got a call from an engineer in Arkansas who had lost a leg.

"I lost my leg below the knee about two months ago, but there's something I don't understand. I'd like your advice," [he said to Ramachandran.]

"What's that?"

"Well, I feel a little embarrassed to tell you this . . . Doctor, every time I have sexual intercourse, I experience sensations in my phantom foot . . . I actually experience my orgasm in my foot. And therefore it's much bigger than it used to be because it's no longer confined to just my genitals."

(Phantoms in the Brain, 36)

I think my first response to this news would have been, 'Congratulations.' But for Ramachandran it was a revelation. Remember: as far as the somatosensory cortex is concerned, the genitals and the feet are right next to each other. It makes perfect sense that, in some people, there's an overlap between the genital region and the foot region. The result? Your feet would be an erogenous zone.