In the mid-20th century, Canadian neurosurgeon Wilder Penfield discovered that the entire surface of a person's body is mapped on the surface of his or her brain. When one touches a certain body part, say one's foot, neurons in the part of one's brain mapped for the foot respond. A decade ago, building on Penfield's work, Dr. Tim Pons of the National Institutes of Health and his colleagues found while working with monkeys that, over time, sensory information coming from, say, the face could invade cells of that part of the brain mapped for a dysfunctional body part, such as a paralyzed arm. That is, the brain began modifying the Penfield map when part of it was no longer receiving impulses. Ramachandran wondered if this phenomenon could explain phantom-limb syndrome. To get an answer, he needed a human being, who, unlike a monkey, could describe what he was feeling.
That is how I came to meet Tom [Sorenson, who lost his left arm above the elbow in a car accident]. I called him up right away and asked whether he would like to participate in a study. Although initially shy and reticent in his mannerisms, Tom soon became eager to participate in our experiment. I was careful not to tell him what we hoped to find, so as not to bias his responses. Even though he was distressed by "itching" and painful sensations in his phantom fingers, he was cheerful, apparently pleased that he had survived the accident.
With Tom seated comfortably in my basement laboratory, I placed a blindfold over his eyes because I didn't want him to see where I was touching him. Then I took an ordinary Q-tip and started stroking various parts of his body surface, asking him to tell me where he felt the sensations. (My graduate student, who was watching, thought I was crazy.)
I swabbed his cheek. "What do you feel?"
"You are touching my cheek."
"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."
"Really? Are you sure?"
"Yes, I can feel it both places."
"How about here?" I stroked his lower jaw with the swab.
"That's my missing pinkie."
I soon found a complete map of Tom's phantom hand—on his face! I realized that what I was seeing was perhaps a direct perceptual correlate of the remapping that Tim Pons had seen in his monkeys. For there is no other way of explaining why touching an area so far away from the stump—namely, the face—should generate sensations in the phantom hand; the secret lies in the peculiar mapping of body parts in the brain, with the face lying right beside the hand.
I continued this procedure until I had explored Tom's entire body surface. When I touched his chest, right shoulder, right leg, or lower back, he felt sensations in those places and not in the phantom. But I also found a second, beautifully laid out "map" of his missing hand—tucked into his left upper arm a few inches above the line of amputation. Stroking the skin surface on this second map also evoked precisely localized sensations on the individual fingers: Touch here and he says, "Oh, that's my thumb," and so on.