Read Online Phantoms in the Brain: Probing the Mysteries of the Human Mind by V. S. Ramachandran, Sandra Blakeslee - Free Book Online
Authors: V. S. Ramachandran, Sandra Blakeslee
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adolescence and into old age? And even if the maps are already there at birth, to what extent can they be modified in the adult?5
It was these questions that prompted Tim Pons and his colleagues to embark on their research. Their strategy was to record signals from the brains of monkeys who had undergone dorsal rhizotomy—a procedure in which all the nerve fibers carrying sensory information from one arm into the spinal cord are completely severed.6 Eleven years after the surgery, they anesthetized the animals, opened their skulls and recorded from the somatosensory map. Since the monkey's paralyzed arm was not sending messages to the brain, you would not expect to record any sig−
nals when you touch the monkey's useless hand and record from the "hand area" of the brain. There should be a big patch of silent cortex corresponding to the affected hand.
25
Indeed, when the researchers stroked the useless hand, there was no activity in this region. But to their amazement they found that when they touched the monkey's face, the cells in the brain corresponding to the
"dead" hand started firing vigorously. (So did cells corresponding to the face, but those were expected to fire.) It appeared that sensory information from the monkey's face not only went to the face area of the cortex, as it would in a normal animal, but it had also invaded the territory of the paralyzed hand!
The implications of this finding are astonishing: It means that you can change the map; you can alter the brain circuitry of an adult animal, and connections can be modified over distances spanning a centimeter or more.
Upon reading Pons's paper, I thought, "My God! Might this be an explanation for phantom limbs?" What did the monkey actually "feel" when its face was being stroked? Since its "hand" cortex was also being excited, did it perceive sensations as arising from the useless hand as well as the face? Or would it use higher brain centers to reinterpret the sensations correctly as arising from the face alone? The monkey of course was silent on the subject.
It takes years to train a monkey to carry out even very simple tasks, let alone signal what part of its body is being touched. Then it occurred to me that you don't have to use a monkey. Why not answer the same question by touching the face of a human patient who has lost an arm? I telephoned my colleagues Dr. Mark Johnson and Dr. Rita Finkelstein in orthopedic surgery and asked, "Do you have any patients who have recently lost an arm?"
That is how I came to meet Tom. 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."
"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."
"Really? Are you sure?"
"Yes. I can feel it both places."
"How about here?" I stroked his lower jaw with the swab.
26
cluster of points on the face that evoke sensations in the phantom and a second cluster on the upper arm, corresponding to the two body parts that are represented on either side (above and below) of the hand representation in the
It was these questions that prompted Tim Pons and his colleagues to embark on their research. Their strategy was to record signals from the brains of monkeys who had undergone dorsal rhizotomy—a procedure in which all the nerve fibers carrying sensory information from one arm into the spinal cord are completely severed.6 Eleven years after the surgery, they anesthetized the animals, opened their skulls and recorded from the somatosensory map. Since the monkey's paralyzed arm was not sending messages to the brain, you would not expect to record any sig−
nals when you touch the monkey's useless hand and record from the "hand area" of the brain. There should be a big patch of silent cortex corresponding to the affected hand.
25
Indeed, when the researchers stroked the useless hand, there was no activity in this region. But to their amazement they found that when they touched the monkey's face, the cells in the brain corresponding to the
"dead" hand started firing vigorously. (So did cells corresponding to the face, but those were expected to fire.) It appeared that sensory information from the monkey's face not only went to the face area of the cortex, as it would in a normal animal, but it had also invaded the territory of the paralyzed hand!
The implications of this finding are astonishing: It means that you can change the map; you can alter the brain circuitry of an adult animal, and connections can be modified over distances spanning a centimeter or more.
Upon reading Pons's paper, I thought, "My God! Might this be an explanation for phantom limbs?" What did the monkey actually "feel" when its face was being stroked? Since its "hand" cortex was also being excited, did it perceive sensations as arising from the useless hand as well as the face? Or would it use higher brain centers to reinterpret the sensations correctly as arising from the face alone? The monkey of course was silent on the subject.
It takes years to train a monkey to carry out even very simple tasks, let alone signal what part of its body is being touched. Then it occurred to me that you don't have to use a monkey. Why not answer the same question by touching the face of a human patient who has lost an arm? I telephoned my colleagues Dr. Mark Johnson and Dr. Rita Finkelstein in orthopedic surgery and asked, "Do you have any patients who have recently lost an arm?"
That is how I came to meet Tom. 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."
"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."
"Really? Are you sure?"
"Yes. I can feel it both places."
"How about here?" I stroked his lower jaw with the swab.
26
cluster of points on the face that evoke sensations in the phantom and a second cluster on the upper arm, corresponding to the two body parts that are represented on either side (above and below) of the hand representation in the
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