Read Online It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind by David A. Rosenbaum - Free Book Online Page B
Authors: David A. Rosenbaum
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fingers
lost
neural space in the process as they became responsive to stimulation of the highly practiced middle finger. One zone’s gain was, therefore, another zone’s loss. 20
Neural plasticity has also been found for modalities other than touch. People who go blind have been found to develop sound sensitivity in brain regions that would have otherwise been devoted to vision. 21 The neurons in the so-called visual cortex that have this crossover property lie next to the primary auditory cortex, consistent with the view that neural plasticity mainly reflects infiltration by neural neighbors.
Plasticity and Experience
Does neural plasticity have consequences for experience, or is it just something picked up by neurophysiologists in laboratories—a curious phenomenon, perhaps, but not of any practical value? Many findings indicate that the malleability of neural representations affects experience outside the lab. Some of these findings relate to the fact that when more brain area comes to be devoted to a function, better perception or performance results. For perception, this translates to better detection or discrimination. For performance, it translates to finer coordination or dexterity.
Consider the super-sized thumb of the sensory homunculus, shown in Figure 2 . The thumb is a very sensitive area. When you try to determine whether someone is touching you with one toothpick or two, your discrimination threshold is small on your thumb. You’re very sensitive there and canpick up a small difference. Only one other part of the body is more sensitive than the thumb when it’s subjected to two-point discrimination tests: the tongue.
Sensitivity to touch as measured through two-point discrimination is related to how much brain tissue is devoted to that body part. The tongue has lots of brain representation and so does the thumb. But the abdomen, whose two-point discrimination threshold is much larger, is much less “brainy,” with much less neural territory devoted to the abdomen on a per-square-centimeter basis than to the tongue or thumb. The thigh and calf are likewise scantily represented in the somatosensory cortex. 22 What this shows is that there is a relation between the amount of neural territory devoted to or associated with touch and the fineness of tactile discrimination that the associated patch of skin can support.
This relation is mediated by experience in humans. This was shown in studies of violin players. People who spend long periods practicing the violin develop enlarged receptive fields for the somatosensory cortex on the side of the brain receiving inputs from the left hand (the hand used for fingering on the violin). 23 Similarly, blind individuals who learn to read Braille develop greater tactile sensitivity as a result of this experience. This change is partly due to the growth of tactile sensitivity in
visual
areas. Visual areas—so named because they’re typically tuned to visual inputs but not to other kinds of inputs—become touch sensitive in Braille readers. 24
Strange Consequences of Neural Plasticity
Sometimes, when an area that was responsive to input of one kind is usurped by input of another kind, experience can change in strange ways. The researcher who has done the most to explain these strange effects is Vilayanur Ramachandran, a psychologist at the University of California, San Diego. He has shown that a couple of bizarre phenomena are attributable to neural plasticity. 25
One strange phenomenon is that touch on one part of the body can elicit feelings in other, absent body parts. Ramachandran discovered this while interacting with a man whose arm had been amputated. Acting on reports from the man, Ramachandran asked him to indicate what he felt when Ramachandran touched him gently on his face. “As I’ve told you,” the man said, “when my face is touched, I also feel my hand being touched.” “Do you mean the hand that’s no longer there?” Ramachandran asked.
lost
neural space in the process as they became responsive to stimulation of the highly practiced middle finger. One zone’s gain was, therefore, another zone’s loss. 20
Neural plasticity has also been found for modalities other than touch. People who go blind have been found to develop sound sensitivity in brain regions that would have otherwise been devoted to vision. 21 The neurons in the so-called visual cortex that have this crossover property lie next to the primary auditory cortex, consistent with the view that neural plasticity mainly reflects infiltration by neural neighbors.
Plasticity and Experience
Does neural plasticity have consequences for experience, or is it just something picked up by neurophysiologists in laboratories—a curious phenomenon, perhaps, but not of any practical value? Many findings indicate that the malleability of neural representations affects experience outside the lab. Some of these findings relate to the fact that when more brain area comes to be devoted to a function, better perception or performance results. For perception, this translates to better detection or discrimination. For performance, it translates to finer coordination or dexterity.
Consider the super-sized thumb of the sensory homunculus, shown in Figure 2 . The thumb is a very sensitive area. When you try to determine whether someone is touching you with one toothpick or two, your discrimination threshold is small on your thumb. You’re very sensitive there and canpick up a small difference. Only one other part of the body is more sensitive than the thumb when it’s subjected to two-point discrimination tests: the tongue.
Sensitivity to touch as measured through two-point discrimination is related to how much brain tissue is devoted to that body part. The tongue has lots of brain representation and so does the thumb. But the abdomen, whose two-point discrimination threshold is much larger, is much less “brainy,” with much less neural territory devoted to the abdomen on a per-square-centimeter basis than to the tongue or thumb. The thigh and calf are likewise scantily represented in the somatosensory cortex. 22 What this shows is that there is a relation between the amount of neural territory devoted to or associated with touch and the fineness of tactile discrimination that the associated patch of skin can support.
This relation is mediated by experience in humans. This was shown in studies of violin players. People who spend long periods practicing the violin develop enlarged receptive fields for the somatosensory cortex on the side of the brain receiving inputs from the left hand (the hand used for fingering on the violin). 23 Similarly, blind individuals who learn to read Braille develop greater tactile sensitivity as a result of this experience. This change is partly due to the growth of tactile sensitivity in
visual
areas. Visual areas—so named because they’re typically tuned to visual inputs but not to other kinds of inputs—become touch sensitive in Braille readers. 24
Strange Consequences of Neural Plasticity
Sometimes, when an area that was responsive to input of one kind is usurped by input of another kind, experience can change in strange ways. The researcher who has done the most to explain these strange effects is Vilayanur Ramachandran, a psychologist at the University of California, San Diego. He has shown that a couple of bizarre phenomena are attributable to neural plasticity. 25
One strange phenomenon is that touch on one part of the body can elicit feelings in other, absent body parts. Ramachandran discovered this while interacting with a man whose arm had been amputated. Acting on reports from the man, Ramachandran asked him to indicate what he felt when Ramachandran touched him gently on his face. “As I’ve told you,” the man said, “when my face is touched, I also feel my hand being touched.” “Do you mean the hand that’s no longer there?” Ramachandran asked.
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