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 A
Authors: David A. Rosenbaum
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than neurons aroused by belly brushes, and so on. The thumb’s representation in this part of the brain is especially large ( Figure 2 ). What Merzenich expected was that the sizes of the brain regions from which he recorded would remain more or less the same in the brains of all the monkeys he studied.
To his surprise, Merzenich and his coworkers found that brains of different monkeys varied considerably in how much of their brains were devoted to the same functions. Though the animals were roughly the same age and size, the sizes of their brain regions devoted to any given function varied considerably.
What was going on here, Merzenich and his coworkers wondered? One possibility was that different animals were genetically disposed to have differently sized brain regions. According to this view, one animal, through inheritance, had lots of brain space for touch on its
index
finger; another animal, owing to
its
genetic constitution, had more neural real estate devoted to touches on its
middle
finger, and so on. According to this nature-rather-than-nurture view,what determined the amount of brain space for a function was genetic determination. According to another view, the nurture-as-well-as-nature view, the differences in the sizes of the brain zones reflected experience as well as genes.
FIGURE 2. Cartoon of the amount of space (number of neurons) responsive to touch on different parts of the body.
To decide between these hypotheses, Merzenich and his colleagues performed experiments in which they altered the experiences of their experimental subjects. Their logic was straightforward: If the organization of the brain is fixed, experience shouldn’t change it.
In one study, Merzenich et al. amputated the middle finger of a monkey to see how that would affect the monkey’s somatosensory cortexes. This manipulation provided a particularly strong test of the experience hypothesis and so was deemed ethically acceptable.
Merzenich and his colleagues found that cells in the somatosensory cortex that had previously responded to touch on the middle finger but not to touch on the adjacent ring or index finger became responsive to touch on the still-present adjacent fingers after the middle finger was removed. In other words, the middle-finger region of the somatosensory cortex, which had been unresponsive to touch on the ring or index finger, became responsive to touch on the ring and index digits after the middle finger was removed.
How should you interpret this result? The best interpretation, I believe, is that it’s a jungle in there. A neuron engaged in transmitting signals from the
index
finger to the brain, or a neuron engaged in transmitting signals from the
ring
finger to the brain, can be said to have always sought to make contact with the middle-finger region. That region was next to the place where thoseneurons mainly projected. But something kept those neurons from extending their tendrils to the middle-finger zone. Keeping them out were the neural tendrils from the middle finger. After surgical removal of the middle finger, afferents to the middle-finger region of the somatosensory cortex no longer delivered signals to that cortical zone, so index-finger inputs and ring-finger inputs had less competition for entry to the middle-finger area. Once they made that entry and could do so on repeated occasions, they could form reliable connections to it.
Do such changes occur only after unfortunate events like amputations? The answer, happily, is no. Enlargements of neural territories also accompany practice. Merzenich and his colleagues demonstrated practice effects in other, more benign experiments. In one, they gave monkeys practice on a task requiring fine tactile discrimination by the middle finger. After prolonged practice on this task, more brain tissue in the somatosensory cortex became responsive to touch on that finger. Other neighboring regions that were previously responsive to touch from the adjacent
To his surprise, Merzenich and his coworkers found that brains of different monkeys varied considerably in how much of their brains were devoted to the same functions. Though the animals were roughly the same age and size, the sizes of their brain regions devoted to any given function varied considerably.
What was going on here, Merzenich and his coworkers wondered? One possibility was that different animals were genetically disposed to have differently sized brain regions. According to this view, one animal, through inheritance, had lots of brain space for touch on its
index
finger; another animal, owing to
its
genetic constitution, had more neural real estate devoted to touches on its
middle
finger, and so on. According to this nature-rather-than-nurture view,what determined the amount of brain space for a function was genetic determination. According to another view, the nurture-as-well-as-nature view, the differences in the sizes of the brain zones reflected experience as well as genes.
FIGURE 2. Cartoon of the amount of space (number of neurons) responsive to touch on different parts of the body.
To decide between these hypotheses, Merzenich and his colleagues performed experiments in which they altered the experiences of their experimental subjects. Their logic was straightforward: If the organization of the brain is fixed, experience shouldn’t change it.
In one study, Merzenich et al. amputated the middle finger of a monkey to see how that would affect the monkey’s somatosensory cortexes. This manipulation provided a particularly strong test of the experience hypothesis and so was deemed ethically acceptable.
Merzenich and his colleagues found that cells in the somatosensory cortex that had previously responded to touch on the middle finger but not to touch on the adjacent ring or index finger became responsive to touch on the still-present adjacent fingers after the middle finger was removed. In other words, the middle-finger region of the somatosensory cortex, which had been unresponsive to touch on the ring or index finger, became responsive to touch on the ring and index digits after the middle finger was removed.
How should you interpret this result? The best interpretation, I believe, is that it’s a jungle in there. A neuron engaged in transmitting signals from the
index
finger to the brain, or a neuron engaged in transmitting signals from the
ring
finger to the brain, can be said to have always sought to make contact with the middle-finger region. That region was next to the place where thoseneurons mainly projected. But something kept those neurons from extending their tendrils to the middle-finger zone. Keeping them out were the neural tendrils from the middle finger. After surgical removal of the middle finger, afferents to the middle-finger region of the somatosensory cortex no longer delivered signals to that cortical zone, so index-finger inputs and ring-finger inputs had less competition for entry to the middle-finger area. Once they made that entry and could do so on repeated occasions, they could form reliable connections to it.
Do such changes occur only after unfortunate events like amputations? The answer, happily, is no. Enlargements of neural territories also accompany practice. Merzenich and his colleagues demonstrated practice effects in other, more benign experiments. In one, they gave monkeys practice on a task requiring fine tactile discrimination by the middle finger. After prolonged practice on this task, more brain tissue in the somatosensory cortex became responsive to touch on that finger. Other neighboring regions that were previously responsive to touch from the adjacent
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