The Brain That Changes Itself (2007)
By Norman Doidge
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Merzenich and Jenkins also showed that individual neurons got more selective with training. Each neuron in a brain map for the sense of touch has a “receptive field,” a segment on the skin’s surface that “reports” to it. As the monkeys were trained to feel the disk, the receptive fields of individual neurons got smaller, firing only when small parts of the fingertip touched the disk. Thus, despite the fact that the size of the brain map increased, each neuron in the map became responsible for a smaller part of the skin surface, allowing the animal to have finer touch discrimination. Overall, the map became more precise.
Merzenich and Jenkins also found that as neurons are trained and become more efficient, they can process faster. This means that the speed at which we think is itself plastic. Speed of thought is essential to our survival. Events often happen quickly, and if the brain is slow, it can miss important information. In one experiment Merzenich and Jenkins successfully trained monkeys to distinguish sounds in shorter and shorter spans of time. The trained neurons fired more quickly in response to the sounds, processed them in a shorter time, and needed less time to “rest” between firings. Faster neurons ultimately lead to faster thought—no minor matter—because speed of thought is a crucial component of intelligence. IQ tests, like life, measure not only whether you can get the right answer but how long it takes you to get it.
They also discovered that as they trained an animal at a skill, not only did its neurons fire faster, but because they were faster their signals were clearer. Faster neurons were more likely to fire in sync with each other—becoming better team players—wiring together more and forming groups of neurons that gave off clearer and more powerful signals. This is a crucial point, because a powerful signal has greater impact on the brain. When we want to remember something we have heard we must hear it clearly, because a memory can be only as clear as its original signal.
Finally, Merzenich discovered that paying close attention is essential to long-term plastic change. In numerous experiments he found that lasting changes occurred only when his monkeys paid close attention. When the animals performed tasks automatically, without paying attention, they changed their brain maps, but the changes did not last. We often praise “the ability to multitask.” While you can learn when you divide your attention, divided attention doesn’t lead to abiding change in your brain maps.
When Merzenich was a boy, his mother’s first cousin, a grade-school teacher in Wisconsin, was chosen teacher of the year for the entire United States. After the ceremony at the White House, she visited the Merzenich family in Oregon.
“My mother,” he recalls, “asked the inane question that you’d ask in conversation: ‘What are your most important principles in teaching?’ And her cousin answered, ‘Well, you test them when they come into school, and you figure out whether they are worthwhile. And if they are worthwhile, you really pay attention to them, and you don’t waste time on the ones that aren’t.’ That’s what she said. And you know, in one way or another, that’s reflected in how people have treated children who are different, forever. It’s just so destructive to imagine that your neurological resources are permanent and enduring and cannot be substantially improved and altered.”
Merzenich now became aware of the work of Paula Tallal at Rutgers, who had begun to analyze why children have trouble learning to read. Somewhere between 5 and 10 percent of preschool children have a language disability that makes it difficult for them to read, write, or even follow instructions. Sometimes these children are called dyslexic.
Babies begin talking by practicing consonant-vowel combinations, cooing “da, da, da” and “ba, ba, ba.” In many languages their first words consist of such combinations. In English their first words are often “mama” and “dada,” “pee pee,” and so on. Tallal’s research showed that children with language disabilities have auditory processing problems with common consonant-vowel combinations that are spoken quickly and are called “the fast parts of speech.” The children have trouble hearing them accurately and, as a result, reproducing them accurately.
Merzenich believed that these children’s auditory cortex neurons were firing too slowly, so they couldn’t distinguish between two very similar sounds or be certain, if two sounds occurred close together, which was first and which was second. Often they didn’t hear the beginnings of syllables or the sound changes within syllables. Normally neurons, after they have processed a sound, are ready to fire again after about a 30-millisecond rest. Eighty percent of language-impaired children took at least three times that long, so that they lost large amounts of language information. When their neuron-firing patterns were examined, the signals weren’t clear.
“They were muddy in, muddy out,” says Merzenich. Improper hearing led to weaknesses in all the language tasks, so they were weak in vocabulary, comprehension, speech, reading, and writing. Because they spent so much energy decoding words, they tended to use shorter sentences and failed to exercise their memory for longer sentences. Their language processing was more childlike, or “delayed,” and they still needed practice distinguishing “da, da, da” and “ba, ba, ba.”
When Tallal originally discovered their problems, she feared that “these kids were ‘broken’ and there was nothing you could do” to fix their basic brain defect. But that was before she and Merzenich combined forces.
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