The Brain That Changes Itself (2007)
By Norman Doidge
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In the mid-1960s some scientists were hostile to the very idea of cochlear implants. Some said the project was impossible. Others argued that they would put deaf patients at risk of further damage. Despite the risks, patients volunteered for implants. At first some heard only noise; others heard just a few tones, hisses, and sounds starting and stopping.
Merzenich’s contribution was to use what he had learned from mapping the auditory cortex to determine the kind of input patients needed from the implant to be able to decode speech, and where to implant the electrode. He worked with communication engineers to design a device that could transmit complex speech on a small number of bandwidth channels and still be intelligible. They developed a highly accurate, multichannel implant that allowed deaf people to hear, and the design became the basis for one of the two primary cochlear implant devices available today.
What Merzenich most wanted, of course, was to investigate plasticity directly. Finally, he decided to do a simple, radical experiment in which he would cut off all sensory input to a brain map and see how it responded. He went to his friend and fellow neuroscientist Jon Kaas, of Vanderbilt University in Nashville, who worked with adult monkeys. A monkey’s hand, like a human’s, has three main nerves: the radial, the median, and the ulnar. The median nerve conveys sensation mostly from the middle of the hand, the other two from either side of the hand. Merzenich cut the median nerve in one of the monkeys to see how the median nerve brain map would respond when all input was cut off. He went back to San Francisco and waited.
Two months later he returned to Nashville. When he mapped the monkey, he saw, as he expected, that the portion of the brain map that serves the median nerve showed no activity when he touched the middle part of the hand. But he was shocked by something else.
When he stroked the outsides of the monkey’s hand—the areas that send their signals through the radial and ulnar nerves—the median nerve map lit up! The brain maps for the radial and ulnar nerves had almost doubled in size and invaded what used to be the median nerve map. And these new maps were topographical. This time he and Kaas, writing up the findings, called the changes “spectacular” and used the word “plasticity” to explain the change, though they put it in quotes.
The experiment demonstrated that if the median nerve was cut, other nerves, still brimming with electrical input, would take over the unused map space to process their input. When it came to allocating brain-processing power, brain maps were governed by competition for precious resources and the principle of use it or lose it.
The competitive nature of plasticity affects us all. There is an endless war of nerves going on inside each of our brains. If we stop exercising our mental skills, we do not just forget them: the brain map space for those skills is turned over to the skills we practice instead. If you ever ask yourself, “How often must I practice French, or guitar, or math to keep on top of it?” you are asking a question about competitive plasticity. You are asking how frequently you must practice one activity to make sure its brain map space is not lost to another.
Competitive plasticity in adults even explains some of our limitations. Think of the difficulty most adults have in learning a second language. The conventional view now is that the difficulty arises because the critical period for language learning has ended, leaving us with a brain too rigid to change its structure on a large scale. But the discovery of competitive plasticity suggests there is more to it. As we age, the more we use our native language, the more it comes to dominate our linguistic map space. Thus it is also because our brain is plastic—and because plasticity is competitive—that it is so hard to learn a new language and end the tyranny of the mother tongue.
But why, if this is true, is it easier to learn a second language when we are young? Is there not competition then too? Not really. If two languages are learned at the same time, during the critical period, both get a foothold. Brain scans, says Merzenich, show that in a bilingual child all the sounds of its two languages share a single large map, a library of sounds from both languages.
Competitive plasticity also explains why our bad habits are so difficult to break or “unlearn.” Most of us think of the brain as a container and learning as putting something in it. When we try to break a bad habit, we think the solution is to put something new into the container. But when we learn a bad habit, it takes over a brain map, and each time we repeat it, it claims more control of that map and prevents the use of that space for “good” habits. That is why “unlearning” is often a lot harder than learning, and why early childhood education is so important—it’s best to get it right early, before the “bad habit” gets a competitive advantage.
Merzenich’s next experiment, ingeniously simple, made plasticity famous among neuroscientists and eventually did more to win over skeptics than any plasticity experiment before or since.
He mapped a monkey’s hand map in the brain. Then he amputated the monkey’s middle finger. After a number of months he remapped the monkey and found that the brain map for the amputated finger had disappeared and that the maps for the adjacent fingers had grown into the space that had originally mapped for the middle finger. Here was the clearest possible demonstration that brain maps are dynamic, that there is a competition for cortical real estate, and that brain resources are allocated according to the principle of use it or lose it.
Merzenich also noticed that animals of a particular species may have similar maps, but they are never identical. Micromapping allowed him to see differences that Penfield, with larger electrodes, could not. He also found that the maps of normal body parts change every few weeks. Every time he mapped a normal monkey’s face, it was unequivocally different. Plasticity doesn’t require the provocation of cut nerves or amputations. Plasticity is a normal phenomenon, and brain maps are constantly changing. When he wrote up this new experiment, Merzenich finally took the word “plasticity” out of quotes. Yet despite the elegance of his experiment, opposition to Merzenich’s ideas did not melt away overnight.
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