The Brain That Changes Itself (2007)
By Norman Doidge
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He laughs when he says it. “Let me tell you what happened when I began to declare that the brain was plastic. I received hostile treatment. I don’t know how else to put it. I got people saying things in reviews such as, ‘This would be really interesting if it could possibly be true, but it could not be.’ It was as if I just made it up.”
Because Merzenich was arguing that brain maps could alter their borders and location and change their functions well into adulthood, localizationists opposed him. “Almost everybody I knew in the mainstream of neuroscience,” he says, “thought that this was sort of semi-serious stuff—that the experiments were sloppy, that the effects described were uncertain. But actually the experiment had been done enough times that I realized that the position of the majority was arrogant and indefensible.”
One of the major figures who voiced doubts was Torsten Wiesel. Despite the fact that Wiesel had shown that plasticity exists in the critical period, he still opposed the idea that it existed in adults, and wrote that he and Hubel “firmly believed that once cortical connections were established in their mature form, they stayed in place permanently.” He had indeed won the Nobel Prize for establishing where visual processing occurs, a finding considered one of localizationism’s greatest triumphs. Wiesel now accepts adult plasticity and has gracefully acknowledged in print that for a long time he was wrong and that Merzenich’s pioneering experiments ultimately led him and his colleagues to change their minds. Hardcore localizationists took notice when a man of Wiesel’s stature changed his mind.
“The most frustrating thing,” says Merzenich, “was that I saw that neuroplasticity had all kinds of potential implications for medical therapeutics—for the interpretation of human neuropathology and psychiatry. And nobody paid any attention.”
Since plastic change is a process, Merzenich realized he would only really be able to understand it if he could see it unfolding in the brain over time. He cut a monkey’s median nerve and then did multiple mappings over a number of months.
The first mapping, immediately after he cut the nerve, showed, as he expected, that the brain map for the median nerve was completely silent when the middle of the hand was stroked. But when he stroked the part of the hand served by the outside nerves, the silent median nerve portion of the map lit up immediately. Maps for the outside nerves, the radial and ulnar nerves, now appeared in the median map space. These maps sprang up so quickly, it was as though they had been hidden there all along, since early development, and now they were “unmasked.”
On the twenty-second day Merzenich mapped the monkey again. The radial and ulnar maps, which had been lacking in detail when they first appeared, had grown more refined and detailed and had now expanded to occupy almost the entire median nerve map. (A primitive map lacks detail; a refined map has a lot and thus conveys more information.)
By the 144th day the whole map was every bit as detailed as a normal map.
By doing multiple mappings over time, Merzenich observed that the new maps were changing their borders, becoming more detailed, and even moving around the brain. In one case he even saw a map disappear altogether, like Atlantis.
It seemed reasonable to assume that if totally new maps were forming, then new connections must have been forming among neurons. To help understand this process, Merzenich invoked the ideas of Donald O. Hebb, a Canadian behavioral psychologist who had worked with Penfield. In 1949 Hebb proposed that learning linked neurons in new ways. He proposed that when two neurons fire at the same time repeatedly (or when one fires, causing another to fire), chemical changes occur in both, so that the two tend to connect more strongly. Hebb’s concept—actually proposed by Freud sixty years before—was neatly summarized by neuroscientist Carla Shatz: Neurons that fire together wire together.
Hebb’s theory thus argued that neuronal structure can be altered by experience. Following Hebb, Merzenich’s new theory was that neurons in brain maps develop strong connections to one another when they are activated at the same moment in time. And if maps could change, thought Merzenich, then there was reason to hope that people born with problems in brain map–processing areas—people with learning problems, psychological problems, strokes, or brain injuries—might be able to form new maps if he could help them form new neuronal connections, by getting their healthy neurons to fire together and wire together.
Starting in the late 1980s, Merzenich designed or participated in brilliant studies to test whether brain maps are time based and whether their borders and functioning can be manipulated by “playing” with the timing of input to them.
In one ingenious experiment, Merzenich mapped a normal monkey’s hand, then sewed together two of the monkey’s fingers, so that both fingers moved as one. After several months of allowing the monkey to use its sewn fingers, the monkey was remapped. The two maps of the originally separate fingers had now merged into a single map. If the experimenters touched any point on either finger, this new single map would light up. Because all the movements and sensations in those fingers always occurred simultaneously, they’d formed the same map. The experiment showed that timing of the input to the neurons in the map was the key to forming it—neurons that fired together in time wired together to make one map.
Other scientists tested Merzenich’s findings on human beings. Some people are born with their fingers fused, a condition called syndactyly or “webbed-finger syndrome.” When two such people were mapped, the brain scan found that they each had one large map for their fused fingers instead of two separate ones.
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