The Brain That Changes Itself (2007)
By Norman Doidge
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When Hubel and Wiesel examined the brain map for that blind eye, they made one more unexpected discovery about plasticity. The part of the kitten’s brain that had been deprived of input from the shut eye did not remain idle. It had begun to process visual input from the open eye, as though the brain didn’t want to waste any “cortical real estate” and had found a way to rewire itself—another indication that the brain is plastic in the critical period. For this work Hubel and Wiesel received the Nobel Prize. Yet even though they had discovered plasticity in infancy, they remained localizationists, defending the idea that the adult brain is hardwired by the end of infancy to perform functions in fixed locations.
The discovery of the critical period became one of the most famous in biology in the second half of the twentieth century. Scientists soon showed that other brain systems required environmental stimuli to develop. It also seemed that each neural system had a different critical period, or window of time, during which it was especially plastic and sensitive to the environment, and during which it had rapid, formative growth. Language development, for instance, has a critical period that begins in infancy and ends between eight years and puberty. After this critical period closes, a person’s ability to learn a second language without an accent is limited. In fact, second languages learned after the critical period are not processed in the same part of the brain as is the native tongue.
The notion of critical periods also lent support to ethologist Konrad Lorenz’s observation that goslings, if exposed to a human being for a brief period of time, between fifteen hours and three days after birth, bonded with that person, instead of with their mother, for life. To prove it, he got goslings to bond to him and follow him around. He called this process “imprinting.” In fact, the psychological version of the critical period went back to Freud, who argued that we go through developmental stages that are brief windows of time, during which we must have certain experiences to be healthy; these periods are formative, he said, and shape us for the rest of our lives.
Critical-period plasticity changed medical practice. Because of Hubel and Wiesel’s discovery, children born with cataracts no longer faced blindness. They were now sent for corrective surgery as infants, during their critical period, so their brains could get the light required to form crucial connections. Microelectrodes had shown that plasticity is an indisputable fact of childhood. And they also seemed to show that, like childhood, this period of cerebral suppleness is short-lived.
Merzenich’s first glimpse of adult plasticity was accidental. In 1968, after completing his doctorate, he went to do a postdoc with Clinton Woolsey, a researcher in Madison, Wisconsin, and peer of Penfield’s. Woolsey asked Merzenich to supervise two neurosurgeons, Drs. Ron Paul and Herbert Goodman. The three decided to observe what happens in the brain when one of the peripheral nerves in the hand is cut and then starts to regenerate.
It is important to understand that the nervous system is divided into two parts. The first part is the central nervous system (the brain and spinal cord), which is the command-and-control center of the system; it was thought to lack plasticity. The second part is the peripheral nervous system, which brings messages from the sense receptors to the spinal cord and brain and carries messages from the brain and spinal cord to the muscles and glands. The peripheral nervous system was long known to be plastic; if you cut a nerve in your hand, it can “regenerate” or heal itself.
Each neuron has three parts. The dendrites are treelike branches that receive input from other neurons. These dendrites lead into the cell body, which sustains the life of the cell and contains its DNA. Finally the axon is a living cable of varying lengths (from microscopic lengths in the brain, to some that can run down to the legs and reach up to six feet long). Axons are often compared to wires because they carry electrical impulses at very high speeds (from 2 to 200 miles per hour) toward the dendrites of neighboring neurons.
A neuron can receive two kinds of signals: those that excite it and those that inhibit it. If a neuron receives enough excitatory signals from other neurons, it will fire off its own signal. When it receives enough inhibitory signals, it becomes less likely to fire. Axons don’t quite touch the neighboring dendrites. They are separated by a microscopic space called a synapse. Once an electrical signal gets to the end of the axon, it triggers the release of a chemical messenger, called a neurotransmitter, into the synapse. The chemical messenger floats over to the dendrite of the adjacent neuron, exciting or inhibiting it. When we say that neurons “rewire” themselves, we mean that alterations occur at the synapse, strengthening and increasing, or weakening and decreasing, the number of connections between the neurons.
Merzenich, Paul, and Goodman wanted to investigate a well-known but mysterious interaction between the peripheral and central nervous systems. When a large peripheral nerve (which consists of many axons) is cut, sometimes in the process of regeneration the “wires get crossed.” When axons reattach to the axons of the wrong nerve, the person may experience “false localization,” so that a touch on the index finger is felt in the thumb. Scientists assumed that this false localization occurred because the regeneration process “shuffled” the nerves, sending the signal from the index finger to the brain map for the thumb.
The model scientists had of the brain and the nervous system was that each point on the body surface had a nerve that passed signals directly to a specific point on the brain map, anatomically hardwired at birth. Thus a nerve branch for the thumb always passed its signals directly to the spot on the sensory brain map for the thumb. Merzenich and the group accepted this “point-to-point” model of the brain map and innocently set out to document what was happening in the brain during this shuffling of nerves.
They micromapped the hand maps in the brains of several adolescent monkeys, cut a peripheral nerve to the hand, and immediately sewed the two severed ends close together but not quite touching, hoping the many axonal wires in the nerve would get crossed as the nerve regenerated itself. After seven months they remapped the brain. Merzenich assumed they would see a very disturbed, chaotic brain map. Thus, if the nerves for the thumb and the index finger had been crossed, he expected that touching the index finger would generate activity in the map area for the thumb. But he saw nothing of the kind. The map was almost normal.
“What we saw,” says Merzenich, “was absolutely astounding. I couldn’t understand it.” It was topographically arranged as though the brain had unshuffled the signals from the crossed nerves.
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