The Brain That Changes Itself (2007)
By Norman Doidge
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Penfield spent years mapping the sensory and motor parts of the brain, while performing brain surgery on cancer and epilepsy patients who could be conscious during the operation, because there are no pain receptors in the brain. Both the sensory and motor maps are part of the cerebral cortex, which lies on the brain’s surface and so is easily accessible with a probe. Penfield discovered that when he touched a patient’s sensory brain map with an electric probe, it triggered sensations that the patient felt in his body. He used the electric probe to help him distinguish the healthy tissue he wanted to preserve from the unhealthy tumors or pathological tissue he needed to remove.
Normally, when one’s hand is touched, an electrical signal passes to the spinal cord and up to the brain, where it turns on cells in the map that make the hand feel touched. Penfield found he could also make the patient feel his hand was touched by turning on the hand area of the brain map electrically. When he stimulated another part of the map, the patient might feel his arm being touched; another part, his face. Each time he stimulated an area, he asked his patients what they’d felt, to make sure he didn’t cut away healthy tissue. After many such operations he was able to show where on the brain’s sensory map all parts of the body’s surface were represented.
He did the same for the motor map, the part of the brain that controls movement. By touching different parts of this map, he could trigger movements in a patient’s leg, arm, face, and other muscles.
One of the great discoveries Penfield made was that sensory and motor brain maps, like geographical maps, are topographical, meaning that areas adjacent to each other on the body’s surface are generally adjacent to each other on the brain maps. He also discovered that when he touched certain parts of the brain, he triggered long-lost childhood memories or dreamlike scenes—which implied that higher mental activities were also mapped in the brain.
The Penfield maps shaped several generations’ view of the brain. But because scientists believed that the brain couldn’t change, they assumed, and taught, that the maps were fixed, immutable, and universal—the same in each of us—though Penfield himself never made either claim.
Merzenich discovered that these maps are neither immutable within a single brain nor universal but vary in their borders and size from person to person. In a series of brilliant experiments he showed that the shape of our brain maps changes depending upon what we do over the course of our lives. But in order to prove this point he needed a tool far finer than Penfield’s electrodes, one that would be able to detect changes in just a few neurons at a time.
While an undergraduate at the University of Portland, Merzenich and a friend used electronic lab equipment to demonstrate the storm of electrical activity in insects’ neurons. These experiments came to the attention of a professor who admired Merzenich’s talent and curiosity and recommended him for graduate school at both Harvard and Johns Hopkins. Both accepted him. Merzenich opted for Hopkins to do his Ph.D. in physiology under one of the great neuroscientists of the time, Vernon Mountcastle, who in the 1950s was demonstrating that the subtleties of brain architecture could be discovered by studying the electrical activity of neurons using a new technique: micromapping with pin-shaped microelectrodes.
Microelectrodes are so small and sensitive that they can be inserted inside or beside a single neuron and can detect when an individual neuron fires off its electrical signal to other neurons. The neuron’s signal passes from the microelectrode to an amplifier and then to an oscilloscope screen, where it appears as a sharp spike. Merzenich would make most of his major discoveries with microelectrodes.
This momentous invention allowed neuroscientists to decode the communication of neurons, of which the adult human brain has approximately 100 billion. Using large electrodes as Penfield did, scientists could observe thousands of neurons firing at once. With microelectrodes, scientists could “listen in on” one or several neurons at a time as they communicated with one another. Micromapping is still about a thousand times more precise than the current generation of brain scans, which detect bursts of activity that last one second in thousands of neurons. But a neuron’s electrical signal often lasts a thousandth of a second, so brain scans miss an extraordinary amount of information. Yet micromapping hasn’t replaced brain scans because it requires an extremely tedious kind of surgery, conducted under a microscope with microsurgical instruments.
Merzenich took to this technology right away. To map the area of the brain that processes feeling from the hand, Merzenich would cut away a piece of a monkey’s skull over the sensory cortex, exposing a 1- to 2-millimeter strip of brain, then insert a microelectrode beside a sensory neuron. Next, he would tap the monkey’s hand until he touched a part—say, the tip of a finger—that caused that neuron to fire an electrical signal into the microelectrode. He would record the location of the neuron that represented the fingertip, establishing the first point on the map. Then he would remove the microelectrode, reinsert it near another neuron, and tap different parts of the hand, until he located the part that turned on that neuron. He did this until he’d mapped the entire hand. A single mapping might require five hundred insertions and take several days, and Merzenich and his colleagues did thousands of these laborious surgeries to make their discoveries.
At about this time, a crucial discovery was made that would forever affect Merzenich’s work. In the 1960s, just as Merzenich was beginning to use microelectrodes on the brain, two other scientists, who had also worked at Johns Hopkins with Mountcastle, discovered that the brain in very young animals is plastic. David Hubel and Torsten Wiesel were micromapping the visual cortex to learn how vision is processed. They’d inserted microelectrodes into the visual cortex of kittens and discovered that different parts of the cortex processed the lines, orientations, and movements of visually perceived objects. They also discovered that there was a “critical period,” from the third to the eighth week of life, when the newborn kitten’s brain had to receive visual stimulation in order to develop normally. In the crucial experiment Hubel and Wiesel sewed shut one eyelid of a kitten during its critical period, so the eye got no visual stimulation. When they opened this shut eye, they found that the visual areas in the brain map that normally processed input from the shut eye had failed to develop, leaving the kitten blind in that eye for life. Clearly the brains of kittens during the critical period were plastic, their structure literally shaped by experience.
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