Authors: Nicholas Carr
IT’S 1968. I’M
nine years old, a run-of-the-mill suburban kid playing in a patch of woods near my family’s home. Marshall McLuhan and Norman Mailer are on prime-time TV, debating the intellectual and moral implications of what Mailer describes as “man’s acceleration into a super-technological world.”
is having its first theatrical run, leaving moviegoers befuddled, bemused, or just plain annoyed. And in a quiet laboratory at the University of Wisconsin in Madison, Michael Merzenich is cutting a hole in a monkey’s skull.
Twenty-six years old, Merzenich has just received a doctorate in physiology from Johns Hopkins, where he studied under Vernon Mountcastle, a pioneering neuroscientist. He has come to Wisconsin to do postdoctoral research in brain mapping. It’s been known for years that every area of a person’s body is represented by a corresponding area in the cerebral cortex, the brain’s wrinkled outer layer. When certain nerve cells in the skin are stimulated—by being touched or pinched, say—they send an electric pulse through the spinal cord to a particular cluster of neurons in the cortex, which translates the touch or the pinch into a conscious sensation. In the 1930s, the Canadian neurosurgeon Wilder Penfield had used electrical probes to draw the first sensory maps of people’s brains. But Penfield’s probes were crude instruments, and his maps, while groundbreaking in their time, lacked precision. Merzenich is using a new kind of probe, the hair-thin microelectrode, to create much finer maps that will, he hopes, provide new insight into the brain’s structure.
Once he has removed a piece of the monkey’s skull and exposed a small portion of its brain, he threads a microelectrode into the area of the cortex that registers sensations from one of the animal’s hands. He begins tapping that hand in different places until the neuron beside the tip of the electrode fires. After methodically inserting and reinserting the electrode thousands of times over the course of a few days, he ends up with a “micromap” showing in minute detail, down to the individual nerve cell, how the monkey’s brain processes what its hand feels. He repeats the painstaking exercise with five more monkeys.
Merzenich proceeds to the second stage of his experiment. Using a scalpel, he makes incisions in the hands of the animals, severing the sensory nerve. He wants to find out how the brain reacts when a peripheral nerve system is damaged and then allowed to heal. What he discovers astounds him. The nerves in the monkeys’ hands grow back in a haphazard fashion, as expected, and their brains, also as expected, become confused. When, for example, Merzenich touches the lower joint of a finger on one monkey’s hand, the monkey’s brain tells the animal that the sensation is coming from the tip of the finger. The signals have been crossed, the brain map scrambled. But when Merzenich conducts the same sensory tests a few months later, he finds that the mental confusion has been cleared up. What the monkeys’ brains tell them is happening to their hands now matches what’s really happening. The brains, Merzenich realizes, have reorganized themselves. The animals’ neural pathways have woven themselves into a new map that corresponds to the new arrangement of nerves in their hands.
At first, he can’t believe what he’s seen. Like every other neuroscientist, he’s been taught that the structure of the adult brain is fixed. Yet in his lab he has just seen the brains of six monkeys undergo rapid and extensive restructuring at the cellular level. “I knew it was astounding reorganization, but I couldn’t explain it,” Merzenich will later recall. “Looking back on it, I realized that I had seen evidence of neuroplasticity. But I didn’t know it at the time. I simply didn’t know what I was seeing. And besides, in mainstream neuroscience, nobody would believe that plasticity was occurring on this scale.”
Merzenich publishes the results of his experiment in an academic journal.
Nobody pays much heed. But he knows he’s onto something, and over the course of the next three decades he conducts many more tests on many more monkeys, all of which point to the existence of broad plasticity in the brains of mature primates. In a 1983 paper documenting one of the experiments, Merzenich declares flatly, “These results are completely contrary to a view of sensory systems as consisting of a series of hardwired machines.”
At first dismissed, Merzenich’s meticulous work finally begins to receive serious notice in the neurological community. It ends up setting off a wholesale reevaluation of accepted theories about how our brains work. Researchers uncover a trail of experiments, dating back to the days of William James and Sigmund Freud, that record examples of plasticity. Long ignored, the old research is now taken seriously.
As brain science continues to advance, the evidence for plasticity strengthens. Using sensitive new brain-scanning equipment, as well as microelectrodes and other probes, neuroscientists conduct more experiments, not only on lab animals but on people. All of them confirm Merzenich’s discovery. They also reveal something more: The brain’s plasticity is not limited to the somatosensory cortex, the area that governs our sense of touch. It’s universal. Virtually all of our neural circuits—whether they’re involved in feeling, seeing, hearing, moving, thinking, learning, perceiving, or remembering—are subject to change. The received wisdom is cast aside.
THE ADULT BRAIN,
it turns out, is not just plastic but, as James Olds, a professor of neuroscience who directs the Krasnow Institute for Advanced Study at George Mason University, puts it, “very plastic.”
Or, as Merzenich himself says, “massively plastic.”
The plasticity diminishes as we get older—brains do get stuck in their ways—but it never goes away. Our neurons are always breaking old connections and forming new ones, and brand-new nerve cells are always being created. “The brain,” observes Olds, “has the ability to reprogram itself on the fly, altering the way it functions.”
We don’t yet know all the details of how the brain reprograms itself, but it has become clear that, as Freud proposed, the secret lies mainly in the rich chemical broth of our synapses. What goes on in the microscopic spaces between our neurons is exceedingly complicated, but in simple terms it involves various chemical reactions that register and record experiences in neural pathways. Every time we perform a task or experience a sensation, whether physical or mental, a set of neurons in our brains is activated. If they’re in proximity, these neurons join together through the exchange of synaptic neurotransmitters like the amino acid glutamate.
As the same experience is repeated, the synaptic links between the neurons grow stronger and more plentiful through both physiological changes, such as the release of higher concentrations of neurotransmitters, and anatomical ones, such as the generation of new neurons or the growth of new synaptic terminals on existing axons and dendrites. Synaptic links can also weaken in response to experiences, again as a result of physiological and anatomical alterations. What we learn as we live is embedded in the ever-changing cellular connections inside our heads. The chains of linked neurons form our minds’ true “vital paths.” Today, scientists sum up the essential dynamic of neuroplasticity with a saying known as Hebb’s rule: “Cells that fire together wire together.”
One of the simplest yet most powerful demonstrations of how synaptic connections change came in a series of experiments that the biologist Eric Kandel performed in the early 1970s on a type of large sea slug called
. (Sea creatures make particularly good subjects for neurological tests because they tend to have simple nervous systems and large nerve cells.) Kandel, who would earn a Nobel Prize for his work, found that if you touch a slug’s gill, even very lightly, the gill will immediately and reflexively recoil. But if you touch the gill repeatedly, without causing any harm to the animal, the recoiling instinct will steadily diminish. The slug will become habituated to the touch and learn to ignore it. By monitoring slugs’ nervous systems, Kandel discovered that “this learned change in behavior was paralleled by a progressive weakening of the synaptic connections” between the sensory neurons that “feel” the touch and the motor neurons that tell the gill to retract. In a slug’s ordinary state, about ninety percent of the sensory neurons in its gill have connections to motor neurons. But after its gill is touched just forty times, only ten percent of the sensory cells maintain links to the motor cells. The research “showed dramatically,” Kandel wrote, that “synapses can undergo large and enduring changes in strength after only a relatively small amount of training.”
The plasticity of our synapses brings into harmony two philosophies of the mind that have for centuries stood in conflict: empiricism and rationalism. In the view of empiricists, like John Locke, the mind we are born with is a blank slate, a “tabula rasa.” What we know comes entirely through our experiences, through what we learn as we live. To put it into more familiar terms, we are products of nurture, not nature. In the view of rationalists, like Immanuel Kant, we are born with built-in mental “templates” that determine how we perceive and make sense of the world. All our experiences are filtered through these inborn templates. Nature predominates.
experiments revealed, as Kandel reports, “that both views had merit—in fact they complemented each other.” Our genes “specify” many of “the connections among neurons—that is, which neurons form synaptic connections with which other neurons and when.” Those genetically determined connections form Kant’s innate templates, the basic architecture of the brain. But our experiences regulate the strength, or “long-term effectiveness,” of the connections, allowing, as Locke had argued, the ongoing reshaping of the mind and “the expression of new patterns of behavior.”
The opposing philosophies of the empiricist and the rationalist find their common ground in the synapse. The New York University neuroscientist Joseph LeDoux explains in his book
that nature and nurture “actually speak the same language. They both ultimately achieve their mental and behavioral effects by shaping the synaptic organization of the brain.”
The brain is not the machine we once thought it to be. Though different regions are associated with different mental functions, the cellular components do not form permanent structures or play rigid roles. They’re flexible. They change with experience, circumstance, and need. Some of the most extensive and remarkable changes take place in response to damage to the nervous system. Experiments show, for instance, that if a person is struck blind, the part of the brain that had been dedicated to processing visual stimuli—the visual cortex—doesn’t just go dark. It is quickly taken over by circuits used for audio processing. And if the person learns to read Braille, the visual cortex will be redeployed for processing information delivered through the sense of touch.
“Neurons seem to ‘want’ to receive input,” explains Nancy Kanwisher of MIT’s McGovern Institute for Brain Research: “When their usual input disappears, they start responding to the next best thing.”
Thanks to the ready adaptability of neurons, the senses of hearing and touch can grow sharper to mitigate the effects of the loss of sight. Similar alterations happen in the brains of people who go deaf: their other senses strengthen to help make up for the loss of hearing. The area in the brain that processes peripheral vision, for example, grows larger, enabling them to see what they once would have heard.
Tests on people who have lost arms or legs in accidents also reveal how extensively the brain can reorganize itself. The areas in the victims’ brains that had registered sensations in their lost limbs are quickly taken over by circuits that register sensations from other parts of their bodies. In studying a teenage boy who had lost his left arm in a car crash, the neurologist V. S. Ramachandran, who heads the Center for Brain and Cognition at the University of California at San Diego, discovered that when he had the young man close his eyes and then touched different parts of his face, the patient believed that it was his missing arm that was being touched. At one point, Ramachandran brushed a spot beneath the boy’s nose and asked, “Where do you feel that?” The boy replied, “On my left pinky. It tingles.” The boy’s brain map was in the process of being reorganized, the neurons redeployed for new uses.
As a result of such experiments, it’s now believed that the sensations of a “phantom limb” felt by amputees are largely the result of neuroplastic changes in the brain.
Our expanding understanding of the brain’s adaptability has led to the development of new therapies for conditions that used to be considered untreatable.
Doidge, in his 2007 book
The Brain That Changes Itself
, tells the story of a man named Michael Bernstein who suffered a severe stroke when he was fifty-four, damaging an area in the right half of his brain that regulated movement in the left side of his body. Through a traditional program of physical therapy, he recovered some of his motor skills, but his left hand remained crippled and he had to use a cane to walk. Until recently, that would have been the end of the story. But Bernstein enrolled in a program of experimental therapy, run at the University of Alabama by a pioneering neuroplasticity researcher named Edward Taub. For as many as eight hours a day, six days a week, Bernstein used his left hand and his left leg to perform routine tasks over and over again. One day he might wash the pane of a window. The next day he might trace the letters of the alphabet. The repeated actions were a means of coaxing his neurons and synapses to form new circuits that would take over the functions once carried out by the circuits in the damaged area in his brain. In a matter of weeks, he regained nearly all of the movement in his hand and his leg, allowing him to return to his everyday routines and throw away his cane. Many of Taub’s other patients have experienced similarly strong recoveries.