The Shallows

by Nicholas Carr

  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 Aplysia. (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.”19

  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.

  The Aplysia 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.”20 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 Synaptic Self 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.”21

  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.22 “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.”23 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.24 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.25 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.

  Much of the early evidence of neuroplasticity came through the study of the brain’s reaction to injuries, whether the severing of the nerves in the hands of Merzenich’s monkeys or the loss of sight, hearing, or a limb by human beings. That led some scientists to wonder whether the malleability of the adult brain might be limited to extreme situations. Perhaps, they theorized, plasticity is essentially a healing mechanism, triggered by trauma to the brain or the sensory organs. Further experiments have shown that that’s not the case. Extensive, perpetual plasticity has been documented in healthy, normally functioning nervous systems, leading neuroscientists to conclude that our brains are always in flux, adapting to even small shifts in our circumstances and behavior. “We have learned that neuroplasticity is not only possible but that it is constantly in action,” writes Mark Hallett, head of the Medical Neurology Branch of the National Institutes of Health. “That is the way we adapt to changing conditions, the way we learn new facts, and the way we develop new skills.”26

  “Plasticity,” says Alvaro Pascual-Leone, a top neurology researcher at Harvard Medical School, is “the normal ongoing state of the nervous system throughout the life span.” Our brains are constantly changing in response to our experiences and our behavior, reworking their circuitry with “each sensory input, motor act, association, reward signal, action plan, or [shift of] awareness.” Neuroplasticity, argues Pascual-Leone, is one of the most important products of evolution, a trait that enables the nervous system “to escape the restrictions of its own genome and thus adapt to environmental pressures, physiologic changes, and experiences.”27 The genius of our brain’s co
nstruction is not that it contains a lot of hardwiring but that it doesn’t. Natural selection, writes the philosopher David Buller in Adapting Minds, his critique of evolutionary psychology, “has not designed a brain that consists of numerous prefabricated adaptations” but rather one that is able “to adapt to local environmental demands throughout the lifetime of an individual, and sometimes within a period of days, by forming specialized structures to deal with those demands.”28 Evolution has given us a brain that can literally change its mind—over and over again.

  Our ways of thinking, perceiving, and acting, we now know, are not entirely determined by our genes. Nor are they entirely determined by our childhood experiences. We change them through the way we live—and, as Nietzsche sensed, through the tools we use. Years before Edward Taub opened his rehabilitation clinic in Alabama, he conducted a famous experiment on a group of right-handed violinists. Using a machine that monitors neural activity, he measured the areas of their sensory cortex that processed signals from their left hands, the hands they used to finger the strings of their instruments. He also measured the same cortical areas in a group of right-handed volunteers who had never played a musical instrument. He found that the brain areas of the violinists were significantly larger than those of the nonmusicians. He then measured the size of the cortical areas that processed sensations from the subjects’ right hands. Here, he found no differences between the musicians and the nonmusicians. Playing a violin, a musical tool, had resulted in substantial physical changes in the brain. That was true even for the musicians who had first taken up their instruments as adults.

  When scientists have trained primates and other animals to use simple tools, they’ve discovered just how profoundly the brain can be influenced by technology. Monkeys, for instance, were taught how to use rakes and pliers to take hold of pieces of food that would otherwise have been out of reach. When researchers monitored the animals’ neural activity throughout the course of the training, they found significant growth in the visual and motor areas involved in controlling the hands that held the tools. But they discovered something even more striking as well: the rakes and pliers actually came to be incorporated into the brain maps of the animals’ hands. The tools, so far as the animals’ brains were concerned, had become part of their bodies. As the researchers who conducted the experiment with the pliers reported, the monkeys’ brains began to act “as if the pliers were now the hand fingers.”29

  It’s not just repeated physical actions that can rewire our brains. Purely mental activity can also alter our neural circuitry, sometimes in far-reaching ways. In the late 1990s, a group of British researchers scanned the brains of sixteen London cab drivers who had between two and forty-two years of experience behind the wheel. When they compared the scans with those of a control group, they found that the taxi drivers’ posterior hippocampus, a part of the brain that plays a key role in storing and manipulating spatial representations of a person’s surroundings, was much larger than normal. Moreover, the longer a cab driver had been on the job, the larger his posterior hippocampus tended to be. The researchers also discovered that a portion of the drivers’ anterior hippocampus was smaller than average, apparently a result of the need to accommodate the enlargement of the posterior area. Further tests indicated that the shrinking of the anterior hippocampus might have reduced the cabbies’ aptitude for certain other memorization tasks. The constant spatial processing required to navigate London’s intricate road system, the researchers concluded, is “associated with a relative redistribution of gray matter in the hippocampus.”30

  Another experiment, conducted by Pascual-Leone when he was a researcher at the National Institutes of Health, provides even more remarkable evidence of the way our patterns of thought affect the anatomy of our brains. Pascual-Leone recruited people who had no experience playing a piano, and he taught them how to play a simple melody consisting of a short series of notes. He then split the participants into two groups. He had the members of one group practice the melody on a keyboard for two hours a day over the next five days. He had the members of the other group sit in front of a keyboard for the same amount of time but only imagine playing the song—without ever touching the keys. Using a technique called transcranial magnetic stimulation, or TMS, Pascual-Leone mapped the brain activity of all the participants before, during, and after the test. He found that the people who had only imagined playing the notes exhibited precisely the same changes in their brains as those who had actually pressed the keys.31 Their brains had changed in response to actions that took place purely in their imagination—in response, that is, to their thoughts. Descartes may have been wrong about dualism, but he appears to have been correct in believing that our thoughts can exert a physical influence on, or at least cause a physical reaction in, our brains. We become, neurologically, what we think.

  MICHAEL GREENBERG, IN a 2008 essay in the New York Review of Books, found the poetry in neuroplasticity. He observed that our neurological system, “with its branches and transmitters and ingeniously spanned gaps, has an improvised quality that seems to mirror the unpredictability of thought itself.” It’s “an ephemeral place that changes as our experience changes.”32 There are many reasons to be grateful that our mental hardware is able to adapt so readily to experience, that even old brains can be taught new tricks. The brain’s adaptability hasn’t just led to new treatments, and new hope, for those suffering from brain injury or illness. It provides all of us with a mental flexibility, an intellectual litheness, that allows us to adapt to new situations, learn new skills, and in general expand our horizons.

  But the news is not all good. Although neuroplasticity provides an escape from genetic determinism, a loophole for free thought and free will, it also imposes its own form of determinism on our behavior. As particular circuits in our brain strengthen through the repetition of a physical or mental activity, they begin to transform that activity into a habit. The paradox of neuroplasticity, observes Doidge, is that, for all the mental flexibility it grants us, it can end up locking us into “rigid behaviors.”33 The chemically triggered synapses that link our neurons program us, in effect, to want to keep exercising the circuits they’ve formed. Once we’ve wired new circuitry in our brain, Doidge writes, “we long to keep it activated.”34 That’s the way the brain fine-tunes its operations. Routine activities are carried out ever more quickly and efficiently, while unused circuits are pruned away.

  Plastic does not mean elastic, in other words. Our neural loops don’t snap back to their former state the way a rubber band does; they hold onto their changed state. And nothing says the new state has to be a desirable one. Bad habits can be ingrained in our neurons as easily as good ones. Pascual-Leone observes that “plastic changes may not necessarily represent a behavioral gain for a given subject.” In addition to being “the mechanism for development and learning,” plasticity can be “a cause of pathology.”35

  It comes as no surprise that neuroplasticity has been linked to mental afflictions ranging from depression to obsessive-compulsive disorder to tinnitus. The more a sufferer concentrates on his symptoms, the deeper those symptoms are etched into his neural circuits. In the worst cases, the mind essentially trains itself to be sick. Many addictions, too, are reinforced by the strengthening of plastic pathways in the brain. Even very small doses of addictive drugs can dramatically alter the flow of neurotransmitters in a person’s synapses, resulting in long-lasting alterations in brain circuitry and function. In some cases, the buildup of certain kinds of neurotransmitters, such as dopamine, a pleasure-producing cousin to adrenaline, seems to actually trigger the turning on or off of particular genes, bringing even stronger cravings for the drug. The vital paths turn deadly.

  The potential for unwelcome neuroplastic adaptations also exists in the everyday, normal functioning of our minds. Experiments show that just as the brain can build new or stronger circuits through physical or mental practice, those circuits can weaken or dissolve with neglect. “If we stop
exercising our mental skills,” writes Doidge, “we do not just forget them: the brain map space for those skills is turned over to the skills we practice instead.”36 Jeffrey Schwartz, a professor of psychiatry at UCLA’s medical school, terms this process “survival of the busiest.”37 The mental skills we sacrifice may be as valuable, or even more valuable, than the ones we gain. When it comes to the quality of our thought, our neurons and synapses are entirely indifferent. The possibility of intellectual decay is inherent in the malleability of our brains.

  That doesn’t mean that we can’t, with concerted effort, once again redirect our neural signals and rebuild the skills we’ve lost. What it does mean is that the vital paths in our brains become, as Monsieur Dumont understood, the paths of least resistance. They are the paths that most of us will take most of the time, and the farther we proceed down them, the more difficult it becomes to turn back.

  A Digression On What The Brain Thinks About When It Thinks About Itself

  THE FUNCTION OF the brain, Aristotle believed, was to keep the body from overheating. A “compound of earth and water,” brain matter “tempers the heat and seething of the heart,” he wrote in The Parts of Animals, a treatise on anatomy and physiology. Blood rises from the “fiery” region of the chest until it reaches the head, where the brain reduces its temperature “to moderation.” The cooled blood then flows back down through the rest of the body. The process, suggested Aristotle, was akin to that which “occurs in the production of showers. For when vapor steams up from the earth under the influence of heat and is carried into the upper regions, so soon as it reaches the cold air that is above the earth, it condenses again into water owing to the refrigeration, and falls back to the earth as rain.” The reason man has “the largest brain in proportion to his size” is that “the region of the heart and of the lung is hotter and richer in blood in man than in any other animal.” It seemed obvious to Aristotle that the brain could not possibly be “the organ of sensation,” as Hippocrates and others had conjectured, since “when it is touched, no sensation is produced.” In its insensibility, “it resembles,” he wrote, “the blood of animals and their excrement.”1