In March, a Berlin newspaper reported that Nietzsche “feels better than ever” and, thanks to his typewriter, “has resumed his writing activities.”
But the device had a subtler effect on his work. One of Nietzsche’s closest friends, the writer and composer Heinrich Köselitz, noticed a change in the style of his writing. Nietzsche’s prose had become tighter, more telegraphic. There was a new forcefulness to it, too, as though the machine’s power—its “iron”—was, through some mysterious metaphysical mechanism, being transferred into the words it pressed into the page. “Perhaps you will through this instrument even take to a new idiom,” Köselitz wrote in a letter, noting that, in his own work, “my ‘thoughts’ in music and language often depend on the quality of pen and paper.”
“You are right,” Nietzsche replied. “Our writing equipment takes part in the forming of our thoughts.”2
WHILE NIETZSCHE WAS learning to type on his writing ball in Genoa, five hundred miles to the northeast a young medical student named Sigmund Freud was working as a neurophysiology researcher in a Vienna laboratory. His specialty was dissecting the nervous systems of fish and crustaceans. Through his experiments, he came to surmise that the brain, like other bodily organs, is made up of many separate cells. He later extended his theory to suggest that the gaps between the cells—the “contact barriers,” as he termed them—play an essential role in governing the functions of the mind, shaping our memories and our thoughts. At the time, Freud’s conclusions lay outside the mainstream of scientific opinion. Most doctors and researchers believed that the brain was not cellular in construction but rather consisted of a single, continuous fabric of nerve fibers. And even among those who shared Freud’s view that the brain was made of cells, few paid any attention to what might be going on in the spaces between those cells.3
Engaged to be wed and in need of a more substantial income, Freud soon abandoned his career as a researcher and went into private practice as a psychoanalyst. But subsequent studies bore out his youthful speculations. Armed with ever more powerful microscopes, scientists confirmed the existence of discrete nerve cells. They also discovered that those cells—our neurons—are both like and unlike the other cells in our bodies. Neurons have central cores, or somas, which carry out the functions common to all cells, but they also have two kinds of tentacle-like appendages—axons and dendrites—that transmit and receive electric pulses. When a neuron is active, a pulse flows from the soma to the tip of the axon, where it triggers the release of chemicals called neurotransmitters. The neurotransmitters flow across Freud’s contact barrier—the synapse, we now call it—and attach themselves to a dendrite of a neighboring neuron, triggering (or suppressing) a new electric pulse in that cell. It’s through the flow of neurotransmitters across synapses that neurons communicate with one another, directing the transmission of electrical signals along complex cellular pathways. Thoughts, memories, emotions—all emerge from the electrochemical interactions of neurons, mediated by synapses.
During the twentieth century, neuroscientists and psychologists also came to more fully appreciate the astounding complexity of the human brain. Inside our skulls, they discovered, are some 100 billion neurons, which take many different shapes and range in length from a few tenths of a millimeter to a few feet.4 A single neuron typically has many dendrites (though only one axon), and dendrites and axons can have a multitude of branches and synaptic terminals. The average neuron makes about a thousand synaptic connections, and some neurons can make a hundred times that number. The millions of billions of synapses inside our skulls tie our neurons together into a dense mesh of circuits that, in ways that are still far from understood, give rise to what we think, how we feel, and who we are.
Even as our knowledge of the physical workings of the brain advanced during the last century, one old assumption remained firmly in place: most biologists and neurologists continued to believe, as they had for hundreds of years, that the structure of the adult brain never changed. Our neurons would connect into circuits during childhood, when our brains were malleable, and as we reached maturity the circuitry would become fixed. The brain, in the prevailing view, was something like a concrete structure. After being poured and shaped in our youth, it hardened quickly into its final form. Once we hit our twenties, no new neurons were created, no new circuits forged. We would, of course, continue to store new memories throughout our lives (and lose some old ones), but the only structural change the brain would go through in adulthood was a slow process of decay as the body aged and nerve cells died.
Although the belief in the adult brain’s immutability was deeply and widely held, there were a few heretics. A handful of biologists and psychologists saw in the rapidly growing body of brain research indications that even the adult brain was malleable, or “plastic.” New neural circuits could form throughout our lives, they suggested, and old ones might grow stronger or weaker or wither away entirely. The British biologist J. Z. Young, in a series of lectures broadcast by the BBC in 1950, argued that the structure of the brain might in fact be in a constant state of flux, adapting to whatever task it’s called on to perform. “There is evidence that the cells of our brains literally develop and grow bigger with use, and atrophy or waste away with disuse,” he said. “It may be therefore that every action leaves some permanent print upon the nervous tissue.”5
Young was not the first to propose such an idea. Seventy years earlier, the American psychologist William James had expressed a similar intuition about the brain’s adaptability. The “nervous tissue,” he wrote in his landmark Principles of Psychology, “seems endowed with a very extraordinary degree of plasticity.” As with any other physical compound, “either outward forces or inward tensions can, from one hour to another, turn that structure into something different from what it was.” James quoted, approvingly, an analogy that the French scientist Léon Dumont had drawn, in an earlier essay about the biological consequences of habit, between the actions of water on land and the effects of experience on the brain: “Flowing water hollows out a channel for itself which grows broader and deeper; and when it later flows again, it follows the path traced by itself before. Just so, the impressions of outer objects fashion for themselves more and more appropriate paths in the nervous system, and these vital paths recur under similar external stimulation, even if they have been interrupted for some time.”6 Freud, too, ended up taking the contrarian position. In “Project for a Scientific Psychology,” a manuscript he wrote in 1895 but never published, he argued that the brain, and in particular the contact barriers between neurons, could change in response to a person’s experiences.7
Such speculations were dismissed, often contemptuously, by most brain scientists and physicians. They remained convinced that the brain’s plasticity ended with childhood, that the “vital paths,” once laid, could not be widened or narrowed, much less rerouted. They stood with Santiago Ramón y Cajal, the eminent Spanish physician, neuroanatomist, and Nobel laureate, who in 1913 declared, with a tone that left little room for debate, “In the adult [brain] centres, the nerve paths are something fixed, ended, immutable. Everything may die, nothing may be regenerated.”8 In his younger days, Ramón y Cajal had himself expressed doubts about the orthodox view—he had suggested, in 1894, that the “organ of thought is, within certain limits, malleable, and perfectible by well-directed mental exercise”9 —but in the end he embraced the conventional wisdom and became one of its most eloquent and authoritative defenders.
The conception of the adult brain as an unchanging physical apparatus grew out of, and was buttressed by, an Industrial Age metaphor that represented the brain as a mechanical contraption. Like a steam engine or an electric dynamo, the nervous system was made up of many parts, and each had a specific and set purpose that contributed in some essential way to the successful operation of the whole. The parts could not change, in shape or function, because that would lead, immediately and inexorably, to the breakdown of the machine. Different regions of the bra
in, and even individual circuits, played precisely defined roles in processing sensory inputs, directing the movements of muscles, and forming memories and thoughts; and those roles, established in childhood, were not susceptible to alteration. When it came to the brain, the child was indeed, as Wordsworth had written, the father to the man.
The mechanical conception of the brain both reflected and refuted the famous theory of dualism that René Descartes had laid out in his Meditations of 1641. Descartes claimed that the brain and the mind existed in two separate spheres: one material, one ethereal. The physical brain, like the rest of the body, was a purely mechanical instrument that, like a clock or a pump, acted according to the movements of its component parts. But the workings of the brain, argued Descartes, did not explain the workings of the conscious mind. As the essence of the self, the mind existed outside of space, beyond the laws of matter. Mind and brain could influence each other (through, as Descartes saw it, some mysterious action of the pineal gland), but they remained entirely separate substances. At a time of rapid scientific advance and social upheaval, Descartes’ dualism came as a comfort. Reality had a material side, which was the realm of science, but it also had a spiritual side, which was the realm of theology—and never the twain shall meet.
As reason became the new religion of the Enlightenment, the notion of an immaterial mind lying outside the reach of observation and experiment seemed increasingly tenuous. Scientists rejected the “mind” half of Cartesian dualism even as they embraced Descartes’ idea of the brain as a machine. Thought, memory, and emotion, rather than being the emanations of a spirit world, came to be seen as the logical and predetermined outputs of the physical operations of the brain. Consciousness was simply a by-product of those operations. “The word Mind is obsolete,” one prominent neurophysiologist ultimately declared.10 The machine metaphor was extended, and further reinforced, by the arrival of the digital computer—a “thinking machine”—in the middle of the twentieth century. That’s when scientists and philosophers began referring to our brain circuits, and even our behavior, as being “hardwired,” just like the microscopic circuits etched into the silicon substrate of a computer chip.
As the idea of the unchangeable adult brain hardened into dogma, it turned into a kind of “neurological nihilism,” according to the research psychiatrist Norman Doidge. Because it created “a sense that treatment for many brain problems was ineffective or unwarranted,” Doidge explains, it left those with mental illnesses or brain injuries little hope of treatment, much less cure. And as the idea “spread through our culture,” it ended up “stunting our overall view of human nature. Since the brain could not change, human nature, which emerges from it, seemed necessarily fixed and unalterable as well.”11 There was no regeneration; there was only decay. We, too, were stuck in the frozen concrete of our brain cells—or at least in the frozen concrete of received wisdom.
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.”12 2001 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.”13
Merzenich publishes the results of his experiment in an academic journal.14 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.”15 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.
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br /> 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.”16 Or, as Merzenich himself says, “massively plastic.”17 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.18 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.”
The Shallows Page 3
