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JUST ADD WORLD

HOW TO GROW A GOOD BRAIN

Brains are not born into the world as blank slates. Instead, they arrive pre-equipped with expectations. Consider the birth of a baby chicken: moments after hatching, it wobbles around on its little legs and can clumsily run and dodge. In its environment, it simply doesn’t have time to spend months or years learning how to move around.

Human infants, as well, come to the table with a good deal of preprogramming. Take the fact that we come pre-equipped to absorb language. Or that babies will mimic an adult sticking out her tongue, a feat requiring a sophisticated ability to translate vision into motor action.¹ Or that fibers from your eye don’t need to learn how to find their targets deep in the brain; they simply follow molecular cues and hit their goal—every time. For all this sort of hardwiring, we can thank our genes.

But genetic hardwiring does not provide the whole story, especially for humans. The system’s organization is too complex, and the genes are far too few. Even when you take into account the slicing and dicing that produces many different flavors of the same gene, the number of neurons and their connections vastly outstrips the number of genetic combinations.

So we know that the details of brain wiring involve more than the genetics. And two centuries ago, thinkers began to correctly suspect that the details of experience carried importance. In 1815, the physiologist Johann Spurzheim proposed that the brain, like the muscles, could be increased by exercise: his idea was that blood carried with it the nutrition for growth and that it was “carried in greater abundance to the parts which are excited.”² By 1874, Charles Darwin wondered if this basic idea might explain why rabbits in the wild had larger brains than domestic rabbits: he suggested that the wild hares were forced to use their wits and senses more than the domesticated ones and that the size of their brains followed.³

In the 1960s, researchers began to study in earnest whether the brain could change in measurable ways as a direct result of experience. The simplest way to examine the question was to raise rats in different environments—for example, a rich environment packed with toys and running wheels, or the deprived environment of an empty and solitary cage.⁴ The results were striking: the environment altered the rats’ brain structure, and the structure correlated with the animals’ capacity for learning and memory. The rats raised in enriched environments performed better at tasks and were found at autopsy to have long, lush dendrites (the treelike branches growing from the cell body).⁵ In contrast, rats from the deprived environments were poor learners and had abnormally shrunken neurons. This same effect of environment is found in birds, monkeys, and other mammals.⁶ To the brain, context matters.

A neuron normally grows like a branched tree, allowing it to connect to other neurons. In an enriched environment, branches grow more lavishly. In a deprived environment, branches shrivel.

Does the same happen in humans? In the early 1990s, researchers in California realized they could take advantage of autopsies to compare the brains of those who completed high school with those who completed college. In analogy to the animal studies, they found that an area involved in language comprehension contained more elaborate dendrites in the college educated.⁷

So the first lesson is that the fine structure of the brain reflects the environment to which it is exposed. And this is not just about dendrites. As we’ll learn shortly, world experience modulates almost every measurable detail of the brain, from the molecular scale to overall brain anatomy.

EXPERIENCE NECESSARY

Why was Einstein Einstein? Surely genetics mattered, but he is affixed to our history books because of every experience he’d had: the exposure to cellos, the physics teacher he had in his senior year, the rejection of a girl he loved, the patent office in which he worked, the math problems he was praised for, the stories he read, and millions of further experiences—all of which shaped his nervous system into the biological machinery we distinguish as Albert Einstein. Each year, there are thousands of other children with his potential but who are exposed to cultures, economic conditions, or family structures that don’t give sufficiently positive feedback. And we don’t call them Einsteins.

If DNA were the only thing that mattered, there would be no particular reason to build meaningful social programs to pour good experiences into children and protect them from bad experiences. But brains require the right kind of environment if they are to correctly develop. When the first draft of the Human Genome Project came to completion at the turn of the millennium, one of the great surprises was that humans have only about twenty thousand genes.⁸ This number came as a surprise to biologists: given the complexity of the brain and the body, it had been assumed that hundreds of thousands of genes would be required.

So how does the massively complicated brain, with its eighty-six billion neurons, get built from such a small recipe book? The answer pivots on a clever strategy implemented by the genome: build incompletely and let world experience refine. Thus, for humans at birth, the brain is remarkably unfinished, and interaction with the world is necessary to complete it.

Consider the sleep-wake cycle. This internal clock, known as the circadian rhythm, runs roughly on a twenty-four-hour cycle. However, if you descend into a cave for several days—where there are no clues to the light and dark cycles of the surface—your circadian rhythm would drift in a range between twenty-one and twenty-seven hours. This exposes the brain’s simple solution: build a non-exact clock and then calibrate it to the sun’s cycle. With this elegant trick, there is no need to genetically code a perfectly wound clock. The world does the winding.

The flexibility of the brain allows the events in your life to stitch themselves directly into the neural fabric. It’s a great trick on the part of Mother Nature, allowing the brain to learn languages, ride bicycles, and grasp quantum physics, all from the seeds of a small collection of genes. Our DNA is not a blueprint; it is merely the first domino that kicks off the show.

From this viewpoint, it is easy to understand why some of the most common problems of vision—such as the inability to see depth correctly—develop from imbalances in the pattern of activity delivered to the visual cortex by the two eyes. For example, when children are born cross-eyed or wall-eyed, the activity from the two eyes is not well correlated (as it would be with aligned eyes). If the problem is not addressed, the child will not develop normal stereo vision—that is, the capacity to determine depth from the small differences between what the two eyes are seeing. One eye will grow progressively weaker, often to the point of blindness. We’ll return to this later to understand why and what can be done about it. For now, the important point is that the development of normal visual circuits relies on normal visual input. It is experience-dependent.

So genetic instructions play only a minor role in the detailed assembly of cortical connections. It couldn’t be any other way: With twenty thousand genes and 200 trillion connections between neurons, how could the details possibly be prespecified? That model could never have worked. Instead, neuronal networks require interaction with the world for their proper development.⁹

NATURE’S GREAT GAMBLE

On September 29, 1812, a baby was born who would inherit the grand ducal throne of Baden, Germany. Unfortunately, the baby died seventeen days later. That was the end of that.

Or was it? Sixteen years later, a young man named Kaspar Hauser showed up in Nuremberg, Germany. He carried a note that explained he had been given away as a child, and he apparently knew only a few sentences, including “I want to be a cavalryman, as my father was.” He attracted widespread attention and the audiences of powerful people; many began to suspect he was the hereditary prince of Baden, switched in those first weeks with a dying baby in a nefarious plot by those who stood to inherit the throne.

The story grew famous beyond the royal intrigue: Kaspar became the exemplar of a feral child. According to his own telling, Kaspar had spent his entire youth alone in a dark cell. It was only a meter wide, two meters long, and one and a half meters high. It had a straw bed and a small wooden horse. Each morning he awoke to discover some bread and water, nothing more. He saw no one enter or leave. Occasionally the water he drank would taste a bit different, and then he would grow sleepy afterward—and when he awoke his hair would be cut and his nails clipped. But it was not until just before his release that he had direct contact with another human, a man who taught him how to write but always kept his face hidden from view.

Kaspar Hauser’s story aroused international attention. He grew up to write prolifically and touchingly about his childhood. His story lives on today in plays, books, and music; it is perhaps history’s most famous story of a feral childhood.

But Kaspar’s claim was almost certainly false. Beyond the extensive historical analysis that rules it out, there is a neurobiological reason: a child raised without human interaction does not grow up to walk, speak, write, lecture, and thrive, like the successful Kaspar. After a century of popular press about Kaspar, the psychiatrist Karl Leonhard put a fine point on it:

If he had been living since childhood under the conditions he describes, he would not have developed beyond the condition of an idiot; indeed he would not have remained alive long. His tale is so full of absurdities that it is astonishing that it was ever believed and is even today still believed by many people.¹⁰

After all, despite some genetic pre-specification, nature’s approach to growing a brain relies on receiving a vast set of experiences, such as social interaction, conversation, play, exposure to the world, and the rest of the landscape of normal human affairs. The strategy of interaction with the world allows the colossal machinery of the brain to take shape from a relatively small set of instructions. It’s an ingenious approach for unpacking a brain (and body) from a single microscopic egg.

But this strategy is also a gamble. It’s a slightly risky approach—one in which the brain-shaping work is partially relegated to world experience rather than hardwiring. After all, what if a child is actually born into Kaspar’s story and has an infancy characterized by total parental neglect?

Tragically, we know the answer to this question. In one example in July 2005, police in Plant City, Florida, pulled up outside a dilapidated house to perform an investigation. They had been alerted by a neighbor who had seen a girl in the window on a few occasions but had never seen the girl exit the house and had never seen any adults with her in the window.

The officers knocked on the door for a while and were eventually greeted by a woman. They told the woman they had a warrant to search for her daughter inside the house. They walked down the hallways, probed several rooms, and finally entered a small bedroom. There was the girl. One of the officers vomited.

Danielle, a feral child discovered in 2005 in Florida. Although the photograph displays a child’s beautiful face, the behaviors and expressions inherent to normal human interaction are absent in her: she missed the critical window for proper input from the world.

Danielle Crockett, an undersized girl of almost seven years old, had been locked away in a dark closet for her entire childhood. She was flecked with fecal matter and cockroaches. Beyond basic sustenance, she had never received physical affection, never engaged in normal conversation, and in all likelihood had never been let outdoors. She was fully incapable of speech. When she met the police officers (and later the social workers and psychologists), she appeared to look right through them; she had no glimmer of recognition or indications of normal human interaction. She could not chew solid food, did not know how to use a toilet, could not nod yes or no, and one year later had not mastered the use of a sippy cup. After many tests, physicians were able to verify that she had no genetic problems such as cerebral palsy, autism, or Down syndrome. Instead, the normal development of her brain had been derailed by severe social deprivation.

Despite the best attempts of doctors and social workers, the prognosis for Danielle is poor; the likely scenario is that she will live in a nursing home and may eventually be able to live without diapers.¹¹ Heartbreakingly, hers is a real-life Kaspar Hauser story, with the real-life consequences.

Danielle’s outcome is grim because the human brain arrives in the world unfinished. Proper development requires proper input. The brain absorbs experience to unpack its programs, and only during a rapidly closing window of time. Once the window is missed, it is difficult or impossible to reopen.

Danielle’s story is paralleled by a set of animal experiments in the early 1970s. Harry Harlow, a scientist at the University of Wisconsin, used monkeys to study the bonding between mothers and their children. He had an active scientific career, but when his wife died of cancer in 1971, Harlow sank into a depression. He continued to work, but his friends and co-workers sensed that he was not the same. He turned his scientific interests to the study of depression.

Using monkeys to model human depression, he developed a study of isolation. He put a baby monkey into a steel-walled cage with no windows. A two-way mirror allowed Harlow to look in but prevented the monkey from seeing out. Harlow tried this with a monkey for thirty days. Then another monkey for six months. Other monkeys were incarcerated for a full year.

Because the baby monkeys never had the chance to develop normal bonds (they were put in the cage shortly after birth), they emerged with deep-seated disturbances. Those who were isolated the longest ended up much like Danielle: they showed no normal interaction with other monkeys and did not engage in recreation, cooperation, or competition. They barely moved. Two would no longer eat.

Harlow also noted that the monkeys were incapable of having normal sexual relations. Even so, he took some of the isolated females and had them impregnated to see how these disturbed monkeys would interact with children of their own. The results were disastrous. The isolated monkeys were completely unable to raise children. In the best cases, they ignored the children entirely; in the worst cases, they injured them.¹²

The lesson of Harlow’s monkeys is the same as the lesson from Danielle: Mother Nature’s strategy of unpacking a brain relies on proper world experience. Without it, the brain becomes malformed and pathological. Like a tree that needs nutrient-rich soil to arborize, a brain requires the rich soil of social and sensory interaction.

With this background in hand, we now see that the brain leverages its environment to shape itself. But how, exactly, does it absorb the world—especially from inside its dark cave? What happens when a person loses an arm or goes deaf? Does a blind person actually enjoy better hearing? And what does any of this have to do with why we dream?