Livewired. David Eagleman

their healthy child—not because he was going to die, but because he was no longer going to live a normal life. They went through anger and denial. Their normal changed. Finally, during a three-week hospital stay, the neurologists had to allow that this problem was bigger than they knew how to handle at the local hospital.

      So the family took an air ambulance flight from their home in Albuquerque, New Mexico, to Johns Hopkins hospital in Baltimore. It was here, in the pediatric intensive care unit, that they came to understand that Matthew had Rasmussen’s encephalitis, a rare, chronic inflammatory disease. The problem with the disease is that it affects not just a small bit of the brain but an entire half. Valerie and Jim explored their options and were alarmed to learn there was only one known treatment for Matthew’s condition: a hemispherectomy, or the surgical removal of an entire half of the brain. “I can’t tell you anything the doctors said after that,” Valerie told me. “One just shuts down, like everyone’s talking a foreign language.”

      Valerie and Jim tried other approaches, but they proved fruitless. When Valerie called Johns Hopkins hospital to schedule the hemispherectomy some months later, the doctor asked her, “Are you sure?”

      “Yes,” she said.

      “Can you look in the mirror every day and know you’ve chosen what you’ve needed to do?”

      Valerie and Jim couldn’t sleep beneath the crushing anxiety. Could Matthew survive the surgery? Was it even possible to live with half of the brain missing? And even if so, would the removal of one hemisphere be so debilitating as to offer Matthew a life on terms not worth taking?

      But there were no more options. A normal life couldn’t be lived in the shadow of multiple seizures each day. They found themselves weighing Matthew’s assured disadvantages against an uncertain surgical outcome.

      Matthew’s parents flew him to the hospital in Baltimore. Under a small child-sized mask, Matthew drifted away into the anesthesia. A blade carefully opened a slit in his shaved scalp. A bone drill cut a circular burr hole in his skull.

      Working patiently over the course of several hours, the surgeon removed half of the delicate pink material that underpinned Matthew’s intellect, emotion, language, sense of humor, fears, and loves. The extracted brain tissue, useless outside its biological milieu, was banked in small containers. The empty half of Matthew’s skull slowly filled up with cerebrospinal fluid, appearing in neuroimaging as a black void.²

       Half of Matthew’s brain was surgically removed.

      In the recovery room, his parents drank hospital coffee and waited for Matthew to open his eyes. What would their son be like now? Who would he be with only half a brain?

      Of all the objects our species has discovered on the planet, nothing rivals the complexity of our own brains. The human brain consists of eighty-six billion cells called neurons: cells that shuttle information rapidly in the form of traveling voltage spikes.³ Neurons are densely connected to one another in intricate, forest-like networks, and the total number of connections between the neurons in your head is in the hundreds of trillions (around 0.2 quadrillion). To calibrate yourself, think of it this way: there are twenty times more connections in a cubic millimeter of cortical tissue than there are human beings on the entire planet.

      But it’s not the number of parts that make a brain interesting; it’s the way those parts interact.

      In textbooks, media advertisements, and popular culture, the brain is typically portrayed as an organ with different regions dedicated to specific tasks. This area here exists for vision, that swath there is necessary for knowing how to use tools, this region becomes active when resisting candy, and that spot lights up when mulling over a moral conundrum. All the areas can be neatly labeled and categorized.

      But that textbook model is inadequate, and it misses the most interesting part of the story. The brain is a dynamic system, constantly altering its own circuitry to match the demands of the environment and the capabilities of the body. If you had a magical video camera with which to zoom in to the living, microscopic cosmos inside the skull, you would witness the neurons’ tentacle-like extensions grasping around, feeling, bumping against one another, searching for the right connections to form or forgo, like citizens of a country establishing friendships, marriages, neighborhoods, political parties, vendettas, and social networks. Think of the brain as a living community of trillions of intertwining organisms. Much stranger than the textbook picture, the brain is a cryptic kind of computational material, a living three-dimensional textile that shifts, reacts, and adjusts itself to maximize its efficiency. The elaborate pattern of connections in the brain—the circuitry—is full of life: connections between neurons ceaselessly blossom, die, and reconfigure. You are a different person than you were at this time last year, because the gargantuan tapestry of your brain has woven itself into something new.

      When you learn something—the location of a restaurant you like, a piece of gossip about your boss, that addictive new song on the radio—your brain physically changes. The same thing happens when you experience a financial success, a social fiasco, or an emotional awakening. When you shoot a basketball, disagree with a colleague, fly into a new city, gaze at a nostalgic photo, or hear the mellifluous tones of a beloved voice, the immense, intertwining jungles of your brain work themselves into something slightly different from what they were a moment before. These changes sum up to our memories: the outcome of our living and loving. Accumulating over minutes and months and decades, the innumerable brain changes tally up to what we call you.

      Or at least the you right now. Yesterday you were marginally different. And tomorrow you’ll be someone else again.

LIFE’S OTHER SECRET

      In 1953, Francis Crick burst into the Eagle and Child pub. He announced to the startled swillers that he and James Watson had just discovered the secret of life: they had deciphered the double-helical structure of DNA. It was one of the great pub-crashing moments of science.

      But it turns out that Crick and Watson had discovered only half the secret. The other half you won’t find written in a sequence of DNA base pairs, and you won’t find it written in a textbook. Not now, not ever.

      Because the other half is all around you. It is every bit of experience you have with the world: the textures and tastes, the caresses and car accidents, the languages and love stories.⁴

      To appreciate this, imagine you were born thirty thousand years ago. You have exactly your same DNA, but you slide out of the womb and open your eyes onto a different time period. What would you be like? Would you relish dancing in pelts around the fire while marveling at stars? Would you bellow from a treetop to warn of approaching saber-toothed tigers? Would you be anxious about sleeping outdoors when rain clouds bloomed overhead?

      Whatever you think you’d be like, you’re wrong. It’s a trick question.

      Because you wouldn’t be you. Not even vaguely. This caveman with identical DNA might look a bit like you, as a result of having the same genomic recipe book. But the caveman wouldn’t think like you. Nor would the caveman strategize, imagine, love, or simulate the past and future quite as you do.

      Why? Because the caveman’s experiences are different from yours. Although DNA is a part of the story of your life, it is only a small part. The rest of the story involves the rich details of your experiences and your environment, all of which sculpt the vast, microscopic tapestry of your brain cells and their connections. What we think of as you is a vessel of experience into which is poured a small sample of space and time. You imbibe your local culture and technology through your senses. Who you are owes as much to your surroundings as it does to the DNA inside you.

      Contrast this story with a Komodo dragon born today and a Komodo dragon born thirty thousand years ago. Presumably it would be more difficult to tell them apart by any measure of their behavior.

      What’s