does what it does. This book lays out that framework, allowing us to better understand who we are, how we came to be, and where we’re going.
Once we get in the mode of thinking about livewiring, our current hardwired machines seem hopelessly inadequate for our future. After all, in traditional engineering, everything important is carefully designed. When a car company remodels the chassis of a vehicle, it spends months producing the engine to fit. But imagine changing the bodywork any way you’d like and letting the engine reconfigure itself to match. As we’ll see, once we understand the principles of livewiring, we can draft off Mother Nature’s genius to fabricate new machines: devices that dynamically determine their own circuitry by optimizing themselves to their inputs and learning from experience.
The thrill of life is not about who we are but about who we are in the process of becoming. Similarly, the magic of our brain lies not in its constituent elements but in the way those elements unceasingly reweave themselves to form a dynamic, electric, living fabric.
Just a handful of pages into this book, your brain has already changed: these symbols on the page have orchestrated millions of tiny changes across the vast seas of your neural connections, crafting you into someone just slightly different than you were at the beginning of the chapter.
2
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
