Brains of higher complexity are also more creative. A complex brain can combine past experiences in new ways to deal with things that it has never encountered before; for example, you can climb an unfamiliar hill or staircase without tripping because you’ve climbed similar ones in the past. Complex brains may adjust faster to changing environments that require different body budgeting. It’s one reason that humans can live successfully in so many climates and social structures. If you have to move from the equator to Northern Europe, or from a laid-back culture to one with strict rules, you’ll adapt more swiftly with a complex brain in your head.
On top of that, higher complexity may make a brain more resilient to injury. If one collection of neurons stops working, other collections may serve in its place. That’s one reason complex brains may be favored by natural selection. Pocketknife Brain would not have this capability; lost neurons would be more likely to mean lost functionality.
Human brains may be some of the most complex brains on Earth, but they aren’t the only ones with high complexity. Intelligent behavior has emerged many times in different species with differently structured brains. Take the octopus, for example, whose complex brain is distributed throughout its body. Octopuses can solve puzzles and even dismantle their tanks in aquariums. Bird brains can be complex too. Some bird species can use simple tools and have a bit of language ability, even though their neurons are not organized into a cerebral cortex. The highly complex human brain isn’t a pinnacle of evolution, remember; it’s just well adapted to the environments we inhabit.
High complexity may be a prerequisite for so much that makes you human, but by itself it does not empower a human brain to make a human mind. Your Paleolithic ancestors needed more than a highly complex brain to pick up a hunk of rock and imagine a future hand axe within it. Likewise, you need more than high complexity to look at a piece of paper, a piece of metal, and a piece of plastic, which are all physically different, and treat them all as having a similar function, like serving as money. High complexity helps you climb an unfamiliar staircase, but you need more than high complexity to understand what it means for someone to climb a social ladder to gain power and influence. We also need more than high complexity to contemplate the nature of a human brain and to invent the many creative metaphors for what a brain is like, such as the triune brain, Systems 1 and 2, and mental organs. These feats of imagination require a high level of complexity packaged in a really big brain, as well as other factors that you’ll learn about in the coming lessons.
A brain network is not a metaphor, as I mentioned earlier; it’s the best scientific description of a brain today. It allows us to consider how one physical structure reconfigures in an instant to integrate vast amounts of information efficiently. It reveals similarities and differences between various kinds of brains by quantifying their complexity. It even helps us understand how a brain might compensate when it’s damaged.
Still, I’ve relied on a few metaphors to explain the network. For example, the word wiring is a metaphor. Neurons aren’t literally wired together—they’re separated by the small gaps we called synapses, and chemicals complete the connections. Neurons are also not trees with branches and trunks. And your brain most likely doesn’t have airports inside it.
Metaphors are wonderful for explaining complex topics in simple, familiar terms. A metaphor’s simplicity, however, can become its greatest failing if people treat the metaphor as an explanation. In biology, for example, genes are sometimes described as “blueprints.” If you take this metaphor literally, you might think that particular genes always have the same basic function; say, to make a specific characteristic or body part. (They don’t.) Physicists sometimes say that light travels in waves, a metaphor that invites us to assume that space, like an ocean, contains some substance for those waves to move through. (It doesn’t.) Metaphors provide the illusion of knowledge, so they must be used with care.
The complex network in your head may not be a metaphor, but my description here is necessarily incomplete. Your brain is more than just neurons. It includes blood vessels and various fluids that I haven’t talked about. It also includes other kinds of brain cells, called glial cells, that function in ways that scientists don’t fully understand yet. Your brain network may even extend, surprisingly, into your gut and intestines, where scientists have found microbes that communicate with your brain via neurotransmitters.
As scientists learn more about the brain and its interconnections, we may discover better ways to describe its structure and function. Until then, understanding the brain as a complex network allows us to ponder how a human brain creates a human mind without any need for an allegedly rational and oversize neocortex. If human brain evolution has a crowning achievement, it is the complexity of its crown.
Lesson No.
3
Little Brains Wire Themselves to Their World
HAVE YOU EVER noticed that many newborn animals are more competent than newborn humans? A newborn garter snake can slither on its own almost instantly. Horses can walk shortly after birth, and an infant chimp can cling to its mother’s hair. In comparison, human newborns are pretty pathetic. They can’t even control their limbs. It takes weeks before they can swat their tiny hands with intent. Many animals emerge from the egg or womb with brains that are more fully wired to control their bodies, but little human brains are born under construction. They don’t take on their full adult structure and function until they finish their principal wiring, a process that takes about twenty-five years.
Why did we evolve this way, to be born with our brain wiring only partially complete? No one knows for sure (though plenty of scientists have been happy to speculate). What we can learn is where those wiring instructions come from after birth and what advantages this arrangement affords us.
Scholars usually discuss this issue in terms of nature versus nurture—which aspects of humanity are built into our genes before birth and which ones we learn from our culture. But this distinction is illusory. We cannot attribute causes to genes alone or to the environment alone, because the two are like lovers in a fiery tango—so deeply entwined that it’s unhelpful to call them separate names like nature and nurture.
To a remarkable extent, a baby’s genes are guided and regulated by the surrounding environment. The brain areas that are most centrally involved in vision, for example, develop normally after birth only if a baby’s retinas are regularly exposed to light. An infant’s brain also learns to locate sounds in the world based on the specific shape of the baby’s ear. To make matters even stranger, a baby’s body requires some additional genes that sneak in from the outside world. These tiny visitors travel inside of bacteria and other critters and affect the brain in ways that scientists are only beginning to understand.
Caregivers play a critical role in wiring a baby’s brain.
A baby’s wiring instructions come not only from the physical environment but also from the social environment, from caregivers and people like you and me. When you cradle a newborn girl in your arms, you present your face to her at just the right distance to teach her brain to process and recognize faces. When you expose her to boxes and buildings, you’re training her visual system to see edges and corners. Many other social things we do with a baby, like cuddling and talking and making eye contact in key moments, sculpt her brain in necessary and irrevocable ways. Genes play a key role in building a baby’s brain wiring, and they also open the door for us to wire her newborn brain in the context of her culture.
As information travels from the world into the newborn brain, some neurons fire together more frequently than others, causing gradual brain changes that we’ve called plasticity. These changes nudge the infant’s brain toward higher complexity via two processes we’ll call tuning and pruning.
Tuning means strengthening the connections between neurons, particularly connections that are used frequently or are important for budgeting the resources of your body (water, salt, glucos
e, and so on). If we think again of neurons as little trees, tuning means that the branch-like dendrites become bushier. It also means that the trunk-like axon develops a thicker coating of myelin, a fatty “bark” that’s like the insulation around electrical wires, which makes signals travel faster. Well-tuned connections are more efficient at carrying and processing information than poorly tuned ones and are therefore more likely to be reused in the future. This means the brain is more likely to recreate certain neural patterns that include those well-tuned connections. As neuroscientists like to say, “Neurons that fire together, wire together.”
Meanwhile, less-used connections weaken and die off. This is the process of pruning, the neural equivalent of “If you don’t use it, you lose it.” Pruning is critical in a developing brain, because little humans are born with many more connections than they will ultimately use. A human embryo creates twice as many neurons as an adult brain needs, and infant neurons are quite a bit bushier than neurons in an adult brain. Unused connections are helpful at the outset. They enable a brain to tailor itself to diverse environments. But over the longer term, unused connections are a burden, metabolically speaking—they don’t contribute anything worthwhile, so it’s a waste of energy for the brain to maintain them. The good news is that pruning these extra connections makes room for more learning—that is, for more useful connections to be tuned.
Tuning and pruning happen continuously and often simultaneously, driven by the physical and social world outside the infant’s head and by the growth and activity in the infant’s body. Both processes also continue throughout life. Your bushy dendrites keep sprouting new buds, and your brain tunes and prunes them. Buds that aren’t tuned disappear within a couple of days.
Let’s look at three examples of tuning and pruning that set newborn brains on a path to develop into typical adult brains. These examples demonstrate how our unfinished wiring completes itself in the months and years after we’re born, driven by wiring instructions that arrive from the outside world.
First, consider how you manage your body budget. When you’re hungry, you can open the fridge. When you’re tired, you can go to bed. When you’re cold, you can put on a coat. When you’re agitated, you can take deep breaths to calm your nerves. Babies can’t do any of these things by themselves. They can’t even burp without help.
That’s where caregivers come in. They regulate the baby’s physical environment and therefore her body budget by feeding her, setting sleep times (or trying to!), and wrapping her in blankets and cuddles. These actions help the baby’s brain maintain its body budget, so her internal systems operate efficiently and she stays alive and healthy.
If caregivers do these activities effectively, the baby’s brain is free to tune and prune itself to perform healthy body budgeting. Little by little, the caregivers’ roles diminish as the infant’s brain becomes more capable of controlling her body, enabling her to fall asleep without being held or to stuff a bit of banana into her mouth without smearing it on her face. It may take years before the little brain can put on a sweater by herself or make her own breakfast, but eventually she’ll have primary responsibility for her own body budget.
Little brains are also wired by what caregivers don’t do. If you don’t let a baby fall asleep on her own and instead rock her to sleep every night, her brain might not learn how to fall asleep without help. When an infant is crying for a long time and you don’t check in regularly, her brain may learn that the world is unreliable and unsafe while her body budget goes untended.
Things change once she’s a toddler, however. Her toddler brain has to learn to calm her body after a tantrum and, eventually, to body-budget without a tantrum in the first place. When my daughter was little, I found it helpful to give her space so her brain could learn to soothe her body. In general, toddlers learn to tend their own body budgets better when their caregivers create learning opportunities for them instead of hovering and taking care of their every need. A big challenge of parenting is knowing when to step in and when to step back.
Our second example of tuning and pruning concerns how you learn to pay attention. Have you ever been in a crowd, not really attending to the conversations sprouting around you, and then someone speaks your name and you immediately orient to it? (Scientists call this the “cocktail party effect.”) Your adult brain can effortlessly focus on one thing and ignore others, similar to a spotlight in the darkness. That’s because your brain network contains smaller communities of neurons whose main job is to focus on certain details as important and ignore other details as irrelevant. Your brain focuses its spotlight of attention continually and automatically, and often you’re unaware that it’s happening.
We do need help sometimes to focus our spotlight—that’s why noise-canceling headphones sell so well. But the newborn brain doesn’t have a spotlight. It has more of a lantern, illuminating a wide area in its physical environment. Newborn brains don’t know what’s important and what’s not, so they cannot focus as adults do. They still lack the wiring that narrows their lantern into a spotlight.
Again, the missing ingredient comes from caregivers in the social world. They constantly guide the baby’s attention to things of interest. A mother picks up a toy dog and looks at it. She looks at her little boy, then back at the dog, guiding the baby’s gaze. She turns to her son and says with intent, “What a cute little doggie,” in a singsong tone. The mother’s speech and the back-and-forth switching of gaze, which scientists call sharing attention, alert the baby that the toy dog is significant—that is, the toy could affect his body budget, so he should care about and learn about the dog.
Little by little, sharing attention teaches an infant which parts of the environment matter and which parts don’t. The infant brain is then able to construct its own environment of what is relevant to its body budget and what can be ignored. Scientists call this environment a niche. Every animal has a niche, and it creates that niche as it senses the world, makes worthwhile movements, and regulates its body budget. Adult humans have a gigantic niche, perhaps the largest of any creature. Your niche extends far beyond your immediate surroundings to include events around the world, past, present, and future.
After months of practice sharing attention with caregivers, an infant will learn to elicit shared attention from them. He will look at them as a way of asking whether something is in his niche and what it might mean for his body budget. In this manner, the infant learns to focus attention even more effectively on things that matter.
Our third example of tuning and pruning is how your senses develop. In the first few months of life, babies are bathed in all kinds of sounds, including the sound of people speaking. Newborns, with their lantern of attention, take in all the sounds around them. When tested in a lab, newborns can distinguish a wide range of language sounds, including those that they don’t hear very often. But over time, tuning and pruning will wire the baby’s brain based on the vocal sounds he hears more regularly. Sounds that are frequent cause certain neural connections to be tuned, and the baby’s brain starts to treat those sounds as part of its niche. Sounds that are rare are treated as noise to be ignored, and eventually, related neural connections fall out of use and are pruned away.
Scientists think this sort of pruning may be one reason why children have an easier time learning languages than adults do. Different spoken languages use different sets of sounds. For example, Greek and Spanish have a handful of vowel sounds, while Danish has twenty or more (depending on how they’re counted). If people interacted with you in multiple languages when you were a baby, then your brain was likely tuned and pruned to hear and distinguish the sounds in those languages. If you heard only one language as a baby, you’d need to relearn the ability to hear and distinguish sounds outside your language, which is hard.
This process works similarly for seeing faces. When you were a baby, you learned to recognize the people around you. Your infant brain was tuned and pruned to detect fine diff
erences in their faces so you could tell them apart. But there’s a catch—people tend to live around others of the same ethnicity, so babies are often not exposed to a wide array of facial features. That means the baby’s brain does not tune itself to detect those different features. Scientists think this is one reason why it can be harder for you to remember the faces of people of an ethnicity different from your own or to tell one face from another. Fortunately, you can quickly retune your brain and restore this ability by looking at lots of diverse faces; it’s much easier than retuning to the sounds of a foreign language.
These examples of hearing language and seeing faces focus on a single sense, but you live in a multisensory world. For example, when you kiss someone, you are enveloped in a unified experience that combines the sight of a face, the sound of breathing, the feel, taste, and scent of luscious lips, and the racing of your heart. Your brain assembles these sensations into a cohesive whole. Scientists call this process sensory integration.
Sensory integration itself is tuned and pruned as babies grow. A newborn at first can’t recognize his mother by her face, because he hasn’t learned what a face is, and his visual system isn’t fully formed. He might know a bit about how his mother sounds, and he can smell her breast milk. If you put a newborn on his mother’s belly, he will wriggle up to her breast by following the aroma. Soon, he learns to recognize his mother by different combinations of all his senses together. His little brain absorbs each pattern of sight, smell, sound, touch, and taste, plus sensations from inside his body, and learns its meaning: the person who regulates his body budget is here. Sensory integration conjures his first feeling of trust. It’s part of the neural foundation for attachment.
Seven and a Half Lessons About the Brain Page 4