These ideas, if taken seriously, could shake the foundations of all sorts of sacred institutions in our society. In the law, for example, attorneys plead that their clients’ emotions overwhelmed their reason in the heat of passion, and therefore they aren’t fully to blame for their actions. But feeling distressed is not evidence of being irrational or that your so-called emotional brain has hijacked your supposed rational brain. Distress can be evidence that your whole brain is expending resources toward an anticipated payoff.
Many other social institutions are steeped in the idea of a mind at war with itself. In economics, models for investor behavior assume a sharp distinction between the rational and the emotional. In politics, we have leaders with clear conflicts of interest, such as past lobbying work in industries that they now oversee, who believe they can easily set aside their emotions and make rational decisions for the good of the people. Beneath these lofty ideas lurks the myth of the triune brain.
You have one brain, not three. To move past Plato’s ancient battle, we might need to fundamentally rethink what it means to be rational, what it means to be responsible for our actions, and perhaps even what it means to be human.
Lesson No.
2
Your Brain Is a Network
THE BRAINS ON THIS PLANET have been pondering brains for thousands of years. Aristotle believed the brain was a cooling chamber for the heart, sort of like the radiator in your car. Philosophers in the Middle Ages maintained that certain brain cavities housed the human soul. In the nineteenth century, a popular idea called phrenology portrayed the brain as a jigsaw puzzle, where each piece produced a different human quality, such as self-esteem, destructiveness, or love.
A cooling chamber, a house for the soul, a jigsaw puzzle—these are all just metaphors invented to help us understand what brains are and how they work.
Today, we remain surrounded by so-called facts about the brain that are also just metaphors. If you’ve heard that the left side of your brain is logical and the right side is creative, that’s just a metaphor. So is the idea that your brain has a “System 1” for quick, instinctive responses and a “System 2” for slower, more thoughtful processing, concepts discussed in the book Thinking, Fast and Slow by psychologist Daniel Kahneman. (Kahneman is very clear that Systems 1 and 2 are metaphors about the mind; but they are often mistaken for brain structures.) Some scientists describe the human mind as a collection of “mental organs” for fear, empathy, jealousy, and other psychological tools that evolved for survival, but the brain itself isn’t structured like that. Your brain also does not “light up” with activity, as if some parts are on and others off. It does not “store” memories like computer files to be retrieved and opened later. These ideas are metaphors that emerged from beliefs about the brain that are now outdated.
If real brains don’t work like any of these metaphors suggest they do, and the triune brain is a myth, then what kind of brain do we actually have that makes us the kind of animal that we are? What kind of brain gives us our ability to cooperate, our capacity for language, and our talent to guess what other people are thinking or feeling? What kind of a brain is necessary to create a human mind?
The answer to these questions begins with an important insight. Your brain is a network—a collection of parts that are connected to function as a single unit. You are surely familiar with other networks that surround us. The internet is a network of connected devices. An anthill is a network of underground locations connected by tunnels. Your social network is a collection of connected people. Your brain, in turn, is a network of 128 billion neurons connected as a single, massive, and flexible structure.
A brain network is not a metaphor. It’s a description that comes from the best available science about how brains evolved, how they’re structured, and how they function. And as you will see, this network structure will take us one step closer to understanding what makes your brain able to create your mind.
How do 128 billion individual neurons become a single brain network? Generally speaking, each neuron looks like a little tree, with bushy branches at the top, a long trunk, and roots at the bottom. (Yes, I know, I’m using a metaphor!) The bushy branches, which are called dendrites, receive signals from other neurons, and the trunk, which is called the axon, sends signals to other neurons through its roots.
Your 128 billion neurons continually fire off communications to each other, day and night. When a neuron fires, an electrical signal races down its trunk to its roots. This signal causes the roots to release chemicals into the gaps between neurons, called synapses. The chemicals travel across synapses and attach to another neuron’s bushy top, causing that neuron to fire as well, and voilà, one neuron has passed information to another.
This arrangement of dendrites, axons, and synapses knits your 128 billion individual neurons into a network. To make things simpler, I’ll refer to this whole arrangement as the “wiring” of your brain.
Neurons and their wiring
Your brain network is always on. Your neurons never just sit around waiting for something in the outside world to make them fire. Instead, all of your neurons chat constantly with one another through their wiring. Their communications may become stronger or weaker depending on what’s happening in the world and in your body, but the conversation never stops until you die.
Communication in your brain is a balancing act between speed and cost. Each neuron directly passes information to just a few thousand other neurons and receives information from a few thousand others, give or take, yielding over five hundred trillion neuron-to-neuron connections. That’s a really big number, but it would be considerably larger if every neuron spoke directly to every other neuron in the network. Such a structure would require so many more connections that your brain would run out of resources to sustain itself.
So you have a more frugal wiring arrangement that is sort of like the global air-travel system. (Yep, here comes another metaphor.) The air-travel system is a network of about seventeen thousand airports around the world. Whereas your brain carries electrical and chemical signals, this network carries passengers (and, if we’re lucky, our luggage). Each airport runs direct flights to some other airports but not to every other airport. If every airport sent flights to every other, air traffic would increase by billions more flights per year, and the whole system would run out of fuel and pilots and runways and ultimately collapse. Instead, some airports take the burden off the rest by serving as hubs. There’s no direct flight from Lincoln, Nebraska, to Rome, Italy, so you first fly from Lincoln to a hub like Newark International Airport in New Jersey, then hop onto a second, longer-distance flight from the hub to Rome. You might even take three flights and pass through two hubs on your journey. The hub system is flexible and scalable, and it forms the backbone of international travel. It allows all airports to participate globally, even while many of them focus on local flights.
Your brain network is organized in much the same way. Its neurons are grouped into clusters that are like airports. Most of the connections in and out of a cluster are local, so, like an airport, the cluster serves mostly local traffic. In addition, some clusters serve as hubs for communication. They are densely connected to many other clusters, and some of their axons reach far across the brain and act as long-distance connections. Brain hubs, like airport hubs, make a complicated system efficient. They allow most neurons to participate globally even as they focus more locally. Hubs form the backbone of communication throughout the brain.
Clusters of neurons connected by hubs
Hubs are supercritical infrastructure. When a major airport hub like Newark or London’s Heathrow goes down, flight delays and cancellations ripple across the world. So imagine what happens when a brain hub goes down. Hub damage is associated with depression, schizophrenia, dyslexia, chronic pain, dementia, Parkinson’s disease, and other disorders. Hubs are points of vulnerability because they are points of efficiency—they make it possible to run a human brain in a
human body without depleting a body budget.
You can thank natural selection for this lean and potent hub structure. Scientists speculate that over evolutionary time, neurons organized into this kind of network because it’s powerful and fast yet energy-efficient and still small enough to fit inside your skull.
Your brain network is not static—it changes continuously. Some changes are extremely fast. Your brain wiring is bathed in chemicals that complete the local connections between neurons. These chemicals, such as glutamate, serotonin, and dopamine, are called neurotransmitters, and they make it easier or harder for signals to pass across synapses. They’re like the airport staff—ticket agents, security screeners, ground crew—who can speed up or slow down the flow of passengers within an airport and without whom we can’t travel at all. These network changes happen instantaneously and continually, even as your physical brain structure seems unchanged. In addition, some of these chemicals, such as serotonin and dopamine, can also act on other neurotransmitters to dial up or dial down their effects. When brain chemicals act in this way, we call them neuromodulators. They are like the weather between airports. When it’s clear, planes fly quickly. When it’s stormy, flights are grounded or rerouted. Neuromodulators and neurotransmitters together allow your brain’s single structure to take on trillions of different patterns of activity.
Other network changes are relatively slower. Just as airports build or renovate their terminals, your brain is constantly under construction. Neurons die, and in some parts of the human brain, neurons are born. Connections become more or less numerous, and they become stronger when neurons fire together and weaker when they don’t. These changes are examples of what scientists call plasticity, and they occur throughout your life. Anytime you learn something—a new friend’s name or an interesting fact from the news—the experience becomes encoded in your wiring so you can remember it, and over time, these encodings can change that wiring.
Your network is also dynamic in another way. As neurons change conversation partners, a single neuron can take on different roles. For example, your ability to see is so intimately tied to an area of the brain called the occipital cortex that the area is routinely called the visual cortex; however, its neurons routinely carry information about hearing and touch. In fact, if you blindfold people with typical vision for a few days and teach them to read braille, neurons in their visual cortex become more devoted to the sense of touch. Remove the blindfold, and the effect disappears after twenty-four hours. Similarly, when babies are born with dense cataracts, meaning the brain receives no visual input, neurons in the visual cortex become repurposed for other senses.
Some neurons in your brain are so flexibly connected that their main job is to have many jobs. An example is one part of your famous prefrontal cortex, called the dorsomedial prefrontal cortex. This brain region is always engaged in body budgeting, but it’s also regularly involved in memory, emotion, perception, decision-making, pain, moral judgments, imagination, language, empathy, and more.
Overall, no neuron has a single psychological function, though a neuron may be more likely to contribute to some functions than others. Even when scientists name a brain area after a function, like “visual cortex” or the “language network,” the name tends to reflect the scientist’s focus at the time rather than any exclusive job performed by that part of the brain. I’m not saying that every neuron can do everything, but any neuron can do more than one thing, just like a single airport can launch planes, sell tickets, and serve crappy food.
It’s also the case that different groups of neurons can produce the same result. Try this right now: Reach for something in front of you, like your phone or a chocolate bar. Draw your hand back, and reach for it again in exactly the same way. Even a simple reaching action like this, when done more than once, can be guided by different sets of neurons. This phenomenon is called degeneracy.
Scientists suspect that all biological systems have degeneracy. In genetics, for example, the same eye color can be produced by different combinations of genes. Your sense of smell works by degeneracy too, and so does your immune system. Transportation systems have degeneracy as well. You can fly from London to Rome on different airlines, on different flights, on different models of airplane, in different seats, with different flight attendants. Copilots can take over for pilots. Degeneracy in the brain means that your actions and experiences can be created in multiple ways. Each time you feel afraid, for example, your brain may construct that feeling with various sets of neurons.
We’ve now seen how helpful it is to understand the brain as a network. This perspective captures so much of the brain’s dynamic behavior—slow changes by plasticity, faster changes by neurotransmitters and neuromodulators, and the flexibility of neurons with multiple jobs.
A network organization has another advantage as well. It furnishes a brain with a special characteristic that is key to creating a human mind. This characteristic is called complexity. It is a brain’s ability to configure itself into an enormous number of distinct neural patterns.
In general, a system with complexity is made of many interacting parts that collaborate and coordinate to create a multitude of patterns of activity. The world’s air-travel system has complexity because its parts—the ticket agents, air traffic controllers, pilots, planes, ground crew, and so on—depend on one another to make the whole system function. The behavior of a complex system is more than the sum of its parts.
Complexity empowers a brain to act flexibly in all kinds of situations. It opens a door so we can think abstractly, have a rich, spoken language, imagine a future very different from the present, and have the creativity and innovation to construct airplanes and suspension bridges and robot vacuum cleaners. Complexity also helps us contemplate the whole world beyond our immediate surroundings, even outer space, and care about the past and the future to an extent that other animals do not. Complexity alone doesn’t give us these capabilities; many other animals have complex brains too. But complexity is a critical ingredient for these capabilities, and the human brain has it in abundance.
In the brain’s case, what constitutes complexity? Picture billions of neurons, each one sending signals to other specific neurons all at once, using neurotransmitters, neuromodulators, and other dynamic bells and whistles. That whole picture is one “pattern” of brain activity. Complexity means your brain can create massive numbers of different patterns by combining bits and pieces of old patterns it has made before. The result is a brain that runs its body efficiently in a world full of constantly changing situations by recalling patterns that helped in the past and generating new ones to try.
A system has higher or lower complexity depending on how much information it can manage by reconfiguring itself. The world’s air-travel system is highly complex in this way. Passengers can fly almost anywhere by different combinations of flights. If a new airport opens, the system can reconfigure to accommodate it. If an airport is damaged by a tornado, travel will be disrupted for a time, but ultimately the airlines can route around the problem. A system with lower complexity, in contrast, could not reconfigure itself as well. The air-travel system would have lower complexity if any given route had just one flight plan or if all planes were forced to fly in and out of a single hub. If that hub was lost, the whole air-travel system would grind to a halt.
We can explore higher and lower complexity by considering two imaginary human brains that are less complex than yours. The first imaginary brain has about 128 billion neurons like yours does, but every neuron is connected to every other. When one neuron receives a signal to change its firing rate, all the other neurons eventually change in kind, because they are all connected. We’ll call this one Meatloaf Brain because its structure is so uniform. Functionally speaking, Meatloaf Brain has lower complexity than yours because at any point in time, its 128 billion elements are effectively just a single element.
A second imaginary brain also
has 128 billion neurons, but it’s carved into puzzle pieces that serve dedicated functions—seeing, hearing, smelling, tasting, touching, thinking, feeling, and so on—like the brain imagined by phrenologists in the nineteenth century. This brain is like a collection of specialized tools that work together, so we’ll call it Pocketknife Brain. Pocketknife Brain has higher complexity than Meatloaf Brain but much lower complexity than your brain, because each tool adds little to the total number of patterns that Pocketknife Brain can make. A real pocketknife with, say, fourteen tools can open into about sixteen thousand possible patterns (214 to be precise), and adding a fifteenth tool merely doubles the total. Your brain’s neurons, however, have multiple functions that increase the number of patterns exponentially. If you had a fourteen-tool pocketknife and added one additional function to each tool—say, making the blade serve as a crude bottle opener, using the screwdriver to punch holes, and so on—the total number of patterns leaps from sixteen thousand (214) to over four million (314). In other words, when existing brain parts become more flexible, the result is much more complexity than we get by accumulating new parts.
Meatloaf Brain and Pocketknife Brain may have some advantages, but a brain with high complexity beats them both.
Brains of higher complexity can remember more. A brain doesn’t store memories like files in a computer—it reconstructs them on demand with electricity and swirling chemicals. We call this process remembering but it’s really assembling. A complex brain can assemble many more memories than either Meatloaf Brain or Pocketknife Brain could. And each time you have the same memory, your brain may have assembled it with a different collection of neurons. (That’s degeneracy.)
Seven and a Half Lessons About the Brain Page 3