The Brain: The Story of You

by David Eagleman

Given the brain’s centrality to our lives, I used to wonder why our society so rarely talks about it, preferring instead to fill our airwaves with celebrity gossip and reality shows. But I now think this lack of attention to the brain can be taken not as a shortcoming, but as a clue: we’re so trapped inside our reality that it is inordinately difficult to realize we’re trapped inside anything.

You become who you are not because of what grows in your brain, but because of what is removed.

Without an environment with emotional care and cognitive stimulation, the human brain cannot develop normally.

A mature pleasure-seeking system coupled with an immature orbitofrontal cortex means that teens are not only emotionally hypersensitive, but also less able to control their emotions than adults.

who we are as a teenager is not simply the result of a choice or an attitude; it is the product of a period of intense and inevitable neural change.

Every four months your red blood cells are entirely replaced, for instance, and your skin cells are replaced every few weeks. Within about seven years every atom in your body will be replaced by other atoms. Physically, you are constantly a new you. Fortunately, there may be one constant that links all these different versions of your self together: memory. Perhaps memory can serve as the thread that makes you who you are. It sits at the core of your identity, providing a single, continuous sense of self.

Your memory of an event is represented by the unique constellation of cells involved in the details you experience.

The enemy of memory isn’t time; it’s other memories. Each new event needs to establish new relationships among a finite number of neurons.

On the flip side, they found that negative psychological factors like loneliness, anxiety, depression, and proneness to psychological distress were related to more rapid cognitive decline. Positive traits like conscientiousness, purpose in life, and keeping busy were protective.

Scientists often debate the detailed definition of consciousness, but it’s easy enough to pin down what we’re talking about with the help of a simple comparison: when you’re awake you have consciousness, and when you’re in deep sleep you don’t.

Consciousness emerges when neurons are coordinating with one another in complex, subtle, mostly independent rhythms. In slow-wave sleep, neurons are more synchronized with one another, and consciousness is absent.

You don’t perceive objects as they are. You perceive them as you are.

Your interpretation of physical objects has everything to do with the historical trajectory of your brain – and little to do with the objects themselves.

Our perception of reality has less to do with what’s happening out there, and more to do with what’s happening inside our brain.

It feels as though you have direct access to the world through your senses. You can reach out and touch the material of the physical world – like this book or the chair you’re sitting on. But this sense of touch is not a direct experience. Although it feels like the touch is happening in your fingers, in fact it’s all happening in the mission control center of the brain. It’s the same across all your sensory experiences. Seeing isn’t happening in your eyes; hearing isn’t taking place in your ears; smell isn’t happening in your nose. All of your sensory experiences are taking place in storms of activity within the computational material of your brain. Here’s the key: the brain has no access to the world outside. Sealed within the dark, silent chamber of your skull, your brain has never directly experienced the external world, and it never will.

There are a hundred billion neurons in the human brain, and each neuron sends tens or hundreds of electrical pulses to thousands of other neurons every second of your life.

Everything you experience – every sight, sound, smell – rather than being a direct experience, is an electrochemical rendition in a dark theater.

One of neuroscience’s unsolved puzzles is known as the “binding problem”: how is the brain able to produce a single, unified picture of the world, given that vision is processed in one region, hearing in another, touch in another, and so on? While the problem is still unsolved, the common currency among neurons – as well as their massive interconnectivity – promises to be at the heart of the solution.

In Mike’s case, forty years of blindness meant that the territory of his visual system (what we would normally call the visual cortex) had been largely taken over by his remaining senses, such as hearing and touch.

In fact, the brain generates its own reality, even before it receives information coming in from the eyes and the other senses. This is known as the internal model.

When you’re confronted with the hollow side of a mask (right), it still looks like it’s coming towards you. What we see is strongly influenced by our expectations.

Twenty percent of the calories we consume are used to power the brain. So brains try to operate in the most energy-efficient way possible, and that means processing only the minimum amount of information from our senses that we need to navigate the world.

Color is an interpretation of wavelengths, one that only exists internally.

Humans detect a tiny fraction of the information carried on the electromagnetic spectrum. The rainbow-colored slice marked “visible light” is made of the same stuff as the rest of the spectrum, but it’s the only part for which we come equipped with biological receptors.

Not only is there no color, there’s also no sound: the compression and expansion of air is picked up by the ears, and turned into electrical signals. The brain then presents these signals to us as mellifluous tones and swishes and clatters and jangles. Reality is also odorless: there’s no such thing as smell outside our brains. Molecules floating through the air bind to receptors in our nose and are interpreted as different smells by our brain. The real world is not full of rich sensory events; instead, our brains light up the world with their own sensuality.

Synesthesia is a condition in which senses (or in some cases concepts) are blended. There are many different kinds of synesthesia. Some taste words. Some see sounds as colors. Some hear visual motion. About 3% of the population has some form of synesthesia.

Schizophrenia is a disorder of her brain function, causing her to hear voices, or see things others don’t see, or believe that other people are reading her thoughts.

So what is reality? It’s like a television show that only you can see, and you can’t turn it off. The good news is that it happens to be broadcasting the most interesting show you could ask for: edited, personalized, and presented just for you.

Even with your eyes closed, you know where your limbs are: is your left arm up or down? Are your legs straight or bent? Is your back straight or slumped? This capacity to know the state of your muscles is called proprioception.

An EEG, short for electroencephalogram, is a method for eavesdropping on the overall electrical activity that arises from the activity of neurons. Small electrodes placed on the surface of the scalp pick up on “brain waves”, the colloquial term for the averaged electrical signals produced by the underlying detailed neural chatter.

different types of brain waves: Delta waves (below 4 Hz) occur during sleep; Theta waves (4–7 Hz) are associated with sleep, deep relaxation, and visualization; Alpha waves (8–13 Hz) occur when we are relaxed and calm; Beta waves (13–38 Hz) are seen when we are actively thinking and problem solving. Other ranges of brain waves have been identified as important since then, including Gamma waves (39–100 Hz) which are involved in concentrated mental activity, such as reasoning and planning. Our overall brain activity is a mix of all these different frequencies, but depending on what we’re doing we’ll exhibit some more than others.

In the early days of learning a new motor skill, the cerebellum plays a particularly important role, coordinating the flow of movements required for accuracy and perfect timing.

As a skill becomes hardwired, it sinks below the level of conscious control. At that point, we can perform a task automatically and without thinking about it – that is, without conscious awareness.

The connections between neurons are called synapses. These connections are where chemicals called neurotransmitters carry signals between neurons. But synaptic connections are not all of the same strength: depending on their history of activity, they can become stronger or weaker.

dilated eyes are a biological sign of female arousal.

Consciousness gets involved when the unexpected happens, when we need to work out what to do next. Although the brain tries to tick along as long as possible on autopilot, it’s not always possible in a world that throws curveballs.

We mostly walk around in our own mental worlds, passing strangers in the street without registering any details about them. But when something challenges our unconscious expectations, conscious attention comes online to try to build a rapid model of what’s happening.

Unlike computers, the brain runs on conflict between different possibilities, all of which try to out-compete the others. And there are always multiple options.

One pundit suggested that the button to launch nuclear missiles should be implanted in the chest of the President’s best friend. That way, if he chose to launch nukes, he’d have to inflict physical violence on his friend, tearing him open. That consideration would recruit emotional networks into the decision. When making life-and-death decisions, unchecked reason can be dangerous; our emotions are a powerful and often insightful constituency, and we’d be remiss to exclude them from the parliamentary voting. The world would not be better if we all behaved like robots.

Your valuation of everything around you is changeable, because quite often our predictions don’t match what actually happens. The key to effective learning lies in tracking this prediction error: the difference between the expected outcome of a choice and the outcome that actually occurred. In today’s case, my brain has a prediction about how rewarding the park is going to be. If we run into friends there and it turns out even better than I thought, that raises the appraisal the next time I’m making such a decision. On the other hand, if the swings are broken and it rains, that lowers my appraisal the next time around.

The dopamine acts as an error corrector: a chemical appraiser that always works to make your appraisals as updated as they can be. That way, you can prioritize your decisions based on your optimized guesses about the future.

Every year the US spends $20 billion on the War on Drugs; globally, the total is over $100 billion. But the investment hasn’t worked. Since the war began, drug use has expanded. Why hasn’t the expenditure succeeded? The difficulty with drug supply is that it’s like a water balloon: if you push it down in one place, it comes up somewhere else. Instead of attacking supply, the better strategy is to address demand. And drug demand is in the brain of the addict. Some people argue that drug addiction is about poverty and peer pressure. Those do play a role, but at the core of the issue is the biology of the brain. In laboratory experiments, rats will self-administer drugs, continually hitting the delivery lever at the expense of food and drink. The rats aren’t doing that because of finances or social coercion. They’re doing it because the drugs tap into fundamental reward circuitry in their brains. The drugs effectively tell the brain that this decision is better than all the other things it could be doing.

Decision making lies at the heart of everything: who we are, what we do, how we perceive the world around us. Without the ability to weigh alternatives, we would be hostages to our most basic drives. We wouldn’t be able to wisely navigate the now, or plan our future lives. Although you have a single identity, you’re not of a single mind: instead, you are a collection of many competing drives. By understanding how choices battle it out in the brain, we can learn to make better decisions for ourselves, and for our society.

The brain comes with inborn instincts to detect who’s trustworthy, and who isn’t.

my facial muscles reflect what I’m feeling, and your neural machinery takes advantage of that. When you’re trying to understand what I’m feeling, you try on my facial expression. You don’t mean to do it – it happens rapidly and unconsciously – but that automatic mirroring of my expression gives you a rapid estimate of what I’m likely to be feeling. This is a powerful trick for your brain to gain a better understanding of me and make better predictions about what I’ll do. As it turns out, it’s just one trick of many.

In the early weeks and months of solitary confinement you’re reduced to an animal-like state. I mean, you are an animal in a cage, and the majority of your hours are spent pacing. And the animal-like state eventually transforms into a more plant-like state: your mind starts to slow down and your thoughts become repetitive. Your brain turns on itself and becomes the source of your worst pain and your worst torture. I’d relive every moment of my life, and eventually you run out of memories. You’ve told them all to yourself so many times. And it doesn’t take that long.

Genocide is only possible when dehumanization happens on a massive scale, and the perfect tool for this job is propaganda: it keys right into the neural networks that understand other people, and dials down the degree to which we empathize with them.

our brains can be manipulated by political agendas to dehumanize other people, which can then lead to the darkest side of human acts.

Education plays a key role in preventing genocide. Only by understanding the neural drive to form ingroups and outgroups – and the standard tricks by which propaganda plugs into this drive – can we hope to interrupt the paths of dehumanization that end in mass atrocity. In this age of digital hyperlinking, it’s more important than ever to understand the links between humans. Human brains are fundamentally wired to interact: we’re a splendidly social species. Although our social drives can sometimes be manipulated, they also sit squarely at the center of the human success story.

sensory substitution, which refers to feeding sensory information through unusual sensory channels such as vision through touch. The brain figures out what to do with the information, because it doesn’t care how the data finds its way in.