Why does Arnold Schwarzenegger have an accent but Milakunis doesn't. And why is there a correlation between how tall a person is and how much his salary is likely to be Why does an elderly person have a hard time learning a new language but no trouble learning the name and face of a new movie star. What would we mean by saying that you are born as many people but die as a single one. Welcome to the inner cosmos with me, David Eagleman. I'm a neuroscientist and author at Stanford, and in these episodes we sail deeply into our three pound universe to understand why and how our lives look the way they do. Today's episode is about brain plasticity, which is the ability of the brain to modify itself and how this changes throughout your lifetime. So we're going to address why it is harder to teach an old dog new tricks and ask whether that is always true. So let's start fifty years ago. There was a psychologist named Hans Lucas Touber at Mit and he got curious about what had happened to soldiers who had sustained head injuries in World War two. Now, this was in the nineteen seventies, so the war had been almost thirty years earlier. So he tracked down five hundred and twenty men who had sustained brain damage during the battles, and some had fared well in their recovery, but others didn't have such good outcomes, and Toyber wanted to understand what the difference was. So he scoured all the records and he looked for the things that correlated with good outcomes and bad outcomes. And you know what he found. The younger the soldier was when he got the head injury, the better he was now. And the older the soldier, the more permanent the damage. Why it's because younger brains are more flexible. There's more brain plasticity, which means the ability of the circuitry to reconfigure itself, and so if there's damage, a young brain can do what it needs to to rewrite its circuitry and get itself back on track. Now you know that I love analogies, So here's Today's brains are like a map of Europe where you look at the borders between the countries. So think of a young brain like Europe five thousand years ago, and if you can imagine different historical trajectories that could have happened, the borders could have evolved in many ways. There's nothing fundamental about where the borders between the countries sit today, But today, after millennia of human history, the maps are more settled into place. Now that humans have had centuries to clang swords and discharge rifles, you get these territory borders that are kind of stubborn to change. So think of the borders between France and Italy and Switzerland. These are totally arbitrary lines, but they're not likely to change now because you don't have roving bands of marauders anymore or bearded conquerors leading big horse armies. These things have been replaced with the United Nations and international rules of engagement, and economies have gotten increased seemly dependent on information and expertise rather than on treasures that you can go pillage. So even in the face of trade arguments and immigration debates, the boundaries between European countries are really hard to move. For the most part, the nations have settled into place, so the land mass began with lots of possibilities for where the borders sat essentially infinite possibility, but with time the potential has narrowed, the map solidified into place, and now it's not so easy to make big changes. The brain matures like Europe through years of border disputes within neural networks, the maps become increasingly solidified. So as a result, brain injury is really dangerous for the elderly, but it's less dangerous for the young because an older brain can't easily reassign settled territories for new tasks. But a brain that's at the dawn of its wars can still reimagine its maps. Okay, so think about the trajectory of a human life. So imagine a young baby born somewhere sometime. When she's first born. Her brain has unbelievable flexibility wherever and whenever she drops out of the womb. She will soak up the local language. She'll pick up on the subtle details of her culture and what to wear and how to act. She will absorb the local religious beliefs. She'll learn all of the rules around her. She'll learn how to gather massive amounts of information, and depending on her generation, that might be by unrolling a scroll or flipping through the pages of a book or swiping the screen of a small rectangle. But by the time she's grown, that story of flexibility has changed somewhat. Her brain isn't so flexible now. She belongs to a particular political party, and she's unlikely to change. She plays the piano reasonably well, but she doesn't have any particular interest in studying violin or other instruments. She likes to cook, and all of her dishes exploit combinations of the fourteen ingredients that she is used to. She spends her online time with a vanishingly small fraction of the billions of available web pages. She has a respectable golf game, but she doesn't have any curiosity about other sports. She lives in a city of eight million people, but she only has three close friends. She isn't particularly interested in the science that she didn't already learn in school. When she goes to the store, she passes racks of shirts until she finds the kind that she always wears, and she selects two of them in her standard colors. Her haircut is the same as it was since she was a teenager. Okay, so this sort of life trajectory underscores a general point, and we've all seen this before, which is that babies are born with not many built in skills, and they have a ton of plasticity. They can learn anything, while adults have mastered specific tasks, but at the expense of flexibility. So there's a trade off between adaptability and efficiency. So as your brain gets good at certain jobs, it becomes less able to tackle others. Now, just to be clear, this is not to say that adam dults aren't intelligent. In fact, it's just the opposite. An adult can do all kinds of things that a baby can't. An adult can run a company, or fix an air conditioner, or plan a business takeover precisely because the adult's brain understands things about the functioning of the world that a child's brain just can't understand. So the way we capture this concept is to say that the baby's brain has fluid intelligence, meaning they can learn anything, while adult brains have crystallized intelligence. So, for example, a few episodes ago, I mentioned a story about the violinist Yittsawk Pearlman, in which a fan told him that he would give his life to play like that, and Pearlman said, I did what Pearlman was pointing to is a fact of life. To get good at one thing is to close the door on other things. So Pearlman went from presumably being able to tackle any instrument to being superlative at one at the expense of everything else. It's unlikely that Pearlman could also be a professional baseball player in the same lifetime. Why because you have only one single life, and what you devote yourself to sends you down particular roads. But that means that all the other roads will forever remain untrodden by you. And this is what the philosopher Martin Heidegger was pointing to when he said, quote every man is born as many men and dies as a single one end quote. You're born with lots of possibility, but when you die, you are just the limited you. Now, from the point of view of your neural networks, what does it mean to descend into pattern and habit? So here's another analogy to help us picture this. Imagine two villages a few miles apart. So people interested in going from one settlement over to the other one, they take all possible paths. Some travelers walk the scenic route along the ridgetops, but others prefer the shade of the cliff side, and some people move among the slippery rocks by the river, and others take the riskier but faster route through the woods. Okay, with time and experience, one route ends up proving more popular. Maybe it makes better sense or it's faster. But eventually that path becomes grooved where the most people have walked, and it starts to become the standard. After some years go by, the local government lays down roadways, and after a few decades that expands into highways, and now the way to get from here to there is really nailed down. So you started with broad optionality, but eventually that gets reduced to the standard path. And this is what happens inside brains. They begin with almost infinite possible routes through the neural networks, but with time the practiced pathways become difficult to exit, the unused paths become thinned away. So through decades of experience, the brain comes to physically represent your world, and your decisions follow the remaining hard paved paths. I mean, just think about what it would be like if you were born with your genes and your brain in a different part of the world. Maybe a different generation a thousand years ago or maybe a thousand years from now. You would function and thrive in whatever environment you drop into, but you'd speak a totally different language, you'd have a different religion, you would believe different things about the world. But as it happens to have turned out, you were born in your hometown, and you grew up with your language and your parents, and so all of the us that could have been ended up getting thinned away. Now that might sound sad to lose the optionality, but the upside of a solidifying brain is that you end up with lightning fast ways of solving problems. Now, the downside is that it's harder to attack new problems with wild, unstructured inventiveness. Now, from the neuroscience point of view, there's also a second reason why older brains are less flexible, and this is beyond the diminishing optionality in the pathways issue. When older brains make changes, they do so only in small spots. In contrast, baby's brains modify across vast territories, and this is because of chemicals in the brain called neurotransmitters that are broadcast broadly in a baby's brain, So in an infant brain, these chemicals like acetylcholine. They transmit announcements throughout the brain, saying, Hey, something important just happen, And this allows pathways and connections to change and modify. So a baby's brain is changeable throughout its territory, and over years its understanding of the world comes into focus like a polaroid photograph. But an adult brain changes only little bits at a time. It keeps most of its connections locked into place to hold on to what has been learned, and only small areas are made flexible via a combination lock of the right neurotransmitters. So an adult brain is like a point to list artist who modifies the color of only a few dots in an almost finished painting. Now you might wonder what does it feel like to be inside the massively flexible brain of a baby. I mean, we were all there as infants, but we can't remember that. So what is it like to be so plastic, so uninhibited and learning about a wide range of novel events. Well, you can probably get close to understanding it by considering other situations in your life in which your awareness plasticity are firing on all cylinders. So when you are traveling in a new land. You drink in all the sights and sounds and smells of the foreign country. You are experiencing lots of novelty and more learning and more distributed attention. After all, at home, you don't pay much attention to much of anything going on. Why because it's predictable. You know what to expect there. But when you're the traveler, you overflow with attention and your brain is changing much more. So think about it like this. When you are highly engaged and paying attention, you are like a baby again. So the difference is between a baby's very fluid brain and an adult's crystallized brain. This is easy to into it, but the neural transition from one to the other does happened in a smooth line. Instead, it's like a door that swings closed and once it shuts, large scale change is over. And this is the concept of this sensitive period. So to understand the sensitive period, consider an infant named Matthew, who is from my hometown. He started having epileptic seizures as a very young boy, and by the time he was six, these seizures were happening with increasing frequency, so he could be having multiple seizures in an hour, and his parents tried everything they could do to figure out what is going on here, and they finally found out that he had something called Rasmussen's encephalitis, which is an inflammation that affects an entire half of his brain. And so they searched everywhere for a solution, and they came to find out that really the only solution is a radical neurosurgery called a hemispherectomy. And in this surgery, an entire half of the brain is removed, just taken out, and that empty half of the skull fills up with cerebra spinal fluid, and if you do brain imaging, what you see is just blackness in that half of the head. Now, this is a horrifying thing for a parent to put their child through. But the completely amazing thing is that kids who get the surgery generally turn out to be just fine. They sometimes have a slight limp on the other side of their body because the left side of the body is controlled by the right side of the brain and vice versa, but other than that, they don't have any particular signs that tell anyone that they only have half of their brain remaining. Now I'm going to talk more about hemispherectomy is in a future episode. But the thing I want to emphasize here is that this kind of surgery is recommended only if the patient is less than let's say eight years old. Matthew was six when he went under the knife, which is nearing old age for this surgery. If a child is older, let's say an adolescent, he will have to function in life by bending tasks to fit what his brain can do, rather than counting on his brain to adapt to the tasks. So the thing to note here is that there is a door that closes at about eight years old, where your brain before that is so flexible that even if it only has half the real estate available, it can readjust to take care of all the functions that it needs. Before eight, you're fine. After probably not so fine now. This kind of age limit. This is seen in so many aspects of brain function. For example, sometimes there are heartbreaking cases in which a child is so profoundly neglected through her childhood without conversation and affection that she will end up incapable of speech. And if that child is found after a certain age, let's say about seven, she will be incapable of ever learning speech. Even when a team of psychologists come in and work with her for years to try to teach her language, it's too late. I told the story of one such girl, Danielle, in my book Live Wired, and I'll return to her story in a future episode. But the thing I want to emphasize for today is that the door for learning language closes. And I'm not talking about learning a second language or dealing with an accent or something like that. I'm talking about the idea of language. What language is, as in, how do I put words together to label things in the outside world and communicate with someone else this way, with a particular grammar and sentence structure and so on. If a child does not get language in the formative years, it becomes too late. Her maps have largely stabilized into place and they can't be changed anymore. So children like Matthew who got a hemisphectomy, or a child like Danielle who didn't get language in time, they tell the same story, which is that brains are really flexible at the beginning in this window of time known as the sensitive period, and as this period passes, the neural geography becomes more difficult to change. So as seen with children like Danielle, a young child's brain needs to hear lots of language during the sensitive period, and without that input, the neurons don't arrange themselves to capture the fundamental concepts of language. And by the way, it's a side note, you might wonder what happens with a deaf baby who doesn't hear any auditory input, and the answer is, as long as the parents present sign language to the baby, her brain will wire up correctly for communication. The deaf baby will employ her hands to babble, making resemblances to sign language, and the same way that a hearing baby exposed to language will babble with her vocal cords. If there is input to pick up on, the baby will do so. As long as that input arrives within the sensitive period. After that door swing shut, it's too late to learn the fundamentals of communication. So there's a window for acquiring the ability to communicate, and there are also windows for more subtle aspects of language, like accents. So take the actress Mila Kunis. She speaks American English with no discernible accent, so you probably didn't know that she was born in Ukraine and lived there, not speaking a word of English until the age of seven. Now, in contrast, Arnold Schwarzenegger, who's been in American filmmaking since his early twenties, he has a very strong Austrian accent. Why because he didn't move to America until he was twenty one, and that meant his use of English began too late. Brain wise, generally, if you arrive in a new country during your first seven years, your fluency in the new tongue will be as high as a native speakers, because your window of sensitivity for obtaining the sounds that's still open. If you immigrate when you're eight to ten years old, you have a slightly more difficult time blending in, but you'll be close. If you're past your teen years. When you move like Arnold was, your fluency is likely to remain low, and you're going to have an accent that reveals your history. So your ability to sonically morph into a different culture is a door that remains open for only about a decade. And let's take another example. Take something like vision. So imagine a child is born with misaligned eyes where one eye is pointing straight but the other eye points inwarder outward. This is known as strabismus, and colloquially it's often called being cross eyed or walleyed. What you do clinically is you fix the extra ocular muscles so the eyes can point the same direction, and then you cover the good eye for a while, which allows the weak eye to fight to regain its lost territory. But no out that the good eye has to be patched. You have to do this technique within this sensitive period about the first six years, otherwise it's too late. The vision will never be recoverable after that from the weakey. After six years, the dirt roads in the brain have been paved into highways and you can't now modify them. So this influence of developmental timing. You see this across all the senses. I talked in earlier episodes about how the body maps readjust if you have an amputation or when you learn a new musical instrument, but across the board, these kinds of changes happen more in young brains than in old brains, just like Mila Kunis with her unaccented speech. So we find that Jitzak Peerlman took up the violin at a very young age. If you were to take up the violin for the first time in your teenage years, there's no possibility that you would ever become a pearlman. Even if you worked really hard to rack up the same number of hours of practice. Your brain is already behind in the race. It has grown too solidified by the time you start doing your first piscata as a teenager. So acquiring vision and language and violin proficiency, this all depends on input from the world, and if a severely neglected child like Danielle doesn't receive this input, she can't later the ability to learn language, to possess vision, to interact socially, to walk normally, to have normal neurodevelopment. This is all limited to the years of young childhood, and after a certain point these abilities are lost. The brain needs to experience the proper input within the right window to achieve its most useful connectivity. Now, as a result of this diminishing flexibility, we are highly influenced by the events that happen in our childhoods. So here's a really interesting example. Consider the correlation between how tall a man is and how much salary he will command. In America, each additional inch of height translates into a one point eight percent increase in take home pay. Why is that well, the popular assumption is that this stems from discrimination in hiring practices. Everyone wants to hire the tall guy because of his commanding presence. But it turns out there is a deeper reason. The best indicator of a male's future salary is how tall he was at the age of sixteen. However, tall he grows after that doesn't change the outcome. Now, how do we understand that? Could it be some effect of nutritional differences between people? Know because when the researchers correlated with height at ages seven or eleven, the effect wasn't as strong. Instead, it's that the teenage years are a time when social status is being worked out, and as a result, who you are as an adult strongly depends on who you were then. In fact, studies that track thousands of children into adulthood find that socially oriented careers like sales or managing other people show the strongest effect of teenage height, and other careers like blue collar work or artistic trades are less influenced. So how people treat you during your formative years has an enormous impact. On your comportment in the world in terms of self esteem and confidence and leadership. Here's another example. Think about Oprah Winfrey, who is worth the bid two point eight billion dollars. So I was a little surprised when I read that she has a deep rooted fear of ending up homeless and penniless. But it's because of the path that got her here. Before she was an empress of the media, she was an impoverished child in Mississippi. She was born to a teenage single mother. So who she was then influenced who she is now. The Great Aristotle noted this twenty four hundred years ago. He said, quote, the habits we form from childhood make no small difference, but rather they make all the difference. Okay, So to capture the idea of the sensitive period, I introduced the metaphor of a door swinging shut. But now we're ready to take the analogy to the next level. It's not one door, it's a bunch of different doors which swing shut at different times. So let's look at an example of that. Sometimes the brain is so impressionable in its earliest days that it can sometimes get into hot water. For example, the baby goose hatches from its egg, and it establishes a parental relationship with the first animate object that it sees. And this is a sufficient strategy in most cases because that first sight is usually its mother. But it can get fooled in the wrong circumstances, and this was shown in the nineteen thirties by the zoologist Conrad Lorenz, who didn't have to work hard for the geese to imprint on him. Instead, he just needed to show up in the right window of plasticity right after they hatched, and then they would imprint on him and follow him around. So that's an example of a fast swinging door for geese to imprint on their parent. But the geese can still learn other things later in life, such as where the is or where to best seek food, or the identities of other geese that they meet in adulthood. So sensitive periods are different for different tasks of the brain, and not all brain regions are equally plastic in terms of how flexibly they begin and how long they retain their adaptability. So is there a pattern to which areas solidify first. So some years ago some colleagues of mine did some research, so they looked a little bit of damage to the retina at the back of the eye in an adult and looked at how that cause changes in the back of the brain and the visual cortex. And they assumed that because the visual cortex was not getting any information from that little patch of eye, that it would readjust you'd see plasticity. And to their surprise, they found no measurable changes in the visual cortex. The part of the cortex that was inactive because it wasn't getting any data stayed inactive. It didn't get taken over by the surrounding areas. Now, given the history of brain plasticity studies even in adults, that was a little bit unexpected. After all, you still have a lot of flexibility in the parts of your brain that drive the body or feel from the body, and this is what allows you to learn how to hang gliders, snowboard even in your later years. So what was the difference between the studies involving your visual system versus the systems that drive your body or feel from your body. Why are the patterns in the primary visual cortex locked into place after a short window of a few years, while the parts of your body involved in moving your sensing continue to learn The answer is that different areas of the brain operate on different schedules of plasticity. Some neural networks are unyielding and others are highly pliable. Some sensitive periods are really brief and others are long. Okay, So is there a general principle at work behind this diversity. One possibility is that the different sensitive periods are caused by different underlying learning strategies in different parts of the brain. So, in this view, some regions are geared to learn throughout life because they're meant to encode changeable details of the world. So think of vocabulary words, or the ability to learn new map directions, or the visual recognition of people's faces. These are tasks for which you want to retain flexibility. But in contrast, other brain areas are involved in really stable relationships, like the building blocks of vision, or how to chew food food or the general rules of grammar, and these areas require a faster lockdown. But how could the brain know and advance the order in which to solidify things? Is that genetically encoded? Possibly some aspects are, but I've previously published a new hypothesis about this, which is that the degree of plasticity in a brain region reflects how much the data change or are likely to change in the outside world. So let me explain this. If the incoming data aren't changing, the system hardens around that. But if the data are constantly changing, then the system remains flexible for that area. So as a result, stable data solidify. First, let me give an example. Take information from the ears versus information from the body. So areas encoding the base six sounds of the world. Like the primary auditory cortex, these become resistant to change. They stiffen rapidly. And that's why, as I spoke about in a previous episode thirty five, a baby born in America and a baby born in Japan will learn how to hear different possible sounds, and by nine months of age, their brain is locking down on those sounds in their language. But in contrast, the parts of your brain involved in navigating your body and feeling from your body, these remain more plastics throughout your life. Why because body plans change throughout your life. You get heavier or skinnier, you put on boots or slippers, or you're on crutches, or you jump on a bicycle or a scooter or a trampoline, and that's why you can pick up something new as an adult, like windsurfing. The statistics of the language you're surrounding with that doesn't change much, but your body's feedback from the world does change, and as a result, the auditory cortex tightens down, but less so for your body plan. So now let's zoom into a single sense like vision. This is really cool because in low level visual areas, what's called the primary visual cortex, the neurons and code basic properties of the world like edges and colors and angles. But you have these higher areas of visual cortex that are involved in more particular items like the layout of your street, or the sleek look of this year's sports car, or the arrangement of apps on your cell phone. Now, the information in the low level areas that becomes established first, and the successive layers wire up on top of those foundations, so the possible angles of a line, these are fixed in place. But you can still learn the face of the latest movie star. So there's a hierarchy of flexibility where the representations at the bottom are learned first. These reflect the basic statistics of the visual world, and those are unlikely to change, these remain stable so that the higher order patterns, which can change more rapidly, they can be learned. Okay, so let's do an analogy. If you are building a library, you want to nail down the basics first. You establish the positions of the shelves, and you set up the Dewey decimal system for organization and maybe the workflow for checking out the books. Once that's all nailed down, then it's straightforward to maintain a flexible inventory of books. You expand the offerings and exciting categories, you reduce outdated volumes, and you constantly test out new titles. This is the same thing in the brain. The primary visual cortex gets all nailed down and set up first, and higher order areas of the brain can try out new things and remain flexible. So there's no single answer to whether the brain is plastic as we get older. It depends on what brain area we're talking about. Plasticity declines with age, but across the brain it declines differently, steeply or shallowly, depending on its function. Now, interestingly, this hypothesis that the amount of plasticity reflects the variance in the outside world. This has an analogy I think in genetics in ways that science is still working to understand, genomes seem to lock in some parts of their nucleotide sequences the actgs. They lock in some parts more than others, and they protect them against mutation, and conversely, other regions of the chromes are much more variable. So, roughly speaking, the variability of a genetic sequence mirrors the variability of features in the outside world. For example, skin pigment genes are highly variable because humans find themselves at different latitudes and need to change the pigmentation to absorb enough vitamin D. But in contrast, the genes that code for proteins that break down sugar, these are really stable because that is a critical and unchanging energy source. So by analogy in the brain, I hypothesize that in the future we may be able to quantify the variability of mental and social and behavioral functions in human life, and we can put to the test this hypothesis that the most flexible circuits of the brain mirror the most variable parts of our environment. Okay, so where does all this talk about brain plasticity put us. Well, often what we find is that adults envy children. Why because children have the ability to absorb languages at an extraordinary rate, and they can think of magically bizarre approaches to any problem, and they can celebrate the novelty of every experience. But older brains have more closed doors, which is why Toyber's World War Two veterans fared worse if they were older, and why Arnold Schwarzenegger retains his accent. And by analogy, the older a city is is the more its infrastructure becomes resistant to shift. So look at something like Rome. The city of Rome can't untangle its windy roads to resemble the grid work of Manhattan because too much history has glued its snaking roots into place. Just like developing humans, cities deepen their tracks along early roads, and so adults often wish they could have the plasticity that they used to. In nineteen eighty four, at the age of thirty five, the physicist my friend Alan Lightman wrote a short essay in The New York Times titled Elapsed Expectations, in which he lamented the perceived stiffening of his mind. Here's what he said. Quote. The limber years for scientists, as for athletes, generally come at a young age. Isaac Newton was in his early twenties when he discovered the law of gravity, Albert Einstein was twenty six when he formulated special relativity, and James Clerk Maxwell had polished off electromagnetic theory and retired to the country. By thirty five. Lightman goes on to say, quote, when I hit thirty five myself some months ago, I went through the unpleasant but irresistible exercise of summing up my career in physics. By this age or another few years, the most creative achievements are finished and visible. You've either got the stuff and used it, or you haven't. End quote. So Lightman was concerned that his brain plasticity was stiffening into place, and these same sentiments were echoed by the physicist James Gates in a television interview I saw. He said, quote, there's a saying that old physicists accept new ideas when they die. It's the next general that brings new ideas to their full fruition. When you get to be an old physicist like me, you know a lot of stuff, and it acts like a ballast on a ship. It pulls you down. You have all the weight of these other things that you know, and sometimes an idea like a small ferry or a sprite passes by and you say, ah, I don't know what that is, but it can't be very important. Well, sometimes it is end quote. So this kind of lamentation is typical of people as they age. But happily, although brain plasticity diminishes over the years, it is still present. Live wiring is not solely the privilege of the young. Neural reconfiguration is an ongoing process that lasts throughout our lives. We form new ideas, we accumulate fresh information, we remember people and events that we're seeing now. So going back to this analogy, despite having decreased flexibility, the city of Rome still evolves. Rome now isn't what it was twenty years ago. Today its statues are ringed with cell phone towers and internet cafes. Although the rudiments of the city are difficult to change, Rome nonetheless advances all its finer points according to new circumstances, just like the library changes its stock of books while its architecture remains largely set. And you see this in so many neuroscience studies. For example, when adults learn a new task like juggling, you can see major changes in their brains. If they take up a new musical instrument, you see these major changes. If they become a London taxi driver and memorize enormous maps of London, you can see these changes. And all of these involve adult plasticity. One really stunning example emerged recently from this nun study called the Religious Order Study, which is a multi decade investigation of hundreds of Catholic nuns living in convents. So all these sisters agreed to regularly test their cognitive function and share their medical records, and when they die, they donate their brains. So amazingly, many of these nuns never displayed any cognitive decline. They were sharp as a whip, but yet their brains at autopsy were riddled with Alzheimer's disease. In other words, their neural networks were physically degenerating, but their performance was not. Now what could explain that, well, the key is that the nuns and their convents had to consistently use their wits until their final days. They had responsibilities and chores and social lives and arguments and game nights and group discussions and so on. So unlike typical retirees, they didn't PLoP onto a couch in front of a television set. Because they had active mental lives, their brains were forced to constantly build new bridges, even as some of their neural roadways were physically falling apart. What stunning is that a third of the nuns seemed to have had the molecular pathology of Alzheimer's without the expected cognitive symptoms. An active mental life, even in the very elderly. This makes new connections in the brain. So learning can happen in any age, and the question is why is it slower than as the brain matures. Well, one reason, as we've discussed, is that many of the swinging doors have closed. But there's another way to look at this. Remember that brain changes are driven by the difference between your internal model and what actually happens in the world, So brains make change only when something is unexpected. As you learn all this and figure this out, your brain becomes less challenged through time, it becomes more settled into place. For example, when you're a child, your internal model tells you that all people believe everything that you believe, and as world experience teaches you the difference between your predictions and your experience, your networks are constantly having to adjust to address that growing gap. Or consider what happens when you start a new job. At first, everything is new, from your coworkers to your responsibilities, to your approaches. You have all this brain plasticity during the first days and weeks as you incorporate your new gig into your internal model. But after a while you become proficient at your job, so skill replaces flexibility. And by the way, as an analogy, we can see this pattern in the way that nations settle into place. Consider the amendments to the constitution of any country. Almost all the change happens near the beginning, while the nation is learning the strategies of running itself, and later constitutions congeal into place and amendments slow down. So take the US Constitution. Twelve of the amendments took place in the first thirteen years, and after that there were a maximum of four changes in any twenty year period, and most periods had no changes at all. And the latest change, ratifying the t twenty seventh Amendment, that was in nineteen ninety two, and the Constitution has been at a standstill since then. In this way, nations steadily diminish their adaptation to the world, because what they do is they profusely modify at the beginning, and with time they settle on a working model that offers what the country needs to be operational. And in this same way, the brain's solidification reflects its success in understanding the world. Neural networks lock themselves more deeply into place, not because of fading function, but because they've had success in figuring things out. So would you really want the plasticity of a child again? Although having a sponge like brain that absorbs everything that sounds appealing, the game of life is largely about figuring out the rules. What we lose in modifiability, we gain in expertise our hard won neural networks. They might not be correct about everything, or even internally consistent, but they add up to life experience to know how to an approach to the world. A child simply doesn't have the capacity to run a company, or write about deep ideas or lead a nation. If plasticity didn't decline, you couldn't lock down the conventions of the world. You would never learn the streets of your neighborhood or people's names, or how to do a job, or how to navigate a social life. You wouldn't be able to hold a meaningful conversation, or ride a bike or obtain food for yourself. If you had total flexibility, you would have the helplessness of an in And don't forget that locking things down this isn't just about skills that you learn. Locking things down is what allows you to retain memories. Every single thing you remember in your life, every bit of your story, is stored in the exact patterns of your neural networks. So just imagine that you had the opportunity to swallow a capsule that would renew the brain plasticity even infant. This would give you the capacity to reprogram your neural networks, to learn new languages rapidly and adopt new accents and new views of physics. But you'd forget everything that came before. Your memories of your childhood would be erased and overwritten. Memories of your first lover, your first trip to Disneyland, your interaction with your parents all would fade like a dream after waking. Would it be worth it to you? You know? While I was working on my book LiveWire, I was struck by thinking about a horror scenario in the future of warfare, because in warfare, countries want to injure their enemies, not kill them, because it requires a lot more resource from the other side. They have to attend to the wounded. So imagine a biological weapon that implements broad brain plasticity. Again, no one is physically hurt, but the troops are propelled back to the state of infants. They forget their ability to walk into talk all if their memories are wiped. When they're returned home by their commanders, they have no remembrance of their families or friends, or spouses or children. Technically, they're still fine, they can still learn. Again, nothing is damaged, only their mental lives. The part we can't see. These have been factory reset back to their original state. Now, this scene is so horrific because fundamentally, who you are is the sum total of your neural circuits. Who you are is stored in the exact configuration of the forest of eighty six billion neurons. So if you said, hey, I want the plasticity of a child again, that comes at the cost of who you are now and what you know. So what we've seen today is that the flexibility of a brain declines with age, and we saw several examples where this can be sudden, like the closing of a door, such that if you don't do something before you're six or eight or thirteen, your brain simply can't learn it later. And we also saw that there's not just one door that closes in the brain, but instead different doors close at different rates. So, for example, you have to be exposed to language in your formative early years to understand the concept of language. But the question of whether you retain an accent after moving somewhere, that's something that comes many years later. And of course, you can learn a foreign language at any age, even though it becomes more difficult. And I just want to remind us about relevance here. In episode thirty five, I talked about what sticks in your brain and what does not stick in your brain, And the bottom line is that your brain has to care about something in order to learn it. You have to have the right cocktail of neurotransmitters present, and that correlates with curiosity and relevance to you. So you can definitely learn a language at any age. The issue is simply do you have enough motivation. If so, you can learn it. The problem is that as people get older, there's typically less motivation to get really good at something like a foreign language, because you can get by with the language that you have, or you figure out the minimum of that language that you need to learn, so you can get a few things and you don't really care as much about impressing people with your fluency. But the mechanisms are still available and you can learn it if it's important to you. And I think one of the tricks in life is figuring out how to fool yourself using your psychology to influence your biology by reminding yourself, hey, here's why this would be important to me, and it can be anything. It can be this is going to impress the person I want to go on a date with, or this will make me proud of myself, or this will get me that promotion that I really want, or this will get me into medical school, or this will make my parents prouder. Whatever it is that drives you, it's really useful to clarify that to yourself, to explicitly specify to yourself why this task matters, and then you'll have that at write cocktail of neurotransmitters to make that information stick because in the end we get to be the sculptors of our own brains. Go to eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcasts at eagleman dot com with questions or discussions, and I'm making sporadic episodes in which I address those until next time. I'm David Eagleman, and this is Inner Cosmos.

Why does Arnold Schwarzenegger have an accent but Milakunis doesn't. And why is there a correlation between how tall a person is and how much his salary is likely to be Why does an elderly person have a hard time learning a new language but no trouble learning the name and face of a new movie star. What would we mean by saying that you are born as many people but die as a single one. Welcome to the inner cosmos with me, David Eagleman. I'm a neuroscientist and author at Stanford, and in these episodes we sail deeply into our three pound universe to understand why and how our lives look the way they do. Today's episode is about brain plasticity, which is the ability of the brain to modify itself and how this changes throughout your lifetime. So we're going to address why it is harder to teach an old dog new tricks and ask whether that is always true. So let's start fifty years ago. There was a psychologist named Hans Lucas Touber at Mit and he got curious about what had happened to soldiers who had sustained head injuries in World War two. Now, this was in the nineteen seventies, so the war had been almost thirty years earlier. So he tracked down five hundred and twenty men who had sustained brain damage during the battles, and some had fared well in their recovery, but others didn't have such good outcomes, and Toyber wanted to understand what the difference was. So he scoured all the records and he looked for the things that correlated with good outcomes and bad outcomes. And you know what he found. The younger the soldier was when he got the head injury, the better he was now. And the older the soldier, the more permanent the damage. Why it's because younger brains are more flexible. There's more brain plasticity, which means the ability of the circuitry to reconfigure itself, and so if there's damage, a young brain can do what it needs to to rewrite its circuitry and get itself back on track. Now you know that I love analogies, So here's Today's brains are like a map of Europe where you look at the borders between the countries. So think of a young brain like Europe five thousand years ago, and if you can imagine different historical trajectories that could have happened, the borders could have evolved in many ways. There's nothing fundamental about where the borders between the countries sit today, But today, after millennia of human history, the maps are more settled into place. Now that humans have had centuries to clang swords and discharge rifles, you get these territory borders that are kind of stubborn to change. So think of the borders between France and Italy and Switzerland. These are totally arbitrary lines, but they're not likely to change now because you don't have roving bands of marauders anymore or bearded conquerors leading big horse armies. These things have been replaced with the United Nations and international rules of engagement, and economies have gotten increased seemly dependent on information and expertise rather than on treasures that you can go pillage. So even in the face of trade arguments and immigration debates, the boundaries between European countries are really hard to move. For the most part, the nations have settled into place, so the land mass began with lots of possibilities for where the borders sat essentially infinite possibility, but with time the potential has narrowed, the map solidified into place, and now it's not so easy to make big changes. The brain matures like Europe through years of border disputes within neural networks, the maps become increasingly solidified. So as a result, brain injury is really dangerous for the elderly, but it's less dangerous for the young because an older brain can't easily reassign settled territories for new tasks. But a brain that's at the dawn of its wars can still reimagine its maps. Okay, so think about the trajectory of a human life. So imagine a young baby born somewhere sometime. When she's first born. Her brain has unbelievable flexibility wherever and whenever she drops out of the womb. She will soak up the local language. She'll pick up on the subtle details of her culture and what to wear and how to act. She will absorb the local religious beliefs. She'll learn all of the rules around her. She'll learn how to gather massive amounts of information, and depending on her generation, that might be by unrolling a scroll or flipping through the pages of a book or swiping the screen of a small rectangle. But by the time she's grown, that story of flexibility has changed somewhat. Her brain isn't so flexible now. She belongs to a particular political party, and she's unlikely to change. She plays the piano reasonably well, but she doesn't have any particular interest in studying violin or other instruments. She likes to cook, and all of her dishes exploit combinations of the fourteen ingredients that she is used to. She spends her online time with a vanishingly small fraction of the billions of available web pages. She has a respectable golf game, but she doesn't have any curiosity about other sports. She lives in a city of eight million people, but she only has three close friends. She isn't particularly interested in the science that she didn't already learn in school. When she goes to the store, she passes racks of shirts until she finds the kind that she always wears, and she selects two of them in her standard colors. Her haircut is the same as it was since she was a teenager. Okay, so this sort of life trajectory underscores a general point, and we've all seen this before, which is that babies are born with not many built in skills, and they have a ton of plasticity. They can learn anything, while adults have mastered specific tasks, but at the expense of flexibility. So there's a trade off between adaptability and efficiency. So as your brain gets good at certain jobs, it becomes less able to tackle others. Now, just to be clear, this is not to say that adam dults aren't intelligent. In fact, it's just the opposite. An adult can do all kinds of things that a baby can't. An adult can run a company, or fix an air conditioner, or plan a business takeover precisely because the adult's brain understands things about the functioning of the world that a child's brain just can't understand. So the way we capture this concept is to say that the baby's brain has fluid intelligence, meaning they can learn anything, while adult brains have crystallized intelligence. So, for example, a few episodes ago, I mentioned a story about the violinist Yittsawk Pearlman, in which a fan told him that he would give his life to play like that, and Pearlman said, I did what Pearlman was pointing to is a fact of life. To get good at one thing is to close the door on other things. So Pearlman went from presumably being able to tackle any instrument to being superlative at one at the expense of everything else. It's unlikely that Pearlman could also be a professional baseball player in the same lifetime. Why because you have only one single life, and what you devote yourself to sends you down particular roads. But that means that all the other roads will forever remain untrodden by you. And this is what the philosopher Martin Heidegger was pointing to when he said, quote every man is born as many men and dies as a single one end quote. You're born with lots of possibility, but when you die, you are just the limited you. Now, from the point of view of your neural networks, what does it mean to descend into pattern and habit? So here's another analogy to help us picture this. Imagine two villages a few miles apart. So people interested in going from one settlement over to the other one, they take all possible paths. Some travelers walk the scenic route along the ridgetops, but others prefer the shade of the cliff side, and some people move among the slippery rocks by the river, and others take the riskier but faster route through the woods. Okay, with time and experience, one route ends up proving more popular. Maybe it makes better sense or it's faster. But eventually that path becomes grooved where the most people have walked, and it starts to become the standard. After some years go by, the local government lays down roadways, and after a few decades that expands into highways, and now the way to get from here to there is really nailed down. So you started with broad optionality, but eventually that gets reduced to the standard path. And this is what happens inside brains. They begin with almost infinite possible routes through the neural networks, but with time the practiced pathways become difficult to exit, the unused paths become thinned away. So through decades of experience, the brain comes to physically represent your world, and your decisions follow the remaining hard paved paths. I mean, just think about what it would be like if you were born with your genes and your brain in a different part of the world. Maybe a different generation a thousand years ago or maybe a thousand years from now. You would function and thrive in whatever environment you drop into, but you'd speak a totally different language, you'd have a different religion, you would believe different things about the world. But as it happens to have turned out, you were born in your hometown, and you grew up with your language and your parents, and so all of the us that could have been ended up getting thinned away. Now that might sound sad to lose the optionality, but the upside of a solidifying brain is that you end up with lightning fast ways of solving problems. Now, the downside is that it's harder to attack new problems with wild, unstructured inventiveness. Now, from the neuroscience point of view, there's also a second reason why older brains are less flexible, and this is beyond the diminishing optionality in the pathways issue. When older brains make changes, they do so only in small spots. In contrast, baby's brains modify across vast territories, and this is because of chemicals in the brain called neurotransmitters that are broadcast broadly in a baby's brain, So in an infant brain, these chemicals like acetylcholine. They transmit announcements throughout the brain, saying, Hey, something important just happen, And this allows pathways and connections to change and modify. So a baby's brain is changeable throughout its territory, and over years its understanding of the world comes into focus like a polaroid photograph. But an adult brain changes only little bits at a time. It keeps most of its connections locked into place to hold on to what has been learned, and only small areas are made flexible via a combination lock of the right neurotransmitters. So an adult brain is like a point to list artist who modifies the color of only a few dots in an almost finished painting. Now you might wonder what does it feel like to be inside the massively flexible brain of a baby. I mean, we were all there as infants, but we can't remember that. So what is it like to be so plastic, so uninhibited and learning about a wide range of novel events. Well, you can probably get close to understanding it by considering other situations in your life in which your awareness plasticity are firing on all cylinders. So when you are traveling in a new land. You drink in all the sights and sounds and smells of the foreign country. You are experiencing lots of novelty and more learning and more distributed attention. After all, at home, you don't pay much attention to much of anything going on. Why because it's predictable. You know what to expect there. But when you're the traveler, you overflow with attention and your brain is changing much more. So think about it like this. When you are highly engaged and paying attention, you are like a baby again. So the difference is between a baby's very fluid brain and an adult's crystallized brain. This is easy to into it, but the neural transition from one to the other does happened in a smooth line. Instead, it's like a door that swings closed and once it shuts, large scale change is over. And this is the concept of this sensitive period. So to understand the sensitive period, consider an infant named Matthew, who is from my hometown. He started having epileptic seizures as a very young boy, and by the time he was six, these seizures were happening with increasing frequency, so he could be having multiple seizures in an hour, and his parents tried everything they could do to figure out what is going on here, and they finally found out that he had something called Rasmussen's encephalitis, which is an inflammation that affects an entire half of his brain. And so they searched everywhere for a solution, and they came to find out that really the only solution is a radical neurosurgery called a hemispherectomy. And in this surgery, an entire half of the brain is removed, just taken out, and that empty half of the skull fills up with cerebra spinal fluid, and if you do brain imaging, what you see is just blackness in that half of the head. Now, this is a horrifying thing for a parent to put their child through. But the completely amazing thing is that kids who get the surgery generally turn out to be just fine. They sometimes have a slight limp on the other side of their body because the left side of the body is controlled by the right side of the brain and vice versa, but other than that, they don't have any particular signs that tell anyone that they only have half of their brain remaining. Now I'm going to talk more about hemispherectomy is in a future episode. But the thing I want to emphasize here is that this kind of surgery is recommended only if the patient is less than let's say eight years old. Matthew was six when he went under the knife, which is nearing old age for this surgery. If a child is older, let's say an adolescent, he will have to function in life by bending tasks to fit what his brain can do, rather than counting on his brain to adapt to the tasks. So the thing to note here is that there is a door that closes at about eight years old, where your brain before that is so flexible that even if it only has half the real estate available, it can readjust to take care of all the functions that it needs. Before eight, you're fine. After probably not so fine now. This kind of age limit. This is seen in so many aspects of brain function. For example, sometimes there are heartbreaking cases in which a child is so profoundly neglected through her childhood without conversation and affection that she will end up incapable of speech. And if that child is found after a certain age, let's say about seven, she will be incapable of ever learning speech. Even when a team of psychologists come in and work with her for years to try to teach her language, it's too late. I told the story of one such girl, Danielle, in my book Live Wired, and I'll return to her story in a future episode. But the thing I want to emphasize for today is that the door for learning language closes. And I'm not talking about learning a second language or dealing with an accent or something like that. I'm talking about the idea of language. What language is, as in, how do I put words together to label things in the outside world and communicate with someone else this way, with a particular grammar and sentence structure and so on. If a child does not get language in the formative years, it becomes too late. Her maps have largely stabilized into place and they can't be changed anymore. So children like Matthew who got a hemisphectomy, or a child like Danielle who didn't get language in time, they tell the same story, which is that brains are really flexible at the beginning in this window of time known as the sensitive period, and as this period passes, the neural geography becomes more difficult to change. So as seen with children like Danielle, a young child's brain needs to hear lots of language during the sensitive period, and without that input, the neurons don't arrange themselves to capture the fundamental concepts of language. And by the way, it's a side note, you might wonder what happens with a deaf baby who doesn't hear any auditory input, and the answer is, as long as the parents present sign language to the baby, her brain will wire up correctly for communication. The deaf baby will employ her hands to babble, making resemblances to sign language, and the same way that a hearing baby exposed to language will babble with her vocal cords. If there is input to pick up on, the baby will do so. As long as that input arrives within the sensitive period. After that door swing shut, it's too late to learn the fundamentals of communication. So there's a window for acquiring the ability to communicate, and there are also windows for more subtle aspects of language, like accents. So take the actress Mila Kunis. She speaks American English with no discernible accent, so you probably didn't know that she was born in Ukraine and lived there, not speaking a word of English until the age of seven. Now, in contrast, Arnold Schwarzenegger, who's been in American filmmaking since his early twenties, he has a very strong Austrian accent. Why because he didn't move to America until he was twenty one, and that meant his use of English began too late. Brain wise, generally, if you arrive in a new country during your first seven years, your fluency in the new tongue will be as high as a native speakers, because your window of sensitivity for obtaining the sounds that's still open. If you immigrate when you're eight to ten years old, you have a slightly more difficult time blending in, but you'll be close. If you're past your teen years. When you move like Arnold was, your fluency is likely to remain low, and you're going to have an accent that reveals your history. So your ability to sonically morph into a different culture is a door that remains open for only about a decade. And let's take another example. Take something like vision. So imagine a child is born with misaligned eyes where one eye is pointing straight but the other eye points inwarder outward. This is known as strabismus, and colloquially it's often called being cross eyed or walleyed. What you do clinically is you fix the extra ocular muscles so the eyes can point the same direction, and then you cover the good eye for a while, which allows the weak eye to fight to regain its lost territory. But no out that the good eye has to be patched. You have to do this technique within this sensitive period about the first six years, otherwise it's too late. The vision will never be recoverable after that from the weakey. After six years, the dirt roads in the brain have been paved into highways and you can't now modify them. So this influence of developmental timing. You see this across all the senses. I talked in earlier episodes about how the body maps readjust if you have an amputation or when you learn a new musical instrument, but across the board, these kinds of changes happen more in young brains than in old brains, just like Mila Kunis with her unaccented speech. So we find that Jitzak Peerlman took up the violin at a very young age. If you were to take up the violin for the first time in your teenage years, there's no possibility that you would ever become a pearlman. Even if you worked really hard to rack up the same number of hours of practice. Your brain is already behind in the race. It has grown too solidified by the time you start doing your first piscata as a teenager. So acquiring vision and language and violin proficiency, this all depends on input from the world, and if a severely neglected child like Danielle doesn't receive this input, she can't later the ability to learn language, to possess vision, to interact socially, to walk normally, to have normal neurodevelopment. This is all limited to the years of young childhood, and after a certain point these abilities are lost. The brain needs to experience the proper input within the right window to achieve its most useful connectivity. Now, as a result of this diminishing flexibility, we are highly influenced by the events that happen in our childhoods. So here's a really interesting example. Consider the correlation between how tall a man is and how much salary he will command. In America, each additional inch of height translates into a one point eight percent increase in take home pay. Why is that well, the popular assumption is that this stems from discrimination in hiring practices. Everyone wants to hire the tall guy because of his commanding presence. But it turns out there is a deeper reason. The best indicator of a male's future salary is how tall he was at the age of sixteen. However, tall he grows after that doesn't change the outcome. Now, how do we understand that? Could it be some effect of nutritional differences between people? Know because when the researchers correlated with height at ages seven or eleven, the effect wasn't as strong. Instead, it's that the teenage years are a time when social status is being worked out, and as a result, who you are as an adult strongly depends on who you were then. In fact, studies that track thousands of children into adulthood find that socially oriented careers like sales or managing other people show the strongest effect of teenage height, and other careers like blue collar work or artistic trades are less influenced. So how people treat you during your formative years has an enormous impact. On your comportment in the world in terms of self esteem and confidence and leadership. Here's another example. Think about Oprah Winfrey, who is worth the bid two point eight billion dollars. So I was a little surprised when I read that she has a deep rooted fear of ending up homeless and penniless. But it's because of the path that got her here. Before she was an empress of the media, she was an impoverished child in Mississippi. She was born to a teenage single mother. So who she was then influenced who she is now. The Great Aristotle noted this twenty four hundred years ago. He said, quote, the habits we form from childhood make no small difference, but rather they make all the difference. Okay, So to capture the idea of the sensitive period, I introduced the metaphor of a door swinging shut. But now we're ready to take the analogy to the next level. It's not one door, it's a bunch of different doors which swing shut at different times. So let's look at an example of that. Sometimes the brain is so impressionable in its earliest days that it can sometimes get into hot water. For example, the baby goose hatches from its egg, and it establishes a parental relationship with the first animate object that it sees. And this is a sufficient strategy in most cases because that first sight is usually its mother. But it can get fooled in the wrong circumstances, and this was shown in the nineteen thirties by the zoologist Conrad Lorenz, who didn't have to work hard for the geese to imprint on him. Instead, he just needed to show up in the right window of plasticity right after they hatched, and then they would imprint on him and follow him around. So that's an example of a fast swinging door for geese to imprint on their parent. But the geese can still learn other things later in life, such as where the is or where to best seek food, or the identities of other geese that they meet in adulthood. So sensitive periods are different for different tasks of the brain, and not all brain regions are equally plastic in terms of how flexibly they begin and how long they retain their adaptability. So is there a pattern to which areas solidify first. So some years ago some colleagues of mine did some research, so they looked a little bit of damage to the retina at the back of the eye in an adult and looked at how that cause changes in the back of the brain and the visual cortex. And they assumed that because the visual cortex was not getting any information from that little patch of eye, that it would readjust you'd see plasticity. And to their surprise, they found no measurable changes in the visual cortex. The part of the cortex that was inactive because it wasn't getting any data stayed inactive. It didn't get taken over by the surrounding areas. Now, given the history of brain plasticity studies even in adults, that was a little bit unexpected. After all, you still have a lot of flexibility in the parts of your brain that drive the body or feel from the body, and this is what allows you to learn how to hang gliders, snowboard even in your later years. So what was the difference between the studies involving your visual system versus the systems that drive your body or feel from your body. Why are the patterns in the primary visual cortex locked into place after a short window of a few years, while the parts of your body involved in moving your sensing continue to learn The answer is that different areas of the brain operate on different schedules of plasticity. Some neural networks are unyielding and others are highly pliable. Some sensitive periods are really brief and others are long. Okay, So is there a general principle at work behind this diversity. One possibility is that the different sensitive periods are caused by different underlying learning strategies in different parts of the brain. So, in this view, some regions are geared to learn throughout life because they're meant to encode changeable details of the world. So think of vocabulary words, or the ability to learn new map directions, or the visual recognition of people's faces. These are tasks for which you want to retain flexibility. But in contrast, other brain areas are involved in really stable relationships, like the building blocks of vision, or how to chew food food or the general rules of grammar, and these areas require a faster lockdown. But how could the brain know and advance the order in which to solidify things? Is that genetically encoded? Possibly some aspects are, but I've previously published a new hypothesis about this, which is that the degree of plasticity in a brain region reflects how much the data change or are likely to change in the outside world. So let me explain this. If the incoming data aren't changing, the system hardens around that. But if the data are constantly changing, then the system remains flexible for that area. So as a result, stable data solidify. First, let me give an example. Take information from the ears versus information from the body. So areas encoding the base six sounds of the world. Like the primary auditory cortex, these become resistant to change. They stiffen rapidly. And that's why, as I spoke about in a previous episode thirty five, a baby born in America and a baby born in Japan will learn how to hear different possible sounds, and by nine months of age, their brain is locking down on those sounds in their language. But in contrast, the parts of your brain involved in navigating your body and feeling from your body, these remain more plastics throughout your life. Why because body plans change throughout your life. You get heavier or skinnier, you put on boots or slippers, or you're on crutches, or you jump on a bicycle or a scooter or a trampoline, and that's why you can pick up something new as an adult, like windsurfing. The statistics of the language you're surrounding with that doesn't change much, but your body's feedback from the world does change, and as a result, the auditory cortex tightens down, but less so for your body plan. So now let's zoom into a single sense like vision. This is really cool because in low level visual areas, what's called the primary visual cortex, the neurons and code basic properties of the world like edges and colors and angles. But you have these higher areas of visual cortex that are involved in more particular items like the layout of your street, or the sleek look of this year's sports car, or the arrangement of apps on your cell phone. Now, the information in the low level areas that becomes established first, and the successive layers wire up on top of those foundations, so the possible angles of a line, these are fixed in place. But you can still learn the face of the latest movie star. So there's a hierarchy of flexibility where the representations at the bottom are learned first. These reflect the basic statistics of the visual world, and those are unlikely to change, these remain stable so that the higher order patterns, which can change more rapidly, they can be learned. Okay, so let's do an analogy. If you are building a library, you want to nail down the basics first. You establish the positions of the shelves, and you set up the Dewey decimal system for organization and maybe the workflow for checking out the books. Once that's all nailed down, then it's straightforward to maintain a flexible inventory of books. You expand the offerings and exciting categories, you reduce outdated volumes, and you constantly test out new titles. This is the same thing in the brain. The primary visual cortex gets all nailed down and set up first, and higher order areas of the brain can try out new things and remain flexible. So there's no single answer to whether the brain is plastic as we get older. It depends on what brain area we're talking about. Plasticity declines with age, but across the brain it declines differently, steeply or shallowly, depending on its function. Now, interestingly, this hypothesis that the amount of plasticity reflects the variance in the outside world. This has an analogy I think in genetics in ways that science is still working to understand, genomes seem to lock in some parts of their nucleotide sequences the actgs. They lock in some parts more than others, and they protect them against mutation, and conversely, other regions of the chromes are much more variable. So, roughly speaking, the variability of a genetic sequence mirrors the variability of features in the outside world. For example, skin pigment genes are highly variable because humans find themselves at different latitudes and need to change the pigmentation to absorb enough vitamin D. But in contrast, the genes that code for proteins that break down sugar, these are really stable because that is a critical and unchanging energy source. So by analogy in the brain, I hypothesize that in the future we may be able to quantify the variability of mental and social and behavioral functions in human life, and we can put to the test this hypothesis that the most flexible circuits of the brain mirror the most variable parts of our environment. Okay, so where does all this talk about brain plasticity put us. Well, often what we find is that adults envy children. Why because children have the ability to absorb languages at an extraordinary rate, and they can think of magically bizarre approaches to any problem, and they can celebrate the novelty of every experience. But older brains have more closed doors, which is why Toyber's World War Two veterans fared worse if they were older, and why Arnold Schwarzenegger retains his accent. And by analogy, the older a city is is the more its infrastructure becomes resistant to shift. So look at something like Rome. The city of Rome can't untangle its windy roads to resemble the grid work of Manhattan because too much history has glued its snaking roots into place. Just like developing humans, cities deepen their tracks along early roads, and so adults often wish they could have the plasticity that they used to. In nineteen eighty four, at the age of thirty five, the physicist my friend Alan Lightman wrote a short essay in The New York Times titled Elapsed Expectations, in which he lamented the perceived stiffening of his mind. Here's what he said. Quote. The limber years for scientists, as for athletes, generally come at a young age. Isaac Newton was in his early twenties when he discovered the law of gravity, Albert Einstein was twenty six when he formulated special relativity, and James Clerk Maxwell had polished off electromagnetic theory and retired to the country. By thirty five. Lightman goes on to say, quote, when I hit thirty five myself some months ago, I went through the unpleasant but irresistible exercise of summing up my career in physics. By this age or another few years, the most creative achievements are finished and visible. You've either got the stuff and used it, or you haven't. End quote. So Lightman was concerned that his brain plasticity was stiffening into place, and these same sentiments were echoed by the physicist James Gates in a television interview I saw. He said, quote, there's a saying that old physicists accept new ideas when they die. It's the next general that brings new ideas to their full fruition. When you get to be an old physicist like me, you know a lot of stuff, and it acts like a ballast on a ship. It pulls you down. You have all the weight of these other things that you know, and sometimes an idea like a small ferry or a sprite passes by and you say, ah, I don't know what that is, but it can't be very important. Well, sometimes it is end quote. So this kind of lamentation is typical of people as they age. But happily, although brain plasticity diminishes over the years, it is still present. Live wiring is not solely the privilege of the young. Neural reconfiguration is an ongoing process that lasts throughout our lives. We form new ideas, we accumulate fresh information, we remember people and events that we're seeing now. So going back to this analogy, despite having decreased flexibility, the city of Rome still evolves. Rome now isn't what it was twenty years ago. Today its statues are ringed with cell phone towers and internet cafes. Although the rudiments of the city are difficult to change, Rome nonetheless advances all its finer points according to new circumstances, just like the library changes its stock of books while its architecture remains largely set. And you see this in so many neuroscience studies. For example, when adults learn a new task like juggling, you can see major changes in their brains. If they take up a new musical instrument, you see these major changes. If they become a London taxi driver and memorize enormous maps of London, you can see these changes. And all of these involve adult plasticity. One really stunning example emerged recently from this nun study called the Religious Order Study, which is a multi decade investigation of hundreds of Catholic nuns living in convents. So all these sisters agreed to regularly test their cognitive function and share their medical records, and when they die, they donate their brains. So amazingly, many of these nuns never displayed any cognitive decline. They were sharp as a whip, but yet their brains at autopsy were riddled with Alzheimer's disease. In other words, their neural networks were physically degenerating, but their performance was not. Now what could explain that, well, the key is that the nuns and their convents had to consistently use their wits until their final days. They had responsibilities and chores and social lives and arguments and game nights and group discussions and so on. So unlike typical retirees, they didn't PLoP onto a couch in front of a television set. Because they had active mental lives, their brains were forced to constantly build new bridges, even as some of their neural roadways were physically falling apart. What stunning is that a third of the nuns seemed to have had the molecular pathology of Alzheimer's without the expected cognitive symptoms. An active mental life, even in the very elderly. This makes new connections in the brain. So learning can happen in any age, and the question is why is it slower than as the brain matures. Well, one reason, as we've discussed, is that many of the swinging doors have closed. But there's another way to look at this. Remember that brain changes are driven by the difference between your internal model and what actually happens in the world, So brains make change only when something is unexpected. As you learn all this and figure this out, your brain becomes less challenged through time, it becomes more settled into place. For example, when you're a child, your internal model tells you that all people believe everything that you believe, and as world experience teaches you the difference between your predictions and your experience, your networks are constantly having to adjust to address that growing gap. Or consider what happens when you start a new job. At first, everything is new, from your coworkers to your responsibilities, to your approaches. You have all this brain plasticity during the first days and weeks as you incorporate your new gig into your internal model. But after a while you become proficient at your job, so skill replaces flexibility. And by the way, as an analogy, we can see this pattern in the way that nations settle into place. Consider the amendments to the constitution of any country. Almost all the change happens near the beginning, while the nation is learning the strategies of running itself, and later constitutions congeal into place and amendments slow down. So take the US Constitution. Twelve of the amendments took place in the first thirteen years, and after that there were a maximum of four changes in any twenty year period, and most periods had no changes at all. And the latest change, ratifying the t twenty seventh Amendment, that was in nineteen ninety two, and the Constitution has been at a standstill since then. In this way, nations steadily diminish their adaptation to the world, because what they do is they profusely modify at the beginning, and with time they settle on a working model that offers what the country needs to be operational. And in this same way, the brain's solidification reflects its success in understanding the world. Neural networks lock themselves more deeply into place, not because of fading function, but because they've had success in figuring things out. So would you really want the plasticity of a child again? Although having a sponge like brain that absorbs everything that sounds appealing, the game of life is largely about figuring out the rules. What we lose in modifiability, we gain in expertise our hard won neural networks. They might not be correct about everything, or even internally consistent, but they add up to life experience to know how to an approach to the world. A child simply doesn't have the capacity to run a company, or write about deep ideas or lead a nation. If plasticity didn't decline, you couldn't lock down the conventions of the world. You would never learn the streets of your neighborhood or people's names, or how to do a job, or how to navigate a social life. You wouldn't be able to hold a meaningful conversation, or ride a bike or obtain food for yourself. If you had total flexibility, you would have the helplessness of an in And don't forget that locking things down this isn't just about skills that you learn. Locking things down is what allows you to retain memories. Every single thing you remember in your life, every bit of your story, is stored in the exact patterns of your neural networks. So just imagine that you had the opportunity to swallow a capsule that would renew the brain plasticity even infant. This would give you the capacity to reprogram your neural networks, to learn new languages rapidly and adopt new accents and new views of physics. But you'd forget everything that came before. Your memories of your childhood would be erased and overwritten. Memories of your first lover, your first trip to Disneyland, your interaction with your parents all would fade like a dream after waking. Would it be worth it to you? You know? While I was working on my book LiveWire, I was struck by thinking about a horror scenario in the future of warfare, because in warfare, countries want to injure their enemies, not kill them, because it requires a lot more resource from the other side. They have to attend to the wounded. So imagine a biological weapon that implements broad brain plasticity. Again, no one is physically hurt, but the troops are propelled back to the state of infants. They forget their ability to walk into talk all if their memories are wiped. When they're returned home by their commanders, they have no remembrance of their families or friends, or spouses or children. Technically, they're still fine, they can still learn. Again, nothing is damaged, only their mental lives. The part we can't see. These have been factory reset back to their original state. Now, this scene is so horrific because fundamentally, who you are is the sum total of your neural circuits. Who you are is stored in the exact configuration of the forest of eighty six billion neurons. So if you said, hey, I want the plasticity of a child again, that comes at the cost of who you are now and what you know. So what we've seen today is that the flexibility of a brain declines with age, and we saw several examples where this can be sudden, like the closing of a door, such that if you don't do something before you're six or eight or thirteen, your brain simply can't learn it later. And we also saw that there's not just one door that closes in the brain, but instead different doors close at different rates. So, for example, you have to be exposed to language in your formative early years to understand the concept of language. But the question of whether you retain an accent after moving somewhere, that's something that comes many years later. And of course, you can learn a foreign language at any age, even though it becomes more difficult. And I just want to remind us about relevance here. In episode thirty five, I talked about what sticks in your brain and what does not stick in your brain, And the bottom line is that your brain has to care about something in order to learn it. You have to have the right cocktail of neurotransmitters present, and that correlates with curiosity and relevance to you. So you can definitely learn a language at any age. The issue is simply do you have enough motivation. If so, you can learn it. The problem is that as people get older, there's typically less motivation to get really good at something like a foreign language, because you can get by with the language that you have, or you figure out the minimum of that language that you need to learn, so you can get a few things and you don't really care as much about impressing people with your fluency. But the mechanisms are still available and you can learn it if it's important to you. And I think one of the tricks in life is figuring out how to fool yourself using your psychology to influence your biology by reminding yourself, hey, here's why this would be important to me, and it can be anything. It can be this is going to impress the person I want to go on a date with, or this will make me proud of myself, or this will get me that promotion that I really want, or this will get me into medical school, or this will make my parents prouder. Whatever it is that drives you, it's really useful to clarify that to yourself, to explicitly specify to yourself why this task matters, and then you'll have that at write cocktail of neurotransmitters to make that information stick because in the end we get to be the sculptors of our own brains. Go to eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcasts at eagleman dot com with questions or discussions, and I'm making sporadic episodes in which I address those until next time. I'm David Eagleman, and this is Inner Cosmos.