Speaker 1: Why does Arnold Schwarzenegger have an accent but Milakunis doesn't.
Speaker 1: And why is there a correlation between how tall a
Speaker 1: person is and how much his salary is likely to
Speaker 1: be Why does an elderly person have a hard time
Speaker 1: learning a new language but no trouble learning the name
Speaker 1: and face of a new movie star. What would we
Speaker 1: mean by saying that you are born as many people
Speaker 1: but die as a single one. Welcome to the inner
Speaker 1: cosmos with me, David Eagleman. I'm a neuroscientist and author
Speaker 1: at Stanford, and in these episodes we sail deeply into
Speaker 1: our three pound universe to understand why and how our
Speaker 1: lives look the way they do. Today's episode is about
Speaker 1: brain plasticity, which is the ability of the brain to
Speaker 1: modify itself and how this changes throughout your lifetime. So
Speaker 1: we're going to address why it is harder to teach
Speaker 1: an old dog new tricks and ask whether that is
Speaker 1: always true. So let's start fifty years ago. There was
Speaker 1: a psychologist named Hans Lucas Touber at Mit and he
Speaker 1: got curious about what had happened to soldiers who had
Speaker 1: sustained head injuries in World War two. Now, this was
Speaker 1: in the nineteen seventies, so the war had been almost
Speaker 1: thirty years earlier. So he tracked down five hundred and
Speaker 1: twenty men who had sustained brain damage during the battles,
Speaker 1: and some had fared well in their recovery, but others
Speaker 1: didn't have such good outcomes, and Toyber wanted to understand
Speaker 1: what the difference was. So he scoured all the records
Speaker 1: and he looked for the things that correlated with good
Speaker 1: outcomes and bad outcomes. And you know what he found.
Speaker 1: The younger the soldier was when he got the head injury,
Speaker 1: the better he was now. And the older the soldier,
Speaker 1: the more permanent the damage. Why it's because younger brains
Speaker 1: are more flexible. There's more brain plasticity, which means the
Speaker 1: ability of the circuitry to reconfigure itself, and so if
Speaker 1: there's damage, a young brain can do what it needs
Speaker 1: to to rewrite its circuitry and get itself back on track.
Speaker 1: Now you know that I love analogies, So here's Today's
Speaker 1: brains are like a map of Europe where you look
Speaker 1: at the borders between the countries. So think of a
Speaker 1: young brain like Europe five thousand years ago, and if
Speaker 1: you can imagine different historical trajectories that could have happened,
Speaker 1: the borders could have evolved in many ways. There's nothing
Speaker 1: fundamental about where the borders between the countries sit today,
Speaker 1: But today, after millennia of human history, the maps are
Speaker 1: more settled into place. Now that humans have had centuries
Speaker 1: to clang swords and discharge rifles, you get these territory
Speaker 1: borders that are kind of stubborn to change. So think
Speaker 1: of the borders between France and Italy and Switzerland. These
Speaker 1: are totally arbitrary lines, but they're not likely to change
Speaker 1: now because you don't have roving bands of marauders anymore
Speaker 1: or bearded conquerors leading big horse armies. These things have
Speaker 1: been replaced with the United Nations and international rules of engagement,
Speaker 1: and economies have gotten increased seemly dependent on information and
Speaker 1: expertise rather than on treasures that you can go pillage.
Speaker 1: So even in the face of trade arguments and immigration debates,
Speaker 1: the boundaries between European countries are really hard to move.
Speaker 1: For the most part, the nations have settled into place,
Speaker 1: so the land mass began with lots of possibilities for
Speaker 1: where the borders sat essentially infinite possibility, but with time
Speaker 1: the potential has narrowed, the map solidified into place, and
Speaker 1: now it's not so easy to make big changes. The
Speaker 1: brain matures like Europe through years of border disputes within
Speaker 1: neural networks, the maps become increasingly solidified. So as a result,
Speaker 1: brain injury is really dangerous for the elderly, but it's
Speaker 1: less dangerous for the young because an older brain can't
Speaker 1: easily reassign settled territories for new tasks. But a brain
Speaker 1: that's at the dawn of its wars can still reimagine
Speaker 1: its maps. Okay, so think about the trajectory of a
Speaker 1: human life. So imagine a young baby born somewhere sometime.
Speaker 1: When she's first born. Her brain has unbelievable flexibility wherever
Speaker 1: and whenever she drops out of the womb. She will
Speaker 1: soak up the local language. She'll pick up on the
Speaker 1: subtle details of her culture and what to wear and
Speaker 1: how to act. She will absorb the local religious beliefs.
Speaker 1: She'll learn all of the rules around her. She'll learn
Speaker 1: how to gather massive amounts of information, and depending on
Speaker 1: her generation, that might be by unrolling a scroll or
Speaker 1: flipping through the pages of a book or swiping the
Speaker 1: screen of a small rectangle. But by the time she's grown,
Speaker 1: that story of flexibility has changed somewhat. Her brain isn't
Speaker 1: so flexible now. She belongs to a particular political party,
Speaker 1: and she's unlikely to change. She plays the piano reasonably well,
Speaker 1: but she doesn't have any particular interest in studying violin
Speaker 1: or other instruments. She likes to cook, and all of
Speaker 1: her dishes exploit combinations of the fourteen ingredients that she
Speaker 1: is used to. She spends her online time with a
Speaker 1: vanishingly small fraction of the billions of available web pages.
Speaker 1: She has a respectable golf game, but she doesn't have
Speaker 1: any curiosity about other sports. She lives in a city
Speaker 1: of eight million people, but she only has three close friends.
Speaker 1: She isn't particularly interested in the science that she didn't
Speaker 1: already learn in school. When she goes to the store,
Speaker 1: she passes racks of shirts until she finds the kind
Speaker 1: that she always wears, and she selects two of them
Speaker 1: in her standard colors. Her haircut is the same as
Speaker 1: it was since she was a teenager. Okay, so this
Speaker 1: sort of life trajectory underscores a general point, and we've
Speaker 1: all seen this before, which is that babies are born
Speaker 1: with not many built in skills, and they have a
Speaker 1: ton of plasticity. They can learn anything, while adults have
Speaker 1: mastered specific tasks, but at the expense of flexibility. So
Speaker 1: there's a trade off between adaptability and efficiency. So as
Speaker 1: your brain gets good at certain jobs, it becomes less
Speaker 1: able to tackle others. Now, just to be clear, this
Speaker 1: is not to say that adam dults aren't intelligent. In fact,
Speaker 1: it's just the opposite. An adult can do all kinds
Speaker 1: of things that a baby can't. An adult can run
Speaker 1: a company, or fix an air conditioner, or plan a
Speaker 1: business takeover precisely because the adult's brain understands things about
Speaker 1: the functioning of the world that a child's brain just
Speaker 1: can't understand. So the way we capture this concept is
Speaker 1: to say that the baby's brain has fluid intelligence, meaning
Speaker 1: they can learn anything, while adult brains have crystallized intelligence. So,
Speaker 1: for example, a few episodes ago, I mentioned a story
Speaker 1: about the violinist Yittsawk Pearlman, in which a fan told
Speaker 1: him that he would give his life to play like that,
Speaker 1: and Pearlman said, I did what Pearlman was pointing to
Speaker 1: is a fact of life. To get good at one
Speaker 1: thing is to close the door on other things. So
Speaker 1: Pearlman went from presumably being able to tackle any instrument
Speaker 1: to being superlative at one at the expense of everything else.
Speaker 1: It's unlikely that Pearlman could also be a professional baseball
Speaker 1: player in the same lifetime. Why because you have only
Speaker 1: one single life, and what you devote yourself to sends
Speaker 1: you down particular roads. But that means that all the
Speaker 1: other roads will forever remain untrodden by you. And this
Speaker 1: is what the philosopher Martin Heidegger was pointing to when
Speaker 1: he said, quote every man is born as many men
Speaker 1: and dies as a single one end quote. You're born
Speaker 1: with lots of possibility, but when you die, you are
Speaker 1: just the limited you. Now, from the point of view
Speaker 1: of your neural networks, what does it mean to descend
Speaker 1: into pattern and habit? So here's another analogy to help
Speaker 1: us picture this. Imagine two villages a few miles apart.
Speaker 1: So people interested in going from one settlement over to
Speaker 1: the other one, they take all possible paths. Some travelers
Speaker 1: walk the scenic route along the ridgetops, but others prefer
Speaker 1: the shade of the cliff side, and some people move
Speaker 1: among the slippery rocks by the river, and others take
Speaker 1: the riskier but faster route through the woods. Okay, with
Speaker 1: time and experience, one route ends up proving more popular.
Speaker 1: Maybe it makes better sense or it's faster. But eventually
Speaker 1: that path becomes grooved where the most people have walked,
Speaker 1: and it starts to become the standard. After some years
Speaker 1: go by, the local government lays down roadways, and after
Speaker 1: a few decades that expands into highways, and now the
Speaker 1: way to get from here to there is really nailed down.
Speaker 1: So you started with broad optionality, but eventually that gets
Speaker 1: reduced to the standard path. And this is what happens
Speaker 1: inside brains. They begin with almost infinite possible routes through
Speaker 1: the neural networks, but with time the practiced pathways become
Speaker 1: difficult to exit, the unused paths become thinned away. So
Speaker 1: through decades of experience, the brain comes to physically represent
Speaker 1: your world, and your decisions follow the remaining hard paved paths.
Speaker 1: I mean, just think about what it would be like
Speaker 1: if you were born with your genes and your brain
Speaker 1: in a different part of the world. Maybe a different
Speaker 1: generation a thousand years ago or maybe a thousand years
Speaker 1: from now. You would function and thrive in whatever environment
Speaker 1: you drop into, but you'd speak a totally different language,
Speaker 1: you'd have a different religion, you would believe different things
Speaker 1: about the world. But as it happens to have turned out,
Speaker 1: you were born in your hometown, and you grew up
Speaker 1: with your language and your parents, and so all of
Speaker 1: the us that could have been ended up getting thinned away.
Speaker 1: Now that might sound sad to lose the optionality, but
Speaker 1: the upside of a solidifying brain is that you end
Speaker 1: up with lightning fast ways of solving problems. Now, the
Speaker 1: downside is that it's harder to attack new problems with wild,
Speaker 1: unstructured inventiveness. Now, from the neuroscience point of view, there's
Speaker 1: also a second reason why older brains are less flexible,
Speaker 1: and this is beyond the diminishing optionality in the pathways issue.
Speaker 1: When older brains make changes, they do so only in
Speaker 1: small spots. In contrast, baby's brains modify across vast territories,
Speaker 1: and this is because of chemicals in the brain called
Speaker 1: neurotransmitters that are broadcast broadly in a baby's brain, So
Speaker 1: in an infant brain, these chemicals like acetylcholine. They transmit
Speaker 1: announcements throughout the brain, saying, Hey, something important just happen,
Speaker 1: And this allows pathways and connections to change and modify.
Speaker 1: So a baby's brain is changeable throughout its territory, and
Speaker 1: over years its understanding of the world comes into focus
Speaker 1: like a polaroid photograph. But an adult brain changes only
Speaker 1: little bits at a time. It keeps most of its
Speaker 1: connections locked into place to hold on to what has
Speaker 1: been learned, and only small areas are made flexible via
Speaker 1: a combination lock of the right neurotransmitters. So an adult
Speaker 1: brain is like a point to list artist who modifies
Speaker 1: the color of only a few dots in an almost
Speaker 1: finished painting. Now you might wonder what does it feel
Speaker 1: like to be inside the massively flexible brain of a baby.
Speaker 1: I mean, we were all there as infants, but we
Speaker 1: can't remember that. So what is it like to be
Speaker 1: so plastic, so uninhibited and learning about a wide range
Speaker 1: of novel events. Well, you can probably get close to
Speaker 1: understanding it by considering other situations in your life in
Speaker 1: which your awareness plasticity are firing on all cylinders. So
Speaker 1: when you are traveling in a new land. You drink
Speaker 1: in all the sights and sounds and smells of the
Speaker 1: foreign country. You are experiencing lots of novelty and more
Speaker 1: learning and more distributed attention. After all, at home, you
Speaker 1: don't pay much attention to much of anything going on.
Speaker 1: Why because it's predictable. You know what to expect there.
Speaker 1: But when you're the traveler, you overflow with attention and
Speaker 1: your brain is changing much more. So think about it
Speaker 1: like this. When you are highly engaged and paying attention,
Speaker 1: you are like a baby again. So the difference is
Speaker 1: between a baby's very fluid brain and an adult's crystallized brain.
Speaker 1: This is easy to into it, but the neural transition
Speaker 1: from one to the other does happened in a smooth line. Instead,
Speaker 1: it's like a door that swings closed and once it shuts,
Speaker 1: large scale change is over. And this is the concept
Speaker 1: of this sensitive period. So to understand the sensitive period,
Speaker 1: consider an infant named Matthew, who is from my hometown.
Speaker 1: He started having epileptic seizures as a very young boy,
Speaker 1: and by the time he was six, these seizures were
Speaker 1: happening with increasing frequency, so he could be having multiple
Speaker 1: seizures in an hour, and his parents tried everything they
Speaker 1: could do to figure out what is going on here,
Speaker 1: and they finally found out that he had something called
Speaker 1: Rasmussen's encephalitis, which is an inflammation that affects an entire
Speaker 1: half of his brain. And so they searched everywhere for
Speaker 1: a solution, and they came to find out that really
Speaker 1: the only solution is a radical neurosurgery called a hemispherectomy.
Speaker 1: And in this surgery, an entire half of the brain
Speaker 1: is removed, just taken out, and that empty half of
Speaker 1: the skull fills up with cerebra spinal fluid, and if
Speaker 1: you do brain imaging, what you see is just blackness
Speaker 1: in that half of the head. Now, this is a
Speaker 1: horrifying thing for a parent to put their child through.
Speaker 1: But the completely amazing thing is that kids who get
Speaker 1: the surgery generally turn out to be just fine. They
Speaker 1: sometimes have a slight limp on the other side of
Speaker 1: their body because the left side of the body is
Speaker 1: controlled by the right side of the brain and vice versa,
Speaker 1: but other than that, they don't have any particular signs
Speaker 1: that tell anyone that they only have half of their
Speaker 1: brain remaining. Now I'm going to talk more about hemispherectomy
Speaker 1: is in a future episode. But the thing I want
Speaker 1: to emphasize here is that this kind of surgery is
Speaker 1: recommended only if the patient is less than let's say
Speaker 1: eight years old. Matthew was six when he went under
Speaker 1: the knife, which is nearing old age for this surgery.
Speaker 1: If a child is older, let's say an adolescent, he
Speaker 1: will have to function in life by bending tasks to
Speaker 1: fit what his brain can do, rather than counting on
Speaker 1: his brain to adapt to the tasks. So the thing
Speaker 1: to note here is that there is a door that
Speaker 1: closes at about eight years old, where your brain before
Speaker 1: that is so flexible that even if it only has
Speaker 1: half the real estate available, it can readjust to take
Speaker 1: care of all the functions that it needs. Before eight,
Speaker 1: you're fine. After probably not so fine now. This kind
Speaker 1: of age limit. This is seen in so many aspects
Speaker 1: of brain function. For example, sometimes there are heartbreaking cases
Speaker 1: in which a child is so profoundly neglected through her
Speaker 1: childhood without conversation and affection that she will end up
Speaker 1: incapable of speech. And if that child is found after
Speaker 1: a certain age, let's say about seven, she will be
Speaker 1: incapable of ever learning speech. Even when a team of
Speaker 1: psychologists come in and work with her for years to
Speaker 1: try to teach her language, it's too late. I told
Speaker 1: the story of one such girl, Danielle, in my book
Speaker 1: Live Wired, and I'll return to her story in a
Speaker 1: future episode. But the thing I want to emphasize for
Speaker 1: today is that the door for learning language closes. And
Speaker 1: I'm not talking about learning a second language or dealing
Speaker 1: with an accent or something like that. I'm talking about
Speaker 1: the idea of language. What language is, as in, how
Speaker 1: do I put words together to label things in the
Speaker 1: outside world and communicate with someone else this way, with
Speaker 1: a particular grammar and sentence structure and so on. If
Speaker 1: a child does not get language in the formative years,
Speaker 1: it becomes too late. Her maps have largely stabilized into
Speaker 1: place and they can't be changed anymore. So children like
Speaker 1: Matthew who got a hemisphectomy, or a child like Danielle
Speaker 1: who didn't get language in time, they tell the same story,
Speaker 1: which is that brains are really flexible at the beginning
Speaker 1: in this window of time known as the sensitive period,
Speaker 1: and as this period passes, the neural geography becomes more
Speaker 1: difficult to change. So as seen with children like Danielle,
Speaker 1: a young child's brain needs to hear lots of language
Speaker 1: during the sensitive period, and without that input, the neurons
Speaker 1: don't arrange themselves to capture the fundamental concepts of language.
Speaker 1: And by the way, it's a side note, you might
Speaker 1: wonder what happens with a deaf baby who doesn't hear
Speaker 1: any auditory input, and the answer is, as long as
Speaker 1: the parents present sign language to the baby, her brain
Speaker 1: will wire up correctly for communication. The deaf baby will
Speaker 1: employ her hands to babble, making resemblances to sign language,
Speaker 1: and the same way that a hearing baby exposed to
Speaker 1: language will babble with her vocal cords. If there is
Speaker 1: input to pick up on, the baby will do so.
Speaker 1: As long as that input arrives within the sensitive period.
Speaker 1: After that door swing shut, it's too late to learn
Speaker 1: the fundamentals of communication. So there's a window for acquiring
Speaker 1: the ability to communicate, and there are also windows for
Speaker 1: more subtle aspects of language, like accents. So take the
Speaker 1: actress Mila Kunis. She speaks American English with no discernible accent,
Speaker 1: so you probably didn't know that she was born in
Speaker 1: Ukraine and lived there, not speaking a word of English
Speaker 1: until the age of seven. Now, in contrast, Arnold Schwarzenegger,
Speaker 1: who's been in American filmmaking since his early twenties, he
Speaker 1: has a very strong Austrian accent. Why because he didn't
Speaker 1: move to America until he was twenty one, and that
Speaker 1: meant his use of English began too late. Brain wise, generally,
Speaker 1: if you arrive in a new country during your first
Speaker 1: seven years, your fluency in the new tongue will be
Speaker 1: as high as a native speakers, because your window of
Speaker 1: sensitivity for obtaining the sounds that's still open. If you
Speaker 1: immigrate when you're eight to ten years old, you have
Speaker 1: a slightly more difficult time blending in, but you'll be close.
Speaker 1: If you're past your teen years. When you move like
Speaker 1: Arnold was, your fluency is likely to remain low, and
Speaker 1: you're going to have an accent that reveals your history.
Speaker 1: So your ability to sonically morph into a different culture
Speaker 1: is a door that remains open for only about a decade.
Speaker 1: And let's take another example. Take something like vision. So
Speaker 1: imagine a child is born with misaligned eyes where one
Speaker 1: eye is pointing straight but the other eye points inwarder outward.
Speaker 1: This is known as strabismus, and colloquially it's often called
Speaker 1: being cross eyed or walleyed. What you do clinically is
Speaker 1: you fix the extra ocular muscles so the eyes can
Speaker 1: point the same direction, and then you cover the good
Speaker 1: eye for a while, which allows the weak eye to
Speaker 1: fight to regain its lost territory. But no out that
Speaker 1: the good eye has to be patched. You have to
Speaker 1: do this technique within this sensitive period about the first
Speaker 1: six years, otherwise it's too late. The vision will never
Speaker 1: be recoverable after that from the weakey. After six years,
Speaker 1: the dirt roads in the brain have been paved into
Speaker 1: highways and you can't now modify them. So this influence
Speaker 1: of developmental timing. You see this across all the senses.
Speaker 1: I talked in earlier episodes about how the body maps
Speaker 1: readjust if you have an amputation or when you learn
Speaker 1: a new musical instrument, but across the board, these kinds
Speaker 1: of changes happen more in young brains than in old brains,
Speaker 1: just like Mila Kunis with her unaccented speech. So we
Speaker 1: find that Jitzak Peerlman took up the violin at a
Speaker 1: very young age. If you were to take up the
Speaker 1: violin for the first time in your teenage years, there's
Speaker 1: no possibility that you would ever become a pearlman. Even
Speaker 1: if you worked really hard to rack up the same
Speaker 1: number of hours of practice. Your brain is already behind
Speaker 1: in the race. It has grown too solidified by the
Speaker 1: time you start doing your first piscata as a teenager.
Speaker 1: So acquiring vision and language and violin proficiency, this all
Speaker 1: depends on input from the world, and if a severely
Speaker 1: neglected child like Danielle doesn't receive this input, she can't
Speaker 1: later the ability to learn language, to possess vision, to
Speaker 1: interact socially, to walk normally, to have normal neurodevelopment. This
Speaker 1: is all limited to the years of young childhood, and
Speaker 1: after a certain point these abilities are lost. The brain
Speaker 1: needs to experience the proper input within the right window
Speaker 1: to achieve its most useful connectivity. Now, as a result
Speaker 1: of this diminishing flexibility, we are highly influenced by the
Speaker 1: events that happen in our childhoods. So here's a really
Speaker 1: interesting example. Consider the correlation between how tall a man
Speaker 1: is and how much salary he will command. In America,
Speaker 1: each additional inch of height translates into a one point
Speaker 1: eight percent increase in take home pay. Why is that well,
Speaker 1: the popular assumption is that this stems from discrimination in
Speaker 1: hiring practices. Everyone wants to hire the tall guy because
Speaker 1: of his commanding presence. But it turns out there is
Speaker 1: a deeper reason. The best indicator of a male's future
Speaker 1: salary is how tall he was at the age of sixteen. However,
Speaker 1: tall he grows after that doesn't change the outcome. Now,
Speaker 1: how do we understand that? Could it be some effect
Speaker 1: of nutritional differences between people? Know because when the researchers
Speaker 1: correlated with height at ages seven or eleven, the effect
Speaker 1: wasn't as strong. Instead, it's that the teenage years are
Speaker 1: a time when social status is being worked out, and
Speaker 1: as a result, who you are as an adult strongly
Speaker 1: depends on who you were then. In fact, studies that
Speaker 1: track thousands of children into adulthood find that socially oriented
Speaker 1: careers like sales or managing other people show the strongest
Speaker 1: effect of teenage height, and other careers like blue collar
Speaker 1: work or artistic trades are less influenced. So how people
Speaker 1: treat you during your formative years has an enormous impact.
Speaker 1: On your comportment in the world in terms of self
Speaker 1: esteem and confidence and leadership. Here's another example. Think about
Speaker 1: Oprah Winfrey, who is worth the bid two point eight
Speaker 1: billion dollars. So I was a little surprised when I
Speaker 1: read that she has a deep rooted fear of ending
Speaker 1: up homeless and penniless. But it's because of the path
Speaker 1: that got her here. Before she was an empress of
Speaker 1: the media, she was an impoverished child in Mississippi. She
Speaker 1: was born to a teenage single mother. So who she
Speaker 1: was then influenced who she is now. The Great Aristotle
Speaker 1: noted this twenty four hundred years ago. He said, quote,
Speaker 1: the habits we form from childhood make no small difference,
Speaker 1: but rather they make all the difference. Okay, So to
Speaker 1: capture the idea of the sensitive period, I introduced the
Speaker 1: metaphor of a door swinging shut. But now we're ready
Speaker 1: to take the analogy to the next level. It's not
Speaker 1: one door, it's a bunch of different doors which swing
Speaker 1: shut at different times. So let's look at an example
Speaker 1: of that. Sometimes the brain is so impressionable in its
Speaker 1: earliest days that it can sometimes get into hot water.
Speaker 1: For example, the baby goose hatches from its egg, and
Speaker 1: it establishes a parental relationship with the first animate object
Speaker 1: that it sees. And this is a sufficient strategy in
Speaker 1: most cases because that first sight is usually its mother.
Speaker 1: But it can get fooled in the wrong circumstances, and
Speaker 1: this was shown in the nineteen thirties by the zoologist
Speaker 1: Conrad Lorenz, who didn't have to work hard for the
Speaker 1: geese to imprint on him. Instead, he just needed to
Speaker 1: show up in the right window of plasticity right after
Speaker 1: they hatched, and then they would imprint on him and
Speaker 1: follow him around. So that's an example of a fast
Speaker 1: swinging door for geese to imprint on their parent. But
Speaker 1: the geese can still learn other things later in life,
Speaker 1: such as where the is or where to best seek food,
Speaker 1: or the identities of other geese that they meet in adulthood.
Speaker 1: So sensitive periods are different for different tasks of the brain,
Speaker 1: and not all brain regions are equally plastic in terms
Speaker 1: of how flexibly they begin and how long they retain
Speaker 1: their adaptability. So is there a pattern to which areas
Speaker 1: solidify first. So some years ago some colleagues of mine
Speaker 1: did some research, so they looked a little bit of
Speaker 1: damage to the retina at the back of the eye
Speaker 1: in an adult and looked at how that cause changes
Speaker 1: in the back of the brain and the visual cortex.
Speaker 1: And they assumed that because the visual cortex was not
Speaker 1: getting any information from that little patch of eye, that
Speaker 1: it would readjust you'd see plasticity. And to their surprise,
Speaker 1: they found no measurable changes in the visual cortex. The
Speaker 1: part of the cortex that was inactive because it wasn't
Speaker 1: getting any data stayed inactive. It didn't get taken over
Speaker 1: by the surrounding areas. Now, given the history of brain
Speaker 1: plasticity studies even in adults, that was a little bit unexpected.
Speaker 1: After all, you still have a lot of flexibility in
Speaker 1: the parts of your brain that drive the body or
Speaker 1: feel from the body, and this is what allows you
Speaker 1: to learn how to hang gliders, snowboard even in your
Speaker 1: later years. So what was the difference between the studies
Speaker 1: involving your visual system versus the systems that drive your
Speaker 1: body or feel from your body. Why are the patterns
Speaker 1: in the primary visual cortex locked into place after a
Speaker 1: short window of a few years, while the parts of
Speaker 1: your body involved in moving your sensing continue to learn
Speaker 1: The answer is that different areas of the brain operate
Speaker 1: on different schedules of plasticity. Some neural networks are unyielding
Speaker 1: and others are highly pliable. Some sensitive periods are really
Speaker 1: brief and others are long. Okay, So is there a
Speaker 1: general principle at work behind this diversity. One possibility is
Speaker 1: that the different sensitive periods are caused by different underlying
Speaker 1: learning strategies in different parts of the brain. So, in
Speaker 1: this view, some regions are geared to learn throughout life
Speaker 1: because they're meant to encode changeable details of the world.
Speaker 1: So think of vocabulary words, or the ability to learn
Speaker 1: new map directions, or the visual recognition of people's faces.
Speaker 1: These are tasks for which you want to retain flexibility.
Speaker 1: But in contrast, other brain areas are involved in really
Speaker 1: stable relationships, like the building blocks of vision, or how
Speaker 1: to chew food food or the general rules of grammar,
Speaker 1: and these areas require a faster lockdown. But how could
Speaker 1: the brain know and advance the order in which to
Speaker 1: solidify things? Is that genetically encoded? Possibly some aspects are,
Speaker 1: but I've previously published a new hypothesis about this, which
Speaker 1: is that the degree of plasticity in a brain region
Speaker 1: reflects how much the data change or are likely to
Speaker 1: change in the outside world. So let me explain this.
Speaker 1: If the incoming data aren't changing, the system hardens around that.
Speaker 1: But if the data are constantly changing, then the system
Speaker 1: remains flexible for that area. So as a result, stable
Speaker 1: data solidify. First, let me give an example. Take information
Speaker 1: from the ears versus information from the body. So areas
Speaker 1: encoding the base six sounds of the world. Like the
Speaker 1: primary auditory cortex, these become resistant to change. They stiffen rapidly.
Speaker 1: And that's why, as I spoke about in a previous
Speaker 1: episode thirty five, a baby born in America and a
Speaker 1: baby born in Japan will learn how to hear different
Speaker 1: possible sounds, and by nine months of age, their brain
Speaker 1: is locking down on those sounds in their language. But
Speaker 1: in contrast, the parts of your brain involved in navigating
Speaker 1: your body and feeling from your body, these remain more
Speaker 1: plastics throughout your life. Why because body plans change throughout
Speaker 1: your life. You get heavier or skinnier, you put on
Speaker 1: boots or slippers, or you're on crutches, or you jump
Speaker 1: on a bicycle or a scooter or a trampoline, and
Speaker 1: that's why you can pick up something new as an adult,
Speaker 1: like windsurfing. The statistics of the language you're surrounding with
Speaker 1: that doesn't change much, but your body's feedback from the
Speaker 1: world does change, and as a result, the auditory cortex
Speaker 1: tightens down, but less so for your body plan. So
Speaker 1: now let's zoom into a single sense like vision. This
Speaker 1: is really cool because in low level visual areas, what's
Speaker 1: called the primary visual cortex, the neurons and code basic
Speaker 1: properties of the world like edges and colors and angles.
Speaker 1: But you have these higher areas of visual cortex that
Speaker 1: are involved in more particular items like the layout of
Speaker 1: your street, or the sleek look of this year's sports car,
Speaker 1: or the arrangement of apps on your cell phone. Now,
Speaker 1: the information in the low level areas that becomes established first,
Speaker 1: and the successive layers wire up on top of those foundations,
Speaker 1: so the possible angles of a line, these are fixed
Speaker 1: in place. But you can still learn the face of
Speaker 1: the latest movie star. So there's a hierarchy of flexibility
Speaker 1: where the representations at the bottom are learned first. These
Speaker 1: reflect the basic statistics of the visual world, and those
Speaker 1: are unlikely to change, these remain stable so that the
Speaker 1: higher order patterns, which can change more rapidly, they can
Speaker 1: be learned. Okay, so let's do an analogy. If you
Speaker 1: are building a library, you want to nail down the
Speaker 1: basics first. You establish the positions of the shelves, and
Speaker 1: you set up the Dewey decimal system for organization and
Speaker 1: maybe the workflow for checking out the books. Once that's
Speaker 1: all nailed down, then it's straightforward to maintain a flexible
Speaker 1: inventory of books. You expand the offerings and exciting categories,
Speaker 1: you reduce outdated volumes, and you constantly test out new titles.
Speaker 1: This is the same thing in the brain. The primary
Speaker 1: visual cortex gets all nailed down and set up first,
Speaker 1: and higher order areas of the brain can try out
Speaker 1: new things and remain flexible. So there's no single answer
Speaker 1: to whether the brain is plastic as we get older.
Speaker 1: It depends on what brain area we're talking about. Plasticity
Speaker 1: declines with age, but across the brain it declines differently,
Speaker 1: steeply or shallowly, depending on its function. Now, interestingly, this
Speaker 1: hypothesis that the amount of plasticity reflects the variance in
Speaker 1: the outside world. This has an analogy I think in
Speaker 1: genetics in ways that science is still working to understand,
Speaker 1: genomes seem to lock in some parts of their nucleotide
Speaker 1: sequences the actgs. They lock in some parts more than others,
Speaker 1: and they protect them against mutation, and conversely, other regions
Speaker 1: of the chromes are much more variable. So, roughly speaking,
Speaker 1: the variability of a genetic sequence mirrors the variability of
Speaker 1: features in the outside world. For example, skin pigment genes
Speaker 1: are highly variable because humans find themselves at different latitudes
Speaker 1: and need to change the pigmentation to absorb enough vitamin D.
Speaker 1: But in contrast, the genes that code for proteins that
Speaker 1: break down sugar, these are really stable because that is
Speaker 1: a critical and unchanging energy source. So by analogy in
Speaker 1: the brain, I hypothesize that in the future we may
Speaker 1: be able to quantify the variability of mental and social
Speaker 1: and behavioral functions in human life, and we can put
Speaker 1: to the test this hypothesis that the most flexible circuits
Speaker 1: of the brain mirror the most variable parts of our environment. Okay,
Speaker 1: so where does all this talk about brain plasticity put us. Well,
Speaker 1: often what we find is that adults envy children. Why
Speaker 1: because children have the ability to absorb languages at an
Speaker 1: extraordinary rate, and they can think of magically bizarre approaches
Speaker 1: to any problem, and they can celebrate the novelty of
Speaker 1: every experience. But older brains have more closed doors, which
Speaker 1: is why Toyber's World War Two veterans fared worse if
Speaker 1: they were older, and why Arnold Schwarzenegger retains his accent.
Speaker 1: And by analogy, the older a city is is the
Speaker 1: more its infrastructure becomes resistant to shift. So look at
Speaker 1: something like Rome. The city of Rome can't untangle its
Speaker 1: windy roads to resemble the grid work of Manhattan because
Speaker 1: too much history has glued its snaking roots into place.
Speaker 1: Just like developing humans, cities deepen their tracks along early roads,
Speaker 1: and so adults often wish they could have the plasticity
Speaker 1: that they used to. In nineteen eighty four, at the
Speaker 1: age of thirty five, the physicist my friend Alan Lightman
Speaker 1: wrote a short essay in The New York Times titled
Speaker 1: Elapsed Expectations, in which he lamented the perceived stiffening of
Speaker 1: his mind. Here's what he said. Quote. The limber years
Speaker 1: for scientists, as for athletes, generally come at a young age.
Speaker 1: Isaac Newton was in his early twenties when he discovered
Speaker 1: the law of gravity, Albert Einstein was twenty six when
Speaker 1: he formulated special relativity, and James Clerk Maxwell had polished
Speaker 1: off electromagnetic theory and retired to the country. By thirty five.
Speaker 1: Lightman goes on to say, quote, when I hit thirty
Speaker 1: five myself some months ago, I went through the unpleasant
Speaker 1: but irresistible exercise of summing up my career in physics.
Speaker 1: By this age or another few years, the most creative
Speaker 1: achievements are finished and visible. You've either got the stuff
Speaker 1: and used it, or you haven't. End quote. So Lightman
Speaker 1: was concerned that his brain plasticity was stiffening into place,
Speaker 1: and these same sentiments were echoed by the physicist James
Speaker 1: Gates in a television interview I saw. He said, quote,
Speaker 1: there's a saying that old physicists accept new ideas when
Speaker 1: they die. It's the next general that brings new ideas
Speaker 1: to their full fruition. When you get to be an
Speaker 1: old physicist like me, you know a lot of stuff,
Speaker 1: and it acts like a ballast on a ship. It
Speaker 1: pulls you down. You have all the weight of these
Speaker 1: other things that you know, and sometimes an idea like
Speaker 1: a small ferry or a sprite passes by and you say, ah,
Speaker 1: I don't know what that is, but it can't be
Speaker 1: very important. Well, sometimes it is end quote. So this
Speaker 1: kind of lamentation is typical of people as they age.
Speaker 1: But happily, although brain plasticity diminishes over the years, it
Speaker 1: is still present. Live wiring is not solely the privilege
Speaker 1: of the young. Neural reconfiguration is an ongoing process that
Speaker 1: lasts throughout our lives. We form new ideas, we accumulate
Speaker 1: fresh information, we remember people and events that we're seeing now.
Speaker 1: So going back to this analogy, despite having decreased flexibility,
Speaker 1: the city of Rome still evolves. Rome now isn't what
Speaker 1: it was twenty years ago. Today its statues are ringed
Speaker 1: with cell phone towers and internet cafes. Although the rudiments
Speaker 1: of the city are difficult to change, Rome nonetheless advances
Speaker 1: all its finer points according to new circumstances, just like
Speaker 1: the library changes its stock of books while its architecture
Speaker 1: remains largely set. And you see this in so many
Speaker 1: neuroscience studies. For example, when adults learn a new task
Speaker 1: like juggling, you can see major changes in their brains.
Speaker 1: If they take up a new musical instrument, you see
Speaker 1: these major changes. If they become a London taxi driver
Speaker 1: and memorize enormous maps of London, you can see these changes.
Speaker 1: And all of these involve adult plasticity. One really stunning
Speaker 1: example emerged recently from this nun study called the Religious
Speaker 1: Order Study, which is a multi decade investigation of hundreds
Speaker 1: of Catholic nuns living in convents. So all these sisters
Speaker 1: agreed to regularly test their cognitive function and share their
Speaker 1: medical records, and when they die, they donate their brains.
Speaker 1: So amazingly, many of these nuns never displayed any cognitive decline.
Speaker 1: They were sharp as a whip, but yet their brains
Speaker 1: at autopsy were riddled with Alzheimer's disease. In other words,
Speaker 1: their neural networks were physically degenerating, but their performance was not.
Speaker 1: Now what could explain that, well, the key is that
Speaker 1: the nuns and their convents had to consistently use their
Speaker 1: wits until their final days. They had responsibilities and chores
Speaker 1: and social lives and arguments and game nights and group
Speaker 1: discussions and so on. So unlike typical retirees, they didn't
Speaker 1: PLoP onto a couch in front of a television set.
Speaker 1: Because they had active mental lives, their brains were forced
Speaker 1: to constantly build new bridges, even as some of their
Speaker 1: neural roadways were physically falling apart. What stunning is that
Speaker 1: a third of the nuns seemed to have had the
Speaker 1: molecular pathology of Alzheimer's without the expected cognitive symptoms. An
Speaker 1: active mental life, even in the very elderly. This makes
Speaker 1: new connections in the brain. So learning can happen in
Speaker 1: any age, and the question is why is it slower
Speaker 1: than as the brain matures. Well, one reason, as we've discussed,
Speaker 1: is that many of the swinging doors have closed. But
Speaker 1: there's another way to look at this. Remember that brain
Speaker 1: changes are driven by the difference between your internal model
Speaker 1: and what actually happens in the world, So brains make
Speaker 1: change only when something is unexpected. As you learn all
Speaker 1: this and figure this out, your brain becomes less challenged
Speaker 1: through time, it becomes more settled into place. For example,
Speaker 1: when you're a child, your internal model tells you that
Speaker 1: all people believe everything that you believe, and as world
Speaker 1: experience teaches you the difference between your predictions and your experience,
Speaker 1: your networks are constantly having to adjust to address that
Speaker 1: growing gap. Or consider what happens when you start a
Speaker 1: new job. At first, everything is new, from your coworkers
Speaker 1: to your responsibilities, to your approaches. You have all this
Speaker 1: brain plasticity during the first days and weeks as you
Speaker 1: incorporate your new gig into your internal model. But after
Speaker 1: a while you become proficient at your job, so skill
Speaker 1: replaces flexibility. And by the way, as an analogy, we
Speaker 1: can see this pattern in the way that nations settle
Speaker 1: into place. Consider the amendments to the constitution of any country.
Speaker 1: Almost all the change happens near the beginning, while the
Speaker 1: nation is learning the strategies of running itself, and later
Speaker 1: constitutions congeal into place and amendments slow down. So take
Speaker 1: the US Constitution. Twelve of the amendments took place in
Speaker 1: the first thirteen years, and after that there were a
Speaker 1: maximum of four changes in any twenty year period, and
Speaker 1: most periods had no changes at all. And the latest change,
Speaker 1: ratifying the t twenty seventh Amendment, that was in nineteen
Speaker 1: ninety two, and the Constitution has been at a standstill
Speaker 1: since then. In this way, nations steadily diminish their adaptation
Speaker 1: to the world, because what they do is they profusely
Speaker 1: modify at the beginning, and with time they settle on
Speaker 1: a working model that offers what the country needs to
Speaker 1: be operational. And in this same way, the brain's solidification
Speaker 1: reflects its success in understanding the world. Neural networks lock
Speaker 1: themselves more deeply into place, not because of fading function,
Speaker 1: but because they've had success in figuring things out. So
Speaker 1: would you really want the plasticity of a child again?
Speaker 1: Although having a sponge like brain that absorbs everything that
Speaker 1: sounds appealing, the game of life is largely about figuring
Speaker 1: out the rules. What we lose in modifiability, we gain
Speaker 1: in expertise our hard won neural networks. They might not
Speaker 1: be correct about everything, or even internally consistent, but they
Speaker 1: add up to life experience to know how to an
Speaker 1: approach to the world. A child simply doesn't have the
Speaker 1: capacity to run a company, or write about deep ideas
Speaker 1: or lead a nation. If plasticity didn't decline, you couldn't
Speaker 1: lock down the conventions of the world. You would never
Speaker 1: learn the streets of your neighborhood or people's names, or
Speaker 1: how to do a job, or how to navigate a
Speaker 1: social life. You wouldn't be able to hold a meaningful conversation,
Speaker 1: or ride a bike or obtain food for yourself. If
Speaker 1: you had total flexibility, you would have the helplessness of
Speaker 1: an in And don't forget that locking things down this
Speaker 1: isn't just about skills that you learn. Locking things down
Speaker 1: is what allows you to retain memories. Every single thing
Speaker 1: you remember in your life, every bit of your story,
Speaker 1: is stored in the exact patterns of your neural networks.
Speaker 1: So just imagine that you had the opportunity to swallow
Speaker 1: a capsule that would renew the brain plasticity even infant.
Speaker 1: This would give you the capacity to reprogram your neural networks,
Speaker 1: to learn new languages rapidly and adopt new accents and
Speaker 1: new views of physics. But you'd forget everything that came before.
Speaker 1: Your memories of your childhood would be erased and overwritten.
Speaker 1: Memories of your first lover, your first trip to Disneyland,
Speaker 1: your interaction with your parents all would fade like a
Speaker 1: dream after waking. Would it be worth it to you?
Speaker 1: You know? While I was working on my book LiveWire,
Speaker 1: I was struck by thinking about a horror scenario in
Speaker 1: the future of warfare, because in warfare, countries want to
Speaker 1: injure their enemies, not kill them, because it requires a
Speaker 1: lot more resource from the other side. They have to
Speaker 1: attend to the wounded. So imagine a biological weapon that
Speaker 1: implements broad brain plasticity. Again, no one is physically hurt,
Speaker 1: but the troops are propelled back to the state of infants.
Speaker 1: They forget their ability to walk into talk all if
Speaker 1: their memories are wiped. When they're returned home by their commanders,
Speaker 1: they have no remembrance of their families or friends, or
Speaker 1: spouses or children. Technically, they're still fine, they can still learn. Again,
Speaker 1: nothing is damaged, only their mental lives. The part we
Speaker 1: can't see. These have been factory reset back to their
Speaker 1: original state. Now, this scene is so horrific because fundamentally,
Speaker 1: who you are is the sum total of your neural circuits.
Speaker 1: Who you are is stored in the exact configuration of
Speaker 1: the forest of eighty six billion neurons. So if you said, hey,
Speaker 1: I want the plasticity of a child again, that comes
Speaker 1: at the cost of who you are now and what
Speaker 1: you know. So what we've seen today is that the
Speaker 1: flexibility of a brain declines with age, and we saw
Speaker 1: several examples where this can be sudden, like the closing
Speaker 1: of a door, such that if you don't do something
Speaker 1: before you're six or eight or thirteen, your brain simply
Speaker 1: can't learn it later. And we also saw that there's
Speaker 1: not just one door that closes in the brain, but
Speaker 1: instead different doors close at different rates. So, for example,
Speaker 1: you have to be exposed to language in your formative
Speaker 1: early years to understand the concept of language. But the
Speaker 1: question of whether you retain an accent after moving somewhere,
Speaker 1: that's something that comes many years later. And of course,
Speaker 1: you can learn a foreign language at any age, even
Speaker 1: though it becomes more difficult. And I just want to
Speaker 1: remind us about relevance here. In episode thirty five, I
Speaker 1: talked about what sticks in your brain and what does
Speaker 1: not stick in your brain, And the bottom line is
Speaker 1: that your brain has to care about something in order
Speaker 1: to learn it. You have to have the right cocktail
Speaker 1: of neurotransmitters present, and that correlates with curiosity and relevance
Speaker 1: to you. So you can definitely learn a language at
Speaker 1: any age. The issue is simply do you have enough motivation.
Speaker 1: If so, you can learn it. The problem is that
Speaker 1: as people get older, there's typically less motivation to get
Speaker 1: really good at something like a foreign language, because you
Speaker 1: can get by with the language that you have, or
Speaker 1: you figure out the minimum of that language that you
Speaker 1: need to learn, so you can get a few things
Speaker 1: and you don't really care as much about impressing people
Speaker 1: with your fluency. But the mechanisms are still available and
Speaker 1: you can learn it if it's important to you. And
Speaker 1: I think one of the tricks in life is figuring
Speaker 1: out how to fool yourself using your psychology to influence
Speaker 1: your biology by reminding yourself, hey, here's why this would
Speaker 1: be important to me, and it can be anything. It
Speaker 1: can be this is going to impress the person I
Speaker 1: want to go on a date with, or this will
Speaker 1: make me proud of myself, or this will get me
Speaker 1: that promotion that I really want, or this will get
Speaker 1: me into medical school, or this will make my parents prouder.
Speaker 1: Whatever it is that drives you, it's really useful to
Speaker 1: clarify that to yourself, to explicitly specify to yourself why
Speaker 1: this task matters, and then you'll have that at write
Speaker 1: cocktail of neurotransmitters to make that information stick because in
Speaker 1: the end we get to be the sculptors of our
Speaker 1: own brains. Go to eagleman dot com slash podcast for
Speaker 1: more information and to find further reading. Send me an
Speaker 1: email at podcasts at eagleman dot com with questions or discussions,
Speaker 1: and I'm making sporadic episodes in which I address those
Speaker 1: until next time. I'm David Eagleman, and this is Inner Cosmos.