December 11, 2023 • 55 mins
Is it always harder to teach an old dog new tricks? Why is an older person slower to learn a new language but able to learn new faces easily? Why does Arnold Schwartzenegger have an accent but Mila Kunis doesn’t, even though neither spoke English as a child? Why is there a correlation between how tall a person is and how much salary they're likely to earn? What would it mean to say that you are born as many people but die as a single one? This week's episode dives into surprises about brain plasticity and why your flexibility changes throughout your lifetime.
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Episode Transcript
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Speaker 1 (00:05):
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
(00:27):
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
(00:51):
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
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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
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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
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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,
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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.
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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,
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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
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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
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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,
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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
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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
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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
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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
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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
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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,
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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,
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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.
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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
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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
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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
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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,
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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,
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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,
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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
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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
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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
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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
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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,
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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,
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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
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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
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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
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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.
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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
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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,
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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.
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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
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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
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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
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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.
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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
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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.
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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
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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
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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
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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
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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
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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,
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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
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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
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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
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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
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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
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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
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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.
(38:24):
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
(38:48):
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,
(39:18):
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
(39:41):
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
(40:04):
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,
(40:27):
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
(40:48):
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
(41:10):
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
(41:31):
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,
(41:53):
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
(42:15):
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.
(42:37):
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
(42:59):
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
(43:24):
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
(43:45):
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
(44:08):
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.
(44:31):
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.
(44:52):
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
(45:15):
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
(45:36):
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
(45:57):
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
(46:18):
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
(46:39):
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
(47:00):
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
(47:22):
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
(47:43):
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
(48:06):
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
(48:31):
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?
(48:52):
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
(49:12):
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
(49:36):
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
(49:59):
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.
(50:22):
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.
(50:48):
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
(51:11):
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,
(51:32):
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,
(51:53):
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.
(52:14):
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
(52:37):
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,
(52:59):
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
(53:20):
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
(53:42):
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
(54:03):
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
(54:26):
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.
(54:47):
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
(55:10):
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
(55:30):
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
(00:27):
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
(00:51):
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
(01:12):
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
(01:36):
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
(01:56):
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,
(02:18):
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.
(02:42):
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,
(03:04):
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
(03:28):
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
(03:51):
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,
(04:14):
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
(04:39):
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
(05:02):
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
(05:23):
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
(05:45):
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
(06:07):
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,
(06:28):
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.
(06:49):
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
(07:11):
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
(07:33):
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
(07:54):
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
(08:17):
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
(08:41):
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
(09:04):
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
(09:26):
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
(09:47):
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.
(10:09):
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
(10:30):
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,
(10:50):
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
(11:14):
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
(11:39):
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
(12:00):
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
(12:22):
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
(12:44):
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.
(13:08):
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
(13:30):
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
(13:50):
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
(14:12):
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.
(14:36):
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
(14:57):
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
(15:17):
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
(15:40):
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,
(16:03):
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,
(16:27):
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
(16:50):
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
(17:12):
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
(17:34):
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
(18:08):
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
(18:28):
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
(18:49):
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
(19:11):
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
(19:33):
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
(19:54):
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
(20:16):
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
(20:36):
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,
(20:58):
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
(21:19):
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
(21:40):
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
(22:01):
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
(22:22):
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,
(22:44):
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.
(23:06):
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.
(23:27):
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
(23:49):
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
(24:11):
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
(24:33):
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
(24:54):
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
(25:16):
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
(25:39):
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
(26:02):
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
(26:25):
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
(26:46):
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
(27:08):
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
(27:31):
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
(27:53):
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
(28:16):
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,
(28:39):
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
(29:00):
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
(29:21):
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
(29:44):
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.
(30:07):
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
(30:30):
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,
(30:53):
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.
(31:15):
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
(31:38):
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
(32:04):
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
(32:27):
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.
(32:50):
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
(33:12):
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.
(33:34):
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
(33:54):
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
(34:15):
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
(34:35):
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
(34:59):
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
(35:21):
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,
(35:42):
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
(36:03):
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
(36:25):
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
(36:48):
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
(37:12):
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
(37:36):
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
(37:59):
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.
(38:24):
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
(38:48):
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,
(39:18):
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
(39:41):
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
(40:04):
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,
(40:27):
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
(40:48):
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
(41:10):
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
(41:31):
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,
(41:53):
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
(42:15):
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.
(42:37):
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
(42:59):
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
(43:24):
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
(43:45):
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
(44:08):
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.
(44:31):
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.
(44:52):
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
(45:15):
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
(45:36):
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
(45:57):
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
(46:18):
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
(46:39):
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
(47:00):
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
(47:22):
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
(47:43):
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
(48:06):
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
(48:31):
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?
(48:52):
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
(49:12):
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
(49:36):
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
(49:59):
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.
(50:22):
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.
(50:48):
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
(51:11):
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,
(51:32):
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,
(51:53):
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.
(52:14):
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
(52:37):
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,
(52:59):
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
(53:20):
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
(53:42):
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
(54:03):
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
(54:26):
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.
(54:47):
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
(55:10):
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
(55:30):
until next time. I'm David Eagleman, and this is Inner Cosmos.
Host
David Eagleman
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