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Inside every head in every home

is the most complex object

we've discovered
in the universe,

the human brain.

I've spent many years of
my life trying to decipher

the mysteries of the brain,

and yet I'm still in awe
every time I hold one.

And that's because although
this marvel of biology

seems so alien to us,
somehow, it is us.

Until recently, activity coursed

through these cells.

This was Barbara.

She had opinions and passions,

she loved, she had her own life,

and this is where
all of that happened,

just as it does
for each one of us.

This 3-pound organ is made up
of hundreds of billions of cells

with a quadrillion connections
between them.

These cells fire trillions of
electrochemical signals

every second of your life.

Somehow, all this wet
biological stuff results

in the experience of being you.

I'm going to explore
a fundamental question

about our lives.

What shapes who you become?

This is the story of how
your life shapes your brain,

and how your brain
shapes your life.

What makes you you?

For a long time, the answer was

an immortal soul, or spirit:

Something that goes
beyond mere matter

and gives you your life
and your identity.

But the modern study of the
brain tells a different story:

Who we are can only be
understood in terms of

the 3-pound organ in our heads.

The story of becoming you

begins with a remarkable fact.

We are born utterly helpless,

and this helplessness
lasts longer in humans

than in any other species.

Compare human babies
to our cousins

across the animal kingdom.

What's mummy doing?

Many newborn animals arrive
ready for the world.

They have life skills
built in right from birth.

A baby zebra can run when
it's just 45 minutes old.

Baby giraffes learn how
to stand within hours.

Dolphins are born swimming.

Now, that would seem
to be an advantage,

but put any of these animals
in an ecosystem

not tailored to them,
and they won't survive.

Oh, look at that animal.

What is that?

In contrast, my son arri is two

and he's still dependent on me,

but one day, he could
live in Alaska

or in the Sahara or on the moon.

In other words, he can
adapt to any environment.

All right.
That was fun, huh?

And that's thanks to the unique
and spectacular way

that the human brain can mold
to fit the world around it.

Humans come to the table

pre-programmed for
certain things,

like absorbing language or
mimicking facial expressions,

but the thing that's
really remarkable about humans

is the degree to which
their brains are unfinished,

and this leads to a period
of prolonged helplessness.

But the plan is simple.

Instead of hard-wiring
everything the way a rhino does,

let life experience wire up
the rest of the brain.

Arrive with something
that's a little bit sloppy

and tune it up on the fly.

We learn on the job.
It sounds risky,

but that's exactly what
young human brains do.

And it gives us
an extraordinary advantage

as we grow and learn and adapt.

What's the secret behind
the flexibility of young brains?

Well, it's not about
growing new cells.

The number of cells is the same
in children and in adults.

Instead, the secret lies in
how those cells are connected.

This 5-year-old
has essentially all of

the brain cells
he's going to have.

But what's happening
inside his head

is very different than what's
happening inside mine.

In the brain of a newborn baby,

the neurons are only
starting to communicate.

But then, over the first
two years of life,

those neurons begin connecting
extremely rapidly,

forming as many as two million
new connections every second.

By age two, a typical neuron has

more than 15,000 connections.

That is almost twice as many
as found in an adult.

So what happens in between?

Well, after the age of two,

the growth is halted.

The process of becoming someone
is about pruning back

the possibilities that
are already present.

You become who you are
not because of

what grows in your brain,
but because of what is removed.

As we grow and learn new skills,

we reduce the number of
connections in our brain...

in favor of focusing
on a smaller number

of stronger connections.

As you learn to read,
your circuitry gets carved

to interpret squiggles
on a page.

The connections go from being
universal to being specific.

Those links you don't
use, you lose.

Over the course of childhood,
brain circuitry is

wired up according to experience

and interaction
with the environment.

But this dependence on
the outside world is a gamble.

The outside world won't always
provide what a brain needs.

I'm going to try...

Ha ha ha!

This is the Jensen
family from Milwaukee.

Carol, bill, their sons tom
and John, and daughter Victoria.

These are no ordinary children:

All three were adopted
from a Romanian orphanage

when they were just 4 years old.

So it was 1996
we came to the United States,

and then that was when I turned
almost 4, on August 5 of 1996.

So, yeah.

In 1989, at the fall
of nicolai ceausescu's regime,

there were 170,000 children
in Romanian orphanages.

The children were often
kept in appalling conditions,

left to cry
without human contact.

You'd walk into a room
and you'd be surrounded

by little kids who
you've never seen before...

And they want to jump in your
arms or sit in your lap

or hold your hand
or walk off with you.

And this sort of indiscriminate
behavior is sort of

the hallmark feature of kids who
have grown up in an institution.

It's so overwhelming
that your tendency is

to get very emotional and so
we'd have to keep that in check

because you didn't want to
do that in front of the kids.

Dr. Charles Nelson witnessed
the children's behavior

and wondered if it went beyond
mere loneliness or distress.

Had all that neglect impacted

the physical structure
of their brains?

Particularly
postnatally, brains need

experience in order to develop.

The developing brain is
seeking out information

and seeking out experiences
and if they don't get those,

the brain doesn't know
how to get wired up

and built correctly.

The kids in the institution,

they have iqs in
the 60s and 70s,

their language is very delayed,

they have severe
attachment problems,

and they show signs of
an underdeveloped brain.

They had small heads and their
eeg activity was very reduced.

Dr. Nelson
began looking at

the electrical activity
in these children's brains.

He was astounded to discover
that the children had

dramatically reduced
neural activity.

But there was more.

He found evidence that
children placed into families

before the age of two
generally recovered.

But for those who didn't
leave the institution

until older than two,
their brain development

was compromised.

The lack of experiences
leads the brain

to wire incorrectly because it
doesn't have any input into it.

And as a result, neurons
don't know which other neurons

they should be
communicating with.

Nelson's work revealed that when

the brain is starved of the
things it needs, like touch,

stimulation, love, a child's
development is stunted.

The only record
the Jensen children have

of their early years is
a couple of polaroids.

In the orphanage,
we really didn't speak any,

like, a known language.

So when, like, my mom
asked the taxi driver

what we were speaking, the taxi
driver was like, "gibberish."

The jensens were all
adopted after the age of two.

In the orphanage, it's hard.

It was rough. It was,
uh, it wasn't easy.

Uh, kids didn't get
much attention.

Almost 20 years have
passed since they left Romania.

They still live with
the consequences

of that early neglect.

Tom has adhd, and
he has learning disabilities,

but not to the same extent
that John has.

For John, it... i think
it started out hard

and it continues to be hard.

We went to doctors,
psychiatrists, physicians,

helping me out through the tough
times that I have right now,

so it's working, so I like it.

With the support
of a loving family,

the Jensen children
are finding ways to cope

with the scars of
their early childhood.

What we experience in our
early years goes a long way

toward defining who we become.

Experience prunes
the brain, but that's not

the only thing that
shapes who you are

because no matter what kind
of life you've led,

the brain is also on
a predetermined schedule

dictated by genetics.

And that means there are
major changes in store.

In our teenage years,

hormones course
around our bodies,

causing dramatic
physical transformations.

But out of sight,
our brains are undergoing

equally monumental changes,

ones that profoundly affect
how we behave

and react to
the world around us.

We all intuit
that teenagers have

a different view on the world
than children or adults.

But what's not always obvious is
that the way teens see the world

is not simply a choice
or an attitude.

Instead, it's the consequence
of a changing brain

that's right on schedule.

Who they are right now is
biological and inevitable.

To get a sense
of the teen brain at work,

we're running an experiment.

With the help of my
graduate student Ricky savjani,

we're going to rig up
volunteers of different ages

to a machine that
measures stress levels

by gauging changes
in their sweat glands.

Hi there.
I'm Dr. eagleman.

Then we have them
sit in a shop window

to be gawked at by passersby.

Ok, cue the curtain.

First up, an adult.

Louisa's stress response
seems to be

holding about steady,
at a pretty low level.

It's clear that she's
responding to people being there

but her stress response
is simply not going up.

But in the same situation,
the teen brain

responds very differently.

Oh, wow, that's a big response.

I've got to auto scale
her heartbeat

'cause it's gone up so much.

Yeah.

Her galvanic skin response
is really high now.

It just keeps going up.

It suggests she's stressed out.

Hi, Zander.
How are you?

I'm good.
Good, good.

The more that
he averts his gaze,

the higher his stress
response is going.

So this is presumably
his response,

is to pretend like
he's not there.

What words would you use
to describe how it felt?

Awkward.
Weird, pretty much.

It was different, having
people just, like, stare at you

and not knowing what
they were thinking.

So why the big
difference in response between

the adults and the teenagers?

The answer involves
an area of the brain

called the medial
prefrontal cortex.

It becomes active when
you think about yourself,

especially the
emotional significance

of a situation to yourself.

As one grows from
childhood into adolescence,

the activity in this area rises,

peaking around 15 years old.

Now, social situations carry
a lot of emotional weight.

In adults, the stress response
from that

feeling of being looked at
is relatively modest.

But in teenagers, that
same experience causes

social emotions to go
into overdrive.

The result is a stress response
of high intensity.

It's not just about
self-consciousness.

The development of the teen
brain has other consequences...

Including poor impulse control.

Parts of the prefrontal cortex
are still developing.

Those parts are
involved in simulating

the consequences of actions,

and that translates into
greater risk-taking.

But what happens when
things calm down?

Once the rush of our
teenage years is over,

do our brains stop changing?

Is who we are fixed in stone
once we reach adulthood?

Well, it's true that
most of the dramatic shifts

in brain structure are done
by our early twenties...

And for a long time researchers
thought that was that.

But I've come to London
to look at a pioneering study

that reveals how
even in adulthood,

our brains can undergo
radical physical changes.

The study of the knowledge is

640 quarter mile radius areas...

24,000 streets and roads
that need to be learnt...

50,000 places of interest.

What is known as "the knowledge"

is a test of recall...

Of all the streets in London.

The driver of every black cab
has to pass it to get a license.

Bayswater road from
marble arch, round park Lane...

It's one of
the world's most difficult

feats of memory, and completing
it usually takes

over 4 years of intensive study.

People will spend
3 to 4 hours a day

reciting pretend journeys.

That starts to make them...
"see it" is the term we use...

how to get around London.

The unique mental challenge
of passing "the knowledge"

made it of particular interest
to a group of neuroscientists.

They began doing brain scans
of the drivers before,

during, and after
the rigorous training.

They were interested
in an area of the brain

called the posterior
hippocampus.

It underpins spatial memory.

At the beginning, it looked
just like everyone else's,

but by the end of the training,
it had grown physically larger.

All the map-memorizing,
all the driving,

all the simulation
of future routes...

this reshaped
their brain anatomy

to match the task at hand.

The longer a cabbie
had been doing his job,

the bigger the change
in that brain region.

Even when we're adults,
our brains can still change.

Something that can be shaped

and hold on to that shape
is what we call plastic,

and that's how it goes
with the adult brain:

Experience changes it,
and it retains that change.

It's malleable,
it has plasticity.

That means that who you are,
and who you can be,

is a work in progress.

Many activities can cause
the brain to transform.

For example, learning
a musical instrument

can produce dramatic changes.

Musicians can learn languages
more quickly

and have improved memory
as a result of the way

years of practice have
altered their brains.

Albert Einstein's brain
was examined after death

by researchers.

They were hunting
for signs of genius.

But instead, they discovered
the brain area

devoted to operating
the fingers of the left hand

was much larger than normal...
all thanks to his

less commonly known passion
for playing the violin.

Hi, Paddington
station. Ok.

In fact,
everything we experience

will alter the physical
structure of our brain

in some way... Meaning that
for as long as we're alive,

our identities aren't fixed,
but constantly changing.

From our jobs
to falling in love,

having kids,

and spending time with friends.

All of these change
the wiring of your brain

to make you who you are,
and who you can become.

But the brain
can also change in ways that

we have no control over...

Ways that can have
a terrible impact

on our personality and actions.

We see this in a letter
discovered by police

in the wake of a violent tragedy

that happened in Austin, Texas,
in the summer of 1966.

25-year-old Charles Whitman
was a model citizen.

He had been an eagle scout,

he'd been honorably
discharged from the military.

He was working as a bank teller

and studying as
an engineering student,

yet what he'd written pointed to

a disturbing change
in his personality.

"I don't really understand
myself these days.

"I'm supposed to be an average,

"reasonable, and intelligent
young man.

"However, lately... i can't
recall when it started...

"I have been a victim
of many unusual

and irrational thoughts."

What Whitman was describing
were thoughts

that would lead to killing.

Death and terror stop the campus

of the university of Texas
in Austin,

as a sniper's bullets force
people to scurry for cover.

A stream of deadly accurate fire

sends bodies crumpling
to the ground everywhere...

On August 1, 1966,
Whitman took an elevator

to the top of the university
of Texas tower.

An armored truck...

There, he started
firing indiscriminately

at the people below.

Victims were
being hit at a rate of

more than one every 3 minutes.

13 people were killed
and 33 wounded,

until Whitman himself was
finally shot dead by the police.

His tower
arsenal included 3 rifles,

a shotgun, two pistols,
and a knife.

When police reached
his house, they discovered that

he'd killed his wife and
his mother the night before.

There was only
one thing more surprising

than this random act
of violence, and that is

there was nothing really about
Charles Whitman that suggested

he would do something like this.

It seemed completely senseless.

But the letter they
found in his home,

written prior to the killings,

suggested a possible
explanation for his actions.

In the note, he made
an unusual request.

"After my death, I wish that an
autopsy would be performed on me

to see if there is any
visible physical disorder."

Whitman's wish was granted.

During the autopsy,
the pathologists found

that Whitman had a brain tumor.

It was about the size
of a nickel,

and it was pressing against
a part of his brain

called the amygdala, which is
involved in fear and aggression.

This pressure on the amygdala
led to a cascade

of consequences
in Whitman's brain,

resulting in him taking actions

that would otherwise be
completely out of character.

His brain matter
had been changing,

and who he was changed with it.

Whitman's example is extreme,

but other less dramatic
changes in the brain

also alter the fabric
of who we are.

Parkinson's disease
can lead some people...

even the most devout...
to lose their faith,

and a medication for Parkinson's
can lead to compulsive gambling.

And it's not just disease
or drugs that can change us.

From the things we consume
to the simple process of aging,

everything contributes
to continually reshaping

the neural networks
that amount to us.

So who we are changes
in the course of our life

as our brain changes.

But thankfully,
there's one constant

that links it all together.

It's a pillar
of our personality...

Memory.

Memory sits right
at the core of our identity...

it gives our lives a narrative,

one that we can share,
one that has meaning.

But there's a problem...

memory isn't always reliable.

To understand how
it alters and why,

I've come up with a little
thought experiment.

Meet 35-year-old Daisy.

So imagine Daisy could
meet her 5-year-old self.

Same person, same
life experiences,

a subset of her memories.

Or what if Daisy could
meet her 85-year-old self,

the same person, but
experiences played out more.

Imagine that Daisy could
meet all of her selves

across the spectrum
of her lifetime.

You might think
that all these Daisys

would share the exact
same memories.

But although their memories
relate back to the same events,

in fact, what they remember is
likely to be quite different.

And that's because of what
a memory actually is.

So a few years ago,
I went out for dinner

to celebrate my friend Cheryl's
birthday with her boyfriend Joe.

I remember it distinctly
because it was so enjoyable.

Everything I experienced
that evening

triggered particular patterns
of activity in my brain,

lighting up constellations
of cells.

The conversation between
Joe and Cheryl.

The smell of the coffee.

The taste of this
little French cake.

All of these constellations
became linked with one another

in a vast associative
network of neurons

that the hippocampus replayed
over and over

until the association
became fixed,

and that was the unique
signature of this experience.

That would become my memory
of Cheryl's birthday.

So is a memory simply like
watching an old video recording

that we just call up and replay?

It feels like that, but in
reality it's quite different.

Memories are actually
brain states from a bygone time

that we have to resurrect.

So here I am 6 months later,

in a totally different city,
and I taste

one of these little
French cakes again,

and it's just like
the one that I had

at Cheryl's birthday party.

And in my brain, this
very specific trigger

lights up a whole web
of associations,

like the lights of
a city coming online,

and suddenly I'm back
in that memory.

But it's not as rich as
I would have imagined.

I know that
Joe and Cheryl were there,

and Cheryl, I think,
was wearing a blue shirt.

Or maybe it was purple.

Actually, maybe it was green.

The memory of Cheryl's
birthday, in my brain,

has started to fade.

Our memories fade gradually,
because our brain only has

a finite number of neurons,

which over time get used
for other memories.

It means that the details
have now become a little hazy.

What matters is that I remember
that we had a great time.

But even that's
not totally certain.

Because in the
intervening months,

Joe and Cheryl have broken up.

And so now I'm wondering... did I
sense any red flags there?

The state of my
emotions right now

changes the network
that corresponds to then.

My present colors my past.

What this means for all of us

is that the same event will
be remembered differently,

depending on where you are
at that point in your life.

So how reliable
are our memories?

How far can they be altered?

And why do our brains
work that way?

The first suggestion about
how vulnerable our memory is

came with
the groundbreaking work

of professor Elizabeth loftus.

She devised this simple
experiment in which volunteers

watched films of car crashes.

She then asked them
a series of questions

to test what they remembered.

So if I ask you
a question, you know,

how fast were the cars going
when they hit each other,

versus how fast
were the cars going

when they smashed
into each other,

witnesses give different
estimates of speed.

They think the cars
were going faster

if you used the word smashed.

And so this was one of my
earliest examples showing that

leading questions can
distort the answers

and can contaminate
a person's memory.

The discovery that existing
memories could be distorted

led loftus to ask
a more radical question.

Would it be possible to implant
entirely false memories?

To find out, she devised
another experiment.

She recruited volunteers
and then had her team

call their families
for information

about events from their past.

The researchers
then put together

a number of stories
about their childhoods.

3 were true.

But one, while
sounding plausible,

was entirely made up.

The story involved the volunteer

getting lost in
a shopping mall as a child...

Then being found by
a kind old person...

And then reunited
with her parents.

When told the 4 stories,

at least a quarter
of the participants

claimed they could remember
being lost in the mall...

even though it never happened.

6 at the time.

You cry or anything like that...

Yeah, no, I did cry.
Cried a lot.

And the experiment
didn't stop there.

They may start to
remember a little bit about it.

But when they come back
a week later,

they're starting
to remember more.

Maybe they'll talk about the
older woman who rescued them.

"I think I heard my name
over a loudspeaker."

Over time,

more and more detail crept
into their false memory.

The old lady was wearing
this crazy hat.

"I had my favorite toy."

"My mom was so mad."

The invention of these
new details that go way beyond

anything we presented to them

as coming from their mother

were pretty impressive to us.

Loftus had discovered
that not only is it possible

to implant entirely new
memories in the brain,

we naturally embrace
and embellish them...

unknowingly weaving fantasy into
the very fabric of who we are.

And we're all susceptible,
even loftus herself.

My mother had drowned
when I was 14.

And years later, I had gone
to a birthday celebration,

and one of my other relatives
started to talk about my mother.

And this relative
started to tell me that

I was the one that found

my mother's body
in the swimming pool.

And he was so convincing that

I went home from that birthday

and I started to think,
maybe I did.

I started to think about other
things that I did remember,

like when the firemen came,
they gave me oxygen,

maybe I needed the oxygen
'cause I was

so upset that I found the body,

and I could almost visualize
my mother in the swimming pool.

But then something
happened that would make loftus

realize her memory
had been tricked.

The relative called
and said, "I made a mistake",

it wasn't you, it was the aunt,
who found the body."

And so I thought, boy, that's...
that's what it feels like, when

you're on your way to developing
such a rich false memory.

Our past is not
a faithful record.

Instead, it's a reconstruction.

In part, it's a mythology.

So what does this mean
for who we are?

Well, think about
your life memories.

Not all the details
are accurate.

Some came from things that
people told you about yourself;

other details were filled in by

what you think
must have happened.

But that's ok.

It's all part of the evolving
story that is you.

So why do we have memories
that are so unreliable?

Well, as strange as it sounds,

memory isn't just used
for recording our past.

It also serves another
important function.

That function would be revealed
by a singular case

that revolutionized
neuroscience.

How long have you
had trouble remembering things?

I don't know myself.
I can't tell you

because I don't remember.

But do you think
it's been more than a year

that you've had this problem?

I think about that 'cause, uh,

this is just a thought that
I'm having myself that, uh...

I possibly have had
an operation or something.

Henry molaison was born in 1926.

His early life was like
any other young boy's,

until his 10th birthday,
when he suffered

his first epileptic seizure.

By 16, his epilepsy
had worsened,

and by 27, he could no longer
function normally.

These major seizures
increased in frequency,

up to the point where his life
was pretty much on hold.

His doctors proposed
an experimental surgery

that would remove
the hippocampus

on both sides of Henry's brain,

leaving two yawning black holes.

The operation proceeded
without incident,

and within a few days,
he recovered.

Henry's epilepsy was cured.

That was when they realized

that he couldn't
remember anything.

For the remaining
55 years of his life,

he never formed a single
new long-term memory.

And there was something more.

What do you think
you'll do tomorrow?

Whatever's beneficial.

You might think when
you said... say to him,

"what... what do you think
you'll do tomorrow?"

He might say, "well, you know,
I'll get up, um, as usual"

and, um, get dressed and shave
and have breakfast,"

but he didn't even
come up with that.

It was like he was absolutely
stuck in the present moment.

Henry molaison's misfortune

had revealed something profound.

The brain areas that underline
memory are the same as those

that are used to simulate
what's coming next.

Both the past and the future
are creations in the brain.

Who we are at any moment in time

is an ongoing narrative.

As we live longer
than ever before,

this presents real problems
for brain health.

Diseases like Alzheimer's
and Parkinson's

attack our brain tissue

and with it, who we are.

But in the same way that
your environment and behavior

shape your brain
when you're younger,

they're just as important
in your later years.

If you don't mind me asking,

how old are you this year?

Oh, I'm just 94.

Ok. For how long
have you been here?

Well, I've been in the
convent over 70 years.

Do people live until their 100s?

We had one I know
that went over 100.

How was she doing
cognitively, at that age?

She was very smart.

She was very smart. Ok.

She sure was, and she
didn't miss anything.

Ha ha! Quite alert!

Across the U.S.,
more than 1,200

nuns, priests, and brothers
have been taking part

in a unique research study

exploring the effects
of aging on the brain.

They really figured the sisters

are a good study group because
we're kind of stable.

You know where to find us.

Each year, the
participants have to provide

detailed records of
how they spend their time.

They also commit to extensive
physical, genetic,

and cognitive tests,
and it doesn't end there.

When I first heard
about the study, I said... Well,

even after I die... ha ha!...

whatever I'm contributing
can still go on.

After they die,
all the participants

will give up... Their brains.

My staff in Chicago is on call
24 hours a day, 7 days a week.

When someone dies in New York,
they call us.

The researchers
carefully examine the brains

for the tell-tale
microscopic evidence

of age-related brain disease.

They're looking to establish
links between

brain degeneration
and cognitive performance.

But the first set of results
was entirely unexpected.

When we first
started publishing it,

a lot of people were surprised.

It turned out
that nearly a third

of the brains tested had
characteristic signs of

full-blown Alzheimer's.

♪ when I fall
on my knees.... ♪

But the cognitive
tests revealed that

the brain owners
had shown no symptoms

of this terrible disease.

Their brains were sick,
yet they remained unaffected.

It didn't make sense.

What had happened?

I have a game on my smartphone...

ruzzle. Ha ha ha!

And some of our sisters

have been involved in
teaching, nursing...

Having responsibilities,

learning new skills,
keeping the brain active.

This was protecting the nuns

from the cognitive symptoms
of the disease.

I am interested
very much in science.

I love scrabble.

I drive the sisters
to the doctors.

mass every day, and if I didn't

have that, I'd be
completely nuts.

Even as parts of
the brain tissue degenerate,

mental and physical
activity can build

new pathways for
solving problems.

This is called
"cognitive reserve."

Think of the brain
like a tool box.

If it's a good tool box, you
have all the tools you need,

so I might choose a ratchet
to disengage this bolt here.

But what if I didn't have
access to this ratchet?

I might be able to find
something else,

like this wrench
would do the trick.

And if I didn't have the wrench,
I could find something else,

like this adjustable wrench.

And it's the same idea in an
active, cognitively fit brain.

Even as parts of
the brain degenerate,

the brain can find
other solutions.

By making sure
our mental tool box

is equipped with a variety
of working tools,

we may be able to slow down
the effects of our aging brains

and hold on to who we are
for as long as possible.

But who we are is more than just
the tasks we can accomplish.

There's something else...
something that's perhaps

the greatest mystery
about how the brain works.

It's the sense of "I," of "me."

I am a conscious being.

When I think about who I am,

it's taking place
inside this head,

through these eyes, from this
particular point of view.

Conscious experience is
at once the most familiar

and the most perplexing
aspect of our identity.

How does the physical
stuff of the brain

equal the mental experience
of being a conscious human?

How do billions
of brain cells produce

the extraordinary,
unique feeling

of being alive, of being me?

I'm made up of
100 billion neurons.

And when I die,
they'll still be there.

But I won't be me anymore;
I'll be dead.

So that means who I am
is not about

the existence of the neurons.

It's about what they do
and how they interact.

Imagine that the neurons
in your brain

are like a collection
of drummers.

If each drummer plays
completely independently,

the noise that emerges
is just that... noise.

But if they start
listening to one another,

something more emerges.

Out of cacophony appears
a rhythm, a performance...

a complex interaction in which
all the drummers are playing

both as individuals
and as something greater.

And in the same way, this is how
the experience of consciousness

arises in the brain.

Billions of interacting
neurons work in concert...

and under the right
circumstances,

they hit a sweet spot,

the place where the singular,
private experience

of being you emerges.

Consciousness is
a performance our brain

puts on for us
throughout our day.

But there comes a time...
in fact, once a day...

when the character of
that drumming changes

and it takes me with it.

The best way for me
to show you how

is for me to go to sleep...

Wearing this.

I'm wearing an
electro-encephalogram, or eeg.

It records my brain's activity,
giving an idea of how my neurons

are interacting while I sleep.

When you go to sleep,
your body seems to shut down,

so you might think that
the drum beat in your brain

would do the same.

But the reality is
quite different.

So at the beginning
of the night,

this is my brain activity:
I was still awake.

But if I go a little bit later,
I have activity

that looks as though
I'm still awake,

but in fact I wasn't;
I was in dream sleep here,

which is a form
of consciousness,

full of vision and sounds

and strange situations
and magnified emotions.

It's me, but a bizarre
form of me.

But things get stranger.

At this part of the night,
I'm in deep sleep.

My brain is still there,
and it's still active,

but I am gone.

In deep sleep, our neurons
become more synchronized.

It's impossible for a complex
rhythm to emerge from this,

which means,
in this brain state,

there's no hope of
conscious experience.

No identity, no personality,

nothing.

I am the relationship
between my neurons.

Change their interaction just
a little bit and I find myself

in a dream world,
or I disappear.

Or I return.

When the neurons find themselves
back in their proper rhythm,

I miraculously come back online.

Consciousness and the sense
of being "me"

somehow emerges from the complex
rhythms of our neurons firing.

But why consciousness
emerges at all

still remains one of
the great unsolved mysteries

of modern science.

After I finished graduate
school, I had the opportunity

to work with one of
my scientific heroes,

Francis crick,
who had co-discovered

the structure of DNA.

By the time I met him,
he had turned his attention to

the question of consciousness...

why does it feel like anything
to be alive.

And I remember when
I went into his office

that he had a lot of writing
on the chalkboard,

but there was one word that
was written in the middle

and it was bigger than the rest.

That word was "meaning."

You see, we know a lot about

the mechanics of neurons and
networks and brain regions,

but what we don't know
about those signals

coursing around in the brain is
why we care about any of them,

why anything carries meaning.

How can the physical
cells in my brain

cause me to care about anything?

The meaning problem
is not yet solved,

but here's what
I think we can say:

The meaning of something
to you is all about

your web of associations,
based on

your whole history
of experiences.

Just imagine I were to take
a piece of cloth

and put some
colored pigments on it,

and then put that in front
of your visual system.

Is that likely
to trigger memories

and fire up your imagination?

Well, probably not, because it's
just a piece of cloth, right?

Here it is... pigments
arranged on a cloth

in the pattern of
a national flag.

Presumably this triggers
something for you,

but the meaning is unique to
your history of experiences.

We don't perceive objects
as they are.

We perceive them as we are.

Each of us is
on our own trajectory...

steered by our genes and our
experiences... and as a result,

every brain has a different
neural reality.

Brains end up being
as unique as snowflakes.

Your story plays out
across a lifetime.

Trillions of new connections are

continually forming
and re-forming

as we learn and create memories

and become who we are.

The unique connections
in your brain

mean no one like you
has ever existed...

Or will ever exist again.

For me, the wonder
of the human brain is that

from a vast network
of physical pieces and parts,

the experience of being you
or me emerges.

And because the physical stuff
is changing, we are, too.

We're not fixed.

From cradle to grave,
we are works in progress.

"The brain with David eagleman"

Next time on "the brain,"

I'm going to investigate the
unexpected ways that your brain

secretly controls everything
that you do.

That was amazing.

How it allows
great athletic achievement

and makes decisions
without your awareness.

This is the story of everything
the brain does

that remains hidden from us.

It's the story
of who is really in control.

"The brain with David eagleman"

is available on DVD.

The companion book is

also available.

To order, visit
shoppbs. Org