All language subtitles for NOVA.S50E09.Your.Brain.Perception.Deception.1080p.WEB.h264-BAE_track3_[eng]

♪ ♪

♪ ♪

ANIL SETH:
The brain is one of
the most complex objects

that we know of in the universe.

BOBBY KASTHURI:
There are more connections
in your brain

than there are stars
in the Milky Way galaxy.

So, we literally walk around
with about 10,000 galaxies'

worth of neuronal connections

in one of our brains.

HEATHER BERLIN:
That vast web of
connections creates you.

But how?

NANCY KANWISHER:
Figuring out how the brain
implements the mind

is a massive challenge.

♪ ♪

SETH:
It seems as though

the world just pours itself
into the mind

through the transparent windows
of the eyes

and the ears and,
and all our other senses.

♪ ♪

BERLIN:
But is what we see,

hear,

and feel real?

You might think that the reality

outside is actually
what you're perceiving.

And the answer is no,
it really isn't.

Almost at the very first moment,

we are transforming reality.

It feels so real

because we don't know better.

Think about illusions.

ROSA LAFER-SOUSA:
Do you remember the dress?

Of course-- it's like a
celebrity, the dress.

A polarizing debate

that took over the internet.

White and gold.
Blue and black.

SETH:
Illusions are fascinating;

they're like
fractures in the matrix.

BERLIN:
Ow!

Isn't that interesting?
Whoa!

They reveal to us that the way

we perceive things isn't
necessarily the way they are.

BERLIN:
Could you be
the biggest illusion of all?

SUSANA MARTINEZ-CONDE:
Your sense of who you are
is an illusion

as everything else;
you're no exception.

BERLIN:
"Your Brain:
Perception Deception."

Right now, on "NOVA."

♪ ♪

MAN:
Okay, rolling.

♪ ♪

Take three.

BERLIN:
Have you ever thought
about what's real?

(echoing)

Somehow the whole world out
there gets inside my head.

How do I know
what I see, what I hear,

what I feel is right?

It's a question
that's fascinated me

ever since I was a little girl.

I couldn't sleep one night,

and I had this
thought for the first time:

And then I thought, well,
even if I don't have a body,

can I at least keep
my own inner thoughts?

So I asked my dad the next day,

"Dad, where do my thoughts come
from?"

And he said,
"They come from your brain."

(explosion echoes)

♪ ♪

"Your brain."
I was hooked.

This bag of
jelly between my ears,

how does it work?

STANISLAS DEHAENE:
I think it's one of the ultimate
mysteries.

How matter becomes thought.

ANDRÉ FENTON:
To answer that question would be
perhaps

the highest human
achievement to date.

DANIELA SCHILLER:
I mean, forget about scientific
quest.

It's a human quest.

♪ ♪

BERLIN:
To find answers,

I became a neuroscientist
and a psychologist.

I'm Heather Berlin,

and my journey to understand
my brain begins with a question.

How does the world out there,

with all its beauty
and complexity,

get inside our heads?

♪ ♪

Think about it.

Imagine for a second
you're a brain,

sealed inside your skull.

There's no light, no sound.

KASTHURI:
You're a massive collection

of billions and billions
of cells that are living

in this weird pond that
is entirely devoid

of all of the sensations,
and that somehow,

through chemistry
and electricity, all of these

perceptions and memories
of the world

originate in our brains.

BERLIN:
All brains-- from the tiny fish

to the enormous elephant--
contain microscopic cells

called neurons, and one of their
jobs is to translate input

from the external world,
whether that's light,

heat, sound, or
pressure, for instance,

into electrochemical signals
the organism can use to act.

What might be surprising to you
is that

as neurons process sensory
signals,

they create an edited version
of reality,

even on the most basic level.

KASTHURI:
We're deciding to throw away 99%
of the world.

Almost at the very first moment,
we are transforming reality

into something we could use.

BERLIN:
Neurons transform reality
by competing with each other.

When a creature touches, smells,
sees, or hears something,

its sensory neurons fire;
some a little, some a lot,

depending on where the
physical signal is strongest.

But follow those signals
down towards its brain,

you'll see that the weaker ones
get stamped out.

For simple brains,
say, the brain of a crab,

a diffuse light to the eye
becomes a sharp beam.

For more complex brains
like ours,

it's in part what makes
you think

that these two squares
are completely different colors,

but actually, they're identical.

♪ ♪

MARTINEZ-CONDE:
Think about illusions.

First, they're a lot of fun,
but as neuroscientists...

Whoa...

MARTINEZ-CONDE:
...illusions are very important
to us.

Because of this discrepancy
between objective reality

and subjective perception,
we can use these illusions

as a handle to try to
understand what the brain

is doing all the time.

BERLIN:
Susana Martinez-Conde,
along with her partner

and collaborator
Stephen Macknik,

are among the
world's preeminent experts

on illusions and perception,

and what they tell us
about how the brain works.

Ha, now what?

(both laughing)

MARTINEZ-CONDE:
To give a different example,

Adelson's checkerboard illusion,
this is so striking because

you see some of the checks
as dark and others as bright,

but you realize that it is
exactly the same shade of gray.

BERLIN:
Don't believe it?

Look at the squares labeled
A and B.

A looks darker,
right?

Wrong-- that's the illusion.

That's because your brain
is adjusting for the shadow.

MARTINEZ-CONDE:
What's happening is that your
brain is considering

the light source and basically
subtracting that light source

from your resulting perception.

Your brain is performing an
interpretation,

a shortcut, if you will,
to arrive

at a perception.

BERLIN:
If the brain's shortcuts
distort reality this much,

how much of the world
are we really seeing?

STEPHEN MACKNIK:
What you need to understand

is that we really can't see
most of the world around us.

We're effectively blind

to 99.9% of the world around us
at any given time.

If you hold out your thumb
at arm's length...

Mm-hmm.
And you straighten your elbow
and you look at your thumbnail,

your thumbnail is about
one degree of visual angle here,

and it turns out
that that's the only place

we can actually
see with 20/20 vision.

Wow.
And everywhere else,
we're legally blind.

BERLIN:
It might sound hard to believe,

but human vision is really
like this.

You actually only see detail

in about one percent
of your visual field.

That's because only
a tiny portion of the world

can be processed
in detail by the retina.

It feels like I'm seeing
the whole world in 20/20 vision.

And it's almost all completely
made up in your brain,

based on assumptions and
models of how the world works

and just a tiny bit of
high-quality visual information.

Let me demonstrate this
to you.

I know it's
kind of hard to believe...
Mm-hmm.

...because you've been
having your whole life

where you feel like
everything's continuous.
Yeah, show me the data.

(laughs)
Show me the evidence.

Let's look at an eye-tracker
and look at your eyes

and how they actually work.

And if you put your head
in this headrest...

Mm-hmm.
...we'll point the camera
at your eyeballs

and we'll actually be able
to see

where your eyeballs
point during this demonstration.

Feels like "Clockwork Orange."
(chuckles)

"Buck lived at a big house

in the sun-kissed
Santa Clara Valley."

BERLIN:
First up, a reading demo.

Though most of your screen
may be filled with Xs,

to me, it just feels like
normal reading.

I barely see the Xs,

and that's because the display
of letters

is tied to my eye movements.

BERLIN:
Well, it's just, the words are
being

revealed
depending on where I look.

That's right,
so as you move your eyes...

So weird.
...the words are
revealed to you.

But we don't move our eyes
in the same way you do.

So we just see a bunch of Xs
most of the time.

BERLIN:
What turns out to be critical
is my eye movements.

MACKNIK:
So our eye movements program
what part of this

high-quality
piece of visual real estate

we're going to put
where and at what time.

BERLIN:
The human eye moves about
three times per second.

We take it for granted,
but without these movements,

we'd be basically blind,

as Steve is about to show me.

MACKNIK:
In this demonstration,
it's the opposite.

Here we're blocking what you
can possibly see, right?

BERLIN:
Though you may see a whole scene
with a square moving around,

all I see is the square!

I can tell something is around
the edges, but it's blurry.

Whenever I try to look,
the square moves with my eyes

and it's blocked.

BERLIN:
This is so frustrating,
this one.

Who has their hand up in
this image?

BERLIN:
Uh... I think that guy
down there?

MACKNIK:
That's right, but it's very hard
for you to see,

right?
Every time I look at him,
it, yeah.

It disappears because this
block, it blocks it.

MACKNIK:
This actually is interesting

because it's in high-quality
vision wherever you look,

but it's blurry
in the surround.

BERLIN:
Now the scene looks normal
to me,

but mostly blurry to you

because your eye movements
don't match mine.

MACKNIK:
Wherever you happen to look,

you have high-quality
image processing happening

and the surround
is completely blurry.

This kind of represents exactly
what your visual system

looks like all the time
anyway.

So why would our brains
be built this way?

Well, think about what the
alternative is.

What if
we didn't have eye movements?

Well, if we didn't have
eye movements,

and we just wanted to see
the entire world,

we'd need to have our retinas

see everything
in very high quality.

Our brains would be
600 times bigger,

and you gotta remember, the
visual system's our best sense.

This is our richest sense.

So our other senses are,
are even more impoverished.

BERLIN:
Here's how your brain really
sees the world.

It's easy to think it's
like this.

You open your eyes
and the whole world pours in.

But really, it's like this.

Your eyes sample tiny pieces
of the world

and the brain fills in the
rest-- constantly, all the time.

KANWISHER:
We feel like we have

this incredibly rich, wide,
full, detailed percept

of what's going on
moment to moment,

and that's probably pretty
illusory.

of the information
that comes in through our eyes.

BERLIN:
Don't believe it?

Consider this:

your optic nerve is what
connects your eye to your brain,

and its location near the center
of your retina

effectively creates a blind spot

near the center of your
visual field.

And yet, you don't experience
the blind spot-- why?

The brain samples the area
near the blind spot

and fills
in the gap with its best guess.

KASTHURI:
It's probably not fair

to say that we completely
confabulate the world,

it's just
that we probably represent

one percent of it at any
particular moment in time.

So, it's a constant updating
between what I see with,

versus what I remember, versus
what I expect.

And it's that dance between
those three

that actually gives us our
sense of reality.

BERLIN:
And amazingly,
that edited reality--

despite its limitations--

serves us quite well.

KASTHURI:
You might ask,
"If I'm just keeping track

"of one percent of the
information in the world,

how can I drive a car?"

And it turns out that first one
percent

of the information that comes
in from the world

is actually an enormous
amount of information.
(chuckling)

If we had to actually
pay attention

to everything on the road at
the, at one particular time,

it would take minutes,
maybe even longer,

before I decide to turn
the wheel right

or to turn the wheel left.

♪ ♪

BERLIN:
By understanding how
my senses really work,

I'm getting a peek behind
the curtain:

what my brain is really up
to outside of my awareness.

MARTINEZ-CONDE:
Based on this very tiny amount
of information,

we construct this
grand simulation

of the visual world around us.

It feels so real
because we don't know better.

BERLIN:
And most of the time,
we all agree on that simulation.

It's when we don't
that we can learn something.

So do you remember
the dress?
Of course.

Did you see this dress
or this one?

It's a simple question,

but the answer has divided
friends and family.

White and gold.
Blue and black.

I remember it caused
quite the stir, right?

Massive stir.

A polarizing debate

that took over the internet.

♪ ♪

LAFER-SOUSA:
People had existential crises
over this image.

People tweeted things like,

"If that's not white and gold,

my life has been a lie."

Swear on your mother's
grave.

Because that dress is white
and gold.

Out of her (bleep) mind.

LAFER-SOUSA:
Massive arguments.

I watched videos of
people screaming at each other.

GRAYSON DOLAN:
This is white, dude!

ETHAN DOLAN:
White? That is dark blue!

It's purple-blue!

LAFER-SOUSA: I bet there was a
divorce here or there

over this image.

BERLIN: So when you first saw
that dress,

as a, as a vision scientist,
what did you think?

Well, when I first
saw the dress,

I thought it was
blue and black.

And I thought that the internet
was yanking my chain.

Right-- to get the goat
of vision neuroscientists.

Sure.
(chuckles)

But in the morning,
when I looked at my phone,

I saw white and gold.

And now,
of course, I was obsessed.

So I said, "Well,
if this is an ambiguous image,

all I have
to do is disambiguate it."

So I set to work,
I got into Photoshop,

cut out the dress, put it into
a scene with lots of rich cues.

BERLIN:
Hm.

LAFER-SOUSA:
And all of a sudden, boom:

you can see the dress is
white and gold.

Wow.
Now, the pixels,

the pixels that make up the
dress there,

are identical
to the original image.
Okay.

BERLIN: Now, this doesn't work
for everybody,

but for most, the visual context

can make all the difference.

LAFER-SOUSA:
What's different here is, her
skin is tinted blue,

the background has blue light
cast on it,

she's standing
in the shadow of that cube,

and so your brain says,
"Aha, I need to ignore

"some amount of blue light

"that is in this signal that's
hitting my eye

and render
this as white and gold."

BERLIN:
And if we flip things around?

LAFER-SOUSA:
Same dress, pasted it into
this other scene.

Her skin is tinted yellow, the
background has a yellow cast,

she's standing no longer
in the shadow but in the light.

Boom! Blue and black.

Amazing, that's really
amazing.

So again, the dress,

the pixels are exactly
the same.

LAFER-SOUSA:
Identical.

BERLIN:
The dress is a powerful example

of how color really works
in the brain.

Does that mean
we're creating color

in our mind, or does color
actually exist in the world?

Color takes place
in the brain,

and I've prepared a little
illusion for you

that should
convince you of this.

LAFER-SOUSA:
So I have a picture
of four cars here.

I want you to tell me,
what color are these cars?

Let's start on the top left.
BERLIN: Okay, so the one
on the left looks red,

then the one
next to it looks blue.

I'd say the one, the
bottom there,

bottom left looks green,

and then the one next to it
looks orange.

LAFER-SOUSA:
Okay.
Mm-hmm.

What if I told you

that all of those pixels
are not only gray,

they are the same gray?

BERLIN:
How is this possible?

It's because the light
that enters your eyes,

contrary to what you might have
learned in school, is not color.

Color is an interpretation
of your brain.

Here's how it works.

Light shines on the world

and bounces off objects--
this part you know.

And light comes in

different wavelengths,

each corresponding
to a different color.

What you might not have
heard in school

is how those wavelengths change

when they hit different
surfaces--

rough, smooth, wet, et cetera.

This signal that gets into
your eye

is actually a product
of the reflective properties

of the object and the wavelength
of light hitting it.

Then that signal is focused
on the retina,

the back of the eye,

where we have about 130 million
light-sensitive cells.

Three types called cones
are involved in color,

each sensitive to different
wavelengths of light:

long, medium, and short.

But that light
still isn't color.

For that to happen,
our brain has to take

that three-piece code
from the retina

and use the relative response
of the cones to encode color.

It's not until that signal gets
to an area called V4

that we get
a neural representation of color

that corresponds
to our perceptual experience.

So why would
our brains be built this way?

Well, if our brains weren't
built this way,

objects would appear to change
in color all the time,

and that would render color

a pretty useless signal in the
world.

♪ ♪

BERLIN:
That's because objects reflect
different wavelengths

into your eye depending
on the lighting conditions.

If your brain didn't compensate
for this,

a red berry would appear gray
in a cave,

blue at dawn,
and orange at dusk.

But instead, your brain
carefully calibrates

your experience to hold color
constant.

Similarly,
color vision in other animals

is tuned to their needs.

SETH:
Different species have very
different kinds of color vision

that are suited to their
particular environments

and their particular challenges
for staying alive.

BERLIN:
Dogs rely on smell, so they have
fewer types of cones,

and thus see the world like
this.

Birds need to recognize tiny
color differences

from great distances.

so they have an extra type
of cone that allows them

to see more colors than we do.

And bees need to find
flowers rich in nectar,

so they see ultraviolet light
that's invisible to us.

LAFER-SOUSA:
Color provides a
lot of valuable information

about the world,

but only if we can faithfully
extract

something about the object.

So we don't actually see color
as it is in the real world,

we just see it in
terms of how it's useful for us?

Absolutely.

And the dress is probably
the best example of that.

It's a really
powerful demonstration

of how our color machinery
works.

BERLIN:
So why do people see
this image

of the dress differently?

It comes down
to your brain's assumptions

about the lighting conditions.

It seems that the more
time you spend working indoors

under artificial light,
which is predominantly yellow,

the more likely you are to say
the dress is black and blue,

because your brain assumes
it is lit by artificial light

and subtracts out the yellow.

Conversely, if you spend more
time

in natural light,
which is bluer,

you are more likely to see
it as white and gold.

So then what is
the actual color of the dress?

Well, Heather, I happen
to have brought it with me.

(chuckles):
Wow.
So what color is it?

Uh, it's obviously,
I was right, blue and black.

Team blue and black
for the win.
Yes, yes.

I can't believe this is the
actual dress.

BERLIN:
I feel like I'm holding,
like...

It's like a celebrity,
the dress.

I know, it should be in a
museum, not in my closet.
Yes!

(both laugh)

Before the dress,
people hadn't really realized

we differ so much between
individuals.

We're now quite used to the idea

that we all
differ on the outside.

We all have differences in skin
color, in height, in shape.

But just as we all differ
on the outside,

we all
differ on the inside, too.

And this inner diversity
is very important.

It gives us a certain humility
about our own ways of seeing.

BERLIN:
Illusions give us a ringside
seat

to watch how the brain creates
our world.

And it's not
just the visual domain.

Try listening to this.

(staticky computerized voice
playing)

Brainstorm, right?

Simple enough.

Now, listen to this.

(staticky computerized voice
playing)

Green needle.

Okay, so you're thinking,
"What's the big deal?"

But, what if I tell you that the
two audio clips I just played

were exactly identical?

For most people, what you hear
depends on which label you read.

Sounds unbelievable?

Here, try it again,
but this time, just read one.

(staticky computerized voice
playing)

Okay, now read the other
and listen again.

(staticky computerized voice
playing)

Now, when I first encountered
this,

I was floored, too.

Even though
I know what's going on.

When your brain
encounters uncertainty,

it fills in the gaps with its
best guess.

In this case,
we have a degraded audio clip,

and when you're primed with
a certain word to go with it,

your brain automatically
jumps to the best fit.

For most of us, we literally
hear what we want to hear.

And it gets even worse--
let's try one more.

Another internet sensation

that lit up
debates across the country.

(cleaner computerized voice
playing)

Once and for all, is it yanny,
is it laurel?

It's not yanny, it's laurel.
It's yanny!

Did you hear yanny?

(cheering and applauding)

Who heard laurel?

(cheering more loudly)

It is laurel and not yanny.

(remix of computerized voice
playing)

It's like that stupid dress
again

all over, but in audio form!

This is not saying "laurel,"
this is only saying "yanny."

Exactly, it's laurel!

Now, about half of you hear
yanny,

and the other laurel,

and unlike the first illusion,
I can't get most of you

to experience
this one any other way.

You're locked into your version
of reality.

Experts aren't exactly sure why,
but some of us

seem to pay more attention
to the low frequencies, laurel,

and others to the high, yanny.

The divide stems from the fact
that the audio file

is an ambiguous signal made up

of both high and low
frequencies.

But by manipulating the
frequencies,

I might be able to change what
you hear.

(computerized voice playing
at mid frequency)

High...

(voice plays at high frequency)

(voice plays at low frequency)
Low.

All of this goes to show

how much the brain

is an active interpreter
of sensory input.

Our perception of
the external world

is actually much less objective
than we'd like to believe.

Most of the world around us is
very real,

but you just never lived there,
okay?

You lived in your mind,

which is a perception of
that world that's being filtered

through a bunch of salt
water sacks of proteins

and electrochemical signals,
which can't possibly be making

completely accurate
determinations

of what's actually in the
outside world.

Not convinced? Or maybe you're
just asking, "Why?"

Well, try watching for when
the green dot flashes.

Does it line up with the
red dot?

If you are like most people,

the red one always seems just a
little bit ahead.

Now try again.

The red dot and the green dot
are actually perfectly aligned.

That's because some
neuroscientists would say

that it's not your brain's job

to perceive
the world accurately.

Rather, its job is to
predict what happens next.

To a certain extent,
you see what you expect to see:

a predicted path of motion.

And this is what helps
us hit a home run

or flinch from a punch at just
the right moment.

The brain is a predicting
machine.

Given a set of circumstances
in this story at this moment,

what are the likely plausible

next events in the story?

SETH:
The brain is
using sensory information

to calibrate,

update, to fine-tune these
predictions

so they remain tied

to reality in ways that are not
constrained by accuracy,

but that are constrained by how
useful

the brain's perceptual
predictions are

in the business of staying
alive.

BERLIN:
And to keep us alive,
the brain has evolved

to look for signals
of potential danger.

One of the most important
is pain,

and as neuroscientist Theanne
Griffith is about to show me,

sometimes that can be
a kind of illusion, as well.

BERLIN:
So what is this?

GRIFFITH:
This is a thermal grill.
Okay.

This is a machine
that could give us some insight

as to how pain works in your
brain.

All right, this is making me
nervous already

as I'm getting strapped in!
(Griffith laughs)

GRIFFITH:
Don't worry,
it's all an illusion, actually.

Okay.

And it's comprised of these
different metal bars

that are either set
to a cold or warm temperature.

So why don't you go
ahead and touch that first bar?

It's warm, right?

And then the next bar?

Cold.
Mm-hmm.

GRIFFITH:
And then the next one, warm.

You see? So they're alternating
cold, warm, cold, warm.
BERLIN: Mm-hmm.

Now, you want to see what
happens

when you put your hand down?

Not necessarily.
(both laugh)

Go ahead.
Okay.

Okay, here we go.

BERLIN:
Ow!

GRIFFITH:
Right? Isn't that interesting?
Whoa!

Yeah,
what is going on there?

It sort of feels cold at first,
but then...

Then it gets this kind of
burning sensation, right?

Yes, very much so.

BERLIN:
It feels super-hot,
like I'm getting burnt.

So it's not 100% clear
exactly how this is happening.

But what we think might be going
on

is that, basically, your brain

is getting a little bit
confused.

Okay.
It's feeling cold,

and it's also feeling warmth.

And somehow, it's interpreting
these two signals as pain.

BERLIN:
Here's what neuroscientists
think is going on.

In your hands, you have
separate sensors for heat,

cold, and pain.

Normally, when you touch
something slightly cold,

both your cold and pain sensors
are activated,

but the cold ones
override the signals

from the pain sensors,

telling your brain there's
nothing to worry about.

Unless, in this very
unnatural scenario

with the thermal grill,
you happen to be touching

something warm at the same time.

Here, the heat signals
cancel out the cold ones,

leaving you with just
the pain ones activated,

telling your brain, "Ouch!"

So in that respect,
is pain real?

Mm-hmm.
Or is it just an illusion

or a construct of the brain?

That's a really good
question.

So noxious stimuli
is, is a real thing, right?

If you stick
your hand in boiling water,

that's an aversive stimulus.

The perception
of a noxious stimuli is real.
Mm-hmm.

Pain is more of a construct,
right?
Mm-hmm.

And it can vary from
individual to individual.

EMERY BROWN:
Pain is a construct of the
brain.

How do we know that?

You touch a needle, right?
And prick your finger.

We can draw the anatomy
of what just happened.

We have very well-defined
pathways

saying, "This is pain
information."

We don't interpret it as pain
until it hits your brain.

BERLIN: Pain, not unlike the
experience of color,

is a construct of the mind.

Mama! Mama!

BERLIN: But just because pain is
in your brain

doesn't make it any
less critical for survival.

GRIFFITH: Pain is a
very important...

(gasps)
...learning mechanism
for children.

They learn what behaviors
they can engage in that are safe

and what behaviors, well,
they should not engage in

because they could cause
them bodily harm.

And there's, um, uh, different
mutations that people can have

in certain proteins
that make them

completely insensitive to pain.

And so kids do things
like bite on their lips

or on their fingers
when they're very young,

and as they get older,
can engage in risky behavior.

So pain is extremely
important for us to feel.

SETH:
Illusions are fascinating.

They're like fractures in the
matrix.

They reveal to us that the way
we perceive things

isn't necessarily the way they
are.

MACKNIK:
Illusions help us find the
cracks in the mortar

of that world we've built for
ourselves,

and understand what it is our
world is actually made out of

and what the brain is actually
doing.

So most people think of the
brain

reconstructing the world more
or less verbatim.

(dog growling)
But that's just not true.

What it's actually doing is,

it's getting very little
information

and it's using that very little
information

to make a big, grand model
of the world.

MARTINEZ-CONDE:
We cannot process the vast
amount of information

that is constantly bombarding
our senses.

Illusions, you can
think of them as shortcuts.

Shortcuts make us faster,

more efficient
with less resources.

we build this more
complex simulation of reality.

And that simulation
of the world is

what we call consciousness.

BERLIN:
Consciousness.

(alarm buzzing)
We take it for granted,
but every time you wake up,

(alarm stops)
your brain stitches together
all your sensory inputs--

the sound of a distant train...

(train whistle blowing)

...the smell of coffee,

the warmth of the sun--

into an experience of the world.

And that experience,

that awareness of the world,

is what scientists
call consciousness.

In neuroscience,
consciousness is the Holy Grail.

Humans have been fascinated by
consciousness

for thousands of years,
probably much longer than that.
(chuckling)

Take three.

Now, of course,
the word "consciousness"

means a lot of
things to different people.

To some, consciousness means
being awake,

as opposed to asleep.

Or self-aware, or the
contents of my thoughts.

But that's not how we
neuroscientists think about it.

We think of it as something
much more basic--

it's just internal experience.

It feels like
something to see the color red.

To taste a strawberry.

(thunder rumbling)

To hear the crack of thunder.

(thunder crashing)

SETH:
We are complicated
biological creatures,

but the most central
feature of our lives

is that we are
conscious creatures, too.

When I open my eyes,

it's not just that my brain does
some sophisticated processing

of the visual information.

I have an experience.

BERLIN:
During the course of my journey,

I've seen how our experience
of reality

is not what it seems.

If my conscious awareness

is built from my perceptions,
flawed as they may be,

how does that work
and what does it mean?

♪ ♪

So please take a seat.

BERLIN:
Some of the first clues trickled
in from people like this.

LORELLA BATTELLI:
Put your chin on the
chin rest.

BROWN:
A lot of very valuable
information

comes from patients who've had,
part of the brain's damaged.

By piecing these various parts
together, seeing what was lost,

we've come to appreciate
the role

that these various brain regions
play

in the creation of
consciousness.

We're going to
calibrate your eyes first.

SETH:
A powerful example of this

is the phenomenon of blindsight.

BATTELLI:
This is a patient who had a
stroke

in her visual areas,

in the back of the brain.

And this stroke is
affecting her visual field.

BERLIN:
Three years ago,
she felt a pounding in her head.

WOMAN:
I had what
I thought was a migraine.

I actually went
to the emergency room

because I'm walking around with
this area where I can't see.

BERLIN:
The stroke damaged
a piece of the brain

devoted to vision,
leaving her with an apparent

total blind spot.

WOMAN:
That blind spot, it's enough
that if you're driving,

an oncoming car disappears into
it.

It's a little anxiety-producing
and, and things like that.

BERLIN:
In everyday tasks,

her eye movements make up
the difference.

But what happens when
she doesn't move her eyes?

Neuroscientist Lorella
Battelli wants to find out,

so she developed a clever series
of experiments to pin down,

just how blind is
she really in that spot?

BATTELLI:
We're using EyeLink,

which is the eye-tracking system

to make sure she doesn't move
the eyes.

BERLIN:
She keeps her eyes focused on
the center spot.

Every time she hears a beep,

she has to say if those little
dots inside the circle

are moving to the left
or to the right.

Left.

BERLIN:
The eye tracker checks that
she's not shifting her gaze.

WOMAN:
Right.

When you're doing tests like
this,

that blind area, what does that
kind of look like for you?

What does it feel like for you?

When that target pops up
in my blind area,

I don't see it.

(machine beeps)

Left.

BERLIN:
Strangely, even though she says
she doesn't see

anything in the blind spot,

she gets it right more often
than not.

WOMAN:
Right.

BERLIN:
So that some information

is getting in.
Yeah, so...

But they're not consciously
seeing it,

but they can respond
to it in different ways.
Exactly.

BATTELLI:
Even if they say,
"I didn't see anything."

Left.
But you tell them,
"Please, just tell me

whether you saw it or not,"

then their response
would be above chance.

BERLIN:
So just keep your eyes closed,
okay?

And...

BERLIN:
What's going on?

To probe deeper,
Lorella lets me give the patient

a different
version of the challenge.

I put a miniature
screwdriver in her blind spot.

BERLIN:
Okay, I'm going to have
you open your eyes and fixate.

Okay.
Okay.

I see nothing.

You see nothing.

Nothing.
Okay.

BERLIN:
Even though she says

she sees nothing,
look at which tool she picks.

Now turn over there
and look at the objects.

Tell me what you think
you saw.

WOMAN: The screwdriver.
BERLIN: Yep.

BERLIN:
Now try another one.

BERLIN:
Next, I display a tiny wrench.

♪ ♪

Did you see anything?

No.
No? Okay.

Look over there and guess
what you think you, was there.

I think the wrench?
Yep.

WOMAN:
I'm going to guess
the scissors.

Yeah, good job, great,
scissors.

BERLIN:
Time and time again,

she makes the right choice.

Amazing, so, you know,
it seems

to me that you're saying you're
not seeing anything,

yet when
I'm asking you to choose,

you're pretty much getting it
correct,

so something is getting in.

BERLIN:
How is this possible?

It's as if she sees
the tools, but doesn't know it.

BATTELLI:
They actually saw something.

Mm-hmm, certainly.
But they're
not entirely aware of it.

Information is getting in,
affecting our behavior

and how we're responding
to the world around us,

without there being
a conscious perception

of that piece
of visual information.
Correct.

WOMAN:
I'm gonna go
with the hammer again.

BERLIN:
Until patients like this,
we scientists had never seen

perception separate
from conscious experience.

And this tells us that
perception

and consciousness are separate
things in the brain.

But it also has me wondering,

if someone can still use
visual information

without awareness of it,

why do we have
consciousness at all?

What is consciousness for?

A clue might come from babies.

(cooing)

ALISON GOPNIK:
Babies-- everything we know
suggests

that they're born conscious.

They're certainly taking
in information

from the time they're born.

REBECCA SAXE:
They are making rational choices

about what they learn
from extremely early on.

And they are forming memories
of their specific surroundings,

of their parents,

of their important
relationships.

We can see that
in their behavior.

BERLIN:
And all that behavior
burns a lot of fuel.

GOPNIK:
Brains are expensive computing
gadgets.

So while you're just sitting
here,

your brain is using up about 20%
of all the calories

that you have, so
it's using up quite a bit.

But if you think about
a two-year-old,

his brain is using
60% of his calories.

So almost all that food is just
going to keep his brain going.

BERLIN:
To understand why young brains
might need so much more fuel,

check out the connections
in a toddler's brain

versus an adult's.

A two-year-old's brain has about
two quadrillion synapses.

By the time they hit adulthood,
that number is cut in half.

So if you think about the
difference between

the baby brain, the child's
brain, and the adult brain,

the child's brain is more like
back country roads

where you have little,
tiny roads

that are going
from one village to the next.

None of
them are very efficient.

There's not a lot of traffic,

and the traffic doesn't go very
quickly,

but they connect lots
and lots of different places.

And the adult brain is more
like superhighways

that get you from one place
to another very quickly,

and take a lot of traffic,

but don't connect as many
different places.

BERLIN:
As we age,
in the interest of efficiency,

we strengthen the connections
that are useful to us

and prune the rest.

MACKNIK:
You basically take neurons you
don't need, you get rid of them.

And what you have
now is a very lean machine

that does certain things
and it does them very well.

GOPNIK:
We see this early brain
that's very exploratory,

that has lots and lots of
potential,

lots of possibilities.

Not very good at
putting on your jacket

and getting out to
preschool in the morning.

And then we have this
later brain

that's very good at doing
things.

Not so good at changing,

not so good at taking in new
information,

not so good at doing something
new.

BERLIN:
What this suggests is that maybe

what consciousness is for

is choosing what's
important for us to be aware of

at any given moment.

Kind of like a spotlight.

GOPNICK:
For adults,
it's as if consciousness is

this bright spotlight in
one place

and everything around it is
dark.

BERLIN:
While for children and babies,

it would be
more like a flood light,

where nearly everything is
illuminated.

GOPNIK:
You're conscious of a lot more
that's going on.

BERLIN:
Consciousness may
be like an amplifier,

boosting the important
signals over the noise.

Is there any evidence?

This is where FMRI comes in,

a special tool in neuroscience
that takes pictures of the brain

while it's doing something
to map where blood flow

is in high demand.

The result is
a map of brain activity.

So where is consciousness
in the brain

and how does it work?

To find out,
neuroscientists designed

a clever series of
experiments that go like this.

They start by flashing a word

on a screen for about
30 milliseconds.

(computer beeps)

DEHAENE:
You flash this word,

and the person is not
able to see the word at all.

She says, "There was no word."

even though people say
they don't see anything.

So the trick here is to find the
threshold.

Find the timing where sometimes

people consciously see the
image.

DEHAENE:
So if you make now
the word a little bit longer,

suddenly, the person says,

"Oh, well,
there is a word, obviously."

And it's completely visible.

There is really a sort
of all or none phenomenon.

Either you see it or you don't.

And once you've done that,

you've got
a really powerful window

onto the neural correlates of
conscious perception.

BERLIN:
When that happens,

suddenly, a suite of different
parts of the brain

shows a surge in activity:
the parietal cortex,

which integrates the senses,

the anterior cingulate,

which modulates
drive and decision-making,

and the prefrontal cortex,

which handles reasoning
and higher-order cognition.

An ignition of
distributed brain areas

that come online together,
speak to each other,

and broadcast this information
to the rest of the brain.

And this is what
we think is occurring

during conscious perception.

BERLIN:
According to some experts,

this communication
between brain regions

is the signature
of consciousness.

This discovery could have
real-world applications

in matters of life and death.

But that would take one
more step:

figuring out how
to measure consciousness.

♪ ♪

SETH:
In science, when we struggle
to understand

a phenomenon
that seems quite mysterious,

it's often really important
to be able to measure it.

So a few hundred years
ago,

this happened with heat-- you
know, it was the development

of thermometers that catalyzed
our understanding.

Could something work for
consciousness the same way?

Could we have
a consciousness-ometer

that will lead us

to a deeper understanding of
what consciousness is?

BERLIN:
That deeper understanding
could transform

the treatment of people with
brain injuries.

BRIAN EDLOW:
So every year,
over a million people worldwide

will come into an intensive care
unit unresponsive, comatose.

The challenge that we face
is that our bedside exam--

asking the person
to open their eyes,

pinching them and
seeing if they'll respond,

seeing if they move their arms
and their legs--

that bedside exam
is fundamentally limited.

BERLIN:
Limited because it often misses
people

who are actually conscious.

In this case,
we have a healthy volunteer

from the "NOVA" team,

but what if
she were unresponsive?

How would we ever know
if she was conscious?

Brian Edlow's team
at Mass. General Hospital

is testing a new technique
to find out.

So tell me, what are you doing
here?

EDLOW:
We are pinging the brain
with a magnetic pulse

and looking for
an electrical echo.

The ping is transcranial
magnetic stimulation,

or TMS.

The echo is the key;
if it dies out quickly,

the patient is unconscious--
they might be in a coma,

deep sleep, or under anesthesia.

If instead the echo rings out
across the brain

and becomes more complex,

the patient is likely
to be conscious and aware,

even if they appear
unresponsive.

EDLOW:
The analogy that we like
to use

is throwing a pebble in a lake.

So the pebble represents the TMS
pulse to stimulate the brain,

and the brain waves,
the electrical ripples

that emanate from that pulse,
represent the waves in the lake.

The more
complex those waves are,

and the longer duration,

the more likely that person is
to be conscious.

BERLIN:
To detect those brain waves,
Edlow's team uses E.E.G.,

a tool that measures
electrical activity

in the brain, to quantify
the amount of complexity

a patient's brain bounces back.

Here's how it works.

♪ ♪

All neurons,
when poked by a magnet,

will kick back an electrical
signal that looks like this:

a brain wave.

But if the surrounding neurons
aren't healthy,

those brain waves won't get
very far.

It turns out that
in conscious people,

even those
who appear unresponsive,

not only do those brain
waves spread all over the brain,

but they become more complex
too-- it's as if,

to use music as the analogy,

what starts as a single repeated
note by a few neurons

eventually turns into
a coordinated symphony

of millions.

♪ ♪

DEHAENE:
We find is that there
is this explosion of complexity

only when
the person is conscious.

This complexity,

the way different brain areas
speak to each other,

is a signature,
a marker of consciousness.

BERLIN:
Studies of hundreds of
patients in various states--

from deep sleep to anesthesia
to coma--

have enabled scientists
to develop a complexity scale.

A score above
a certain threshold

means you are conscious or have
the capacity for consciousness.

EDLOW:
Multiple studies have now shown

that 15% to 20% of patients who
appear unresponsive,

they don't express themselves on
our behavioral exam,

they are actually conscious.

So, you know,
will this be able

to help
people in those states?

When we speak to families

about what matters to them most,

it is that patient's
current level of consciousness

and their potential for future
recovery of consciousness.

If families were to have

that information,
it could fundamentally affect

the decisions they make about

whether to continue
life-sustaining therapy.

DEHAENE:
Progress in the clinic
is extremely real and fast,

and people realize that the
problem of consciousness

is starting to be solved.

♪ ♪

BERLIN:
Starting to be solved,

because while we have
several clues

about how conscious awareness
might work in the brain,

this is only the beginning.

How all of those pieces
of brain activation add up

to you, a distinct individual
with a sense of self,

is still a mystery.

I know my brain creates
an internal experience

by knitting together bits
of sensory information,

filling in the gaps with its
best guess

of what's out there in the
world,

but what are those guesses
based on?

Memory.

Each of us has a life rich
with experiences to draw from.

Where we were born,
went to school,

who we fell in love with.

Memories are the cornerstone
of our identities,

but as it turns out,

they have
a very shaky foundation.

I could swear by it,

and would pass every
lie detector test, that...

I had met Mother Teresa.

But I hadn't.

Something that I wanted to
happen

but it never did happen.

SCHILLER:
The stories we tell

ourselves, or what we consider
our memory, is a construction.

We create these representations.

And they're very dynamic,
they constantly change.

You're kind of living a revision

of the story of your life,
constantly.

SETH:
The more
often we recall things,

the less objectively accurate

our memories become.

BERLIN:
It turns out that every
time you a recall a memory--

your first kiss,

graduating from college,

the death of a loved one--

the very act of recollection
makes it vulnerable to change.

SCHILLER:
So when you experience
a new event,

it has to be
stored in the brain.

And then, we used to think that

whenever you think about that
event,

you retrieve the same original
memory.

But what we got to
realize in the last few decades

is that whenever
you retrieve a memory,

it goes back
to an unstable state.

BERLIN:
In 2000, memory scientist
Eric Kandel won the Nobel Prize

for showing that each memory
creates new synapses,

connections
that store the memory.

But what happens
when you recall it?

Every time you remember it,

you bring it up into your
working memory

and you perceive it,

and you destroy the long-term
memory.

And you actually have

to recast it into long-term
memory

when you re-remember it.

So every single time
you remember something,

you actually add more
noise to it,

so that it's more and more
and more false

throughout time.

BERLIN:
This mechanism,
called reconsolidation,

was first discovered in rodents,

where neuroscientists witnessed
what happens

when a memory gets recollected:

for the memory to return
to long-term storage,

the connections between neurons
actually have to get rebuilt.

Recent experiments
have suggested

this is likely a mechanism in
human brains, as well,

because certain drugs known
to disrupt reconsolidation

have been shown
to alter human memories.

FENTON:
We're stuck with the problem
of, how do we know what is true?

How do we know what's real?

And maybe

part of the recognition is,
some of those things

don't matter as
much as we think they do.

SCHILLER:
If we think about the fact that
maybe our memories are not

as they originally happened,

it could be a scary thought,
because then, who are we?

I think you need
to think about it as something

more liberating,
because if you're stuck

with original representations,

you're kind
of stuck in the past.

BERLIN:
Just like our perceptions,
our sense of self is dynamic,

built to serve
us in the present.

SETH:
Our experience of, of self

is a construction at all sorts
of different levels.

What the brain is doing,
is interested in,

is weaving
together a kind of story.

FENTON:
The brain is
a storytelling machine, right?

It's a machine that's
designed to make predictions.

KASTHURI:
The narratives
that we tell ourselves

are the biggest illusions
that we ever participate in.

Your sense of who you are is an
illusion,

as everything else--
you're no exception.

BERLIN:
But if even our sense
of self is an illusion,

where does that leave us?

MARTINEZ-CONDE (chuckles):
Trust the illusion,
that's the only thing

that we can be sure of,

that what
we perceive is not what's there.

BERLIN:
So all these years later,

in my quest to understand where
my thoughts come from

and how my brain works,

I've learned that my brain
is an exquisite machine

that perceives reality
in the service of survival,

not accuracy.

The world I carry inside of my
head

is a construction of my
brain

built on bits of sensory
information

woven together with memory

to create a conscious
experience.

Now, to some this might sound
scary,

but to me, it's inspiring.

♪ ♪

SETH:
The simple act of
opening our eyes

and seeing a world,

we should not
take that for granted.

And in realizing

what a miracle of neural
computation

is going on under the hood,

to give us even the simplest
experiences,

I think this adds value,

it adds meaning,
it adds depth to our lives.

♪ ♪

DEHAENE:
I think it's liberating
to understand

that we rise from
this organization of matter.

It means that we can
be a little bit more humble.

We are gorgeous machines
designed by evolution

as well as by our environment,
education, friends, families.

SAXE:
Sometimes when I wonder what I'm
doing with my life,

I think how is it that a spatial

and temporal pattern
of electrical signals

passing between cells in
our brains makes us who we are?

That just being
a part of the team

asking that question
is worth keeping going for.

♪ ♪

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♪ ♪