What Is Reality? The Brain with David Eagleman

The most complex thing we have discovered in the universe...

..is the human brain.

For the past 20 years, I have been trying to understand

how what happens in three pounds of Jell-O-like material

somehow becomes us.

What we feel, what matters to us, our beliefs and our hopes -

everything we are happens in here.

For me, there is one mystery that is absolutely fundamental -

what is reality?

What if I told you that this world around us,

this richly textured world,

were all just an illusion constructed in your head?

What if I said that the real world has no smell or taste?

No sound?

BACKGROUND NOISE CUTS OUT

What if I said there was no colour?

If you could perceive reality as it really is out there,

you wouldn't recognise it at all.

I want to show you how the brain takes in information,

sifts through it to find patterns,

and uses it to build the multi-sensory technicolour show

that is your reality.

When I am in the world, my senses are flooded

with sights and sounds and smells.

It seems obvious that reality is just out there.

There is a person, there is a cab.

All I have to do show up

and my senses let me experience it all.

But there is a twist to this story.

Let me show you something.

So take a look at this middle square here.

Does that look more similar to the light square or the dark?

Well, it looks like a light square, yeah?

You might be surprised that if I move it,

now it looks like a dark square.

-SHE GASPS

-Oh, my God.

It is the same.

-Oh!

-It is surprising, right?

-It is.

-Oh, my goodness.

-Wow.

Seriously?

DAVID LAUGHS

Do you have a guess as to why there is an illusion here?

Well, it seems like there is a shadow,

-so it makes this darker.

-That is exactly right.

Your brain is trying to understand the colours of things

irrespective of the lighting and the shadows.

So somehow it's not about what's hitting your eyes,

it is about your brain's interpretation.

That's...really trippy.

You have just messed up my whole day.

DAVID LAUGHS

Now this is about more than just a visual illusion.

It's about a fact that's central to our lives.

Our perception of reality has less to do with what's happening out there

and more to do with what's happening in here.

To understand what's going on,

we first need to know how information from the world around us

gets into the brain.

It feels as if sights and sounds just stream in

through our eyes and our ears.

But imagine if you could climb inside a human skull.

When you step into the skull,

you will find that there is no way for light or sounds

or smells to get directly in here.

This is a sealed chamber...

..so the brain sits in darkness and silence.

It's in total isolation.

Your brain has never seen the outside world,

but somehow you experience it.

Now this might seem straightforward

because we have portals to the outside world,

like your eyes and ears,

but these aren't just piping in sights and sounds.

Instead, photons of light or air compression waves,

these are getting converted into the common currency of the brain -

electrochemical signals.

These signals travel through

dense networks of brain cells called neurons.

There are 100 billion neurons in the human brain,

and in every second of your life,

each one of these is sending tens or hundreds of electrical pulses

to thousands of other neurons.

And somehow, all of this activity produces your sense of reality.

So whether it is the bark of a dog or the smell of coffee

or a view of a beautiful sunset,

it's all made of the same stuff in here.

And this is the stuff of reality.

But how does the brain turn it into something meaningful?

Well, it does it by sifting through the nonstop stream of incoming data

to find patterns, which are then assembled into a reality.

It's an operation which is the product

of millions of years of evolution.

So efficient, so powerful

that its work seems effortless and instantaneous.

Take, as an example, sight.

The act of seeing feels so natural

that it's hard to appreciate the vast, sophisticated machinery

running under the hood.

For us to see clearly,

many different systems need to be operating in concert.

It's about more than just the eyes.

The best way to understand this

is to look at the extraordinary case of a man who lost his sight...

..and then was given the chance to get it back.

I lost my sight when I was three-and-a-half years old

as the result of a chemical explosion.

Oddly, it didn't seem like it was a big deal.

I guess as a three-and-a-half year old,

my world according to vision was not as well-established

as it would be for somebody who lost their vision later in life.

After over 40 years of blindness,

Mike May had pioneering stem-cell treatment

that would repair the physical damage

that the explosion caused to his eyes.

Cameras were there to witness the moment

when, for the first time, the bandages came off.

'Dr Goodman does the cornea transplant.'

'He peels back the bandages.'

He gets all the way off, and there is this whoosh of light

and bombarding of images onto my eye.

Holy smoke!

In surgical terms, the operation was a total success.

What's across the room over here?

But to Mike, it wasn't.

There was something wrong.

All of a sudden, you turn on this flood of visual information.

It's overwhelming.

My brain is just going, "Oh, my gosh."

So that is how the world proceeded - one image at a time.

Seeing cars as they whizzed by...

..and then I would see a sign ahead of us

and it looked like we were going to smack right into it.

In fact, it's the sign over the freeway

and we are not going to run into it, we are going under it.

That was only the first hour.

It was going to get worse when Mike got home.

If you put four blonde boys together,

all roughly the same height...

I looked at them, I couldn't tell you which two were mine.

Don't go away, I've not finished looking at you.

Mike's new eyes were functioning perfectly

and they were sending signals to the brain just like yours or mine do,

but he couldn't see his sons in any meaningful way.

I had no face recognition whatsoever. None.

When he had been totally blind, Mike was a Paralympic skier.

But his first sighted attempt at skiing was a complete failure.

When I skied for the first time,

because of my depth perception difficulty,

I had no time to figure out the difference between four dark things

on the white snow -

a person, a tree,

a shadow or a hole.

Ten years on, Mike still needs his guide dog to get around.

He can detect light and motion and identify colours,

but he struggles to gauge how far are things are.

He still can't read the expressions on his sons' faces.

He still can't read words on a page.

What Mike's story gives us

is a glimpse of all the elements that have to be in place

for the brain to construct a visual reality.

Many regions of the brain are involved in vision.

They specialise in different aspects, such as motions, edges,

colours, face recognition.

Somehow, the brain weaves all of this together,

unifies it to form what we experience as an image.

In Mike's case, decades of blindness

caused these regions of his brain to be taken over for other tasks,

like hearing and touch.

They just weren't available for him to use,

even when he was given a pair of new eyes.

We often get our best view of how the brain operates

when that operation is disrupted.

-Hey, Brian.

-Hey.

That is why neuroscientists sometimes disrupt things deliberately.

Brian is part of an experiment being conducted by Alyssa Brewer

at the University of California.

-Good to see you.

-Welcome. Are you ready to try the goggles on?

Oh, I am ready.

'Volunteers wear these goggles for weeks at a time.

'Their brains are forced to cope with a new view of the world

'that is dramatically altered.'

What these have inside are two prisms

that take the whole visual world and flip it.

So whatever you see normally on the left side of the world

will now be on the right side of the world.

So as you move through the world, you're going to have a problem

figuring out where things are as you see them on one side

but reach for them on the opposite side.

What the world looks like is this,

but what I'm seeing...

is this.

It is a straightforward change,

but it's also a massive mind mash.

The visual data streaming in through my eyes

no longer makes any intuitive sense,

and I'm struggling.

So, yeah, because the world is left-right flipped,

I know cognitively I am supposed to reach out in the other direction,

but, of course, I have had a lifetime of training

telling me to reach out in a particular direction.

So I feel like this is going to take a little getting used to.

Can you see my hand in your visual field?

Yeah, so it looks like if I reach out this way...

And...this way.

THEY LAUGH

'Even though I'm consciously trying to get it right...'

Over...here, OK.

'..I can't help but respond in a certain way.'

And over here.

There you go. Very good.

-Welcome to the prism world.

-Yes.

Of course, this is all new to me,

but Brian has been wearing his goggles for a week.

So how well has his brain adapted?

It is very difficult to figure out which way to go,

so his motor system, his feeling of touch

is sending him one direction

while his visual system is sending him the other direction.

Brian is doing well.

Unlike me.

I have to consciously reconstruct my reality.

This morning, my brain could rely on automated interactions,

but now it can't.

Interestingly, I've broken out in a sweat and I'm hot,

and I'm super-dizzy and nauseated.

You know what? I've got to take a break.

I'm so sorry. I've got to take these off for a second.

Is that OK?

Boy, that is really nauseating.

We're going to go to the maze down here and see how you guys do

in navigating your way through a spatial map.

-You're going to start out going this way.

-OK.

-Brian, you're going to...

Oh, God! SHE LAUGHS

I'm just going to give him a head start.

So how do I get as good as Brian?

Well, it happens intuitively.

Just look at my hands.

I cross-reference what I see with what I can touch.

In fact, all my senses come into play.

This is what Brian has been doing for the last seven days.

The result is that his brain is now starting to decode

that new visual input automatically.

Brian is not simply getting better at making conscious adjustments -

his whole reality is changing.

If you take those subjects and put goggles on them for two weeks,

you find that it takes them about a week to start behaving normally.

They start being able to figure out how to interact with the world,

constructing a new reality around them,

a new way of dealing with these incoming perceptions.

They say that initially they can tell there is a new left

and an old left, and a new right and old right.

By about a week in, they even lose the concept

of which right and left were the old ones and the new ones.

It is like their whole spatial map of the world is altering.

By two weeks in, they will write well, read without a problem,

do all of our walking tasks and reaching tasks.

Then when we remove their goggles,

it actually takes about a day to go back to normal behaviour.

What this exposes for me is how much effort the brain goes through

to construct our world,

because normally you're walking through the world

and it feels like there's reality out there.

But in fact there is so much work happening behind the scenes

to allow that reality to happen.

Seeing requires an intensive training programme,

but new recruits come on board every day.

We call them babies.

When babies reach out to touch what's in front of them,

they are not just learning what an object feels like,

they're learning how to see.

They're establishing pathways in the brain that will be used

for the rest of their lives.

Because vision is a whole-body experience.

The data coming in from our eyes only means something

if we can cross-reference it.

BABY BABBLES

If from birth you weren't able to interact with the world,

if you couldn't work out through feedback

what the sensory information meant,

in theory, you would never be able to see.

This cross-referencing doesn't stop when we are fully grown.

It continues throughout our lives.

What we touch influences how we see.

Taste is affected by our sense of smell.

Our sight informs how we hear.

Our senses depend on each other,

and our reality is built by comparing these streams of data.

When they're woven together, we get our perception of this moment.

It's an astonishing feat to pull off,

but there's one factor which really adds complication...

..timing.

Al those streams of sensory data

are processed by the brain at different speeds.

For our reality to be constructed, they have to be synchronised.

What do I mean by this?

Well, the easiest way for me to show you is right here at a racetrack.

Set.

When there is a loud sound, it feels as though you react to it instantly.

But you don't.

Watching sprinters in slow motion,

we can see that there is a gap

between the gun going off and their start.

They may train to make this gap as small as possible,

but their biology imposes limits.

Processing that sound

then sending out signals to the muscles to move

will take around two-tenths of a second.

GUN FIRES

And that time really can't be improved on.

In a sport where thousandths of a second can be the difference

between winning and losing, it seems surprisingly slow.

So why do we use a pistol to start sprinters?

Everyone knows that light travels faster than sound,

so why not use a light?

We set up a test to show you.

In the top screen, we're triggered by a light.

In the bottom screen, we're triggered by the gun.

GUN FIRES

You can see that when our start is triggered by a flash of light,

we respond more slowly.

It takes 40 milliseconds longer to process.

Why?

Because the visual system is more complex.

It's bigger - it involves almost a third of the brain.

So while all of the electrical signals inside the brain

travel at the same speed,

the ones related to sight go through more complex processing,

and that takes time.

This isn't just about hearing and seeing.

Every type of sensory information

takes a different amount of time to process.

You will react slower to a touch on the foot than one on the hand.

The astonishing thing is that our brains hide all this.

When I clap my hands, everything seems synchronised. Why?

Well, your brain is pulling off fancy editing tricks.

What it takes to be reality is actually the delayed version

that collects all the information from the senses

before it decides on a story of what happened.

That means you live in the past.

By the time you think the moment "now" occurs...

it's already long gone.

To conjure a reality from all that sensory information,

your brain needs around half a second.

That's the unbridgeable gap between an event occurring...

..and your conscious experience of it.

In that half a second, a lot of things need to happen.

Sometimes it's easy to assume that there is a single spot in the brain

that takes care of this or that function -

like an area for memory or generosity or empathy.

But in fact, the vast networks of the brain

are so much more complex than that.

Think of the brain like a city.

If you were to look out over a city and ask,

"Where is the economy located?"

you'd see that there is no single answer to that.

Instead, the economy emerges as an interaction of all the elements.

So it is with reality.

The raw materials of perception are gathered by our sensory receptors.

They are turned into electrical signals

and transported around our brains along superhighways of neurons.

Processed, they become our reality.

Some parts of Brain City specialise in vision,

other districts care about hearing,

some about touch, and so on.

Even within a sense like vision,

you have streets that specialise in colours, or edges, or motions.

But just like in a city...

no neighbourhood operates in isolation.

Instead, the life of a city depends on the interaction

between residents at all different scales.

And somehow, out of all of this interaction

emerges your personal reality.

Reality is the brain's ultimate construction.

It's based on all the streams of data from our senses,

but it's not dependent on them. How do we know?

Because when you take it all away, reality doesn't stop.

It just gets stranger.

This is Alcatraz.

A jail built on the principle of isolation.

Between its inmates in the rest of society stood not only stone walls...

..but the cold, dangerous waters of the San Francisco Bay.

Prisoners were completely and deliberately cut off.

And there was one place inside the prison

where that seclusion went even further.

This is the Hole.

Prisoners who were sent here were completely isolated

from the outside world.

They had no interactions with people,

there was no sound and there was no light.

Robert Luke was sent to Alcatraz in 1954 for armed robbery.

He was known by the nickname Cold Blue Luke.

Everybody knew about the Dark Hole.

The Dark Hole was a bad place.

Some guys couldn't take that.

I mean, they were in there and within a couple of days

they were banging their heads on the wall.

As punishment for smashing up his cell,

he was sent to the Hole for 29 straight days.

You didn't know how you would act when you got in there.

You didn't want to find out.

When they closed that door...

..there was just nothing there. It is pitch-black.

But it didn't stay that way for long.

Starved of input, Luke's brain started to produce its own reality.

I remember I'd go on these trips.

One I used to remember was flying a kite.

It got pretty real.

They were all in my head.

What Luke felt was something that has also been reported by other prisoners

kept in the same conditions.

Deprived of new sensory information,

they said they went beyond dreaming or daydreaming.

They didn't just imagine pictures...

..they saw.

This testimony goes to the heart of the relationship

between the outside world,

the brain, and what we called reality.

To understand it, we need to look more deeply into the visual system.

This is the thalamus -

one of the brain's major junctions.

Most sensory information connects through here

on its way to the outer surface of the brain, the cortex.

So data collected from the eyes stops here

before going to the visual cortex.

Now, you'd expect a heavy flow of information

from the thalamus to the visual cortex, and there is.

But there is six times as much traffic

flowing in the opposite direction,

and that dwarfs the amount coming in from the eyes.

That suggests that in any one moment,

what we experience as seeing

relies less on the light streaming into our eyes

and more on what is already inside our heads.

Even when brains are unanchored from external data,

they continue to generate their own imagery.

In other words, remove the world, and the show still goes on.

We all have this internally generated reality.

Incredible as it may sound, this world lives inside your brain.

It's constantly updated by information from our senses,

but moment to moment what we experience

isn't what's really out there.

Instead, it's a beautifully rendered simulation.

This is a surprising way to understand how you see the world.

It's called the internal model.

And it's vital to our ability to function.

As I walk down this city street, I seem to automatically

know what things are without having to work out the details.

For example, I don't have to work out the detail of what this rectangular,

metallic thing is, or this giant green, fluffy thing behind me,

or this huge object with reflective panes on it,

or this thing with four appendages.

My brain makes assumptions about what I'm seeing

based on my internal model,

and that's been built up from years of experience of walking city streets

just like this one.

Instead of using my senses to rebuild my reality from scratch every moment,

I'm comparing sensory information

with a model that I've already constructed.

Updating it.

Refining it.

Correcting it.

Our brains are so good at doing this that we're normally unaware of it.

But sometimes, under certain conditions,

we can see the process at work.

Look at this hollow mask of Einstein's face.

Your brain tells you it's coming out at you.

And even when you know it's an illusion

you can't help but fall for it.

What you're seeing is the internal model,

not the raw information that's coming in from your eyes.

Your internal model is built on a lifetime of experience

with faces that stick out.

When you're confronted with one that's hollow,

your model simply sees what it expects to see.

The visual cortex sends its internal expectations to the thalamus

and the thalamus compares those to what's coming in through the eyes.

The difference between the two is what the thalamus sends back

so the cortex can update its model.

Thanks to the internal model, the world out there remains stable

even when I'm moving.

Let me show you what I mean.

So imagine that I really love the scene behind me

and I want to go ahead and capture it so I can view it later.

So I'm going to go ahead and videotape the scene

and I'm checking out all the buildings...

OK. And now I'm going to play this back.

Not surprisingly, the resulting video is nauseating.

So why does this video look so terrible,

given that, when I look at the buildings,

my eyes are making the same jerky movements?

Although you're not generally aware of it,

your eyes move about four times a second.

But your internal model operates under the assumption

that the world outside is stable.

So my eyes aren't taking a video, they're simply gathering

bits of data to update the city that's already inside my head.

Having an internal model helps me make sense of my environment,

and that's its primary function - to navigate the world.

The brain doesn't bother picking up every detail,

just enough to get us through.

But it plays the trick of making us feel as though we've seen it all...

..as another famous experiment shows.

In the 1960s,

the Russian psychologist Paul Yarbus used this painting,

called The Unexpected Visitor, in an experiment.

He devised a way to track the eye movements of volunteers

who were seeing it for the first time.

-Hi, Jennifer.

-Hello.

-I'm going to ask you to put these glasses on.

'We're going to rerun what he did.'

'My volunteers have a few seconds to take in the image.'

Look at this painting,

and I want you to gather what's going on in the scene.

'We can watch in real time exactly where each person's eyes go.'

Tell me what you think is going on in this painting.

I think the man in the brown is the unexpected visitor.

'One brief look is enough for the brain to model the picture.

'But just how detailed is that model?'

-How many children were there?

-Uh, there were two.

OK, so look back at the painting and ask that question again.

-Oh, quite different.

-How many children are there?

I can see three.

'Everyone who'd seen the painting thought they knew what was in it.

'But my specific questions highlighted blanks that the brain

'had never filled in, because the details weren't needed.'

-How many paintings are on the wall in their house?

-Maybe two or three?

OK, look back at the painting and answer that question...

Oh, God, there's a million!

-Yeah, a map and then there's...

-SHE LAUGHS

..seven on the other wall and then one small one and the map.

OK, there's a ton.

This is not a failure of the brain.

It doesn't try to produce a perfect simulation of the world.

The internal model is a hastily drawn approximation

and more details are added on a need-to-know basis.

When you looked at the painting the first time you saw a sort of

rough draft of what was going on,

and when I asked you specific questions, you had to answer those

by turning your attention onto specific parts of the painting,

and only then did you actually see it.

So placing your eyes on an object is no guarantee of seeing it.

But there's something else we're unaware of happening

every time we look at any picture or person or thing.

Any time we look at all.

We might think of colour as a fundamental,

defining quality of the world around us.

After all, it's everywhere.

But here's the startling thing.

In the outside world...

..colour doesn't actually exist.

When electromagnetic radiation hits an object,

some of it bounces off and is captured by our eyes.

We can distinguish between millions of combinations of wavelengths,

but it's only inside our heads that any of this becomes colour.

Add to that the fact that the wavelengths we can detect

are only a small part of what's out there.

You experience reality as it's presented by your senses,

and it doesn't typically strike you that things can be very different.

What we've talking about so far

is what we call the visible spectrum of light, which is a spectrum

of wavelengths that runs from what we call red to violet.

But it turns out that this only constitutes a tiny fraction

of the electromagnetic spectrum.

In fact, less than one ten-trillionth of it.

So all the rest of the spectrum, including radio waves

and microwaves and X-rays and gamma rays,

all of this stuff is flowing through our bodies right now

and we're completely unaware of it

because we don't have any specialised biological receptors

to pick up on it.

So what this means is that the part of reality that we can see

is totally limited by our biology.

And this isn't just about sight.

All our senses are only picking up

a small part of the information that's out there.

DOG SNIFFS

So for a dog, he's tuned in to a whole world of scent molecules

that I'm not.

His experience of smell is as rich as my experience of vision.

In the blind and deaf world of the tick,

the important signals are temperature and body odour.

For cave-dwelling bats, it's all about air compression waves

that allow them to echolocate.

But no-one's having an experience of objective reality,

of the world that really, truly exists.

Instead, each creature perceives only what it has evolved to perceive.

And this isn't just about variation between species.

If we're each experiencing a personal reality,

constructed inside our brains,

how do I know that my reality is at all like yours?

Most of the time it seems as if we operate along the same lines,

as if you and I agree what a blue sky is,

as if the sound of a dog bark provokes the same sort of response

in both of us.

DOG BARKS

But there's a small group of people

whose perception is measurably different from ours.

For me, any time I see a letter or a number or think of a word

or say someone's name, there is a lot of colour associated with that.

Hannah is one of 6,000 people I've studied who have synaesthesia.

I study synaesthesia because it's one of the few conditions in which

it's clear that someone else's reality is different from mine,

and it makes it obvious that how we perceive the world

is not "one size fits all".

In my mind I associate each letter with its own colour.

So, for example, the letter A is always red,

B is always blue,

C is always orange - every time. So they never change.

But what's interesting is

when they're formed into words in different orders,

the configuration of the colours changes and that can be interesting.

So in the word "Hannah", my name, it looks a sunset.

It's yellow fading into red fading into kind of a clear...

like clouds, almost,

and then goes back to red and to yellow.

These experiences come about

because of the simple fact that inside the brain,

all sensory information is made from the same stuff -

electrochemical signals.

Synaesthesia is the result of cross-talk between sensory areas

of the brain.

Think of the blurred borders between city districts.

Synaesthesia shows us that even minute changes in brain wiring

can lead to different realities.

There are different kinds of synaesthesia.

Some people perceive weekdays to have locations in space.

Some taste words.

Others see music.

And every time I meet someone who has this kind of experience,

it's a reminder that from person to person, brain to brain,

our experiences of reality can be quite different.

For a small section of the population,

that difference can be extreme and terrifying.

We all know what it's like to have dreams at night,

to have bizarre, unbidden thoughts that take us on journeys,

sometimes journeys we suffer through.

When we wake up, we're lucky enough to be able to compartmentalise that,

to say, "OK, that was a dream and this is my waking life."

But just imagine what it would be like

if these were more and more intertwined

and it was more and more difficult to tell them apart from one another.

I felt like the houses were communicating with me.

"You are special, you are especially bad.

"Repent. Stop. Go."

You know, kind of...

I did not hear these as words, but I heard them as thoughts

put in my head, but I knew they were the houses' thoughts

and not my thoughts.

I think that explosions are being set off in my brain

and I'm afraid it's going to hurt other people, not just me.

I once had a fantasy that my brains were going to leak out of my ears

and drown people.

What is that? You know?

Elyn Saks is a professor of law

at the University of Southern California.

She's been experiencing schizophrenic episodes since she was 16 years old.

It's scary, it's unpredictable.

It's sort of interesting

cos there are different theories about psychotic symptoms.

For some people they're just random firings of neurons.

I do think they tell the truth about your psychic reality,

so when I saw I've killed hundreds of thousands of people

with my thoughts, that's just an archaic

and extreme way of saying I feel like I'm a bad person.

Schizophrenia is still not fully understood,

but it involves chemical imbalances in the brain which cause problems

in the sending and receiving of signals.

Thanks to medication and therapy,

Elyn has been able to lecture and teach for over 25 years.

So when you were at the bottom in one of your worst psychotic episodes,

you took that to be reality?

I really believe what I think is happening is happening,

and it's terrifying. It's like a waking nightmare -

confusion, bizarre images, violence, terror.

I wouldn't wish it on anyone.

That said, everybody's reality is constructed. Right? You...

filter it through your beliefs and values and issues,

and this is true for people who have mental illness

and for people who don't have mental illness, it's all a spectrum.

Reality differs from person to person.

More than that, it changes from moment to moment.

There are times in all our lives when it can seem enhanced, intensified.

Even the one great constant which we all think we share

and which should never change

somehow becomes stretched and distorted.

I'm talking about time.

Time is something that we rarely stop to consider.

But our brain's experience of time is often quite strange.

It doesn't always seem, in certain situations,

that time is running at an even pace.

Sometimes it runs more slowly or more quickly.

When I was eight years old I fell off the roof of a house about this height

and the fall seemed to me to take a very long time.

But when I got to high school, I learned physics

and I calculated, how long did the fall actually take?

And it turns out it was only 8/10ths of a second.

So that set me off on a quest to understand,

why did it seem to take so long?

And what did this tell me about our perception of reality?

Many people have reported this sensation during moments of terror.

Professional wingsuit flyer Jeb Corliss

experienced it in an extreme way.

-How you doing, Jeb?

-Excellent.

And because he falls for a living,

the event he describes was captured on multiple cameras.

On this day, I decided to aim for a target.

Like, set up balloons and come in and hit balloons.

I was flying towards the balloons.

And as I was coming in to hit the black balloon...

I misjudged.

I impacted flat, solid granite

at 120mph.

Six seconds elapsed between the moment Jeb hit the rock

and the moment he pulled his ripcord.

He broke his leg and both ankles in the fall.

JEB GROANS

From Jeb's perspective, those six seconds seemed to last a long time.

You've got two options.

One is you can not pull, and just be dead right now.

It's really quick, semi-painless, over fast.

Or you can pull, you know, get a parachute over your head,

impact a second time and then bleed to death

while you're waiting for rescue.

These two separate thought processes

felt like minutes of time.

It feels like you're operating so fast that your perception

of everything else seems to slow down.

Everything just gets stretched.

HE GROANS

But what was really happening in Jeb's brain?

I designed an experiment to find out.

It depended on inducing extreme fear in people

by dropping them from 150 feet in the air.

They fell with a digital display strapped to their wrist.

Its numbers were changing

at a rate faster than human vision can normally handle.

If perceptual time did slow,

then they would be able to read the numbers.

But no-one could.

So why did Jeb recall his accident as happening in slow motion?

It was a time distortion on a level I've never experienced before.

I learned later that the rescue took about two and half hours.

But at the time it felt like weeks.

It didn't feel like...

minutes or hours or even days,

it felt like eternities. It felt like forever.

The answer seems to lie with how our memories are made.

In a critical situation, an area of the brain called the amygdala

kicks into high gear.

It commandeers the resources of the rest of the brain,

forcing everything to attend to the situation at hand.

When the amygdala is in play,

memories are laid down with far more detail

than under normal circumstances.

These memories are richer and more vivid.

If you're ever in a similar situation, you have more information

at your disposal to work out how to stay alive.

But there's a fascinating consequence.

When the events are replayed in your memory,

they appear to have taken a longer time.

Ow! Ow-ow-ow!

Jeb's time distortion is something that happened in retrospect.

A trick of the memory that wrote the story of his reality.

The brain is the universe's ultimate storyteller.

We believe whatever our brains serve up to us.

The reality we take for granted requires intensive training

to interpret the world.

It takes time to process sensory information,

so we live in the past.

And because all that information

is ultimately just electrochemical signals

to be sorted, matched, rendered and packaged,

reality is something created inside our head.

Our brain sculpts our reality using the narrow trickle of data

it can gather through the senses,

and from that trickle it tells a story about our world.

It's possible that every brain tells a different narrative.

With seven billion human brains wandering the planet...

..trillions of animal brains...

..no-one is tapped into the full picture.

Each brain carries its own unique model of the world around us.

That is what we experience.

We have no choice.

So what is reality?

It's whatever your brain tells you it is.

Next time on The Brain, I'm going to explore a fundamental question

about our lives.

What makes you...you?

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.

This three-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.

What shapes who you become?

I'm going to explore how your life shapes your brain

and how your brain shapes your life.