0:00:03 > 0:00:06The most complex thing we have discovered in the universe...
0:00:08 > 0:00:09..is the human brain.
0:00:13 > 0:00:16For the past 20 years, I have been trying to understand
0:00:16 > 0:00:21how what happens in three pounds of Jell-O-like material
0:00:21 > 0:00:24somehow becomes us.
0:00:27 > 0:00:32What we feel, what matters to us, our beliefs and our hopes -
0:00:32 > 0:00:36everything we are happens in here.
0:00:42 > 0:00:47For me, there is one mystery that is absolutely fundamental -
0:00:47 > 0:00:48what is reality?
0:00:50 > 0:00:53What if I told you that this world around us,
0:00:53 > 0:00:55this richly textured world,
0:00:55 > 0:00:59were all just an illusion constructed in your head?
0:01:04 > 0:01:09What if I said that the real world has no smell or taste?
0:01:10 > 0:01:12No sound?
0:01:12 > 0:01:14BACKGROUND NOISE CUTS OUT
0:01:16 > 0:01:19What if I said there was no colour?
0:01:23 > 0:01:26If you could perceive reality as it really is out there,
0:01:26 > 0:01:28you wouldn't recognise it at all.
0:01:31 > 0:01:36I want to show you how the brain takes in information,
0:01:36 > 0:01:39sifts through it to find patterns,
0:01:39 > 0:01:45and uses it to build the multi-sensory technicolour show
0:01:45 > 0:01:48that is your reality.
0:02:09 > 0:02:12When I am in the world, my senses are flooded
0:02:12 > 0:02:15with sights and sounds and smells.
0:02:15 > 0:02:18It seems obvious that reality is just out there.
0:02:18 > 0:02:21There is a person, there is a cab.
0:02:21 > 0:02:23All I have to do show up
0:02:23 > 0:02:26and my senses let me experience it all.
0:02:29 > 0:02:32But there is a twist to this story.
0:02:32 > 0:02:34Let me show you something.
0:02:36 > 0:02:38So take a look at this middle square here.
0:02:38 > 0:02:41Does that look more similar to the light square or the dark?
0:02:41 > 0:02:43Well, it looks like a light square, yeah?
0:02:43 > 0:02:45You might be surprised that if I move it,
0:02:45 > 0:02:47now it looks like a dark square.
0:02:47 > 0:02:49- SHE GASPS - Oh, my God.
0:02:49 > 0:02:51It is the same.
0:02:51 > 0:02:54- Oh!- It is surprising, right?- It is.
0:02:54 > 0:02:56- Oh, my goodness.- Wow.
0:02:57 > 0:02:59Seriously?
0:02:59 > 0:03:00DAVID LAUGHS
0:03:00 > 0:03:03Do you have a guess as to why there is an illusion here?
0:03:03 > 0:03:06Well, it seems like there is a shadow,
0:03:06 > 0:03:09- so it makes this darker. - That is exactly right.
0:03:09 > 0:03:12Your brain is trying to understand the colours of things
0:03:12 > 0:03:16irrespective of the lighting and the shadows.
0:03:16 > 0:03:19So somehow it's not about what's hitting your eyes,
0:03:19 > 0:03:21it is about your brain's interpretation.
0:03:21 > 0:03:23That's...really trippy.
0:03:23 > 0:03:25You have just messed up my whole day.
0:03:25 > 0:03:27DAVID LAUGHS
0:03:27 > 0:03:30Now this is about more than just a visual illusion.
0:03:32 > 0:03:35It's about a fact that's central to our lives.
0:03:37 > 0:03:41Our perception of reality has less to do with what's happening out there
0:03:41 > 0:03:43and more to do with what's happening in here.
0:03:48 > 0:03:50To understand what's going on,
0:03:50 > 0:03:54we first need to know how information from the world around us
0:03:54 > 0:03:56gets into the brain.
0:03:59 > 0:04:03It feels as if sights and sounds just stream in
0:04:03 > 0:04:04through our eyes and our ears.
0:04:06 > 0:04:11But imagine if you could climb inside a human skull.
0:04:11 > 0:04:13When you step into the skull,
0:04:13 > 0:04:15you will find that there is no way for light or sounds
0:04:15 > 0:04:18or smells to get directly in here.
0:04:26 > 0:04:28This is a sealed chamber...
0:04:30 > 0:04:34..so the brain sits in darkness and silence.
0:04:36 > 0:04:38It's in total isolation.
0:04:39 > 0:04:42Your brain has never seen the outside world,
0:04:42 > 0:04:45but somehow you experience it.
0:04:45 > 0:04:47Now this might seem straightforward
0:04:47 > 0:04:49because we have portals to the outside world,
0:04:49 > 0:04:51like your eyes and ears,
0:04:51 > 0:04:55but these aren't just piping in sights and sounds.
0:04:55 > 0:04:58Instead, photons of light or air compression waves,
0:04:58 > 0:05:02these are getting converted into the common currency of the brain -
0:05:02 > 0:05:04electrochemical signals.
0:05:08 > 0:05:10These signals travel through
0:05:10 > 0:05:13dense networks of brain cells called neurons.
0:05:18 > 0:05:22There are 100 billion neurons in the human brain,
0:05:22 > 0:05:25and in every second of your life,
0:05:25 > 0:05:30each one of these is sending tens or hundreds of electrical pulses
0:05:30 > 0:05:33to thousands of other neurons.
0:05:33 > 0:05:39And somehow, all of this activity produces your sense of reality.
0:05:39 > 0:05:42So whether it is the bark of a dog or the smell of coffee
0:05:42 > 0:05:45or a view of a beautiful sunset,
0:05:45 > 0:05:48it's all made of the same stuff in here.
0:05:51 > 0:05:54And this is the stuff of reality.
0:05:58 > 0:06:03But how does the brain turn it into something meaningful?
0:06:03 > 0:06:08Well, it does it by sifting through the nonstop stream of incoming data
0:06:08 > 0:06:11to find patterns, which are then assembled into a reality.
0:06:13 > 0:06:15It's an operation which is the product
0:06:15 > 0:06:18of millions of years of evolution.
0:06:18 > 0:06:21So efficient, so powerful
0:06:21 > 0:06:25that its work seems effortless and instantaneous.
0:06:31 > 0:06:34Take, as an example, sight.
0:06:36 > 0:06:40The act of seeing feels so natural
0:06:40 > 0:06:44that it's hard to appreciate the vast, sophisticated machinery
0:06:44 > 0:06:46running under the hood.
0:06:48 > 0:06:50For us to see clearly,
0:06:50 > 0:06:54many different systems need to be operating in concert.
0:06:54 > 0:06:56It's about more than just the eyes.
0:06:59 > 0:07:00The best way to understand this
0:07:00 > 0:07:05is to look at the extraordinary case of a man who lost his sight...
0:07:06 > 0:07:09..and then was given the chance to get it back.
0:07:13 > 0:07:16I lost my sight when I was three-and-a-half years old
0:07:16 > 0:07:19as the result of a chemical explosion.
0:07:19 > 0:07:22Oddly, it didn't seem like it was a big deal.
0:07:22 > 0:07:24I guess as a three-and-a-half year old,
0:07:24 > 0:07:28my world according to vision was not as well-established
0:07:28 > 0:07:31as it would be for somebody who lost their vision later in life.
0:07:33 > 0:07:36After over 40 years of blindness,
0:07:36 > 0:07:39Mike May had pioneering stem-cell treatment
0:07:39 > 0:07:41that would repair the physical damage
0:07:41 > 0:07:44that the explosion caused to his eyes.
0:07:46 > 0:07:48Cameras were there to witness the moment
0:07:48 > 0:07:53when, for the first time, the bandages came off.
0:07:53 > 0:07:55'Dr Goodman does the cornea transplant.'
0:07:58 > 0:08:01'He peels back the bandages.'
0:08:01 > 0:08:06He gets all the way off, and there is this whoosh of light
0:08:06 > 0:08:10and bombarding of images onto my eye.
0:08:10 > 0:08:11Holy smoke!
0:08:13 > 0:08:16In surgical terms, the operation was a total success.
0:08:16 > 0:08:18What's across the room over here?
0:08:18 > 0:08:21But to Mike, it wasn't.
0:08:21 > 0:08:23There was something wrong.
0:08:24 > 0:08:28All of a sudden, you turn on this flood of visual information.
0:08:28 > 0:08:29It's overwhelming.
0:08:32 > 0:08:35My brain is just going, "Oh, my gosh."
0:08:38 > 0:08:41So that is how the world proceeded - one image at a time.
0:08:43 > 0:08:45Seeing cars as they whizzed by...
0:08:46 > 0:08:48..and then I would see a sign ahead of us
0:08:48 > 0:08:51and it looked like we were going to smack right into it.
0:08:51 > 0:08:53In fact, it's the sign over the freeway
0:08:53 > 0:08:56and we are not going to run into it, we are going under it.
0:08:58 > 0:09:00That was only the first hour.
0:09:01 > 0:09:04It was going to get worse when Mike got home.
0:09:05 > 0:09:08If you put four blonde boys together,
0:09:08 > 0:09:10all roughly the same height...
0:09:10 > 0:09:13I looked at them, I couldn't tell you which two were mine.
0:09:13 > 0:09:15Don't go away, I've not finished looking at you.
0:09:17 > 0:09:20Mike's new eyes were functioning perfectly
0:09:20 > 0:09:24and they were sending signals to the brain just like yours or mine do,
0:09:24 > 0:09:28but he couldn't see his sons in any meaningful way.
0:09:30 > 0:09:33I had no face recognition whatsoever. None.
0:09:35 > 0:09:39When he had been totally blind, Mike was a Paralympic skier.
0:09:39 > 0:09:44But his first sighted attempt at skiing was a complete failure.
0:09:44 > 0:09:47When I skied for the first time,
0:09:47 > 0:09:49because of my depth perception difficulty,
0:09:49 > 0:09:54I had no time to figure out the difference between four dark things
0:09:54 > 0:09:55on the white snow -
0:09:55 > 0:10:01a person, a tree,
0:10:01 > 0:10:05a shadow or a hole.
0:10:11 > 0:10:15Ten years on, Mike still needs his guide dog to get around.
0:10:17 > 0:10:21He can detect light and motion and identify colours,
0:10:21 > 0:10:24but he struggles to gauge how far are things are.
0:10:26 > 0:10:30He still can't read the expressions on his sons' faces.
0:10:30 > 0:10:32He still can't read words on a page.
0:10:36 > 0:10:38What Mike's story gives us
0:10:38 > 0:10:41is a glimpse of all the elements that have to be in place
0:10:41 > 0:10:44for the brain to construct a visual reality.
0:10:47 > 0:10:51Many regions of the brain are involved in vision.
0:10:51 > 0:10:56They specialise in different aspects, such as motions, edges,
0:10:56 > 0:10:59colours, face recognition.
0:10:59 > 0:11:02Somehow, the brain weaves all of this together,
0:11:02 > 0:11:06unifies it to form what we experience as an image.
0:11:08 > 0:11:11In Mike's case, decades of blindness
0:11:11 > 0:11:16caused these regions of his brain to be taken over for other tasks,
0:11:16 > 0:11:17like hearing and touch.
0:11:17 > 0:11:20They just weren't available for him to use,
0:11:20 > 0:11:23even when he was given a pair of new eyes.
0:11:34 > 0:11:37We often get our best view of how the brain operates
0:11:37 > 0:11:39when that operation is disrupted.
0:11:42 > 0:11:45- Hey, Brian.- Hey.
0:11:45 > 0:11:50That is why neuroscientists sometimes disrupt things deliberately.
0:11:52 > 0:11:56Brian is part of an experiment being conducted by Alyssa Brewer
0:11:56 > 0:11:58at the University of California.
0:11:58 > 0:12:01- Good to see you.- Welcome. Are you ready to try the goggles on?
0:12:01 > 0:12:02Oh, I am ready.
0:12:02 > 0:12:06'Volunteers wear these goggles for weeks at a time.
0:12:06 > 0:12:09'Their brains are forced to cope with a new view of the world
0:12:09 > 0:12:11'that is dramatically altered.'
0:12:11 > 0:12:14What these have inside are two prisms
0:12:14 > 0:12:16that take the whole visual world and flip it.
0:12:16 > 0:12:18So whatever you see normally on the left side of the world
0:12:18 > 0:12:20will now be on the right side of the world.
0:12:20 > 0:12:23So as you move through the world, you're going to have a problem
0:12:23 > 0:12:26figuring out where things are as you see them on one side
0:12:26 > 0:12:29but reach for them on the opposite side.
0:12:29 > 0:12:32What the world looks like is this,
0:12:32 > 0:12:34but what I'm seeing...
0:12:34 > 0:12:35is this.
0:12:35 > 0:12:37It is a straightforward change,
0:12:37 > 0:12:39but it's also a massive mind mash.
0:12:43 > 0:12:46The visual data streaming in through my eyes
0:12:46 > 0:12:49no longer makes any intuitive sense,
0:12:49 > 0:12:51and I'm struggling.
0:12:54 > 0:12:57So, yeah, because the world is left-right flipped,
0:12:57 > 0:13:01I know cognitively I am supposed to reach out in the other direction,
0:13:01 > 0:13:03but, of course, I have had a lifetime of training
0:13:03 > 0:13:07telling me to reach out in a particular direction.
0:13:07 > 0:13:11So I feel like this is going to take a little getting used to.
0:13:11 > 0:13:13Can you see my hand in your visual field?
0:13:13 > 0:13:15Yeah, so it looks like if I reach out this way...
0:13:15 > 0:13:17And...this way.
0:13:17 > 0:13:18THEY LAUGH
0:13:20 > 0:13:23'Even though I'm consciously trying to get it right...'
0:13:23 > 0:13:25Over...here, OK.
0:13:25 > 0:13:28'..I can't help but respond in a certain way.'
0:13:30 > 0:13:31And over here.
0:13:34 > 0:13:36There you go. Very good.
0:13:36 > 0:13:39- Welcome to the prism world.- Yes.
0:13:40 > 0:13:43Of course, this is all new to me,
0:13:43 > 0:13:46but Brian has been wearing his goggles for a week.
0:13:48 > 0:13:50So how well has his brain adapted?
0:13:55 > 0:13:58It is very difficult to figure out which way to go,
0:13:58 > 0:14:01so his motor system, his feeling of touch
0:14:01 > 0:14:02is sending him one direction
0:14:02 > 0:14:05while his visual system is sending him the other direction.
0:14:16 > 0:14:18Brian is doing well.
0:14:20 > 0:14:22Unlike me.
0:14:23 > 0:14:27I have to consciously reconstruct my reality.
0:14:27 > 0:14:31This morning, my brain could rely on automated interactions,
0:14:31 > 0:14:33but now it can't.
0:14:34 > 0:14:37Interestingly, I've broken out in a sweat and I'm hot,
0:14:37 > 0:14:40and I'm super-dizzy and nauseated.
0:14:40 > 0:14:42You know what? I've got to take a break.
0:14:42 > 0:14:45I'm so sorry. I've got to take these off for a second.
0:14:45 > 0:14:46Is that OK?
0:14:48 > 0:14:50Boy, that is really nauseating.
0:14:53 > 0:14:55We're going to go to the maze down here and see how you guys do
0:14:55 > 0:14:59in navigating your way through a spatial map.
0:14:59 > 0:15:03- You're going to start out going this way.- OK.- Brian, you're going to...
0:15:03 > 0:15:06Oh, God! SHE LAUGHS
0:15:06 > 0:15:08I'm just going to give him a head start.
0:15:08 > 0:15:11So how do I get as good as Brian?
0:15:13 > 0:15:16Well, it happens intuitively.
0:15:18 > 0:15:20Just look at my hands.
0:15:21 > 0:15:24I cross-reference what I see with what I can touch.
0:15:27 > 0:15:30In fact, all my senses come into play.
0:15:30 > 0:15:34This is what Brian has been doing for the last seven days.
0:15:40 > 0:15:43The result is that his brain is now starting to decode
0:15:43 > 0:15:46that new visual input automatically.
0:15:50 > 0:15:55Brian is not simply getting better at making conscious adjustments -
0:15:55 > 0:15:57his whole reality is changing.
0:16:09 > 0:16:12If you take those subjects and put goggles on them for two weeks,
0:16:12 > 0:16:15you find that it takes them about a week to start behaving normally.
0:16:15 > 0:16:19They start being able to figure out how to interact with the world,
0:16:19 > 0:16:21constructing a new reality around them,
0:16:21 > 0:16:24a new way of dealing with these incoming perceptions.
0:16:24 > 0:16:27They say that initially they can tell there is a new left
0:16:27 > 0:16:29and an old left, and a new right and old right.
0:16:31 > 0:16:33By about a week in, they even lose the concept
0:16:33 > 0:16:36of which right and left were the old ones and the new ones.
0:16:36 > 0:16:38It is like their whole spatial map of the world is altering.
0:16:38 > 0:16:42By two weeks in, they will write well, read without a problem,
0:16:42 > 0:16:44do all of our walking tasks and reaching tasks.
0:16:44 > 0:16:46Then when we remove their goggles,
0:16:46 > 0:16:49it actually takes about a day to go back to normal behaviour.
0:16:49 > 0:16:53What this exposes for me is how much effort the brain goes through
0:16:53 > 0:16:55to construct our world,
0:16:55 > 0:16:59because normally you're walking through the world
0:16:59 > 0:17:01and it feels like there's reality out there.
0:17:03 > 0:17:07But in fact there is so much work happening behind the scenes
0:17:07 > 0:17:10to allow that reality to happen.
0:17:10 > 0:17:14Seeing requires an intensive training programme,
0:17:14 > 0:17:18but new recruits come on board every day.
0:17:18 > 0:17:20We call them babies.
0:17:23 > 0:17:26When babies reach out to touch what's in front of them,
0:17:26 > 0:17:29they are not just learning what an object feels like,
0:17:29 > 0:17:31they're learning how to see.
0:17:32 > 0:17:36They're establishing pathways in the brain that will be used
0:17:36 > 0:17:37for the rest of their lives.
0:17:39 > 0:17:43Because vision is a whole-body experience.
0:17:46 > 0:17:50The data coming in from our eyes only means something
0:17:50 > 0:17:53if we can cross-reference it.
0:17:53 > 0:17:54BABY BABBLES
0:17:54 > 0:17:58If from birth you weren't able to interact with the world,
0:17:58 > 0:18:00if you couldn't work out through feedback
0:18:00 > 0:18:03what the sensory information meant,
0:18:03 > 0:18:05in theory, you would never be able to see.
0:18:10 > 0:18:14This cross-referencing doesn't stop when we are fully grown.
0:18:17 > 0:18:20It continues throughout our lives.
0:18:22 > 0:18:26What we touch influences how we see.
0:18:29 > 0:18:32Taste is affected by our sense of smell.
0:18:34 > 0:18:37Our sight informs how we hear.
0:18:39 > 0:18:42Our senses depend on each other,
0:18:42 > 0:18:46and our reality is built by comparing these streams of data.
0:18:48 > 0:18:53When they're woven together, we get our perception of this moment.
0:18:56 > 0:18:58It's an astonishing feat to pull off,
0:18:58 > 0:19:02but there's one factor which really adds complication...
0:19:03 > 0:19:05..timing.
0:19:08 > 0:19:11Al those streams of sensory data
0:19:11 > 0:19:14are processed by the brain at different speeds.
0:19:14 > 0:19:18For our reality to be constructed, they have to be synchronised.
0:19:21 > 0:19:23What do I mean by this?
0:19:23 > 0:19:27Well, the easiest way for me to show you is right here at a racetrack.
0:19:28 > 0:19:29Set.
0:19:32 > 0:19:37When there is a loud sound, it feels as though you react to it instantly.
0:19:40 > 0:19:41But you don't.
0:19:47 > 0:19:50Watching sprinters in slow motion,
0:19:50 > 0:19:52we can see that there is a gap
0:19:52 > 0:19:55between the gun going off and their start.
0:20:02 > 0:20:05They may train to make this gap as small as possible,
0:20:05 > 0:20:09but their biology imposes limits.
0:20:09 > 0:20:11Processing that sound
0:20:11 > 0:20:15then sending out signals to the muscles to move
0:20:15 > 0:20:17will take around two-tenths of a second.
0:20:21 > 0:20:22GUN FIRES
0:20:24 > 0:20:26And that time really can't be improved on.
0:20:30 > 0:20:34In a sport where thousandths of a second can be the difference
0:20:34 > 0:20:37between winning and losing, it seems surprisingly slow.
0:20:41 > 0:20:44So why do we use a pistol to start sprinters?
0:20:44 > 0:20:47Everyone knows that light travels faster than sound,
0:20:47 > 0:20:49so why not use a light?
0:20:54 > 0:20:56We set up a test to show you.
0:20:57 > 0:21:00In the top screen, we're triggered by a light.
0:21:01 > 0:21:05In the bottom screen, we're triggered by the gun.
0:21:05 > 0:21:06GUN FIRES
0:21:06 > 0:21:10You can see that when our start is triggered by a flash of light,
0:21:10 > 0:21:12we respond more slowly.
0:21:16 > 0:21:19It takes 40 milliseconds longer to process.
0:21:21 > 0:21:23Why?
0:21:23 > 0:21:26Because the visual system is more complex.
0:21:28 > 0:21:32It's bigger - it involves almost a third of the brain.
0:21:32 > 0:21:35So while all of the electrical signals inside the brain
0:21:35 > 0:21:38travel at the same speed,
0:21:38 > 0:21:42the ones related to sight go through more complex processing,
0:21:42 > 0:21:43and that takes time.
0:21:50 > 0:21:53This isn't just about hearing and seeing.
0:21:53 > 0:21:56Every type of sensory information
0:21:56 > 0:21:58takes a different amount of time to process.
0:22:02 > 0:22:07You will react slower to a touch on the foot than one on the hand.
0:22:09 > 0:22:13The astonishing thing is that our brains hide all this.
0:22:18 > 0:22:24When I clap my hands, everything seems synchronised. Why?
0:22:24 > 0:22:27Well, your brain is pulling off fancy editing tricks.
0:22:27 > 0:22:32What it takes to be reality is actually the delayed version
0:22:32 > 0:22:35that collects all the information from the senses
0:22:35 > 0:22:38before it decides on a story of what happened.
0:22:38 > 0:22:42That means you live in the past.
0:22:42 > 0:22:45By the time you think the moment "now" occurs...
0:22:45 > 0:22:47it's already long gone.
0:22:47 > 0:22:51To conjure a reality from all that sensory information,
0:22:51 > 0:22:54your brain needs around half a second.
0:22:56 > 0:23:00That's the unbridgeable gap between an event occurring...
0:23:01 > 0:23:03..and your conscious experience of it.
0:23:05 > 0:23:09In that half a second, a lot of things need to happen.
0:23:14 > 0:23:18Sometimes it's easy to assume that there is a single spot in the brain
0:23:18 > 0:23:20that takes care of this or that function -
0:23:20 > 0:23:25like an area for memory or generosity or empathy.
0:23:25 > 0:23:27But in fact, the vast networks of the brain
0:23:27 > 0:23:30are so much more complex than that.
0:23:30 > 0:23:32Think of the brain like a city.
0:23:40 > 0:23:42If you were to look out over a city and ask,
0:23:42 > 0:23:45"Where is the economy located?"
0:23:45 > 0:23:48you'd see that there is no single answer to that.
0:23:48 > 0:23:54Instead, the economy emerges as an interaction of all the elements.
0:23:56 > 0:23:58So it is with reality.
0:23:59 > 0:24:04The raw materials of perception are gathered by our sensory receptors.
0:24:07 > 0:24:09They are turned into electrical signals
0:24:09 > 0:24:14and transported around our brains along superhighways of neurons.
0:24:16 > 0:24:19Processed, they become our reality.
0:24:21 > 0:24:25Some parts of Brain City specialise in vision,
0:24:25 > 0:24:28other districts care about hearing,
0:24:28 > 0:24:30some about touch, and so on.
0:24:33 > 0:24:35Even within a sense like vision,
0:24:35 > 0:24:40you have streets that specialise in colours, or edges, or motions.
0:24:47 > 0:24:48But just like in a city...
0:24:48 > 0:24:52no neighbourhood operates in isolation.
0:24:52 > 0:24:55Instead, the life of a city depends on the interaction
0:24:55 > 0:24:59between residents at all different scales.
0:24:59 > 0:25:02And somehow, out of all of this interaction
0:25:02 > 0:25:05emerges your personal reality.
0:25:11 > 0:25:15Reality is the brain's ultimate construction.
0:25:15 > 0:25:18It's based on all the streams of data from our senses,
0:25:18 > 0:25:22but it's not dependent on them. How do we know?
0:25:22 > 0:25:27Because when you take it all away, reality doesn't stop.
0:25:29 > 0:25:31It just gets stranger.
0:25:35 > 0:25:37This is Alcatraz.
0:25:40 > 0:25:43A jail built on the principle of isolation.
0:25:45 > 0:25:50Between its inmates in the rest of society stood not only stone walls...
0:25:51 > 0:25:55..but the cold, dangerous waters of the San Francisco Bay.
0:25:58 > 0:26:03Prisoners were completely and deliberately cut off.
0:26:07 > 0:26:09And there was one place inside the prison
0:26:09 > 0:26:12where that seclusion went even further.
0:26:19 > 0:26:20This is the Hole.
0:26:20 > 0:26:23Prisoners who were sent here were completely isolated
0:26:23 > 0:26:24from the outside world.
0:26:24 > 0:26:27They had no interactions with people,
0:26:27 > 0:26:29there was no sound and there was no light.
0:26:33 > 0:26:38Robert Luke was sent to Alcatraz in 1954 for armed robbery.
0:26:38 > 0:26:42He was known by the nickname Cold Blue Luke.
0:26:43 > 0:26:46Everybody knew about the Dark Hole.
0:26:46 > 0:26:48The Dark Hole was a bad place.
0:26:51 > 0:26:53Some guys couldn't take that.
0:26:53 > 0:26:55I mean, they were in there and within a couple of days
0:26:55 > 0:26:57they were banging their heads on the wall.
0:26:59 > 0:27:01As punishment for smashing up his cell,
0:27:01 > 0:27:05he was sent to the Hole for 29 straight days.
0:27:08 > 0:27:11You didn't know how you would act when you got in there.
0:27:11 > 0:27:13You didn't want to find out.
0:27:13 > 0:27:16When they closed that door...
0:27:18 > 0:27:21..there was just nothing there. It is pitch-black.
0:27:22 > 0:27:24But it didn't stay that way for long.
0:27:27 > 0:27:33Starved of input, Luke's brain started to produce its own reality.
0:27:36 > 0:27:38I remember I'd go on these trips.
0:27:38 > 0:27:41One I used to remember was flying a kite.
0:27:43 > 0:27:45It got pretty real.
0:27:46 > 0:27:48They were all in my head.
0:27:49 > 0:27:53What Luke felt was something that has also been reported by other prisoners
0:27:53 > 0:27:55kept in the same conditions.
0:28:00 > 0:28:03Deprived of new sensory information,
0:28:03 > 0:28:05they said they went beyond dreaming or daydreaming.
0:28:07 > 0:28:09They didn't just imagine pictures...
0:28:11 > 0:28:13..they saw.
0:28:16 > 0:28:19This testimony goes to the heart of the relationship
0:28:19 > 0:28:22between the outside world,
0:28:22 > 0:28:26the brain, and what we called reality.
0:28:27 > 0:28:31To understand it, we need to look more deeply into the visual system.
0:28:37 > 0:28:38This is the thalamus -
0:28:38 > 0:28:41one of the brain's major junctions.
0:28:41 > 0:28:44Most sensory information connects through here
0:28:44 > 0:28:49on its way to the outer surface of the brain, the cortex.
0:28:49 > 0:28:52So data collected from the eyes stops here
0:28:52 > 0:28:55before going to the visual cortex.
0:28:55 > 0:28:58Now, you'd expect a heavy flow of information
0:28:58 > 0:29:03from the thalamus to the visual cortex, and there is.
0:29:03 > 0:29:06But there is six times as much traffic
0:29:06 > 0:29:08flowing in the opposite direction,
0:29:08 > 0:29:12and that dwarfs the amount coming in from the eyes.
0:29:18 > 0:29:21That suggests that in any one moment,
0:29:21 > 0:29:24what we experience as seeing
0:29:24 > 0:29:27relies less on the light streaming into our eyes
0:29:27 > 0:29:32and more on what is already inside our heads.
0:29:32 > 0:29:35Even when brains are unanchored from external data,
0:29:35 > 0:29:39they continue to generate their own imagery.
0:29:39 > 0:29:43In other words, remove the world, and the show still goes on.
0:29:46 > 0:29:50We all have this internally generated reality.
0:29:51 > 0:29:58Incredible as it may sound, this world lives inside your brain.
0:30:00 > 0:30:04It's constantly updated by information from our senses,
0:30:04 > 0:30:07but moment to moment what we experience
0:30:07 > 0:30:10isn't what's really out there.
0:30:10 > 0:30:14Instead, it's a beautifully rendered simulation.
0:30:16 > 0:30:20This is a surprising way to understand how you see the world.
0:30:20 > 0:30:24It's called the internal model.
0:30:24 > 0:30:27And it's vital to our ability to function.
0:30:29 > 0:30:32As I walk down this city street, I seem to automatically
0:30:32 > 0:30:36know what things are without having to work out the details.
0:30:36 > 0:30:40For example, I don't have to work out the detail of what this rectangular,
0:30:40 > 0:30:44metallic thing is, or this giant green, fluffy thing behind me,
0:30:44 > 0:30:48or this huge object with reflective panes on it,
0:30:48 > 0:30:51or this thing with four appendages.
0:30:51 > 0:30:54My brain makes assumptions about what I'm seeing
0:30:54 > 0:30:57based on my internal model,
0:30:57 > 0:31:00and that's been built up from years of experience of walking city streets
0:31:00 > 0:31:02just like this one.
0:31:03 > 0:31:09Instead of using my senses to rebuild my reality from scratch every moment,
0:31:09 > 0:31:11I'm comparing sensory information
0:31:11 > 0:31:14with a model that I've already constructed.
0:31:16 > 0:31:18Updating it.
0:31:18 > 0:31:20Refining it.
0:31:20 > 0:31:22Correcting it.
0:31:22 > 0:31:27Our brains are so good at doing this that we're normally unaware of it.
0:31:27 > 0:31:30But sometimes, under certain conditions,
0:31:30 > 0:31:33we can see the process at work.
0:31:36 > 0:31:39Look at this hollow mask of Einstein's face.
0:31:41 > 0:31:44Your brain tells you it's coming out at you.
0:31:46 > 0:31:48And even when you know it's an illusion
0:31:48 > 0:31:51you can't help but fall for it.
0:31:56 > 0:31:59What you're seeing is the internal model,
0:31:59 > 0:32:02not the raw information that's coming in from your eyes.
0:32:05 > 0:32:09Your internal model is built on a lifetime of experience
0:32:09 > 0:32:12with faces that stick out.
0:32:12 > 0:32:15When you're confronted with one that's hollow,
0:32:15 > 0:32:18your model simply sees what it expects to see.
0:32:24 > 0:32:28The visual cortex sends its internal expectations to the thalamus
0:32:28 > 0:32:34and the thalamus compares those to what's coming in through the eyes.
0:32:34 > 0:32:38The difference between the two is what the thalamus sends back
0:32:38 > 0:32:42so the cortex can update its model.
0:32:49 > 0:32:54Thanks to the internal model, the world out there remains stable
0:32:54 > 0:32:56even when I'm moving.
0:32:59 > 0:33:00Let me show you what I mean.
0:33:02 > 0:33:04So imagine that I really love the scene behind me
0:33:04 > 0:33:08and I want to go ahead and capture it so I can view it later.
0:33:08 > 0:33:10So I'm going to go ahead and videotape the scene
0:33:10 > 0:33:14and I'm checking out all the buildings...
0:33:14 > 0:33:17OK. And now I'm going to play this back.
0:33:19 > 0:33:22Not surprisingly, the resulting video is nauseating.
0:33:25 > 0:33:27So why does this video look so terrible,
0:33:27 > 0:33:30given that, when I look at the buildings,
0:33:30 > 0:33:33my eyes are making the same jerky movements?
0:33:34 > 0:33:37Although you're not generally aware of it,
0:33:37 > 0:33:41your eyes move about four times a second.
0:33:41 > 0:33:44But your internal model operates under the assumption
0:33:44 > 0:33:47that the world outside is stable.
0:33:47 > 0:33:50So my eyes aren't taking a video, they're simply gathering
0:33:50 > 0:33:56bits of data to update the city that's already inside my head.
0:33:57 > 0:34:01Having an internal model helps me make sense of my environment,
0:34:01 > 0:34:05and that's its primary function - to navigate the world.
0:34:10 > 0:34:13The brain doesn't bother picking up every detail,
0:34:13 > 0:34:16just enough to get us through.
0:34:16 > 0:34:20But it plays the trick of making us feel as though we've seen it all...
0:34:23 > 0:34:26..as another famous experiment shows.
0:34:31 > 0:34:32In the 1960s,
0:34:32 > 0:34:35the Russian psychologist Paul Yarbus used this painting,
0:34:35 > 0:34:40called The Unexpected Visitor, in an experiment.
0:34:40 > 0:34:44He devised a way to track the eye movements of volunteers
0:34:44 > 0:34:47who were seeing it for the first time.
0:34:48 > 0:34:53- Hi, Jennifer.- Hello.- I'm going to ask you to put these glasses on.
0:34:53 > 0:34:55'We're going to rerun what he did.'
0:34:57 > 0:35:00'My volunteers have a few seconds to take in the image.'
0:35:02 > 0:35:04Look at this painting,
0:35:04 > 0:35:08and I want you to gather what's going on in the scene.
0:35:08 > 0:35:13'We can watch in real time exactly where each person's eyes go.'
0:35:16 > 0:35:19Tell me what you think is going on in this painting.
0:35:19 > 0:35:22I think the man in the brown is the unexpected visitor.
0:35:22 > 0:35:26'One brief look is enough for the brain to model the picture.
0:35:26 > 0:35:29'But just how detailed is that model?'
0:35:30 > 0:35:32- How many children were there? - Uh, there were two.
0:35:32 > 0:35:35OK, so look back at the painting and ask that question again.
0:35:35 > 0:35:38- Oh, quite different. - How many children are there?
0:35:38 > 0:35:40I can see three.
0:35:40 > 0:35:44'Everyone who'd seen the painting thought they knew what was in it.
0:35:44 > 0:35:48'But my specific questions highlighted blanks that the brain
0:35:48 > 0:35:52'had never filled in, because the details weren't needed.'
0:35:54 > 0:35:57- How many paintings are on the wall in their house?- Maybe two or three?
0:35:57 > 0:36:00OK, look back at the painting and answer that question...
0:36:00 > 0:36:01Oh, God, there's a million!
0:36:01 > 0:36:04- Yeah, a map and then there's... - SHE LAUGHS
0:36:04 > 0:36:07..seven on the other wall and then one small one and the map.
0:36:07 > 0:36:09OK, there's a ton.
0:36:09 > 0:36:12This is not a failure of the brain.
0:36:12 > 0:36:17It doesn't try to produce a perfect simulation of the world.
0:36:17 > 0:36:20The internal model is a hastily drawn approximation
0:36:20 > 0:36:25and more details are added on a need-to-know basis.
0:36:27 > 0:36:30When you looked at the painting the first time you saw a sort of
0:36:30 > 0:36:32rough draft of what was going on,
0:36:32 > 0:36:36and when I asked you specific questions, you had to answer those
0:36:36 > 0:36:39by turning your attention onto specific parts of the painting,
0:36:39 > 0:36:42and only then did you actually see it.
0:36:43 > 0:36:48So placing your eyes on an object is no guarantee of seeing it.
0:36:50 > 0:36:53But there's something else we're unaware of happening
0:36:53 > 0:36:57every time we look at any picture or person or thing.
0:36:59 > 0:37:02Any time we look at all.
0:37:04 > 0:37:07We might think of colour as a fundamental,
0:37:07 > 0:37:11defining quality of the world around us.
0:37:11 > 0:37:14After all, it's everywhere.
0:37:14 > 0:37:17But here's the startling thing.
0:37:17 > 0:37:19In the outside world...
0:37:21 > 0:37:24..colour doesn't actually exist.
0:37:26 > 0:37:29When electromagnetic radiation hits an object,
0:37:29 > 0:37:32some of it bounces off and is captured by our eyes.
0:37:34 > 0:37:39We can distinguish between millions of combinations of wavelengths,
0:37:39 > 0:37:44but it's only inside our heads that any of this becomes colour.
0:37:46 > 0:37:50Add to that the fact that the wavelengths we can detect
0:37:50 > 0:37:53are only a small part of what's out there.
0:37:57 > 0:38:01You experience reality as it's presented by your senses,
0:38:01 > 0:38:04and it doesn't typically strike you that things can be very different.
0:38:06 > 0:38:08What we've talking about so far
0:38:08 > 0:38:12is what we call the visible spectrum of light, which is a spectrum
0:38:12 > 0:38:18of wavelengths that runs from what we call red to violet.
0:38:20 > 0:38:26But it turns out that this only constitutes a tiny fraction
0:38:26 > 0:38:28of the electromagnetic spectrum.
0:38:28 > 0:38:32In fact, less than one ten-trillionth of it.
0:38:32 > 0:38:35So all the rest of the spectrum, including radio waves
0:38:35 > 0:38:39and microwaves and X-rays and gamma rays,
0:38:39 > 0:38:42all of this stuff is flowing through our bodies right now
0:38:42 > 0:38:45and we're completely unaware of it
0:38:45 > 0:38:48because we don't have any specialised biological receptors
0:38:48 > 0:38:50to pick up on it.
0:38:50 > 0:38:53So what this means is that the part of reality that we can see
0:38:53 > 0:38:57is totally limited by our biology.
0:38:57 > 0:39:00And this isn't just about sight.
0:39:00 > 0:39:02All our senses are only picking up
0:39:02 > 0:39:06a small part of the information that's out there.
0:39:08 > 0:39:10DOG SNIFFS
0:39:12 > 0:39:16So for a dog, he's tuned in to a whole world of scent molecules
0:39:16 > 0:39:17that I'm not.
0:39:19 > 0:39:24His experience of smell is as rich as my experience of vision.
0:39:26 > 0:39:30In the blind and deaf world of the tick,
0:39:30 > 0:39:34the important signals are temperature and body odour.
0:39:34 > 0:39:38For cave-dwelling bats, it's all about air compression waves
0:39:38 > 0:39:41that allow them to echolocate.
0:39:44 > 0:39:47But no-one's having an experience of objective reality,
0:39:47 > 0:39:50of the world that really, truly exists.
0:39:50 > 0:39:57Instead, each creature perceives only what it has evolved to perceive.
0:39:59 > 0:40:02And this isn't just about variation between species.
0:40:02 > 0:40:06If we're each experiencing a personal reality,
0:40:06 > 0:40:08constructed inside our brains,
0:40:08 > 0:40:12how do I know that my reality is at all like yours?
0:40:12 > 0:40:17Most of the time it seems as if we operate along the same lines,
0:40:17 > 0:40:22as if you and I agree what a blue sky is,
0:40:22 > 0:40:26as if the sound of a dog bark provokes the same sort of response
0:40:26 > 0:40:27in both of us.
0:40:27 > 0:40:30DOG BARKS
0:40:35 > 0:40:37But there's a small group of people
0:40:37 > 0:40:41whose perception is measurably different from ours.
0:40:44 > 0:40:49For me, any time I see a letter or a number or think of a word
0:40:49 > 0:40:54or say someone's name, there is a lot of colour associated with that.
0:40:54 > 0:40:59Hannah is one of 6,000 people I've studied who have synaesthesia.
0:41:01 > 0:41:05I study synaesthesia because it's one of the few conditions in which
0:41:05 > 0:41:10it's clear that someone else's reality is different from mine,
0:41:10 > 0:41:13and it makes it obvious that how we perceive the world
0:41:13 > 0:41:15is not "one size fits all".
0:41:17 > 0:41:20In my mind I associate each letter with its own colour.
0:41:22 > 0:41:25So, for example, the letter A is always red,
0:41:25 > 0:41:27B is always blue,
0:41:27 > 0:41:31C is always orange - every time. So they never change.
0:41:31 > 0:41:33But what's interesting is
0:41:33 > 0:41:36when they're formed into words in different orders,
0:41:36 > 0:41:41the configuration of the colours changes and that can be interesting.
0:41:42 > 0:41:46So in the word "Hannah", my name, it looks a sunset.
0:41:48 > 0:41:53It's yellow fading into red fading into kind of a clear...
0:41:53 > 0:41:54like clouds, almost,
0:41:54 > 0:41:57and then goes back to red and to yellow.
0:42:04 > 0:42:06These experiences come about
0:42:06 > 0:42:10because of the simple fact that inside the brain,
0:42:10 > 0:42:14all sensory information is made from the same stuff -
0:42:14 > 0:42:16electrochemical signals.
0:42:19 > 0:42:24Synaesthesia is the result of cross-talk between sensory areas
0:42:24 > 0:42:25of the brain.
0:42:27 > 0:42:30Think of the blurred borders between city districts.
0:42:32 > 0:42:36Synaesthesia shows us that even minute changes in brain wiring
0:42:36 > 0:42:39can lead to different realities.
0:42:43 > 0:42:46There are different kinds of synaesthesia.
0:42:47 > 0:42:52Some people perceive weekdays to have locations in space.
0:42:52 > 0:42:54Some taste words.
0:42:54 > 0:42:56Others see music.
0:42:56 > 0:43:00And every time I meet someone who has this kind of experience,
0:43:00 > 0:43:05it's a reminder that from person to person, brain to brain,
0:43:05 > 0:43:09our experiences of reality can be quite different.
0:43:11 > 0:43:13For a small section of the population,
0:43:13 > 0:43:18that difference can be extreme and terrifying.
0:43:19 > 0:43:23We all know what it's like to have dreams at night,
0:43:23 > 0:43:27to have bizarre, unbidden thoughts that take us on journeys,
0:43:27 > 0:43:29sometimes journeys we suffer through.
0:43:29 > 0:43:33When we wake up, we're lucky enough to be able to compartmentalise that,
0:43:33 > 0:43:37to say, "OK, that was a dream and this is my waking life."
0:43:40 > 0:43:41But just imagine what it would be like
0:43:41 > 0:43:43if these were more and more intertwined
0:43:43 > 0:43:47and it was more and more difficult to tell them apart from one another.
0:43:51 > 0:43:54I felt like the houses were communicating with me.
0:43:56 > 0:43:58"You are special, you are especially bad.
0:43:58 > 0:44:00"Repent. Stop. Go."
0:44:00 > 0:44:02You know, kind of...
0:44:02 > 0:44:05I did not hear these as words, but I heard them as thoughts
0:44:05 > 0:44:07put in my head, but I knew they were the houses' thoughts
0:44:07 > 0:44:08and not my thoughts.
0:44:10 > 0:44:13I think that explosions are being set off in my brain
0:44:13 > 0:44:17and I'm afraid it's going to hurt other people, not just me.
0:44:17 > 0:44:20I once had a fantasy that my brains were going to leak out of my ears
0:44:20 > 0:44:21and drown people.
0:44:23 > 0:44:25What is that? You know?
0:44:25 > 0:44:27Elyn Saks is a professor of law
0:44:27 > 0:44:31at the University of Southern California.
0:44:31 > 0:44:36She's been experiencing schizophrenic episodes since she was 16 years old.
0:44:37 > 0:44:40It's scary, it's unpredictable.
0:44:42 > 0:44:43It's sort of interesting
0:44:43 > 0:44:46cos there are different theories about psychotic symptoms.
0:44:46 > 0:44:49For some people they're just random firings of neurons.
0:44:49 > 0:44:52I do think they tell the truth about your psychic reality,
0:44:52 > 0:44:54so when I saw I've killed hundreds of thousands of people
0:44:54 > 0:44:56with my thoughts, that's just an archaic
0:44:56 > 0:44:58and extreme way of saying I feel like I'm a bad person.
0:45:03 > 0:45:06Schizophrenia is still not fully understood,
0:45:06 > 0:45:10but it involves chemical imbalances in the brain which cause problems
0:45:10 > 0:45:14in the sending and receiving of signals.
0:45:21 > 0:45:23Thanks to medication and therapy,
0:45:23 > 0:45:27Elyn has been able to lecture and teach for over 25 years.
0:45:32 > 0:45:36So when you were at the bottom in one of your worst psychotic episodes,
0:45:36 > 0:45:37you took that to be reality?
0:45:37 > 0:45:40I really believe what I think is happening is happening,
0:45:40 > 0:45:43and it's terrifying. It's like a waking nightmare -
0:45:43 > 0:45:46confusion, bizarre images, violence, terror.
0:45:46 > 0:45:48I wouldn't wish it on anyone.
0:45:48 > 0:45:55That said, everybody's reality is constructed. Right? You...
0:45:55 > 0:46:00filter it through your beliefs and values and issues,
0:46:00 > 0:46:02and this is true for people who have mental illness
0:46:02 > 0:46:06and for people who don't have mental illness, it's all a spectrum.
0:46:09 > 0:46:12Reality differs from person to person.
0:46:15 > 0:46:18More than that, it changes from moment to moment.
0:46:19 > 0:46:24There are times in all our lives when it can seem enhanced, intensified.
0:46:28 > 0:46:32Even the one great constant which we all think we share
0:46:32 > 0:46:34and which should never change
0:46:34 > 0:46:37somehow becomes stretched and distorted.
0:46:38 > 0:46:41I'm talking about time.
0:46:49 > 0:46:53Time is something that we rarely stop to consider.
0:46:53 > 0:46:57But our brain's experience of time is often quite strange.
0:46:57 > 0:47:00It doesn't always seem, in certain situations,
0:47:00 > 0:47:02that time is running at an even pace.
0:47:02 > 0:47:05Sometimes it runs more slowly or more quickly.
0:47:09 > 0:47:13When I was eight years old I fell off the roof of a house about this height
0:47:13 > 0:47:17and the fall seemed to me to take a very long time.
0:47:22 > 0:47:24But when I got to high school, I learned physics
0:47:24 > 0:47:28and I calculated, how long did the fall actually take?
0:47:28 > 0:47:32And it turns out it was only 8/10ths of a second.
0:47:32 > 0:47:34So that set me off on a quest to understand,
0:47:34 > 0:47:37why did it seem to take so long?
0:47:37 > 0:47:41And what did this tell me about our perception of reality?
0:47:43 > 0:47:47Many people have reported this sensation during moments of terror.
0:47:50 > 0:47:52Professional wingsuit flyer Jeb Corliss
0:47:52 > 0:47:55experienced it in an extreme way.
0:47:55 > 0:47:57- How you doing, Jeb?- Excellent.
0:47:58 > 0:48:01And because he falls for a living,
0:48:01 > 0:48:05the event he describes was captured on multiple cameras.
0:48:10 > 0:48:13On this day, I decided to aim for a target.
0:48:13 > 0:48:16Like, set up balloons and come in and hit balloons.
0:48:21 > 0:48:23I was flying towards the balloons.
0:48:26 > 0:48:30And as I was coming in to hit the black balloon...
0:48:30 > 0:48:31I misjudged.
0:48:38 > 0:48:42I impacted flat, solid granite
0:48:42 > 0:48:44at 120mph.
0:48:48 > 0:48:52Six seconds elapsed between the moment Jeb hit the rock
0:48:52 > 0:48:54and the moment he pulled his ripcord.
0:48:56 > 0:49:00He broke his leg and both ankles in the fall.
0:49:01 > 0:49:03JEB GROANS
0:49:05 > 0:49:12From Jeb's perspective, those six seconds seemed to last a long time.
0:49:15 > 0:49:17You've got two options.
0:49:17 > 0:49:23One is you can not pull, and just be dead right now.
0:49:23 > 0:49:27It's really quick, semi-painless, over fast.
0:49:27 > 0:49:32Or you can pull, you know, get a parachute over your head,
0:49:32 > 0:49:35impact a second time and then bleed to death
0:49:35 > 0:49:36while you're waiting for rescue.
0:49:38 > 0:49:41These two separate thought processes
0:49:41 > 0:49:43felt like minutes of time.
0:49:45 > 0:49:48It feels like you're operating so fast that your perception
0:49:48 > 0:49:50of everything else seems to slow down.
0:49:50 > 0:49:52Everything just gets stretched.
0:49:53 > 0:49:56HE GROANS
0:49:56 > 0:50:00But what was really happening in Jeb's brain?
0:50:00 > 0:50:03I designed an experiment to find out.
0:50:04 > 0:50:08It depended on inducing extreme fear in people
0:50:08 > 0:50:12by dropping them from 150 feet in the air.
0:50:14 > 0:50:18They fell with a digital display strapped to their wrist.
0:50:18 > 0:50:19Its numbers were changing
0:50:19 > 0:50:22at a rate faster than human vision can normally handle.
0:50:24 > 0:50:27If perceptual time did slow,
0:50:27 > 0:50:29then they would be able to read the numbers.
0:50:29 > 0:50:31But no-one could.
0:50:33 > 0:50:37So why did Jeb recall his accident as happening in slow motion?
0:50:45 > 0:50:49It was a time distortion on a level I've never experienced before.
0:50:49 > 0:50:52I learned later that the rescue took about two and half hours.
0:50:52 > 0:50:55But at the time it felt like weeks.
0:50:55 > 0:50:57It didn't feel like...
0:50:57 > 0:51:00minutes or hours or even days,
0:51:00 > 0:51:04it felt like eternities. It felt like forever.
0:51:06 > 0:51:11The answer seems to lie with how our memories are made.
0:51:11 > 0:51:15In a critical situation, an area of the brain called the amygdala
0:51:15 > 0:51:17kicks into high gear.
0:51:17 > 0:51:21It commandeers the resources of the rest of the brain,
0:51:21 > 0:51:26forcing everything to attend to the situation at hand.
0:51:26 > 0:51:28When the amygdala is in play,
0:51:28 > 0:51:31memories are laid down with far more detail
0:51:31 > 0:51:33than under normal circumstances.
0:51:38 > 0:51:42These memories are richer and more vivid.
0:51:42 > 0:51:46If you're ever in a similar situation, you have more information
0:51:46 > 0:51:51at your disposal to work out how to stay alive.
0:51:51 > 0:51:53But there's a fascinating consequence.
0:51:53 > 0:51:57When the events are replayed in your memory,
0:51:57 > 0:52:00they appear to have taken a longer time.
0:52:06 > 0:52:09Ow! Ow-ow-ow!
0:52:09 > 0:52:13Jeb's time distortion is something that happened in retrospect.
0:52:15 > 0:52:20A trick of the memory that wrote the story of his reality.
0:52:28 > 0:52:33The brain is the universe's ultimate storyteller.
0:52:33 > 0:52:36We believe whatever our brains serve up to us.
0:52:38 > 0:52:42The reality we take for granted requires intensive training
0:52:42 > 0:52:44to interpret the world.
0:52:46 > 0:52:49It takes time to process sensory information,
0:52:49 > 0:52:51so we live in the past.
0:52:55 > 0:52:57And because all that information
0:52:57 > 0:53:00is ultimately just electrochemical signals
0:53:00 > 0:53:05to be sorted, matched, rendered and packaged,
0:53:05 > 0:53:09reality is something created inside our head.
0:53:15 > 0:53:19Our brain sculpts our reality using the narrow trickle of data
0:53:19 > 0:53:21it can gather through the senses,
0:53:21 > 0:53:26and from that trickle it tells a story about our world.
0:53:26 > 0:53:30It's possible that every brain tells a different narrative.
0:53:33 > 0:53:36With seven billion human brains wandering the planet...
0:53:39 > 0:53:41..trillions of animal brains...
0:53:42 > 0:53:45..no-one is tapped into the full picture.
0:53:48 > 0:53:55Each brain carries its own unique model of the world around us.
0:53:55 > 0:53:57That is what we experience.
0:53:58 > 0:54:00We have no choice.
0:54:04 > 0:54:06So what is reality?
0:54:06 > 0:54:09It's whatever your brain tells you it is.
0:54:13 > 0:54:18Next time on The Brain, I'm going to explore a fundamental question
0:54:18 > 0:54:20about our lives.
0:54:20 > 0:54:22What makes you...you?
0:54:24 > 0:54:27I've spent many years of my life trying to decipher
0:54:27 > 0:54:29the mysteries of the brain,
0:54:29 > 0:54:33and yet I'm still in awe every time I hold one.
0:54:33 > 0:54:36And that's because, although this marvel of biology
0:54:36 > 0:54:42seems so alien to us, somehow, it IS us.
0:54:43 > 0:54:47This three-pound organ is made up of hundreds of billions of cells
0:54:47 > 0:54:51with a quadrillion connections between them.
0:54:51 > 0:54:55These cells fire trillions of electrochemical signals
0:54:55 > 0:54:56every second of your life.
0:54:58 > 0:55:03Somehow all this wet biological stuff
0:55:03 > 0:55:06results in the experience of being you.
0:55:09 > 0:55:12What shapes who you become?
0:55:18 > 0:55:23I'm going to explore how your life shapes your brain
0:55:23 > 0:55:26and how your brain shapes your life.