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Documentary, The Brain with David Eagleman: What Is Reality - BBC Documentary 2016
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00:00The most complex thing we've discovered in the universe is the human brain.
00:13For the past 20 years, I've been trying to understand how what happens in three pounds of jello-like material somehow becomes us.
00:24What we feel, what matters to us, our beliefs and our hopes, everything we are, happens in here.
00:42And for me, there's one mystery that's absolutely fundamental. What is reality?
00:49What if I told you that this world around us, this richly textured world, were all just an illusion constructed in your head?
01:04What if I said that the real world has no smell or taste?
01:10No sound.
01:17What if I said, there's no color?
01:23If you could perceive reality as it really is out there, you wouldn't recognize it at all.
01:28I want to show you how the brain takes in information, sifts through it to find patterns, and uses it to build the multisensory technicolor show that is your reality.
01:48When I'm in the world, my senses are flooded with sights, and sounds, and smells.
02:02And it seems obvious that reality is just out there. There's a person, there's a cab. All I have to do is show up. And my senses let me experience it all.
02:24But there's a twist to this story. Let me show you something.
02:34So take a look at this middle square here. Does that look more similar to the light square or the dark?
02:41Well, it looks like a light square, yeah? You might be surprised if I move it. Now it looks like a dark square.
02:46Now it looks like a dark square.
02:48Oh, my God! See, it is the same.
02:51Oh, it's surprising, right?
02:53It is. Ah! Oh, my goodness.
02:56Wow. You're red.
02:58Seriously?
03:00Do you have a guess as to why there's an illusion here?
03:04Well, it seems like there's a shadow, so it makes this darker.
03:08That's exactly right. Your brain is trying to understand the colors of things irrespective of the lighting and the shadows.
03:16So somehow it's not about what's hitting your eyes. It's about your brain's interpretation.
03:22That's really trippy. You just mess up my whole day.
03:28Now this is about more than just a visual illusion.
03:33It's about a fact that's central to our lives.
03:37Our perception of reality has less to do with what's happening out there and more to do with what's happening in here.
03:43To understand what's going on, we first need to know how information from the world around us gets into the brain.
03:56It feels as if sights and sounds just stream in through our eyes and our ears.
04:05But imagine if you could climb inside a human skull.
04:10When you step into the skull, you'll find there's no way for light or sound or smells to get directly in here.
04:18This is a sealed chamber.
04:29So the brain sits in darkness and in silence.
04:35It's in total isolation.
04:37Your brain's never seen the outside world, but somehow you experience it.
04:45Now this might seem straightforward because we have portals to the outside world like your eyes and ears.
04:51But these aren't just piping in sights and sounds.
04:55Instead, photons of light or air compression waves, these are getting converted into the common currency of the brain.
05:02Electrochemical signals.
05:09These signals travel through dense networks of brain cells called neurons.
05:19There are a hundred billion neurons in the human brain.
05:22And in every second of your life, each one of these is sending tens or hundreds of electrical pulses to thousands of other neurons.
05:33And somehow, all of this activity produces your sense of reality.
05:39So whether it's 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.
05:48And this is the stuff of reality.
05:58But how does the brain turn it into something meaningful?
06:02Well, it does it by sifting through the non-stop stream of incoming data to find patterns which it then assembles into a reality.
06:11It's an operation which is the product of millions of years of evolution, so efficient, so powerful, that its work seems effortless and instantaneous.
06:25Take as an example, sight.
06:37The act of seeing feels so natural that it's hard to appreciate the vast, sophisticated machinery running under the hood.
06:45For us to see clearly, many different systems need to be operating in concert.
06:54It's about more than just the eyes.
06:59The best way to understand this is to look at the extraordinary case of a man who lost his sight.
07:05And then was given the chance to get it back.
07:12I lost my sight when I was three and a half years old as the result of a chemical explosion.
07:19And oddly, it doesn't seem like it was a big deal.
07:23I 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.
07:31After 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.
07:46Cameras were there to witness the moment when, for the first time, the bandages came off.
07:52Dr. Goodman does the cornea transplant.
07:58And he peels back the bandages.
08:00Sure he can.
08:01He gets them all the way off and there's this whoosh of light and bombarding of images onto my eye.
08:10Holy smoke.
08:13In surgical terms, the operation was a total success.
08:17What's across the room over here?
08:18But to Mike, it wasn't.
08:22There was something wrong.
08:24All of a sudden, you turn on this flood of visual information.
08:28It's overwhelming.
08:32My brain is just going, oh my gosh.
08:37So that's how the world proceeded, one image at a time.
08:41Seeing cars as they whizzed by.
08:45And then I would see a sign ahead of us and it looked like we were going to smack right into it.
08:51And in fact, it's a sign over the freeway and we're not going to run into it, we're going under it.
08:57And that was only the first hour.
08:59It was going to get worse when Mike got home.
09:02If you put four blonde boys together all roughly the same height and I looked at them, I couldn't tell you which two were mine.
09:13Don't go away. I'm not finished looking at you.
09:15Mike's new eyes were functioning perfectly and they were sending signals to the brain just like yours or mine do.
09:25But he couldn't see his sons in any meaningful way.
09:28I had no face recognition whatsoever. None.
09:35When he'd been totally blind, Mike was a Paralympic skier.
09:40But his first sighted attempt at skiing was a complete failure.
09:44When I skied for the first time, because of my depth perception, difficulty,
09:50I had no time to figure out the difference between four dark things on the white snow.
09:55A person, a tree, shadow, or a hole.
10:12Ten years on, Mike still needs his guide dog to get around.
10:16He can detect light and motion and identify colors.
10:21But he struggles to gauge how far away things are.
10:25He still can't read the expressions on his son's faces.
10:30He still can't read words on a page.
10:36What 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.
10:44Many regions of the brain are involved in vision.
10:51They specialize in different aspects, such as motion, edges, colors, face recognition.
10:59Somehow the brain weaves all of this together, unifies it, to form what we experience as an image.
11:06In Mike's case, decades of blindness caused these regions of his brain to be taken over for other tasks, like hearing and touch.
11:17They just weren't available for him to use, even when he was given a pair of new eyes.
11:22We often get our best view of how the brain operates when that operation is disrupted.
11:39Hey, Brian.
11:44Hey.
11:46That's why neuroscientists sometimes disrupt things deliberately.
11:53Brian is part of an experiment being conducted by Alyssa Brewer at the University of California.
11:58So good to see you.
12:00Welcome. You ready to try some goggles on here?
12:01Oh, I'm ready.
12:03Volunteers wear these goggles for weeks at a time.
12:05Their brains are forced to cope with a new view of the world that's dramatically altered.
12:11What these have inside are two prisms that take the whole visual world and flip it.
12:16So whatever you see normally on the left side of the world will now be on the right side of the world.
12:20So as you move through the world, you're going to have a problem figuring out where things are as you see them in one side,
12:26but reach for them in the opposite side.
12:28What the world looks like is this.
12:31But what I'm seeing is this.
12:36It's a straightforward change, but it's also a massive mind mash.
12:44The visual data streaming in through my eyes no longer makes any intuitive sense.
12:50And I'm struggling.
12:51So yeah, because the world is left, right flipped, I know cognitively I'm supposed to reach out to the other direction,
13:02but of course I've had a lifetime of training telling me to reach out in a particular direction.
13:08So I feel like this is going to take a little getting used to.
13:11Can you see my hand in your visual field?
13:13Yep. So it looks like if I reach out this way.
13:16And this way.
13:17Even though I'm consciously trying to get it right.
13:24Over here. Okay.
13:26I can't help but respond in a certain way.
13:29Over here.
13:31Over here.
13:35There you go. Very good.
13:37Welcome to the prison world.
13:39Yes.
13:41Of course, this is all new to me.
13:43But Brian's been wearing his goggles for a week.
13:47So how well has his brain adapted?
13:55It's very difficult to figure out which way to go.
13:58And so his motor system and feeling of touch is sending him one direction,
14:02while his visual system is sending him the other direction.
14:17Brian's doing well.
14:19Unlike me.
14:21I have to consciously reconstruct my reality.
14:27This morning my brain could rely on automated interactions.
14:32But now it can't.
14:34Interestingly, I've broken out in a sweat and I'm hot and I'm super dizzy and nauseated.
14:40Oh, you know what?
14:42I've got to take a break.
14:43I'm so sorry.
14:44I've got to take these off for a second.
14:46Is that okay?
14:47Yeah.
14:49Boy, that is really nauseating.
14:53We're going to go out of the maze down here and see how you guys do in navigating your way through a spatial map that's...
15:00David, you're going to start out going forward this way.
15:02Okay.
15:03And Brian, you're going to...
15:04I'm just trying to get my head start.
15:09So how do I get as good as Brian?
15:14Well, it happens intuitively.
15:19Just look at my hands.
15:21I cross-reference what I see with what I can touch.
15:27In fact, all my senses come into play.
15:29This is what Brian's been doing for the last seven days.
15:40And the result is that his brain is now starting to decode that new visual input automatically.
15:51Brian's not simply getting better at making conscious adjustments.
15:55His whole reality is changing.
16:09Now, 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.
16:15They start being able to figure out how to interact with the world or constructing a new reality around them,
16:22a new way of dealing with these incoming perceptions.
16:24And they say that initially they can tell there's a new left and an old left and a new right and an old right.
16:32By about a week in, they even lose the concept of which right and left were the old ones and the new ones.
16:36So it's like the whole spatial map of the world is altering.
16:38And by two weeks in, they're able to write well, read without a problem, do all of our walking tasks and reaching tasks.
16:45And then when they remove the goggles, it actually takes them about a day to go back to normal behavior.
16:50What this exposes for me is how much effort the brain goes through to construct our world.
16:55Because normally you're walking through the world and it feels like there's reality out there.
17:02But in fact, there's so much work happening behind the scenes to allow that reality to happen.
17:10Seeing requires an intensive training program.
17:15But new recruits come on board every day.
17:19We call them babies.
17:20When babies reach out to touch what's in front of them, they're not just learning what an object feels like, they're learning how to see.
17:32They're establishing pathways in the brain that'll be used for the rest of their lives.
17:38Because vision is a whole body experience.
17:42The data coming in from our eyes only means something if we can cross-reference it.
17:55If from birth, you weren't able to interact with the world, if you couldn't work out through feedback what the sensory information meant,
18:03in theory, you'd never be able to see.
18:06This cross-referencing doesn't stop when we're fully grown.
18:18It continues throughout our lives.
18:23What we touch influences how we see.
18:30Taste is affected by our sense of smell.
18:34Our sight informs how we hear.
18:37Our senses depend on each other.
18:42And our reality is built by comparing these streams of data.
18:47When they're woven together, we get our perception of this moment.
18:53It's an astonishing feat to pull off.
18:57But there's one factor which really adds complication.
19:04Timing.
19:08All those streams of sensory data are processed by the brain at different speeds.
19:14For our reality to be constructed, they have to be synchronized.
19:18What do I mean by this?
19:22Well, the easiest way for me to show you is right here at a racetrack.
19:29And set.
19:33When there's a loud sound, it feels as though you react to it instantly.
19:36But you don't.
19:48Watching sprinters in slow motion, we can see that there's a gap between the gun going off and their start.
19:55They may train to make this gap as small as possible.
20:06But their biology imposes limits.
20:10Processing that sound and sending out signals to the muscles to move will take around two tenths of a second.
20:17And that time really can't be improved on.
20:30In a sport where thousandths of a second can be the difference between winning and losing, it seems surprisingly slow.
20:38So why do we use a pistol to start sprinters?
20:44Everyone knows that light travels faster than sound.
20:48So why not use a light?
20:54We set up a test to show you.
20:57In the top screen, we're triggered by a light.
21:01In the bottom screen, we're triggered by the gun.
21:03You can see that when our start is triggered by a flash of light, we respond more slowly.
21:16It takes 40 milliseconds longer to process.
21:20Why?
21:22Because the visual system is more complex.
21:26It's bigger.
21:28It involves almost a third of the brain.
21:31So while all of the electrical signals inside the brain travel at the same speed,
21:37the ones related to sight go through more complex processing.
21:42And that takes time.
21:50And this isn't just about hearing and seeing.
21:53Every type of sensory information takes a different amount of time to process.
21:58You'll react slower to a touch on the foot than one on the hand.
22:09The astonishing thing is that our brains hide all this.
22:13When I clap my hands, everything seems synchronized.
22:23Why?
22:25Well, your brain is pulling off fancy editing tricks.
22:28What it takes to be reality is actually a delayed version.
22:32It collects up all the information from the senses before it decides on a story of what happened.
22:38And that means you live in the past.
22:42By the time you think the moment now occurs, it's already long gone.
22:47To conjure a reality from all that sensory information, your brain needs around half a second.
22:54That's the unbridgeable gap between an event occurring and your conscious experience of it.
23:04In that half a second, a lot of things need to happen.
23:09Sometimes it's easy to assume that there's a single spot in the brain that takes care of this or that function.
23:21Like an area for memory or generosity or empathy.
23:25But in fact, the vast networks of the brain are so much more complex than that.
23:30Think of the brain like a city.
23:32If you were to look out over a city and ask, where is the economy located?
23:46You'd see that there's no single answer to that.
23:49Instead, the economy emerges as an interaction of all the elements.
23:54And so it is with reality.
23:59The raw materials of perception are gathered by our sensory receptors.
24:07They're turned into electrical signals and transported around our brains along superhighways of neurons.
24:15Processed, they become our reality.
24:19Some parts of brain cities specialize in vision.
24:25Other districts care about hearing.
24:28Some about touch and so on.
24:33And even within a sense like vision, you have streets that specialize in colors or edges or motion.
24:40But just like in a city, no neighborhood operates in isolation.
24:52Instead, the life of a city depends on the interaction between residents at all different scales.
24:59And somehow, out of all of this interaction, emerges your personal reality.
25:05Reality is the brain's ultimate construction.
25:15It's based on all the streams of data from our senses.
25:19But it's not dependent on them.
25:22How do we know?
25:24Because when you take it all away, reality doesn't stop.
25:28It just gets stranger.
25:31This is Alcatraz.
25:37A jail built on the principle of isolation.
25:44Between its inmates and the rest of society stood not only stone walls,
25:51but the cold, dangerous waters of the San Francisco Bay.
25:55Prisoners were completely and deliberately cut off.
26:07And there was one place inside the prison where that seclusion went even further.
26:12This is the hole.
26:20Prisoners who were sent here were completely isolated from the outside world.
26:25They had no interactions with people.
26:27There was no sound and there was no light.
26:29Robert Luke was sent to Alcatraz in 1954 for armed robbery.
26:39He was known by the nickname Cold Blue Luke.
26:44Everybody knew about the dark hole.
26:46The dark hole is a bad place.
26:52Some guys couldn't take that.
26:53I mean, they were in there and within a couple of days they were banging their head on the wall.
26:59As punishment for smashing up his cell, he was sent to the hole for 29 straight days.
27:08You didn't know how you would act when you got in there.
27:11You didn't want to find out.
27:14When they closed that door, there was just nothing there.
27:20It's pitch black.
27:22But it didn't stay that way for long.
27:27Starved of input, Luke's brain started to produce its own reality.
27:36I remember when I go on these trips.
27:39One I used to remember was flying a kite.
27:44But it got pretty real.
27:47They were all in my head.
27:48What Luke felt was something that's also been reported by other prisoners kept in the same conditions.
27:59Deprived of new sensory information, they said they went beyond dreaming or daydreaming.
28:07They didn't just imagine pictures.
28:09They saw.
28:17This testimony goes to the heart of the relationship between the outside world, the brain, and what we call reality.
28:23To understand it, we need to look more deeply into the visual system.
28:33This is the thalamus, one of the brain's major junctions.
28:41Most sensory information connects through here on its way to the outer surface of the brain, the cortex.
28:47So data collected from the eyes stops here before going to the visual cortex.
28:55Now, you'd expect a heavy flow of information from the thalamus to the visual cortex.
29:02And there is.
29:03But there's six times as much traffic flowing in the opposite direction.
29:09And that dwarfs the amount coming in from the eyes.
29:16And that suggests that in any one moment, what we experience as seeing relies less on the light streaming into our eyes,
29:26and more on what's already inside our heads.
29:32Even when brains are unanchored from external data, they continue to generate their own imagery.
29:39In other words, remove the world and the show still goes on.
29:46We all have this internally generated reality.
29:50Incredible as it may sound, this world lives inside your brain.
30:01It's constantly updated by information from our senses.
30:05But moment to moment, what we experience isn't what's really out there.
30:10Instead, it's a beautifully rendered simulation.
30:14This is a surprising way to understand how you see the world.
30:21It's called the internal model.
30:24And it's vital to our ability to function.
30:28As I walk down this city street, I seem to automatically know what things are without having to work out the details.
30:36For example, I don't have to work out the detail of what this rectangular metallic thing is,
30:41or this giant green fluffy thing behind me,
30:45or this huge object with reflective panes on it,
30:49or this thing with four appendages.
30:52My brain makes assumptions about what I'm seeing based on my internal model.
30:57And that's been built up from years of experience of walking city streets just like this one.
31:02Instead of using my senses to rebuild my reality from scratch every moment,
31:08I'm comparing sensory information with a model that I've already constructed.
31:17Updating it.
31:19Refining it.
31:21Correcting it.
31:23Our brains are so good at doing this that we're normally unaware of it.
31:28But sometimes, under certain conditions, we can see the process at work.
31:33Look at this hollow mask of Einstein's face.
31:39Your brain tells you it's coming out at you.
31:44And even when you know it's an illusion, you can't help but fall for it.
31:51What you're seeing is the internal model, not the raw information that's coming in from your eyes.
32:03Your internal model is built on a lifetime of experience with faces that stick out.
32:11When you're confronted with one that's hollow, your model simply sees what it expects to see.
32:24The visual cortex sends its internal expectations to the thalamus,
32:29and the thalamus compares those to what's coming in through the eyes.
32:33The difference between the two is what the thalamus sends back, so the cortex can update its model.
32:49Thanks to the internal model, the world out there remains stable, even when I'm moving.
32:56Let me show you what I mean.
33:02So imagine that I really love this scene behind me, and I want to go ahead and capture it so I can view it later.
33:08So I'm going to go ahead and videotape the scene.
33:11And I'm checking out all the buildings.
33:15Okay, and now I'm going to play this back.
33:19Not surprisingly, the resulting video is nauseating.
33:22So why does this video look so terrible?
33:28Given that, when I look at the buildings, my eyes are making the same jerky movements.
33:35Although you're not generally aware of it, your eyes move about four times a second.
33:41But your internal model operates under the assumption that the world outside is stable.
33:46So my eyes aren't taking a video.
33:49They're simply gathering bits of data to update the city that's already inside my head.
33:57Having an internal model helps me make sense of my environment.
34:02And that's its primary function, to navigate the world.
34:05The brain doesn't bother picking up every detail, just enough to get us through.
34:16But it plays the trick of making us feel as though we've seen it all.
34:23As another famous experiment shows.
34:26In the 1960s, the Russian psychologist Paul Yarbis used this painting, called The Unexpected Visitor, in an experiment.
34:41He devised a way to track the eye movements of volunteers who were seeing it for the first time.
34:46Hi, Jennifer. Hello.
34:50I'm going to ask you to put these glasses on.
34:53We're going to re-run what he did.
34:57My volunteers have a few seconds to take in the image.
35:03Dina, look at this painting.
35:06I want you to gather what's going on in the scene.
35:09We can watch in real time, exactly where each person's eyes go.
35:13Tell me what you think is going on in this painting.
35:19I think the man in the brown is the unexpected visitor.
35:23One brief look is enough for the brain to model the picture.
35:27But just how detailed is that model?
35:31How many children are there?
35:32There are two.
35:33Okay, so go ahead and look back at the painting and ask that question again.
35:36Oh, quite different.
35:37How many children are there?
35:39I consider it three.
35:40Everyone who'd seen the painting thought they knew what was in it.
35:45But my specific questions highlighted blanks that the brain had never filled in,
35:50because the details weren't needed.
35:54How many paintings are on the wall in their house?
35:57Maybe two or three.
35:59Okay, go ahead and look back at the painting and answer that question.
36:01Oh, God, there's a million.
36:02Yeah, a map and then another, and then there's seven on the other wall and then one small one in the map.
36:08Okay, there's a ton.
36:10This is not a failure of the brain.
36:13It doesn't try to produce a perfect simulation of the world.
36:17The internal model is a hastily drawn approximation, and more details are added on a need-to-know basis.
36:25When you looked at the painting the first time, you saw sort of a rough draft of what was going on,
36:32and when I asked you specific questions, you had to go and answer those by looking,
36:37by turning your attention onto specific parts of the painting, and only then did you actually see it.
36:42So placing your eyes on an object is no guarantee of seeing it.
36:50But there's something else we're unaware of happening every time we look at any picture, or person, or thing.
37:00Any time we look at all.
37:05We might think of color as a fundamental, defining quality of the world around us.
37:12After all, it's everywhere.
37:15But here's the startling thing.
37:17In the outside world,
37:21color doesn't actually exist.
37:26When electromagnetic radiation hits an object, some of it bounces off and is captured by our eyes.
37:33We can distinguish between millions of combinations of wavelengths,
37:38but it's only inside our heads that any of this becomes color.
37:47Add to that the fact that the wavelengths we can detect are only a small part of what's out there.
37:53You experience reality as it's presented by your senses, and it doesn't typically strike you that things can be very different.
38:05What we've been talking about so far is what we call the visible spectrum of light,
38:11which is a spectrum of wavelengths that runs from what we call red to violet.
38:18But it turns out that this only constitutes a tiny fraction of the electromagnetic spectrum.
38:27In fact, less than one ten trillionth of it.
38:33So all the rest of the spectrum, including radio waves and microwaves and X-rays and gamma rays,
38:40all of this stuff is flowing through our bodies right now, and we're completely unaware of it,
38:45because we don't have any specialized biological receptors to pick up on it.
38:50So what this means is that the part of reality that we can see is totally limited by our biology.
38:58And this isn't just about sight.
39:01All our senses are only picking up a small part of the information that's out there.
39:06So for a dog, he's tuned into a whole world of scent molecules that I'm not.
39:19His experience of smell is as rich as my experience of vision.
39:27In the blind and deaf world of the tick, the important signals are temperature and body odor.
39:33For cave-dwelling bats, it's all about air compression waves that allow them to echolocate.
39:44But no one's having an experience of objective reality of the world that really truly exists.
39:51Instead, each creature perceives only what it has evolved to perceive.
39:56And this isn't just about variation between species.
40:03If we're each experiencing a personal reality constructed inside our brains,
40:09how do I know that my reality is at all like yours?
40:13Most of the time, it seems as if we operate along the same lines.
40:17As if you and I agree what a blue sky is.
40:22As if the sound of a dog bark provokes the same sort of response in both of us.
40:36But there's a small group of people whose perception is measurably different from ours.
40:41For me, anytime I see a letter or a number or think of a word or say someone's name,
40:51there is a lot of color associated with that.
40:54Hannah is one of 6,000 people I've studied who have synesthesia.
40:59I study synesthesia because it's one of the few conditions in which it's clear that someone else's reality is different from mine.
41:10And it makes it obvious that how we perceive the world is not one size fits all.
41:16In my mind, I associate each letter with its own color.
41:20So for example, the letter A is always red, B is always blue, C is always orange, every time.
41:30So they never change.
41:32But what's interesting is when they're formed into words in different orders,
41:37the configuration of the colors changes and that can be sort of interesting.
41:41So in the word Hannah, my name, it looks like a sunset.
41:46It's yellow fading into red, fading into kind of a clear, like clouds almost, and then goes back to red and to yellow.
42:04These experiences come about because of the simple fact that inside the brain,
42:09all sensory information is made from the same stuff, electrochemical signals.
42:20Synesthesia is the result of crosstalk between sensory areas of the brain.
42:28Think of the blurred borders between city districts.
42:30Synesthesia shows us that even minute changes in brain wiring can lead to different realities.
42:43There are different kinds of synesthesia.
42:48Some people perceive weekdays to have locations in space.
42:52Some taste words.
42:54Others see music.
42:56And every time I meet someone who has this kind of experience,
43:01it's a reminder that from person to person, brain to brain,
43:06our experiences of reality can be quite different.
43:12For a small section of the population, that difference can be extreme and terrifying.
43:18We all know what it's like to have dreams at night, to have bizarre, unbidden thoughts that take us on journeys,
43:28sometimes journeys that we suffer through.
43:30But when we wake up, we're lucky enough to be able to compartmentalize that, to say,
43:34okay, that was a dream, and this is my waking life.
43:36But 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.
43:52I felt like the houses were communicating with me.
43:54You are special. You are especially bad. Repent. Stop. Go. You know, kind of, I did not hear these as words,
44:04but I heard them as thoughts put in my head, but I knew they were the house's thoughts and not my thoughts.
44:08I think that explosions are being set off in my brain, and I'm afraid that it's going to hurt other people, not just me.
44:17I once had a fantasy that my brains were going to leak out of my ears and drown people.
44:23What is that? You know?
44:26Ellen Sachs is a professor of law at the University of Southern California.
44:30She's been experiencing schizophrenic episodes since she was 16 years old.
44:38It's scary. It's unpredictable.
44:43It's sort of interesting, because there are different theories about psychotic symptoms.
44:46For some people, they're just random firings of neurons.
44:50I do think they tell the truth about your psychic reality.
44:52So when I say I've killed hundreds of thousands of people with my thoughts,
44:55that's just an archaic and extreme way of saying I feel like I'm a bad person.
45:00I'm a bad person.
45:04Schizophrenia is still not fully understood, but it involves chemical imbalances in the brain,
45:10which cause problems in the sending and receiving of signals.
45:21Thanks to medication and therapy, Ellen has been able to lecture and teach for over 25 years.
45:30So when you were at the bottom in one of your worst psychotic episodes, you took that to be reality?
45:38I really believe what I think is happening is happening.
45:40And it's terrifying. It's like a waking nightmare.
45:43Confusion, bizarre images, violence, terror.
45:46I wouldn't wish it on anyone.
45:49That said, everybody's reality is constructed, right?
45:54You filter it through your beliefs and values and issues.
46:00And this is true for people who have mental illness and for people who don't have mental illness.
46:04It's all a spectrum.
46:09Reality differs from person to person.
46:15And more than that, it changes from moment to moment.
46:18There are times in all our lives when it can seem enhanced, intensified.
46:29Even the one great constant, which we all think we share and which should never change,
46:34somehow becomes stretched and distorted.
46:37I'm talking about time.
46:49Time is something that we rarely stop to consider.
46:53But our brain's experience of time is often quite strange.
46:57It doesn't always seem in certain situations that time is running at an even pace.
47:02Sometimes it runs more slowly or more quickly.
47:09When I was eight years old, I fell off of the roof of the house to about this height.
47:14And the fall seemed to me to take a very long time.
47:22But when I got to high school, I learned physics.
47:25And I calculated how long did the fall actually take.
47:28And it turns out it was only eight tenths of a second.
47:32So that set me off on a quest to understand, why did it seem to take so long?
47:37And what did this tell me about our perception of reality?
47:43Many people have reported this sensation during moments of terror.
47:50Professional wingsuit flyer Jeb Corliss experienced it in an extreme way.
47:54How are you doing, Jeff?
47:59And because he falls for a living, the event he describes was captured on multiple cameras.
48:10On this day, I decided to aim for a target, like set up balloons and come in and hit balloons.
48:15And I was flying towards the balloons.
48:22And I was flying towards the balloons.
48:24And as I was coming in to hit the black balloon, I misjudged.
48:30I impacted flat, solid granite at 120 miles an hour.
48:44Six seconds elapsed between the moment that Jeb hit the rock and the moment he pulled his ripcord.
48:55He broke his leg and both ankles in the fall.
48:59From Jeb's perspective, those six seconds seem to last a long time.
49:12You've got two options.
49:17One is you can not pull and, you know, just be dead right now.
49:24It's like really quick, semi-painless, you know, over fast.
49:28Or you can pull, you know, get a parachute over your head, impact a second time,
49:33and then bleed to death while you're waiting for rescue.
49:38These two separate thought processes felt like minutes of time.
49:45It feels like you're operating so fast that your perception of everything else seems to slow down.
49:51Everything just gets stretched.
49:56But what was really happening in Jeb's brain?
49:59I designed an experiment to find out.
50:04It depended on inducing extreme fear in people by dropping them from 150 feet in the air.
50:14They fell with a digital display strapped to their wrist.
50:18Its numbers were changing at a rate faster than human vision can normally handle.
50:24If perceptual time did slow, then they would be able to read the numbers.
50:30But no one could.
50:33So why did Jeb recall his accident as happening in slow motion?
50:38It was a time distortion on a level I've never experienced before.
50:49I learned later that the rescue took about two and a half hours.
50:53But at the time, it felt like weeks.
50:56I mean, it didn't feel like minutes or hours or even days.
51:00It felt like, it felt like little eternities.
51:02It felt like forever.
51:04The answer seems to lie with how our memories are made.
51:11In a critical situation, an area of the brain called the amygdala kicks into high gear.
51:18It commandeers the resources of the rest of the brain, forcing everything to attend to the situation at hand.
51:25When the amygdala is in play, memories are laid down with far more detail than under normal circumstances.
51:38These memories are richer and more vivid.
51:41If you're ever in a similar situation, you have more information at your disposal to work out how to stay alive.
51:51But there's a fascinating consequence.
51:54When the events are replayed in your memory, they appear to have taken a longer time.
52:00When the events are replayed in your memory, they appear to have taken a longer time.
52:10Jeb's time distortion is something that happened in retrospect.
52:14A trick of the memory that wrote the story of his reality.
52:19The brain is the universe's ultimate storyteller.
52:33We believe whatever our brains serve up to us.
52:37The reality we take for granted requires intensive training to interpret the world.
52:43It takes time to process sensory information, so we live in the past.
52:55And because all that information is ultimately just electrochemical signals to be sorted, matched, rendered and packaged,
53:05reality is something created inside our head.
53:09Our brain sculpts our reality using the narrow trickle of data that it can gather through the senses.
53:22And from that trickle, it tells a story about our world.
53:26It's possible that every brain tells a different narrative.
53:29And with seven billion human brains wandering the planet,
53:37trillions of animal brains,
53:42no one is tapped into the full picture.
53:48Each brain carries its own unique model of the world around us.
53:53That is what we experience.
53:57We have no choice.
54:04So what is reality?
54:06It's whatever your brain tells you it is.
54:13Next time on The Brain, I'm going to explore a fundamental question about our lives.
54:19What makes you, you?
54:23I've spent many years of my life trying to decipher the mysteries of the brain.
54:29And yet, I'm still in awe every time I hold one.
54:33And that's because, although this marvel of biology seems so alien to us,
54:39somehow, it is us.
54:41This three-pound organ is made up of hundreds of billions of cells with a quadrillion connections between them.
54:52These cells fire trillions of electrochemical signals every second of your life.
54:58Somehow, all this wet biological stuff results in the experience of being you.
55:07What shapes who you become?
55:19I'm going to explore how your life shapes your brain.
55:23And how your brain shapes your life.
55:26To be continued...
55:27To be continued...
55:28To be continued...
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