Speaker 1: Why do they use a gun at the Olympics to
Speaker 1: start sprinters? Why not use a flash of light? And
Speaker 1: by the way, how can a sprinter come off the
Speaker 1: blocks after the starting gun but still get disqualified for
Speaker 1: jumping the gun? Or here's another question. If you watch
Speaker 1: someone dribbling a basketball, it looks like when the ball
Speaker 1: hits the ground, the sight and the sound are synchronized,
Speaker 1: and if you back up a ways, it still looks synchronized.
Speaker 1: But when you get to a very particular distance one
Speaker 1: hundred and ten feet, it suddenly goes out of sync.
Speaker 1: Why and what does any of this have to do
Speaker 1: with Robert Oppenheimer, or television broadcasting, or the Emperor Kubla
Speaker 1: Khan or schizophrenia. Welcome to Inner Cosmos with me David Eagleman.
Speaker 1: I'm a neuroscientist and an author at Stanford and in
Speaker 1: these episodes we sail deeply into our three pound universe
Speaker 1: to understand why and how our lives look the way
Speaker 1: they do. Today's episode is about time perception, which is
Speaker 1: an area that I've studied in my lab for years.
Speaker 1: For example, my first episode was on what happens when
Speaker 1: an event seems to go into slow motion? When you're
Speaker 1: in fear for your life. But today we're going to
Speaker 1: talk about a different aspect of time. And specifically, what
Speaker 1: we're going to talk about is that you have a
Speaker 1: three pound mission control center that sits in darkness and
Speaker 1: silence and has to figure out the timing of events
Speaker 1: in the outside world. But this is a massive challenge
Speaker 1: because signals stream in through different senses at different rates,
Speaker 1: and we'll see how and why your brain works so
Speaker 1: hard to pull off editing trail to get the timing right.
Speaker 1: I recently saw the movie Oppenheimer, and if you haven't
Speaker 1: seen it yet, you should. It's terrific. It's about J.
Speaker 1: Robert Oppenheimer, who led the Manhattan Project to build the
Speaker 1: first nuclear bomb. As you may know, the first test
Speaker 1: of the nuclear bomb was in the middle of New
Speaker 1: Mexico in a place called White Sands, which is a
Speaker 1: big empty area. They weren't sure if the nuclear bomb
Speaker 1: was going to work, because, as Oppenheimer often said, theory
Speaker 1: takes you only so far. So after four years of
Speaker 1: work and two billion dollars and hundreds of people on
Speaker 1: this project, they were finally going to get a chance
Speaker 1: to test the first ever nuclear bomb, to see if
Speaker 1: this idea was going to work as the equations predicted.
Speaker 1: So they set up the spot where the bomb would
Speaker 1: be set off, and then they set up a few
Speaker 1: observation sites, with the closest one being ten miles away
Speaker 1: from the explosion. So in the movie we witness a
Speaker 1: reconstruction of this scene, and it's tense because this is
Speaker 1: the culmination of years of work, and this is the
Speaker 1: project that might turn the tide of a world war,
Speaker 1: and this is gonna be the first time to get
Speaker 1: to see if it works. So the countdown clock begins,
Speaker 1: and Oppenheimer and the rest of the scientists put on
Speaker 1: their goggles, and finally the clock hits zero and the
Speaker 1: detonation button is pressed and they watch this giant, white
Speaker 1: and red pillar of fire reach up into the heavens.
Speaker 1: And in the movie it's silent. You just hear their
Speaker 1: heavy breathing as they watch this massive mushrooming cloud of
Speaker 1: incendiary flames, just silence. We see the explosion, but we
Speaker 1: hear nothing, and after about fifty seconds of this, suddenly
Speaker 1: we the audience hear a massive shaking boom. So after
Speaker 1: the movie, I was talking with a friend and he
Speaker 1: hypothesized that the director had done this for cinematic effect.
Speaker 1: The long drawn out silence was done so we could
Speaker 1: appreciate the terrifying success of the race to build a
Speaker 1: nuclear bomb. But I pointed out this was not a
Speaker 1: cinematic effect. This is how it really was and how
Speaker 1: it had to be because light travels ten miles very fast.
Speaker 1: It only takes about point one seconds for the light
Speaker 1: to travel from the bomb to the observation station ten
Speaker 1: miles away. But in contrast, sound travels about a million
Speaker 1: times more slowly. Because remember sound is just traveling by
Speaker 1: pushing around the molecules in the air, compressing them together
Speaker 1: and pulling them apart. And so how long does it
Speaker 1: take sound to travel one mile About five seconds. So
Speaker 1: for the folks at the observation station ten miles away,
Speaker 1: the nuclear explosion was silent for fifty seconds, fifty seconds
Speaker 1: of watching the most terrifying destructive force humans had ever
Speaker 1: made in total silence. And this is also what it
Speaker 1: was like for victims on the ground in Hiroshima and Nagasaki.
Speaker 1: People who were one mile away from ground zero saw
Speaker 1: the mushroom cloud but experienced silence for five seconds, or
Speaker 1: if you were across town twelve miles away. You'd have
Speaker 1: a full minute of watching what must have looked like
Speaker 1: a colossal monstrosity that reached the sky, but it must
Speaker 1: have felt the way it does to watch a television
Speaker 1: with the sound off for a full minute. And of
Speaker 1: course you know about this delay between light and sound
Speaker 1: because of watching lightning, which is followed only later by
Speaker 1: the thunder So it takes five seconds for sound to
Speaker 1: travel a mile, So if you count ten seconds after
Speaker 1: the lightning, that means it's two miles away. Okay, so
Speaker 1: there's this difference between the speed of light and the
Speaker 1: speed of sound. But we're about to see something very weird,
Speaker 1: which is that, unless we're watching something like a bomb
Speaker 1: or a thunderbolt, we don't perceive those time differences. Why
Speaker 1: don't we perceive these normally? Since sound is a million
Speaker 1: times slower, why doesn't someone talking to you from across
Speaker 1: the room look like a badly dubbed film. After all,
Speaker 1: the sight of their mouth and the sound of their
Speaker 1: voice arrives at different times, just like the bomb or
Speaker 1: the thunderbolt. And we're generally very sensitive to small differences
Speaker 1: in timing. So why can't we pick this up? So
Speaker 1: to reach our arms down into this problem. I'll give
Speaker 1: you a do it yourself demonstration that will unmask for
Speaker 1: you the deep weirdness of how much your brain constructs
Speaker 1: your reality. So watch some kid dribbling in basketball. It
Speaker 1: will seem like as the basketball is hitting the ground,
Speaker 1: the sight and the sound of that are synchronized. Now
Speaker 1: what you do is you start backing up. So you
Speaker 1: back up, you back up. They're dribbling, You're watching them,
Speaker 1: and it seems synchronized. The more and more you back up,
Speaker 1: it still seems synchronized until you hit one hundred and
Speaker 1: ten feet and then it doesn't seem synchronized anymore. Then
Speaker 1: suddenly the sight and sound seem off. The basketball visually
Speaker 1: hits the ground and then you hear the thump. It's asynchronous,
Speaker 1: like the badly dubbed movie. So what is going on here?
Speaker 1: Strap in, because we're about to see some things that
Speaker 1: will blow our minds, specifically, how much editing work your
Speaker 1: brain does behind the scenes to try to fix timing
Speaker 1: across the senses until the difference is simply too great
Speaker 1: and it says, okay, forget about it. So let's zoom
Speaker 1: out for a minute and consider the big picture. Of
Speaker 1: what your brain has to do. Your conscious perception requires
Speaker 1: the brain to compare different streams of sensory data against
Speaker 1: one another. You've got these different channels of information coming in.
Speaker 1: You've got vision, hearing, touch, and so on, and your
Speaker 1: brain has to put these together to make a big
Speaker 1: picture of what's happening. But there's something which makes this
Speaker 1: a really big challenge, and that is the issue of timing.
Speaker 1: All of these streams of sensory data are processed by
Speaker 1: the brain at different speeds. So think about sprinters at
Speaker 1: a racetrack. It appears that they get off the blocks
Speaker 1: the instant the gunfires, but it's not actually instantaneous. If
Speaker 1: you watch them in slow motion, you'll see this sizeable
Speaker 1: gap between the bang and the start of their movement.
Speaker 1: It's about one hundred and sixty millisecond because it takes
Speaker 1: time for signals to move through the brain, for the
Speaker 1: bang of the gun to work its way through their
Speaker 1: auditory system and over to their motor system and then
Speaker 1: down the spinal cord into the leg muscles and launched
Speaker 1: them off the blocks. In fact, if they move off
Speaker 1: the blocks before that duration, they're disqualified because they've jumped
Speaker 1: the gun. Because racing commissions know that this signal transmission
Speaker 1: takes time, so the rule is that if you move
Speaker 1: within one hundred milliseconds after the gun, you are disqualified.
Speaker 1: This came up in July of twenty twenty two when
Speaker 1: a hurdler named Devin Allen got off the starting blocks
Speaker 1: too early. He didn't go before the gun went off,
Speaker 1: he went after the gun went off. But he launched
Speaker 1: at ninety nine milliseconds after the gun, and even for
Speaker 1: a great athlete like him, this is below the possible
Speaker 1: reaction time, and there was a storm of activity on
Speaker 1: social media. But the fact is that one's reaction time
Speaker 1: can't be that rapid. Athletes trained to make their gap
Speaker 1: as small as possible, but their biology imposes fundamental limits.
Speaker 1: The brain has to register the sound and then send
Speaker 1: the signals to the motor cortex and down the spinal
Speaker 1: cord to the muscles of the body. Now, in a
Speaker 1: sport where thousandths of a second can be the difference
Speaker 1: between winning and losing, that response seems surprisingly slow. So
Speaker 1: could the delay be shortened if we used, say, a
Speaker 1: flash instead of a pistol to start the racers. After all,
Speaker 1: light travels faster than sounds, so wouldn't that allow them
Speaker 1: to get off the blocks faster? For my television show
Speaker 1: The Brain, I went to the track and I invited
Speaker 1: some fellow sprinters so we could all put this to
Speaker 1: the test. So, in one condition, our sprint was triggered
Speaker 1: by the gun. In a second condition, we were triggered
Speaker 1: by a flash of light. And we filmed this in
Speaker 1: super slow motion so we could compare what happened in
Speaker 1: the two conditions. When we were triggered by the bang
Speaker 1: of a gun, we got off the blocks at about
Speaker 1: one hundred and sixty milliseconds, but when we were triggered
Speaker 1: by the light, we got off the blocks more slowly,
Speaker 1: with a longer delay, about one hundred and ninety milliseconds.
Speaker 1: Now that seems to make no sense given what we
Speaker 1: just talked about, with light moving a million times faster
Speaker 1: than sound. So what gives here To understand what's happening?
Speaker 1: We need to look at the speed of information processing
Speaker 1: on the inside. Visual data goes through more complex processing
Speaker 1: than auditory data, so it takes longer for signals carrying
Speaker 1: the flash to wind their way through the visual system
Speaker 1: then for bang signals to snake their way through the
Speaker 1: auditory system, so light takes longer to trigger a motor response,
Speaker 1: and that's why a pistol is used to start sprinters. Now,
Speaker 1: scientists can prove this by putting electrodes in the auditory cortex,
Speaker 1: which responds to the bang, or the visual cortex, which
Speaker 1: responds to the flash, and you can see that the
Speaker 1: speed of the brain response is faster for the bang.
Speaker 1: But here comes the big mystery. Clap your hands in
Speaker 1: front of you. It looks synchronized. Well, that makes no
Speaker 1: sense because we just saw with the sprinters that your
Speaker 1: auditory system processes information more quickly than your visuals. So
Speaker 1: why aren't you seeing the sound and the sight out
Speaker 1: of sync. The answer, as we'll see in a moment,
Speaker 1: is that even though part of your brain gets the
Speaker 1: information before another part, your consciousness goes through a lot
Speaker 1: of trouble to sync things up. Your perception of the
Speaker 1: outside world is the end result of fancy editing tricks.
Speaker 1: The brain hides the difference in arrival times, and I'll
Speaker 1: explain to you how it does this, but first I'll
Speaker 1: give you a couple more examples. So let's step back
Speaker 1: seventy years to the early days of television broadcasting. The
Speaker 1: engineers realized they could be a stream of sound information
Speaker 1: and they could be a stream of visual information. But
Speaker 1: they were worried about how they could broadcast both the
Speaker 1: sound and the visuals and keep them synchronized with one another.
Speaker 1: And what they realized, quite accidentally, was that they don't
Speaker 1: actually need to keep them perfectly synchronized. As long as
Speaker 1: the sound and the visuals arrive to the viewer within
Speaker 1: about eighty milliseconds of each other. That's about a tenth
Speaker 1: of a second. Your brain does all the work of
Speaker 1: synking these up. And if you've ever seen a movie
Speaker 1: that seems out of sync, it means that the audio
Speaker 1: and the video were more than eighty milliseconds away from
Speaker 1: each other, because your brain can't tell the difference. As
Speaker 1: long as they're within eighty milliseconds of one another, it
Speaker 1: seems perfectly synchronized to you. And this allows us to
Speaker 1: understand what's happening. When you're watching the kid dribbling the basketball.
Speaker 1: It seems that as the ball is hitting the ground,
Speaker 1: everything is synced all the way until you back up
Speaker 1: to one hundred and ten feet and then it's not
Speaker 1: synchronized anymore. Why well, one hundred and ten feet is
Speaker 1: where the speed of light and the speed of sound
Speaker 1: are reaching you at over eighty milliseconds apart. So when
Speaker 1: they reach you at that timing apart from one another,
Speaker 1: your brain can't sync it up anymore. But as long
Speaker 1: as they're within that window, your brain has no problem saying, oh, okay,
Speaker 1: those belong together, I'm gonna synchronize that. So this is
Speaker 1: the question I turned to some years ago. If the
Speaker 1: brain is getting all this information at different speeds, how
Speaker 1: does it know how to synchronize things in consciousness? And
Speaker 1: I just want to make clear how challenging this is
Speaker 1: if you're living in the dark inside the skull, because
Speaker 1: these timing difficulties aren't even restricted to hearing and seeing.
Speaker 1: Every type of sensory information takes a different amount of
Speaker 1: time to process and to complicate things even more, even
Speaker 1: within a sense, there are time differences. For example, a
Speaker 1: bright flash moves through your retina and gets to your
Speaker 1: brain a full eighty milliseconds before a dim flash, or
Speaker 1: as another example, it takes longer for signals to reach
Speaker 1: your brain from your big toe than it does from
Speaker 1: your knows So how does your brain solve all of
Speaker 1: this temporal smearing of information. The only possible answer is
Speaker 1: wacky and somewhat mind blowing. Your brain has to collect
Speaker 1: up all the information from all the senses before it
Speaker 1: decides on a story of what happened. And that means
Speaker 1: your conscious perception is actually a delayed version of what's
Speaker 1: happening in the world. So when you clap your hands,
Speaker 1: your brain gathers the auditory information and the visual information
Speaker 1: and the sensory information of your hands touching, and all
Speaker 1: these stream in at different times, and then it puts
Speaker 1: together its story of what just happened, and it puts
Speaker 1: together what should go with what. And I'll explain in
Speaker 1: a second how it does this. But the key thing
Speaker 1: I want to make clear right now is that in
Speaker 1: order to synchronize everything, you have to wait for all
Speaker 1: of the information to arrive. And the strange consequence of
Speaker 1: all this is that you live in the past. By
Speaker 1: the time you think the moment occurs, it's already long gone.
Speaker 1: When you perceive the clap, it's already over. Your perceptual
Speaker 1: world always lags behind the real world. So think of
Speaker 1: it this way. Your perception of the world is like
Speaker 1: a live television show. Think of something like Saturday Night Live,
Speaker 1: which is not actually live. Instead, those shows are aired
Speaker 1: with a delay of a few seconds in case someone
Speaker 1: cusses or falls down or has a clothing mishap. And
Speaker 1: so it goes with your conscious life. It collects a
Speaker 1: lot of information before it goes live, so there's an
Speaker 1: unbridgable gap between an event occurring in the world and
Speaker 1: your conscious experience of it. So that's why your brain
Speaker 1: can sync the television audio at the video or the
Speaker 1: basketball thump with the visual of it hitting the ground
Speaker 1: or whatever. I need to emphasize that what we're talking
Speaker 1: about here is conscious awareness. You can see from pre
Speaker 1: conscious reactions that your motor system doesn't have to wait
Speaker 1: before its decisions. For example, you can duck out of
Speaker 1: the way of a swinging tree branch before you become
Speaker 1: consciously aware of it, or you can start jumping when
Speaker 1: there's a loud sound before you're consciously aware that there
Speaker 1: was a sound. Your body always tries to act as
Speaker 1: quickly as possible, even before the participation of awareness, but
Speaker 1: your conscious perception takes its time. Now that raises a question,
Speaker 1: what is the use of perception, especially since it lags
Speaker 1: behind reality and it's retrospectively attributed, and it's generally outstripped
Speaker 1: by these automatic, unconscious systems. The most likely answer is
Speaker 1: that your perceptions are like objects that your cognitive systems
Speaker 1: can work with later. So it's important for the brain
Speaker 1: to take sufficient time to settle on its best interpretation
Speaker 1: of what just happened, rather than stick with its initial
Speaker 1: rapid interpretation. Its carefully refined picture of what just happened
Speaker 1: is all that it's going to have to work with later,
Speaker 1: so it invests the time. So this brief waiting period
Speaker 1: before consciousness, this is what allows your sensory systems to
Speaker 1: discount the various delays imposed, But it has the disadvantage
Speaker 1: of pushing your perception into the past. Now, there's a
Speaker 1: distinct survival advantage to operating as close to the present
Speaker 1: as possible. An animal doesn't want to live too far
Speaker 1: in the past. Therefore, it may be that a tenth
Speaker 1: of a second window is the smallest delay that allows
Speaker 1: higher areas of the brain to account for the delays
Speaker 1: create in the first stage of the system while still
Speaker 1: operating near the border of the present. Among other things,
Speaker 1: this strategy of waiting for the slowest information to arrive
Speaker 1: has the advantage of allowing object recognition to be independent
Speaker 1: of lighting conditions. So I mentioned a minute ago about
Speaker 1: bright and dim flashes moving through the system at different speeds.
Speaker 1: So imagine a striped tiger coming toward you under the
Speaker 1: forest canopy, and he's passing through successive patches of sunlight.
Speaker 1: Imagine how difficult recognition would be if the bright and
Speaker 1: the dim parts of the tiger caused incoming signals to
Speaker 1: be perceived at different times. You'd perceive the tiger breaking
Speaker 1: into different space time fragments just before you became the
Speaker 1: tiger's lunch. Somehow, the visual system has evolved to reconcile
Speaker 1: these different speeds of incoming information. After all, it is
Speaker 1: advantageous to recognize tigers regardless of the lighting conditions. Now,
Speaker 1: this hypothesis that the system waits to collect information over
Speaker 1: the window of time that it's all streaming in this
Speaker 1: supplies not only to vision, but more generally, to all
Speaker 1: the senses. So, for example, my lab measured that in
Speaker 1: vision you wait at least a tenth of a second,
Speaker 1: But the size of this window might be different for
Speaker 1: hearing or touch. If I touch your toe and your
Speaker 1: nose at the same time, you will feel those touches
Speaker 1: as simultaneous. And that's really surprising, because the signal from
Speaker 1: your nose reaches your brain well before the signal from
Speaker 1: your toe. Why didn't you feel the nose touch when
Speaker 1: it first arrived. Did your brain wait to see what
Speaker 1: else might be coming up the pipeline of the spinal
Speaker 1: cord until it was sure that it had waited long
Speaker 1: enough for the slower signals from the toe. Strange as
Speaker 1: that sounds, that might be correct, and it may be
Speaker 1: that for a unified sensory perception of the world, you
Speaker 1: have to wait for the slowest overall information to get there,
Speaker 1: and given conduction times along whims, this leads to my
Speaker 1: strange but testable hypothesis that I mentioned in episode thirteen,
Speaker 1: which is that tall people may live further in the
Speaker 1: past than short people because they have to wait for
Speaker 1: the signals all the way from their toes before they
Speaker 1: put together their unified perception. Okay, but how does your
Speaker 1: brain know how to put all these signals together? So
Speaker 1: to think about this, let's turn to the Mongol emperor
Speaker 1: Kublai Khan, who ranged from twelve sixty to twelve ninety four,
Speaker 1: and founded the Yuan dynasty. Now he had conquered the
Speaker 1: largest kingdom the world had ever known. His kingdom reached
Speaker 1: from the Pacific to the Black Sea, and from Siberia
Speaker 1: to modern day Afghanis. His territory covered a fifth of
Speaker 1: the world's inhabited areas, so it was absolutely enormous. He
Speaker 1: situated himself in what is modern day Beijing. And the
Speaker 1: thing to note is that this was back in the
Speaker 1: day before iPhones and telegraphs and trains or email or
Speaker 1: anything like that. And so the question is, how in
Speaker 1: the world could Kubla Khan know his own empire. There's
Speaker 1: no way he could travel even a tiny fraction of
Speaker 1: a territory that large in his entire lifetime. So how
Speaker 1: could the Great Khan know what his empire contained? The
Speaker 1: answer is he hired emissaries like the Venetian traveler Marco Polo,
Speaker 1: and these people would travel out to the distant reaches
Speaker 1: of his empire and they would convey news back to
Speaker 1: him about what was going on in the empire. And
Speaker 1: he hired many emissaries that would go out in different
Speaker 1: directions and bring news back to him about what was
Speaker 1: going on. Now, I've never heard a historian talk about this.
Speaker 1: But I imagine the Great Con must have faced a
Speaker 1: tough problem what events in his empire occurred in which order.
Speaker 1: Because of wars and weather and other issues, people might
Speaker 1: travel at different paces, so the different emissaries would come
Speaker 1: back to him at different times. One emissary comes back
Speaker 1: and reports that a war has just ended, and another
Speaker 1: emissary comes back and reports that a war has just begun,
Speaker 1: and they're talking about the same war. But they got
Speaker 1: back to the capitol at different times. And so the
Speaker 1: question is how did the Great con synchronize all these signals.
Speaker 1: The thing I want to make clear is that the
Speaker 1: timing problem Kubla Khan had is the same problem the
Speaker 1: brain has, which is to say, it's getting all these
Speaker 1: different streams of information that come in at different paces.
Speaker 1: And because your brain is locked up in the dark
Speaker 1: tower of the skull, its only contact with the outside
Speaker 1: world is via the electrical signals exiting and entering along
Speaker 1: the highways of nerve bundles. So you've got touch information
Speaker 1: streaming up the body, You've got auditory coming in, you've
Speaker 1: got visual information coming into the brain. But the issue
Speaker 1: is that all of these things get processed by the
Speaker 1: brain at different speeds and at different places in the brain.
Speaker 1: So your brain faces this enormous challenge how to stitch
Speaker 1: together the incoming signals in the best way to make
Speaker 1: a story about what just happened in the outside world.
Speaker 1: For example, as we just saw, when I clap my hands,
Speaker 1: the auditory signals get processed first, then the visual signals,
Speaker 1: and yet they seem synced up. What this tells us
Speaker 1: is that the brain is somehow pulling off major video
Speaker 1: editing tricks, and we're going to find out how. Now,
Speaker 1: my lab has studied this for years, and the first
Speaker 1: thing to appreciate is that your brain doesn't figure this
Speaker 1: out pass. It does this by involving your own actions.
Speaker 1: Your own actions are the secret to understanding how everything
Speaker 1: gets synchronized. And what I've proposed is that your brain
Speaker 1: goes through so much trouble to get all this timing
Speaker 1: right because of one issue causality. Because one of the
Speaker 1: most fundamental things that any animal does is figure out
Speaker 1: whether it was the one that caused something or not.
Speaker 1: And fundamentally judging what caused what requires looking at the
Speaker 1: order of events. So imagine you're walking in the forest
Speaker 1: and you make a step and you hear a twig crack,
Speaker 1: you can assume that was me because of you. But
Speaker 1: if you hear the twig crack just before you land
Speaker 1: your step, then you'd better be worried about a mountain lion.
Speaker 1: So causality requires a temporal order judgment, which is to say,
Speaker 1: did I put out the action and then I got
Speaker 1: sensory feedback, in which case I'm going to take credit
Speaker 1: for having done that, Versus I got sensory feedback and
Speaker 1: then I did some action, in which case I'm not
Speaker 1: taking credit for that. I had nothing to do with that.
Speaker 1: And what we're often talking about is tens of milliseconds
Speaker 1: one way or the other as the only difference between
Speaker 1: these scenarios. But animals are very sensitive to this, and
Speaker 1: at bottom, this is the challenge that animals have to
Speaker 1: figure out. And as we've seen, the reason this is
Speaker 1: really difficult is because the speed of sensory signals differs
Speaker 1: and also they can change. So, for example, when you
Speaker 1: go from a bright outdoors into a dimly lit room,
Speaker 1: the speed at which your eyes are talking to your
Speaker 1: brain slows down by quite a bit. And what that
Speaker 1: means is that now your vision and your motor actions
Speaker 1: are out of sink a little bit. So if right
Speaker 1: when you walked into the dim room, I threw you
Speaker 1: a ball, you'd probably miss it. I don't know if
Speaker 1: you've ever played volleyball right when the sun's going down,
Speaker 1: but everyone's having good time, and then right as the
Speaker 1: sun's going down, everyone starts getting hit in the face
Speaker 1: with the ball and so on. Because what's happening is
Speaker 1: your time is getting out of sync. Now, now that's
Speaker 1: an example of a short term fast change, but on
Speaker 1: a longer time scale, as you grow from a baby
Speaker 1: to an adult, it takes a longer time to send
Speaker 1: signals out to your hand and get signals back. And
Speaker 1: so all of this led me to think a while ago,
Speaker 1: somehow the brain is having to figure out what its
Speaker 1: expectations are. It's having to modulate on the fly how
Speaker 1: long it expects for signals to come back. And so
Speaker 1: I hypothesize that the brain is always doing this on
Speaker 1: the fly. It's always recalibrating, it's readjusting the expected time
Speaker 1: that it takes for signals to come in. But how
Speaker 1: does it do that? And the answer is by interacting
Speaker 1: with the world. Because whenever you touch things, or you
Speaker 1: kick things, or you do anything like that. Your brain
Speaker 1: is saying, okay, everybody, synchronize your watches. I'm putting out
Speaker 1: an action, and what I expect is that I'm going
Speaker 1: to see it and hear it and feel it all
Speaker 1: at the same time. That's the prior expectation that it
Speaker 1: comes to the table with. And if I hit the
Speaker 1: table and it goes hit knock like that, my brain
Speaker 1: will adjust the timing. In other words, your brain doesn't
Speaker 1: know in advance how long your limbs are, or what
Speaker 1: the lighting level is, or how fast sound travels and
Speaker 1: so on. It just figures out what it needs by
Speaker 1: reaching out and interacting with the world. It just needs
Speaker 1: to embed the single assumption that if it sends out
Speaker 1: an action such as a knock on the table or
Speaker 1: a clap of the hands, all the feedback should be
Speaker 1: assumed to be simultaneous, and any delays between the senses
Speaker 1: should be adjusted until simultaneity is perceived. In other words,
Speaker 1: the best way to predict the expected timing of the
Speaker 1: signals is to interact with the world. Is to go
Speaker 1: out and touch and kick and push and knock on
Speaker 1: the world, and your brain makes the assumption that all
Speaker 1: the feedback is simultaneous. The best way to predict the
Speaker 1: future is to create it. You cause something and all
Speaker 1: the consequences should be synchronized. That's how you keep yourself calibrated. So,
Speaker 1: just to be clear, what this suggests is that if
Speaker 1: the feedback signal arrives with a delay, the brain's going
Speaker 1: to adjust things to make it seem like it happened
Speaker 1: close in time. So I took this hypothesis and in
Speaker 1: my lab we created a very simple experiment. You come
Speaker 1: into the lab and you're seated in front of a button,
Speaker 1: and whenever you hit the button that causes a flash
Speaker 1: of light. You press the button, the light flashes, But
Speaker 1: then we sneakily inject a small delay in there, let's
Speaker 1: say one hundred milliseconds. So you hit the button and
Speaker 1: then the flash of light appears. What happens very quickly
Speaker 1: is that your brain adjusts to the delay. Because you're
Speaker 1: the one causing it, the button and the light are
Speaker 1: interpreted as simultaneous, or at least close to simultaneous. So
Speaker 1: after just a couple of hits, it doesn't feel like
Speaker 1: one hundred millisecond delay. It feels like the button pressed
Speaker 1: and the flash are happening at the same time. Why
Speaker 1: it's because your brain is the one putting out the action,
Speaker 1: so it knows what to expect. And then we pull
Speaker 1: the big trick on you after your brain is used
Speaker 1: to this delay and thinks this is simultaneity. Now you
Speaker 1: press the button, and we make the flash happen immediately,
Speaker 1: no delay. And what is your perception? You think that
Speaker 1: the flash happened before you hit the button, So you
Speaker 1: hit the button, the flash occurs immediately, and you say, WHOA,
Speaker 1: I didn't do that. It flashed just before I hit it. Now,
Speaker 1: this is amazing to witness because this is an illusory
Speaker 1: reversal of action and effect. You hit the button, but
Speaker 1: you believe the consequence happened before your action. Your brain
Speaker 1: gives you the wrong answer because it has adjusted it
Speaker 1: expects a certain delay, and now we've tricked it. Now,
Speaker 1: this is a very simple experiment that you can reproduce
Speaker 1: at home with a little bit of programming, but it's
Speaker 1: a really big deal because it demonstrates that the timing
Speaker 1: you perceive in the world is not what it's truly
Speaker 1: happening out there, but an easily manipulated story put together
Speaker 1: by your brain now. I presented this research at a
Speaker 1: university recently, and a professor there came up to me
Speaker 1: afterwards and asked me about something. He said, they just
Speaker 1: got a new telephone system installed and he types at
Speaker 1: his desk all day and sometimes he turns to the
Speaker 1: phone and dials the number, and his impression was that
Speaker 1: the line starts ringing just before he hits the final number. Well,
Speaker 1: I immediately understood what was happening here, although he wasn't
Speaker 1: aware of it. His computer keyboard has a delay between
Speaker 1: hitting the key and a letter appearing on the screen.
Speaker 1: Typically for computers that's about one hundred milliseconds, but we
Speaker 1: never notice it because we calibrate to the delay. Anyway,
Speaker 1: he now turns to the phone, and this new system
Speaker 1: has a much smaller delay. So when he hits the
Speaker 1: final number on the phone, the ringing starts, but it
Speaker 1: feels like it happens just before he hit the final
Speaker 1: number key. He's experiencing this illusory reversal of action and effect,
Speaker 1: and it's because the delay of the two machines is different.
Speaker 1: He's calibrated to one, and suddenly the delay is different
Speaker 1: on the other. And by the way, you can turn
Speaker 1: this into a game of sorts. My graduate student John
Speaker 1: Jacobson was inspired by this discovery to program an unbeatable
Speaker 1: game of rocks, as or paper. So here's how that goes.
Speaker 1: There's a countdown and then you hit a key to
Speaker 1: register whether you're throwing a rock or as or a paper,
Speaker 1: And just like in the game, the computer presents its
Speaker 1: choice at the same time, or so you think it's
Speaker 1: actually waiting one hundred and fifty milliseconds and seeing what
Speaker 1: you did, But it feels simultaneous to you because your
Speaker 1: brain adjusts to you pressing the button and you're seeing
Speaker 1: the result on the screen. And then every once in
Speaker 1: a while the delay gets dropped, so it feels like
Speaker 1: the computer answered before you, even though it was again
Speaker 1: just registering your press and reacting immediately. You have the
Speaker 1: feeling that you reacted after it and that you answered
Speaker 1: exactly the wrong thing. And by the way, I'll just
Speaker 1: mention that my students and I ran a bunch of
Speaker 1: experiments in which we showed that this temporal recalibration can
Speaker 1: last through time, which, as you may remember from episode
Speaker 1: twenty four, is exactly what happens with the motion after effect.
Speaker 1: When you say, watch a downward waterfall for a while,
Speaker 1: and then you see everything moving upward. It turns out
Speaker 1: if you watch the waterfall and close your eyes and
Speaker 1: then open your eyes later, the illusion still happens. And
Speaker 1: without going into details, this indicates the mechanism by which
Speaker 1: the active recalibration happens under the hood in the neural circuitry. Anyway,
Speaker 1: my lab published some years ago a model in which
Speaker 1: recalibrations of motion are exactly the same mechanism as the
Speaker 1: recalibrations in the time domain. In other words, words, this
Speaker 1: suggests the possibility that the brain uses exactly the same
Speaker 1: circuitry for time and space. I'll skip the details here,
Speaker 1: but for interested parties, I've linked to the paper at
Speaker 1: eagleman dot com slash podcast. Okay, so we can show
Speaker 1: that the brain constantly recalibrates its timing. But remember this
Speaker 1: isn't just a party trick of the brain. It's critical
Speaker 1: to solving the problem of causality. The only way this
Speaker 1: problem can be accurately solved in a brain with lots
Speaker 1: of senses where timing is always changing is by keeping
Speaker 1: the expected time of signals actively calibrated so that you
Speaker 1: can determine before and after, even with these different sensory pathways,
Speaker 1: so your brain's always making this adjustment. But here's something
Speaker 1: really important. Let's return to what we saw with this recalibration.
Speaker 1: A person hits the button, but if we've just removed
Speaker 1: the delay, they say, WHOA, that wasn't me. The light
Speaker 1: flashed before I did anything. And that got me thinking
Speaker 1: about something because I realized I had seen that kind
Speaker 1: of reaction before. So when I saw participants saying that
Speaker 1: wasn't me, I thought that looks really familiar. Specifically, this
Speaker 1: is called credit misattribution, and this is where you cause
Speaker 1: something but you deny that you were the one who
Speaker 1: did it. And this is one of the striking symptoms
Speaker 1: that we see in schizophrenia. A person suffering from schizophrenia
Speaker 1: will often do something and not take credit for it.
Speaker 1: They believe that it was not them who caused the
Speaker 1: thing to happen, and then it's perfectly rational for them
Speaker 1: to cook up a different sort of explanation for it
Speaker 1: and say someone else is doing it, or this was
Speaker 1: caused by a signal from a radio tower or whatever,
Speaker 1: but it's not me that did it. Whatever's going on,
Speaker 1: And so some years ago, I started to hypothesize that
Speaker 1: schizophrenia may actually be a problem with time recalibration. What
Speaker 1: if you're not properly adjusting the timing of your inputs
Speaker 1: to your outputs, you would have a very difficult time
Speaker 1: judging causality. But that's just the beginning. I immediately started
Speaker 1: thinking about another symptom of schizophrenia, which is auditory hallucinations.
Speaker 1: So here's the thing. Under normal circumstances, you're always talking
Speaker 1: to yourself. You have an internally generated voice, and you
Speaker 1: listen to that. And by the way, if you're thinking,
Speaker 1: what internal voice, that's the internal voice. But what happens
Speaker 1: if you get the timing wrong, even by a few milliseconds,
Speaker 1: such that you thought you were hearing the voice just
Speaker 1: before feeling like you generated it, you would have to
Speaker 1: conclude that it was somebody else's voice, not your own.
Speaker 1: And it's all simply because the timing is off between
Speaker 1: generating the voice and listening to it. And if thinking
Speaker 1: led me and my students to run experiments at a
Speaker 1: county mental health facility with people who had schizophrenia, and indeed,
Speaker 1: we found that people with schizophrenia do not recalibrate their
Speaker 1: timing as well as healthy controls do. So when we
Speaker 1: give them a test like hit the button and did
Speaker 1: the flash of light occur before or after, and we
Speaker 1: measure their recalibration, it's much less or it's absent. Now,
Speaker 1: like all science, this is going to require many more studies,
Speaker 1: but if this is the right way to think about schizophrenia,
Speaker 1: it completely changes our approach to it. It means that
Speaker 1: instead of throwing pharmacological solutions at it, which have limited success,
Speaker 1: just imagine if you could give somebody a video game
Speaker 1: that they play for a few minutes and then their
Speaker 1: auditory hallucinations go away because we've recalibrated their timing. So
Speaker 1: this is work I'm pursuing now, and it all began
Speaker 1: with these very simple experiments. So to wrap up today's episode,
Speaker 1: it's an ongoing passion of mind to figure out how
Speaker 1: the brain constructs reality. And there are very few things
Speaker 1: weirder than time. What we saw today is that to
Speaker 1: synchronize the incoming information from the senses, our conscious awareness
Speaker 1: has to lag behind the physical world. But none of
Speaker 1: that is obvious to your perception. And even weirder, the
Speaker 1: order of sensory events in the world is dynamically recalibrated,
Speaker 1: so you can hit a button that causes a flash,
Speaker 1: but under different circumstances, you'll believe that happened before or after,
Speaker 1: and we can easily change that around. And this is
Speaker 1: because your brain can't automatically know what the timing should be,
Speaker 1: especially as sensory timing changes all the time. So your brain,
Speaker 1: living in darkness, constantly interacts with the world to recalibrate
Speaker 1: its timing. It says, I'm going to knock on something now.
Speaker 1: Everyone synchronize your watches. And this matters to the brains
Speaker 1: so much to get all this timing right, because this
Speaker 1: is the basis of judging causality, and when that recalibration
Speaker 1: isn't working well, it makes it difficult to interpret the
Speaker 1: world as we see in schizophrenia. And this episode points
Speaker 1: us back to a fundamental lesson that we've seen a
Speaker 1: lot in earlier episodes, with visual illusions and auditory illusions.
Speaker 1: Reality is not passively received by our brains, but is
Speaker 1: actively constructed. Go to Eagleman dot com slash podcast for
Speaker 1: more information and to find further reading, and send me
Speaker 1: an email at podcast at eagleman dot com with questions
Speaker 1: or discussion, and I'll be making more episodes in which
Speaker 1: I address those. Until next time, I'm David Eagleman, and
Speaker 1: this is Inner Cosmos.