WEBVTT - Ep33 "Why do they start sprinters with a bang instead of a flash?" 0:00:04.600 --> 0:00:07.520 Why do they use a gun at the Olympics to 0:00:07.640 --> 0:00:11.360 start sprinters? Why not use a flash of light? And 0:00:11.400 --> 0:00:14.160 by the way, how can a sprinter come off the 0:00:14.240 --> 0:00:18.759 blocks after the starting gun but still get disqualified for 0:00:18.880 --> 0:00:22.760 jumping the gun? Or here's another question. If you watch 0:00:22.800 --> 0:00:26.200 someone dribbling a basketball, it looks like when the ball 0:00:26.320 --> 0:00:29.880 hits the ground, the sight and the sound are synchronized, 0:00:30.240 --> 0:00:33.040 and if you back up a ways, it still looks synchronized. 0:00:33.560 --> 0:00:36.519 But when you get to a very particular distance one 0:00:36.600 --> 0:00:39.560 hundred and ten feet, it suddenly goes out of sync. 0:00:40.120 --> 0:00:42.839 Why and what does any of this have to do 0:00:42.960 --> 0:00:48.280 with Robert Oppenheimer, or television broadcasting, or the Emperor Kubla 0:00:48.360 --> 0:00:56.960 Khan or schizophrenia. Welcome to Inner Cosmos with me David Eagleman. 0:00:57.080 --> 0:01:00.320 I'm a neuroscientist and an author at Stanford and in 0:01:00.360 --> 0:01:04.400 these episodes we sail deeply into our three pound universe 0:01:04.760 --> 0:01:08.200 to understand why and how our lives look the way 0:01:08.240 --> 0:01:17.880 they do. Today's episode is about time perception, which is 0:01:17.920 --> 0:01:20.200 an area that I've studied in my lab for years. 0:01:20.560 --> 0:01:24.000 For example, my first episode was on what happens when 0:01:24.040 --> 0:01:26.880 an event seems to go into slow motion? When you're 0:01:26.920 --> 0:01:29.720 in fear for your life. But today we're going to 0:01:29.760 --> 0:01:34.240 talk about a different aspect of time. And specifically, what 0:01:34.319 --> 0:01:36.240 we're going to talk about is that you have a 0:01:36.440 --> 0:01:39.920 three pound mission control center that sits in darkness and 0:01:40.000 --> 0:01:43.759 silence and has to figure out the timing of events 0:01:43.800 --> 0:01:47.680 in the outside world. But this is a massive challenge 0:01:48.120 --> 0:01:53.600 because signals stream in through different senses at different rates, 0:01:54.200 --> 0:01:57.280 and we'll see how and why your brain works so 0:01:57.520 --> 0:02:01.840 hard to pull off editing trail to get the timing right. 0:02:03.040 --> 0:02:05.760 I recently saw the movie Oppenheimer, and if you haven't 0:02:05.760 --> 0:02:08.519 seen it yet, you should. It's terrific. It's about J. 0:02:08.800 --> 0:02:12.880 Robert Oppenheimer, who led the Manhattan Project to build the 0:02:12.919 --> 0:02:16.280 first nuclear bomb. As you may know, the first test 0:02:16.400 --> 0:02:18.320 of the nuclear bomb was in the middle of New 0:02:18.360 --> 0:02:21.040 Mexico in a place called White Sands, which is a 0:02:21.040 --> 0:02:25.639 big empty area. They weren't sure if the nuclear bomb 0:02:25.720 --> 0:02:30.440 was going to work, because, as Oppenheimer often said, theory 0:02:30.600 --> 0:02:34.760 takes you only so far. So after four years of 0:02:34.800 --> 0:02:37.920 work and two billion dollars and hundreds of people on 0:02:37.919 --> 0:02:41.320 this project, they were finally going to get a chance 0:02:41.440 --> 0:02:45.000 to test the first ever nuclear bomb, to see if 0:02:45.000 --> 0:02:49.000 this idea was going to work as the equations predicted. 0:02:49.600 --> 0:02:51.920 So they set up the spot where the bomb would 0:02:51.919 --> 0:02:54.680 be set off, and then they set up a few 0:02:54.840 --> 0:02:59.080 observation sites, with the closest one being ten miles away 0:02:59.080 --> 0:03:03.320 from the explosion. So in the movie we witness a 0:03:03.480 --> 0:03:07.960 reconstruction of this scene, and it's tense because this is 0:03:08.000 --> 0:03:11.080 the culmination of years of work, and this is the 0:03:11.400 --> 0:03:14.520 project that might turn the tide of a world war, 0:03:14.880 --> 0:03:16.480 and this is gonna be the first time to get 0:03:16.520 --> 0:03:20.800 to see if it works. So the countdown clock begins, 0:03:21.360 --> 0:03:24.080 and Oppenheimer and the rest of the scientists put on 0:03:24.120 --> 0:03:28.639 their goggles, and finally the clock hits zero and the 0:03:28.680 --> 0:03:33.760 detonation button is pressed and they watch this giant, white 0:03:33.800 --> 0:03:37.840 and red pillar of fire reach up into the heavens. 0:03:38.760 --> 0:03:42.400 And in the movie it's silent. You just hear their 0:03:42.800 --> 0:03:47.760 heavy breathing as they watch this massive mushrooming cloud of 0:03:48.160 --> 0:03:54.720 incendiary flames, just silence. We see the explosion, but we 0:03:54.800 --> 0:03:59.880 hear nothing, and after about fifty seconds of this, suddenly 0:04:00.240 --> 0:04:05.280 we the audience hear a massive shaking boom. So after 0:04:05.320 --> 0:04:07.760 the movie, I was talking with a friend and he 0:04:07.960 --> 0:04:12.600 hypothesized that the director had done this for cinematic effect. 0:04:13.040 --> 0:04:16.360 The long drawn out silence was done so we could 0:04:16.400 --> 0:04:20.359 appreciate the terrifying success of the race to build a 0:04:20.440 --> 0:04:24.560 nuclear bomb. But I pointed out this was not a 0:04:24.600 --> 0:04:27.960 cinematic effect. This is how it really was and how 0:04:28.000 --> 0:04:33.719 it had to be because light travels ten miles very fast. 0:04:33.800 --> 0:04:38.240 It only takes about point one seconds for the light 0:04:38.320 --> 0:04:41.560 to travel from the bomb to the observation station ten 0:04:41.640 --> 0:04:47.000 miles away. But in contrast, sound travels about a million 0:04:47.279 --> 0:04:51.320 times more slowly. Because remember sound is just traveling by 0:04:51.600 --> 0:04:54.800 pushing around the molecules in the air, compressing them together 0:04:54.880 --> 0:04:57.719 and pulling them apart. And so how long does it 0:04:57.760 --> 0:05:04.080 take sound to travel one mile About five seconds. So 0:05:04.200 --> 0:05:07.360 for the folks at the observation station ten miles away, 0:05:07.720 --> 0:05:14.279 the nuclear explosion was silent for fifty seconds, fifty seconds 0:05:14.320 --> 0:05:18.919 of watching the most terrifying destructive force humans had ever 0:05:19.040 --> 0:05:23.840 made in total silence. And this is also what it 0:05:23.880 --> 0:05:27.960 was like for victims on the ground in Hiroshima and Nagasaki. 0:05:28.200 --> 0:05:31.800 People who were one mile away from ground zero saw 0:05:31.800 --> 0:05:36.719 the mushroom cloud but experienced silence for five seconds, or 0:05:36.760 --> 0:05:40.279 if you were across town twelve miles away. You'd have 0:05:40.400 --> 0:05:43.839 a full minute of watching what must have looked like 0:05:44.240 --> 0:05:49.200 a colossal monstrosity that reached the sky, but it must 0:05:49.200 --> 0:05:52.000 have felt the way it does to watch a television 0:05:52.000 --> 0:05:55.839 with the sound off for a full minute. And of 0:05:55.880 --> 0:05:58.760 course you know about this delay between light and sound 0:05:59.200 --> 0:06:03.200 because of watching lightning, which is followed only later by 0:06:03.240 --> 0:06:06.120 the thunder So it takes five seconds for sound to 0:06:06.200 --> 0:06:09.400 travel a mile, So if you count ten seconds after 0:06:09.440 --> 0:06:14.000 the lightning, that means it's two miles away. Okay, so 0:06:14.080 --> 0:06:16.560 there's this difference between the speed of light and the 0:06:16.600 --> 0:06:20.760 speed of sound. But we're about to see something very weird, 0:06:20.960 --> 0:06:24.000 which is that, unless we're watching something like a bomb 0:06:24.080 --> 0:06:28.480 or a thunderbolt, we don't perceive those time differences. Why 0:06:28.520 --> 0:06:32.040 don't we perceive these normally? Since sound is a million 0:06:32.080 --> 0:06:35.719 times slower, why doesn't someone talking to you from across 0:06:35.760 --> 0:06:39.640 the room look like a badly dubbed film. After all, 0:06:39.680 --> 0:06:41.720 the sight of their mouth and the sound of their 0:06:41.800 --> 0:06:45.040 voice arrives at different times, just like the bomb or 0:06:45.040 --> 0:06:49.480 the thunderbolt. And we're generally very sensitive to small differences 0:06:49.520 --> 0:06:53.440 in timing. So why can't we pick this up? So 0:06:53.680 --> 0:06:57.080 to reach our arms down into this problem. I'll give 0:06:57.080 --> 0:07:00.159 you a do it yourself demonstration that will unmask for 0:07:00.200 --> 0:07:04.120 you the deep weirdness of how much your brain constructs 0:07:04.200 --> 0:07:08.960 your reality. So watch some kid dribbling in basketball. It 0:07:09.040 --> 0:07:12.200 will seem like as the basketball is hitting the ground, 0:07:12.280 --> 0:07:15.760 the sight and the sound of that are synchronized. Now 0:07:15.760 --> 0:07:17.640 what you do is you start backing up. So you 0:07:17.760 --> 0:07:20.760 back up, you back up. They're dribbling, You're watching them, 0:07:21.040 --> 0:07:23.960 and it seems synchronized. The more and more you back up, 0:07:24.040 --> 0:07:28.640 it still seems synchronized until you hit one hundred and 0:07:28.720 --> 0:07:32.800 ten feet and then it doesn't seem synchronized anymore. Then 0:07:32.920 --> 0:07:38.040 suddenly the sight and sound seem off. The basketball visually 0:07:38.120 --> 0:07:42.360 hits the ground and then you hear the thump. It's asynchronous, 0:07:42.440 --> 0:07:46.080 like the badly dubbed movie. So what is going on here? 0:07:47.000 --> 0:07:49.560 Strap in, because we're about to see some things that 0:07:49.640 --> 0:07:53.880 will blow our minds, specifically, how much editing work your 0:07:53.920 --> 0:07:57.800 brain does behind the scenes to try to fix timing 0:07:57.840 --> 0:08:01.040 across the senses until the difference is simply too great 0:08:01.040 --> 0:08:04.119 and it says, okay, forget about it. So let's zoom 0:08:04.160 --> 0:08:06.640 out for a minute and consider the big picture. Of 0:08:06.680 --> 0:08:11.920 what your brain has to do. Your conscious perception requires 0:08:11.960 --> 0:08:15.920 the brain to compare different streams of sensory data against 0:08:16.000 --> 0:08:19.920 one another. You've got these different channels of information coming in. 0:08:20.000 --> 0:08:23.720 You've got vision, hearing, touch, and so on, and your 0:08:23.720 --> 0:08:25.880 brain has to put these together to make a big 0:08:25.960 --> 0:08:29.640 picture of what's happening. But there's something which makes this 0:08:29.720 --> 0:08:33.880 a really big challenge, and that is the issue of timing. 0:08:34.360 --> 0:08:37.600 All of these streams of sensory data are processed by 0:08:37.600 --> 0:08:42.320 the brain at different speeds. So think about sprinters at 0:08:42.360 --> 0:08:45.680 a racetrack. It appears that they get off the blocks 0:08:45.800 --> 0:08:50.640 the instant the gunfires, but it's not actually instantaneous. If 0:08:50.679 --> 0:08:54.239 you watch them in slow motion, you'll see this sizeable 0:08:54.480 --> 0:08:58.079 gap between the bang and the start of their movement. 0:08:58.080 --> 0:09:01.720 It's about one hundred and sixty millisecond because it takes 0:09:01.800 --> 0:09:05.880 time for signals to move through the brain, for the 0:09:05.920 --> 0:09:08.120 bang of the gun to work its way through their 0:09:08.200 --> 0:09:11.360 auditory system and over to their motor system and then 0:09:11.440 --> 0:09:14.640 down the spinal cord into the leg muscles and launched 0:09:14.679 --> 0:09:18.640 them off the blocks. In fact, if they move off 0:09:18.640 --> 0:09:23.760 the blocks before that duration, they're disqualified because they've jumped 0:09:23.840 --> 0:09:28.679 the gun. Because racing commissions know that this signal transmission 0:09:28.720 --> 0:09:31.760 takes time, so the rule is that if you move 0:09:31.880 --> 0:09:37.000 within one hundred milliseconds after the gun, you are disqualified. 0:09:37.720 --> 0:09:39.839 This came up in July of twenty twenty two when 0:09:39.880 --> 0:09:43.720 a hurdler named Devin Allen got off the starting blocks 0:09:43.760 --> 0:09:46.800 too early. He didn't go before the gun went off, 0:09:46.800 --> 0:09:50.280 he went after the gun went off. But he launched 0:09:50.360 --> 0:09:54.400 at ninety nine milliseconds after the gun, and even for 0:09:54.440 --> 0:09:58.160 a great athlete like him, this is below the possible 0:09:58.320 --> 0:10:01.240 reaction time, and there was a storm of activity on 0:10:01.280 --> 0:10:04.640 social media. But the fact is that one's reaction time 0:10:05.040 --> 0:10:08.920 can't be that rapid. Athletes trained to make their gap 0:10:08.960 --> 0:10:13.880 as small as possible, but their biology imposes fundamental limits. 0:10:14.440 --> 0:10:17.280 The brain has to register the sound and then send 0:10:17.280 --> 0:10:19.840 the signals to the motor cortex and down the spinal 0:10:19.880 --> 0:10:22.920 cord to the muscles of the body. Now, in a 0:10:23.080 --> 0:10:26.360 sport where thousandths of a second can be the difference 0:10:26.400 --> 0:10:32.720 between winning and losing, that response seems surprisingly slow. So 0:10:32.920 --> 0:10:36.240 could the delay be shortened if we used, say, a 0:10:36.480 --> 0:10:40.160 flash instead of a pistol to start the racers. After all, 0:10:40.679 --> 0:10:43.720 light travels faster than sounds, so wouldn't that allow them 0:10:43.760 --> 0:10:47.680 to get off the blocks faster? For my television show 0:10:47.760 --> 0:10:50.559 The Brain, I went to the track and I invited 0:10:50.600 --> 0:10:53.160 some fellow sprinters so we could all put this to 0:10:53.200 --> 0:10:57.560 the test. So, in one condition, our sprint was triggered 0:10:57.720 --> 0:11:01.800 by the gun. In a second condition, we were triggered 0:11:02.000 --> 0:11:05.280 by a flash of light. And we filmed this in 0:11:05.400 --> 0:11:09.040 super slow motion so we could compare what happened in 0:11:09.080 --> 0:11:12.640 the two conditions. When we were triggered by the bang 0:11:12.679 --> 0:11:14.760 of a gun, we got off the blocks at about 0:11:14.760 --> 0:11:18.400 one hundred and sixty milliseconds, but when we were triggered 0:11:18.400 --> 0:11:22.479 by the light, we got off the blocks more slowly, 0:11:22.679 --> 0:11:25.840 with a longer delay, about one hundred and ninety milliseconds. 0:11:26.240 --> 0:11:28.880 Now that seems to make no sense given what we 0:11:29.040 --> 0:11:32.120 just talked about, with light moving a million times faster 0:11:32.200 --> 0:11:37.160 than sound. So what gives here To understand what's happening? 0:11:37.200 --> 0:11:40.600 We need to look at the speed of information processing 0:11:40.920 --> 0:11:46.400 on the inside. Visual data goes through more complex processing 0:11:46.840 --> 0:11:51.480 than auditory data, so it takes longer for signals carrying 0:11:51.520 --> 0:11:55.040 the flash to wind their way through the visual system 0:11:55.440 --> 0:11:58.480 then for bang signals to snake their way through the 0:11:58.480 --> 0:12:04.880 auditory system, so light takes longer to trigger a motor response, 0:12:04.960 --> 0:12:26.520 and that's why a pistol is used to start sprinters. Now, 0:12:26.600 --> 0:12:30.480 scientists can prove this by putting electrodes in the auditory cortex, 0:12:30.480 --> 0:12:33.600 which responds to the bang, or the visual cortex, which 0:12:33.640 --> 0:12:36.840 responds to the flash, and you can see that the 0:12:36.960 --> 0:12:40.880 speed of the brain response is faster for the bang. 0:12:41.880 --> 0:12:45.880 But here comes the big mystery. Clap your hands in 0:12:45.920 --> 0:12:50.560 front of you. It looks synchronized. Well, that makes no 0:12:50.679 --> 0:12:54.080 sense because we just saw with the sprinters that your 0:12:54.240 --> 0:13:00.880 auditory system processes information more quickly than your visuals. So 0:13:00.920 --> 0:13:04.760 why aren't you seeing the sound and the sight out 0:13:04.800 --> 0:13:08.360 of sync. The answer, as we'll see in a moment, 0:13:08.520 --> 0:13:10.920 is that even though part of your brain gets the 0:13:10.920 --> 0:13:16.160 information before another part, your consciousness goes through a lot 0:13:16.200 --> 0:13:20.560 of trouble to sync things up. Your perception of the 0:13:20.600 --> 0:13:25.040 outside world is the end result of fancy editing tricks. 0:13:25.400 --> 0:13:29.320 The brain hides the difference in arrival times, and I'll 0:13:29.360 --> 0:13:31.440 explain to you how it does this, but first I'll 0:13:31.440 --> 0:13:35.040 give you a couple more examples. So let's step back 0:13:35.120 --> 0:13:40.000 seventy years to the early days of television broadcasting. The 0:13:40.040 --> 0:13:43.960 engineers realized they could be a stream of sound information 0:13:44.320 --> 0:13:47.280 and they could be a stream of visual information. But 0:13:47.320 --> 0:13:50.360 they were worried about how they could broadcast both the 0:13:50.440 --> 0:13:54.160 sound and the visuals and keep them synchronized with one another. 0:13:54.679 --> 0:13:58.280 And what they realized, quite accidentally, was that they don't 0:13:58.400 --> 0:14:02.200 actually need to keep them perfectly synchronized. As long as 0:14:02.240 --> 0:14:06.360 the sound and the visuals arrive to the viewer within 0:14:06.400 --> 0:14:09.520 about eighty milliseconds of each other. That's about a tenth 0:14:09.559 --> 0:14:12.679 of a second. Your brain does all the work of 0:14:12.720 --> 0:14:15.959 synking these up. And if you've ever seen a movie 0:14:16.000 --> 0:14:18.800 that seems out of sync, it means that the audio 0:14:18.880 --> 0:14:22.680 and the video were more than eighty milliseconds away from 0:14:22.720 --> 0:14:26.440 each other, because your brain can't tell the difference. As 0:14:26.480 --> 0:14:29.320 long as they're within eighty milliseconds of one another, it 0:14:29.360 --> 0:14:34.040 seems perfectly synchronized to you. And this allows us to 0:14:34.160 --> 0:14:38.200 understand what's happening. When you're watching the kid dribbling the basketball. 0:14:38.640 --> 0:14:41.160 It seems that as the ball is hitting the ground, 0:14:41.200 --> 0:14:44.200 everything is synced all the way until you back up 0:14:44.240 --> 0:14:46.480 to one hundred and ten feet and then it's not 0:14:46.520 --> 0:14:50.800 synchronized anymore. Why well, one hundred and ten feet is 0:14:50.840 --> 0:14:53.680 where the speed of light and the speed of sound 0:14:53.880 --> 0:14:58.160 are reaching you at over eighty milliseconds apart. So when 0:14:58.200 --> 0:15:01.840 they reach you at that timing apart from one another, 0:15:01.920 --> 0:15:05.200 your brain can't sync it up anymore. But as long 0:15:05.240 --> 0:15:09.560 as they're within that window, your brain has no problem saying, oh, okay, 0:15:09.600 --> 0:15:13.400 those belong together, I'm gonna synchronize that. So this is 0:15:13.440 --> 0:15:15.800 the question I turned to some years ago. If the 0:15:15.920 --> 0:15:19.360 brain is getting all this information at different speeds, how 0:15:19.400 --> 0:15:23.320 does it know how to synchronize things in consciousness? And 0:15:23.360 --> 0:15:26.280 I just want to make clear how challenging this is 0:15:26.360 --> 0:15:29.720 if you're living in the dark inside the skull, because 0:15:29.760 --> 0:15:33.320 these timing difficulties aren't even restricted to hearing and seeing. 0:15:33.640 --> 0:15:37.160 Every type of sensory information takes a different amount of 0:15:37.160 --> 0:15:41.400 time to process and to complicate things even more, even 0:15:41.440 --> 0:15:44.920 within a sense, there are time differences. For example, a 0:15:45.080 --> 0:15:48.640 bright flash moves through your retina and gets to your 0:15:48.640 --> 0:15:53.240 brain a full eighty milliseconds before a dim flash, or 0:15:53.240 --> 0:15:56.920 as another example, it takes longer for signals to reach 0:15:56.960 --> 0:15:59.840 your brain from your big toe than it does from 0:15:59.840 --> 0:16:03.800 your knows So how does your brain solve all of 0:16:03.840 --> 0:16:09.440 this temporal smearing of information. The only possible answer is 0:16:09.520 --> 0:16:14.160 wacky and somewhat mind blowing. Your brain has to collect 0:16:14.320 --> 0:16:18.200 up all the information from all the senses before it 0:16:18.320 --> 0:16:23.440 decides on a story of what happened. And that means 0:16:23.480 --> 0:16:29.040 your conscious perception is actually a delayed version of what's 0:16:29.080 --> 0:16:32.400 happening in the world. So when you clap your hands, 0:16:33.120 --> 0:16:36.560 your brain gathers the auditory information and the visual information 0:16:36.680 --> 0:16:39.680 and the sensory information of your hands touching, and all 0:16:39.760 --> 0:16:43.480 these stream in at different times, and then it puts 0:16:43.520 --> 0:16:47.240 together its story of what just happened, and it puts 0:16:47.280 --> 0:16:50.720 together what should go with what. And I'll explain in 0:16:50.720 --> 0:16:53.160 a second how it does this. But the key thing 0:16:53.200 --> 0:16:55.840 I want to make clear right now is that in 0:16:56.000 --> 0:17:00.480 order to synchronize everything, you have to wait for all 0:17:00.640 --> 0:17:05.639 of the information to arrive. And the strange consequence of 0:17:05.680 --> 0:17:10.080 all this is that you live in the past. By 0:17:10.119 --> 0:17:14.040 the time you think the moment occurs, it's already long gone. 0:17:14.119 --> 0:17:18.760 When you perceive the clap, it's already over. Your perceptual 0:17:18.800 --> 0:17:24.199 world always lags behind the real world. So think of 0:17:24.200 --> 0:17:27.480 it this way. Your perception of the world is like 0:17:27.600 --> 0:17:31.520 a live television show. Think of something like Saturday Night Live, 0:17:32.040 --> 0:17:36.000 which is not actually live. Instead, those shows are aired 0:17:36.080 --> 0:17:39.560 with a delay of a few seconds in case someone 0:17:40.119 --> 0:17:44.000 cusses or falls down or has a clothing mishap. And 0:17:44.080 --> 0:17:47.600 so it goes with your conscious life. It collects a 0:17:47.600 --> 0:17:52.000 lot of information before it goes live, so there's an 0:17:52.119 --> 0:17:56.000 unbridgable gap between an event occurring in the world and 0:17:56.040 --> 0:17:59.680 your conscious experience of it. So that's why your brain 0:17:59.720 --> 0:18:03.200 can sync the television audio at the video or the 0:18:03.520 --> 0:18:06.320 basketball thump with the visual of it hitting the ground 0:18:06.359 --> 0:18:09.359 or whatever. I need to emphasize that what we're talking 0:18:09.400 --> 0:18:13.760 about here is conscious awareness. You can see from pre 0:18:13.880 --> 0:18:17.440 conscious reactions that your motor system doesn't have to wait 0:18:17.520 --> 0:18:21.000 before its decisions. For example, you can duck out of 0:18:21.040 --> 0:18:23.960 the way of a swinging tree branch before you become 0:18:24.040 --> 0:18:27.399 consciously aware of it, or you can start jumping when 0:18:27.440 --> 0:18:31.400 there's a loud sound before you're consciously aware that there 0:18:31.480 --> 0:18:34.320 was a sound. Your body always tries to act as 0:18:34.400 --> 0:18:39.000 quickly as possible, even before the participation of awareness, but 0:18:39.080 --> 0:18:45.040 your conscious perception takes its time. Now that raises a question, 0:18:45.119 --> 0:18:48.640 what is the use of perception, especially since it lags 0:18:48.680 --> 0:18:55.639 behind reality and it's retrospectively attributed, and it's generally outstripped 0:18:55.680 --> 0:18:59.960 by these automatic, unconscious systems. The most likely answer is 0:19:00.080 --> 0:19:04.640 that your perceptions are like objects that your cognitive systems 0:19:04.680 --> 0:19:07.760 can work with later. So it's important for the brain 0:19:08.040 --> 0:19:12.120 to take sufficient time to settle on its best interpretation 0:19:12.240 --> 0:19:15.960 of what just happened, rather than stick with its initial 0:19:16.040 --> 0:19:21.400 rapid interpretation. Its carefully refined picture of what just happened 0:19:21.800 --> 0:19:24.160 is all that it's going to have to work with later, 0:19:24.560 --> 0:19:30.320 so it invests the time. So this brief waiting period 0:19:30.400 --> 0:19:34.199 before consciousness, this is what allows your sensory systems to 0:19:34.880 --> 0:19:39.960 discount the various delays imposed, But it has the disadvantage 0:19:40.000 --> 0:19:43.639 of pushing your perception into the past. Now, there's a 0:19:43.680 --> 0:19:47.080 distinct survival advantage to operating as close to the present 0:19:47.119 --> 0:19:49.840 as possible. An animal doesn't want to live too far 0:19:49.880 --> 0:19:52.800 in the past. Therefore, it may be that a tenth 0:19:52.800 --> 0:19:56.560 of a second window is the smallest delay that allows 0:19:57.040 --> 0:19:59.680 higher areas of the brain to account for the delays 0:19:59.760 --> 0:20:02.840 create in the first stage of the system while still 0:20:02.920 --> 0:20:06.879 operating near the border of the present. Among other things, 0:20:06.920 --> 0:20:10.520 this strategy of waiting for the slowest information to arrive 0:20:11.320 --> 0:20:16.280 has the advantage of allowing object recognition to be independent 0:20:16.400 --> 0:20:19.360 of lighting conditions. So I mentioned a minute ago about 0:20:19.640 --> 0:20:23.320 bright and dim flashes moving through the system at different speeds. 0:20:23.880 --> 0:20:28.560 So imagine a striped tiger coming toward you under the 0:20:28.600 --> 0:20:33.119 forest canopy, and he's passing through successive patches of sunlight. 0:20:33.640 --> 0:20:37.840 Imagine how difficult recognition would be if the bright and 0:20:37.880 --> 0:20:41.600 the dim parts of the tiger caused incoming signals to 0:20:41.640 --> 0:20:45.359 be perceived at different times. You'd perceive the tiger breaking 0:20:45.359 --> 0:20:49.359 into different space time fragments just before you became the 0:20:49.400 --> 0:20:54.960 tiger's lunch. Somehow, the visual system has evolved to reconcile 0:20:55.320 --> 0:20:59.320 these different speeds of incoming information. After all, it is 0:20:59.440 --> 0:21:06.280 advantageous to recognize tigers regardless of the lighting conditions. Now, 0:21:06.320 --> 0:21:10.240 this hypothesis that the system waits to collect information over 0:21:10.280 --> 0:21:13.119 the window of time that it's all streaming in this 0:21:13.160 --> 0:21:15.960 supplies not only to vision, but more generally, to all 0:21:16.040 --> 0:21:20.119 the senses. So, for example, my lab measured that in 0:21:20.240 --> 0:21:22.679 vision you wait at least a tenth of a second, 0:21:23.080 --> 0:21:25.879 But the size of this window might be different for 0:21:26.080 --> 0:21:29.440 hearing or touch. If I touch your toe and your 0:21:29.520 --> 0:21:32.520 nose at the same time, you will feel those touches 0:21:32.520 --> 0:21:36.520 as simultaneous. And that's really surprising, because the signal from 0:21:36.520 --> 0:21:39.920 your nose reaches your brain well before the signal from 0:21:39.960 --> 0:21:43.080 your toe. Why didn't you feel the nose touch when 0:21:43.080 --> 0:21:46.320 it first arrived. Did your brain wait to see what 0:21:46.440 --> 0:21:48.880 else might be coming up the pipeline of the spinal 0:21:48.920 --> 0:21:51.720 cord until it was sure that it had waited long 0:21:51.840 --> 0:21:55.320 enough for the slower signals from the toe. Strange as 0:21:55.320 --> 0:21:57.760 that sounds, that might be correct, and it may be 0:21:57.760 --> 0:22:02.560 that for a unified sensory perception of the world, you 0:22:02.680 --> 0:22:07.000 have to wait for the slowest overall information to get there, 0:22:07.600 --> 0:22:11.800 and given conduction times along whims, this leads to my 0:22:12.160 --> 0:22:16.480 strange but testable hypothesis that I mentioned in episode thirteen, 0:22:16.880 --> 0:22:21.040 which is that tall people may live further in the 0:22:21.080 --> 0:22:24.560 past than short people because they have to wait for 0:22:24.680 --> 0:22:27.480 the signals all the way from their toes before they 0:22:27.520 --> 0:22:33.360 put together their unified perception. Okay, but how does your 0:22:33.359 --> 0:22:37.919 brain know how to put all these signals together? So 0:22:38.000 --> 0:22:42.320 to think about this, let's turn to the Mongol emperor 0:22:42.480 --> 0:22:46.440 Kublai Khan, who ranged from twelve sixty to twelve ninety four, 0:22:46.640 --> 0:22:51.000 and founded the Yuan dynasty. Now he had conquered the 0:22:51.240 --> 0:22:54.679 largest kingdom the world had ever known. His kingdom reached 0:22:54.720 --> 0:22:58.800 from the Pacific to the Black Sea, and from Siberia 0:22:58.840 --> 0:23:03.959 to modern day Afghanis. His territory covered a fifth of 0:23:04.000 --> 0:23:08.679 the world's inhabited areas, so it was absolutely enormous. He 0:23:08.800 --> 0:23:12.639 situated himself in what is modern day Beijing. And the 0:23:12.720 --> 0:23:14.879 thing to note is that this was back in the 0:23:14.960 --> 0:23:18.879 day before iPhones and telegraphs and trains or email or 0:23:18.920 --> 0:23:22.119 anything like that. And so the question is, how in 0:23:22.160 --> 0:23:27.800 the world could Kubla Khan know his own empire. There's 0:23:27.800 --> 0:23:30.760 no way he could travel even a tiny fraction of 0:23:30.800 --> 0:23:35.320 a territory that large in his entire lifetime. So how 0:23:35.359 --> 0:23:39.840 could the Great Khan know what his empire contained? The 0:23:39.960 --> 0:23:45.280 answer is he hired emissaries like the Venetian traveler Marco Polo, 0:23:45.680 --> 0:23:48.760 and these people would travel out to the distant reaches 0:23:48.760 --> 0:23:51.679 of his empire and they would convey news back to 0:23:51.760 --> 0:23:54.719 him about what was going on in the empire. And 0:23:54.800 --> 0:23:57.240 he hired many emissaries that would go out in different 0:23:57.240 --> 0:24:00.840 directions and bring news back to him about what was 0:24:00.880 --> 0:24:04.760 going on. Now, I've never heard a historian talk about this. 0:24:04.880 --> 0:24:08.840 But I imagine the Great Con must have faced a 0:24:08.960 --> 0:24:15.040 tough problem what events in his empire occurred in which order. 0:24:15.840 --> 0:24:18.720 Because of wars and weather and other issues, people might 0:24:18.880 --> 0:24:23.040 travel at different paces, so the different emissaries would come 0:24:23.040 --> 0:24:26.200 back to him at different times. One emissary comes back 0:24:26.240 --> 0:24:29.440 and reports that a war has just ended, and another 0:24:29.480 --> 0:24:33.080 emissary comes back and reports that a war has just begun, 0:24:33.160 --> 0:24:35.760 and they're talking about the same war. But they got 0:24:35.800 --> 0:24:39.520 back to the capitol at different times. And so the 0:24:39.600 --> 0:24:44.080 question is how did the Great con synchronize all these signals. 0:24:44.760 --> 0:24:46.600 The thing I want to make clear is that the 0:24:46.640 --> 0:24:50.800 timing problem Kubla Khan had is the same problem the 0:24:50.920 --> 0:24:54.240 brain has, which is to say, it's getting all these 0:24:54.240 --> 0:24:58.080 different streams of information that come in at different paces. 0:24:58.960 --> 0:25:01.400 And because your brain is locked up in the dark 0:25:01.520 --> 0:25:05.120 tower of the skull, its only contact with the outside 0:25:05.119 --> 0:25:09.119 world is via the electrical signals exiting and entering along 0:25:09.160 --> 0:25:13.360 the highways of nerve bundles. So you've got touch information 0:25:13.480 --> 0:25:16.080 streaming up the body, You've got auditory coming in, you've 0:25:16.119 --> 0:25:19.480 got visual information coming into the brain. But the issue 0:25:19.640 --> 0:25:21.960 is that all of these things get processed by the 0:25:22.000 --> 0:25:25.000 brain at different speeds and at different places in the brain. 0:25:25.440 --> 0:25:29.439 So your brain faces this enormous challenge how to stitch 0:25:29.560 --> 0:25:33.359 together the incoming signals in the best way to make 0:25:33.400 --> 0:25:36.680 a story about what just happened in the outside world. 0:25:37.480 --> 0:25:39.720 For example, as we just saw, when I clap my hands, 0:25:39.720 --> 0:25:43.320 the auditory signals get processed first, then the visual signals, 0:25:43.400 --> 0:25:46.920 and yet they seem synced up. What this tells us 0:25:47.520 --> 0:25:51.080 is that the brain is somehow pulling off major video 0:25:51.200 --> 0:25:54.320 editing tricks, and we're going to find out how. Now, 0:25:54.320 --> 0:25:56.560 my lab has studied this for years, and the first 0:25:56.600 --> 0:25:59.320 thing to appreciate is that your brain doesn't figure this 0:25:59.400 --> 0:26:04.800 out pass. It does this by involving your own actions. 0:26:04.960 --> 0:26:09.080 Your own actions are the secret to understanding how everything 0:26:09.080 --> 0:26:13.040 gets synchronized. And what I've proposed is that your brain 0:26:13.119 --> 0:26:15.960 goes through so much trouble to get all this timing 0:26:16.080 --> 0:26:20.919 right because of one issue causality. Because one of the 0:26:20.920 --> 0:26:25.240