WEBVTT - Ep24 "What does drug withdrawal have in common with heartbreak?" 0:00:04.519 --> 0:00:07.680 Why do you still feel the waves after you get 0:00:07.680 --> 0:00:10.080 off a boat and when you get off the treadmill 0:00:10.119 --> 0:00:12.520 at the gym. Why does everything seem to be flowing 0:00:12.640 --> 0:00:17.280 past you faster? What does heartbreak have in common with 0:00:17.360 --> 0:00:21.520 drug withdrawal? And why after you stare at a waterfall 0:00:21.600 --> 0:00:24.400 for a while, do the rocks on the side seem 0:00:24.440 --> 0:00:27.760 to be crawling upward? Why did people in the nineteen 0:00:27.840 --> 0:00:32.320 eighties think their book pages had some color red in them, 0:00:32.400 --> 0:00:35.519 but no one thought that before or after the eighties? 0:00:35.800 --> 0:00:37.640 And what does any of this have to do with 0:00:37.840 --> 0:00:42.160 the great philosopher Aristotle watching a horse stuck in the river? 0:00:46.120 --> 0:00:49.240 Welcome to Inner Cosmos with me David Eagleman. I'm a 0:00:49.280 --> 0:00:53.080 neuroscientist and author at Stanford and in these episodes we 0:00:53.159 --> 0:00:57.560 sail deeply into our three pound universe to understand why 0:00:57.600 --> 0:01:09.679 and how our lives look the way they do. Today's 0:01:09.720 --> 0:01:14.560 episode is about the way the brain readjusts its circuitry 0:01:14.640 --> 0:01:19.760 on really fast timescales, and why your brain is constantly 0:01:19.840 --> 0:01:23.800 doing this. In the nineteen eighties, tens of thousands of 0:01:23.800 --> 0:01:27.839 people began to notice something really weird when they looked 0:01:27.920 --> 0:01:31.520 at a floppy disk envelope with the black and white 0:01:31.560 --> 0:01:35.440 IBM logo on the front, the letters IBM seemed to 0:01:35.440 --> 0:01:38.760 have a red tint, And the same thing happened when 0:01:38.840 --> 0:01:43.560 people looked at pages in a book, the text seemed 0:01:43.600 --> 0:01:47.640 like it was shaded red. Now, if that's not weird enough, 0:01:47.720 --> 0:01:50.320 check this out. This only happened in the nineteen eighties. 0:01:50.440 --> 0:01:54.440 People didn't perceive a red tint before or after this decade. 0:01:54.800 --> 0:01:59.800 So what was changing about brains during that window. I'll 0:01:59.800 --> 0:02:03.720 tell you the answer. But to understand this, we're first 0:02:03.800 --> 0:02:07.360 going to step back even further in history, twenty four 0:02:07.520 --> 0:02:12.920 hundred years further to Aristotle. Now, Aristotle, as you know, 0:02:13.120 --> 0:02:18.160 is a philosopher and a polymath whose intellectual interests were boundless. 0:02:18.160 --> 0:02:21.400 He wrote about essentially every subject you could imagine. But 0:02:21.480 --> 0:02:24.120 we're coming to Aristotle not because of something he wrote, 0:02:24.160 --> 0:02:28.280 but instead because he was insightful enough to pause and 0:02:28.520 --> 0:02:32.919 notice when something didn't make sense. What happened is that 0:02:33.240 --> 0:02:37.400 someone was trying to cross a fast flowing river on 0:02:37.480 --> 0:02:40.680 his horse, and the horse got stuck halfway in the river. 0:02:41.040 --> 0:02:44.920 So Aristotle, along with some other people, ended up watching 0:02:45.000 --> 0:02:47.720 this rescue operation. The way you might have a bunch 0:02:47.760 --> 0:02:50.880 of people looking at a car crash nowadays. So you 0:02:50.960 --> 0:02:53.680 have all these Greeks all standing there in their togas, 0:02:53.760 --> 0:02:56.640 talking and chatting and watching the horse in the fast 0:02:56.680 --> 0:02:59.920 flowing river. Now most of the people were presumably just 0:03:00.200 --> 0:03:03.720 talking about stuff that made no difference to history. But 0:03:03.880 --> 0:03:08.000 Aristotle stood at the side of the river intently watching 0:03:08.040 --> 0:03:12.640 this horse. Now Here was the key about Aristotle, a 0:03:12.800 --> 0:03:16.480 general characteristic that made him so famous to history. He 0:03:16.639 --> 0:03:22.320 was a good observer. He paid attention. He noticed when 0:03:22.360 --> 0:03:25.680 things were unusual, and when he found something, he would 0:03:25.760 --> 0:03:28.200 sink his teeth into the problem and wouldn't let go. 0:03:28.760 --> 0:03:32.360 And what he noticed here was something that possibly the 0:03:32.400 --> 0:03:34.160 others did too, but they ignored it, and they didn't 0:03:34.200 --> 0:03:37.320 think any further on it. What he noticed is that 0:03:37.400 --> 0:03:39.800 once the horse got unstuck and the man made it 0:03:39.840 --> 0:03:43.840 to the shore, and Aristotle looked away, he noticed that 0:03:43.880 --> 0:03:47.240 everything on the land, the rocks, the huts, the trees, 0:03:47.320 --> 0:03:50.840 everything seemed to be drifting, and they all seemed to 0:03:50.920 --> 0:03:55.839 be moving in the opposite direction of the river. Now 0:03:55.840 --> 0:03:58.720 you may have seen something like this too. The easiest 0:03:58.720 --> 0:04:03.360 way to experience Aristotle's confusion and delight is to stare 0:04:03.400 --> 0:04:07.120 at a waterfall and after you keep your eyes locked 0:04:07.160 --> 0:04:10.160 on it for a while, look over to the rocks 0:04:10.200 --> 0:04:13.320 on the side of the waterfall, and the rocks appear 0:04:13.440 --> 0:04:17.800 to be moving upward. Now, this illusion that he first 0:04:17.839 --> 0:04:20.279 made note of twenty four hundred years ago has come 0:04:20.320 --> 0:04:24.640 to be known as the motion after effect. Why does 0:04:24.680 --> 0:04:29.960 it happen. The activity of particular neurons in your visual 0:04:30.000 --> 0:04:35.159 cortex represents downward motion, and the activity of other neurons 0:04:35.240 --> 0:04:39.840 represents upward motion. And they're always locked in battle, and 0:04:39.920 --> 0:04:43.880 most of the time the competition is evenly pitched, and 0:04:43.920 --> 0:04:47.960 they evenly inhibit each other, and as a result, the 0:04:48.000 --> 0:04:52.520 world appears to you to be moving neither up nor down. So, 0:04:52.640 --> 0:04:56.679 given this, a popular explanation for the motion after effect 0:04:56.839 --> 0:05:01.520 is fatigue. By staring at the downward motion of the waterfall, 0:05:01.880 --> 0:05:04.840 you burn up a good deal of energy in your 0:05:05.160 --> 0:05:10.680 downward coating neurons, and now their vigor is temporarily depleted. 0:05:10.760 --> 0:05:15.280 So the battle tips in favor of the upward encoding neurons, 0:05:15.640 --> 0:05:19.440 and as a result of this unbalanced activity, you perceive 0:05:19.800 --> 0:05:22.839 that the rocks are climbing upward. You have this net 0:05:22.920 --> 0:05:28.000 movement upward. Now, this fatigue hypothesis is very appealing in 0:05:28.040 --> 0:05:33.120 its simplicity, but it's wrong. Why because it can't explain 0:05:33.240 --> 0:05:37.240 some critical facts about the illusion. So imagine you watch 0:05:37.680 --> 0:05:40.960 the downward waterfall for a while and then you close 0:05:41.000 --> 0:05:44.320 your eyes tightly, say, for three hours you close your eyes. 0:05:44.680 --> 0:05:49.159 When you reopen your eyes, you'll see the rocks crawling upward. 0:05:49.600 --> 0:05:52.400 And what that tells us is it's not about a 0:05:52.440 --> 0:05:57.599 temporary energy depletion in neurons. There's something deeper going on here. 0:05:58.560 --> 0:06:01.440 And here's what that deeper thing is is the illusion 0:06:01.520 --> 0:06:06.120 comes about not because of passive fatigue, but instead because 0:06:06.160 --> 0:06:10.880 of an active recalibration in your brain. Your visual system 0:06:10.920 --> 0:06:16.000 is exposed to continuous downward motion, and after a while 0:06:16.160 --> 0:06:21.000 it comes to assume this is the new normal. At first, 0:06:21.400 --> 0:06:25.240 downward motion is dramatic information to your brain, but after 0:06:25.320 --> 0:06:29.360 a while of staring, you receive no new information from 0:06:29.480 --> 0:06:32.960 downwardness here. So, as far as your brain is concerned, 0:06:33.560 --> 0:06:37.040 this is the new reality, a world that flows more 0:06:37.160 --> 0:06:42.720 down than up. So your visual system carefully rebalances its 0:06:42.839 --> 0:06:47.720 expectations to mirror the world to expect more downward than 0:06:47.839 --> 0:06:51.560 upward activity. Now, when you look away from the waterfall 0:06:51.600 --> 0:06:56.160 and toward the cliff side. This recalibrated set point becomes 0:06:56.279 --> 0:07:00.400 obvious because now rocks and trees are flowing upward toward 0:07:00.440 --> 0:07:04.839 the sky. Your set point has shifted. In other words, 0:07:04.839 --> 0:07:10.520 what counts as standing still has shifted. Why well, your 0:07:10.560 --> 0:07:14.200 brain always wants to set up a ground truth so 0:07:14.240 --> 0:07:18.080 that it can be better at detecting change. In this case, 0:07:18.120 --> 0:07:21.120 when your visual field is filled with the site of 0:07:21.160 --> 0:07:26.280 the waterfall, your brain is working to subtract off all 0:07:26.320 --> 0:07:30.239 that downward motion. All that downward motion is no longer 0:07:30.640 --> 0:07:32.920 informative in the way that it was, and so the 0:07:32.960 --> 0:07:36.920 circuitry it adjusts itself so that it can be maximumly 0:07:36.960 --> 0:07:41.160 sensitive to new information. Now, if you watch for it 0:07:41.200 --> 0:07:44.080 the way Aristotle would, you'll see that this kind of 0:07:44.120 --> 0:07:48.040 recalibration happens to you all the time. Think about when 0:07:48.040 --> 0:07:51.200 you're on a small boat on the ocean. You are 0:07:51.360 --> 0:07:53.960 rocking with the waves for a while, and then when 0:07:54.000 --> 0:07:56.680 you get off the boat, the land seems to be 0:07:56.800 --> 0:07:59.040 rocking for a while. It feels as though you're still 0:07:59.120 --> 0:08:01.720 on the water. Now you may have thought about it 0:08:01.760 --> 0:08:04.080 this way, Hey, my body thinks it's still on the water. 0:08:04.400 --> 0:08:08.240 What you're actually feeling is a negative image of the 0:08:08.280 --> 0:08:11.560 water's motion. And you may have noticed this sort of 0:08:11.600 --> 0:08:15.560 illusion if you're a runner. So here's what normally happens. 0:08:15.600 --> 0:08:19.560 Your body sends motor commands to the legs. It says, okay, 0:08:19.560 --> 0:08:23.840 move fast, and correlated with your running is a visual 0:08:23.880 --> 0:08:27.560 sensation where the visual world is flowing by your eyes 0:08:27.680 --> 0:08:31.960 as a result. But when you run on a treadmill 0:08:32.040 --> 0:08:35.520 at the gym, your brain is sending the same signals 0:08:35.520 --> 0:08:38.959 to your legs. Run fast, but now your visual system 0:08:39.360 --> 0:08:43.400 doesn't get the world flowing past. Instead, you're looking at 0:08:43.440 --> 0:08:45.920 the gym wall in front of you the whole time. 0:08:46.600 --> 0:08:51.720 So now when you step off, you experience the treadmill illusion. 0:08:52.360 --> 0:08:55.000 With each step you take toward the locker room, the 0:08:55.040 --> 0:08:58.880 world seems to stream by at a faster pace. It 0:08:58.920 --> 0:09:02.080 looks as though you're moving forward more quickly than you 0:09:02.120 --> 0:09:06.199 really are. Why does it happen Because your brain has 0:09:06.240 --> 0:09:10.000 an expectation about how the act of moving your legs 0:09:10.440 --> 0:09:13.920 should translate to the flow of the visual scene past 0:09:14.000 --> 0:09:17.560 your eyes, and now after the treadmill, it's had to 0:09:17.960 --> 0:09:22.640 adjust that relationship. So now there's an after effect. This 0:09:22.720 --> 0:09:26.760 is just like with Aristotle's horse or the waterfall or 0:09:26.800 --> 0:09:29.240 the feeling that you're still rocking after you get off 0:09:29.280 --> 0:09:34.320 the boat. Your brain is readjusting its expectations about the world. 0:09:34.960 --> 0:09:37.680 In the case of the treadmill, it's how the active 0:09:37.760 --> 0:09:41.000 moving your legs should translate to the flow of the 0:09:41.080 --> 0:09:43.679 visual scene past your eyes, and when you go back 0:09:43.720 --> 0:09:48.480 to the normal world, there's an after effect. As another example, 0:09:48.520 --> 0:09:52.240 you've surely noticed color after effects. If you stare at 0:09:52.280 --> 0:09:54.440 a red dot for a little while and then you 0:09:54.480 --> 0:09:57.040 look somewhere else and blink, you'll see the dot, but 0:09:57.160 --> 0:09:59.720 now it looks green. Or if you stare at something 0:09:59.760 --> 0:10:01.920 bright yellow for a little while and then you look away, 0:10:02.240 --> 0:10:05.720 you'll see the opposite color blue. And these types of 0:10:05.840 --> 0:10:11.560 color after effects can be surprisingly sophisticated. So consider something 0:10:11.640 --> 0:10:16.720 called the McCullough after effect, named after the researcher Celeste McCulloch, 0:10:16.760 --> 0:10:19.920 who discovered this in the nineteen sixties. So really get this. 0:10:20.000 --> 0:10:22.160 I want you to imagine a bunch of black and 0:10:22.160 --> 0:10:24.839 white lines like black white, black white, black white. Now 0:10:24.880 --> 0:10:27.720 imagine a group of these that are horizontal and a 0:10:27.720 --> 0:10:31.480 separate group of these that are vertical. Now, this can't 0:10:31.520 --> 0:10:33.760 just be imagined as an illusion, but has to be 0:10:33.800 --> 0:10:36.200 looked at to work. So please find a picture of 0:10:36.240 --> 0:10:39.400 these lines on eagleman dot com Slash podcast to go 0:10:39.480 --> 0:10:42.200 over there and take a look at this. Okay, So 0:10:42.240 --> 0:10:45.559 look farther down the page and you'll see lines that 0:10:45.600 --> 0:10:47.880 look the same as these black and white lines, but 0:10:47.960 --> 0:10:51.720 now they're colored. The horizontal lines are green and the 0:10:51.800 --> 0:10:54.400 vertical lines are red. And what I want you to 0:10:54.400 --> 0:10:57.319 do is stare at these colored lines for a bit, 0:10:57.440 --> 0:11:00.920 like a few minutes. The green lines go side side, 0:11:00.960 --> 0:11:04.200 the red lines go up and down. Now go back 0:11:04.240 --> 0:11:07.240 to the original black and white lines and you'll see 0:11:07.240 --> 0:11:11.840 something amazing. You'll see that the spaces between the black 0:11:11.880 --> 0:11:16.280 and white horizontal lines now look reddish, and the spaces 0:11:16.360 --> 0:11:20.640 between the black and white vertical lines look greenish. This 0:11:20.679 --> 0:11:23.400 is the McCullock effect. You gotta try this for yourself. 0:11:23.440 --> 0:11:28.000 It's really worthwhile. Now why does it happen. It's because 0:11:28.040 --> 0:11:32.440 when you stared at the colored lines, your brain said, hey, wait, 0:11:32.559 --> 0:11:36.520 for some strange reason, everything horizontal is appearing green to 0:11:36.559 --> 0:11:39.560 me and everything vertical is looking red to me. But 0:11:40.120 --> 0:11:44.360 those features shouldn't really be correlated, and so it adjusts 0:11:44.400 --> 0:11:49.120 itself to cancel out this strange relationship. Going on, and 0:11:49.120 --> 0:11:51.640 when you look at the black and white lines again, 0:11:52.280 --> 0:11:56.439 you're seeing the after effect. The horizontal lines were being 0:11:56.640 --> 0:12:00.720 internally shifted in your brain towards the opposite color red, 0:12:01.080 --> 0:12:05.000 and the vertical lines toward green. And again, I just 0:12:05.040 --> 0:12:07.439 want to be clear, this has nothing to do with fatigue, 0:12:07.760 --> 0:12:11.199 because in nineteen seventy five, two researchers showed that if 0:12:11.240 --> 0:12:14.760 you stare at these red and green lines for fifteen minutes, 0:12:14.800 --> 0:12:17.960 the after effect can last three and a half months. 0:12:18.960 --> 0:12:24.320 So your brain is always doing an active recalibration of 0:12:24.360 --> 0:12:28.240 the world out there. And this active recalibration is why 0:12:28.360 --> 0:12:31.400 in the nineteen eighties many people began to see the 0:12:31.440 --> 0:12:34.520 text in the books appearing to have a red tint 0:12:34.720 --> 0:12:38.280 because at that time the population had just begun to 0:12:38.400 --> 0:12:42.760 use computer monitors to do word processing. And as some 0:12:42.840 --> 0:12:47.720 of you remember, unlike modern monitors, these early monitors only 0:12:47.760 --> 0:12:51.839 displayed one color green, and so you typed a bunch 0:12:51.880 --> 0:12:54.800 of lines of text on the screen and it looked 0:12:54.840 --> 0:12:59.360 like horizontal lines of green on a black background. So 0:12:59.400 --> 0:13:03.520 people would say there at these horizontal green rows for 0:13:03.720 --> 0:13:05.840 hours at a time, all day while they were doing 0:13:05.880 --> 0:13:09.240 their computer work. And so when they picked up a book, 0:13:09.800 --> 0:13:13.520 the horizontal lines of black and white text were shaded 0:13:14.000 --> 0:13:18.079 with the complementary color red. It was the McCullock effect. 0:13:18.559 --> 0:13:24.080 Their brains were adjusting to a world of horizontal green lines, 0:13:24.440 --> 0:13:28.680 and their reality changed accordingly. Now I mentioned that the 0:13:28.679 --> 0:13:32.040 computer users in the nineteen eighties also experienced this illusion 0:13:32.040 --> 0:13:36.040 when they looked at the IBM logo emblazoned on the 0:13:36.080 --> 0:13:38.840 front of the floppy disk sleeve. If any of you 0:13:38.960 --> 0:13:43.160 remember the old IBM logo, it's the three letters spelled 0:13:43.160 --> 0:13:46.800 out of horizontal lines, as though you're looking at IBM 0:13:47.320 --> 0:13:50.680 put through a egg slicer, And so the logo, just 0:13:50.720 --> 0:13:54.680 like the horizontal lines in the book, looked tinged with red, 0:13:55.000 --> 0:13:58.679 and designers at IBM were flummixed about this because they 0:13:58.679 --> 0:14:01.440 had definitely not printed they're black and white design with 0:14:01.480 --> 0:14:04.880 any red in it, and yet everyone was insisting that 0:14:05.040 --> 0:14:09.600 they had. So all these after effects teach us something wild. 0:14:10.200 --> 0:14:12.280 Even though you just open your eyes and look around 0:14:12.360 --> 0:14:15.440 the world and assume you're just seeing reality out there, 0:14:15.880 --> 0:14:20.320 in fact, your brain is doing massive work behind the scenes. 0:14:20.680 --> 0:14:25.240 It's always recalibrating itself on the fly to try to 0:14:25.360 --> 0:14:28.640 improve what it's seeing, to try to get rid of 0:14:28.680 --> 0:14:32.400 information that doesn't matter, so it can surface the information 0:14:32.440 --> 0:14:35.960 that does matter. And this active recalibration of the world 0:14:36.200 --> 0:14:39.600 that's happening behind the scenes. This applies to everything. How 0:14:39.680 --> 0:14:42.960 much motion is in the world, how stable the ground is, 0:14:43.320 --> 0:14:46.400 whether vision flows past us when we move our legs, 0:14:46.760 --> 0:14:50.360 whether lions are infused with color. None of the stuff 0:14:50.400 --> 0:14:55.479 is decided in our genetics. It's all calibrated by our experience. 0:15:12.000 --> 0:15:15.800 Now I want to take this concept of adaptation one 0:15:15.920 --> 0:15:19.120 level deeper. I want to introduce the concept that your 0:15:19.120 --> 0:15:24.760 brain is making things invisible if they are expected. So 0:15:24.840 --> 0:15:27.000 take this as an example. Let's say you take a 0:15:27.120 --> 0:15:30.040 yellow ping pong ball and you cut it exactly in half, 0:15:30.400 --> 0:15:33.840 and you lay one hemisphere over each eye, and what 0:15:33.880 --> 0:15:37.080 you'll see is the whole world is an even blanket 0:15:37.080 --> 0:15:40.720 of yellow color. But within a few moments you don't 0:15:40.720 --> 0:15:44.080 see any color at all. It's as though you're blind. 0:15:44.240 --> 0:15:48.760 Your visual system just assumes that the world has become yellower, 0:15:49.160 --> 0:15:51.880 and so it adapts so that you'll be sensitive to 0:15:52.040 --> 0:15:54.800 other changes. So you should try this. You can find 0:15:54.840 --> 0:15:59.240 any sort of totally featureless scene that's entirely one color 0:15:59.680 --> 0:16:02.040 and fill your eyes with it. Like walk up to 0:16:02.120 --> 0:16:05.160 a wall that's entirely painted red. Walk up real close. 0:16:05.200 --> 0:16:07.680 Your eyes are filled with red, and you'll see the 0:16:07.840 --> 0:16:13.560 color will quickly drain away to neutral. Your brain says, hey, 0:16:13.560 --> 0:16:16.880 there's no more information here. I expect this color, So 0:16:16.880 --> 0:16:21.920 I'm going to adjust things so that becomes invisible and 0:16:21.960 --> 0:16:24.320 I'll tell you something else. The scene doesn't even have 0:16:24.400 --> 0:16:28.880 to be featureless for things to fade away. So in 0:16:28.920 --> 0:16:33.840 eighteen oh four, a Swiss physician named Ignos Troxler noticed 0:16:33.880 --> 0:16:37.000 something really stunning, which is, if you stare at a 0:16:37.200 --> 0:16:40.440 central point in the middle of a bunch of blobs 0:16:40.480 --> 0:16:44.320 of color, all the busyness in the periphery, all those 0:16:44.360 --> 0:16:48.880 blobs will eventually disappear. I have an example of this 0:16:48.960 --> 0:16:51.480 on eagleman dot com slash podcast. Check it out. It's 0:16:51.560 --> 0:16:54.880 really amazing. What you do is you keep your gaze 0:16:54.920 --> 0:16:57.560 fixed right in the middle of the picture, and you'll 0:16:57.560 --> 0:17:00.800 see there's all these smooth blobs of color, and within 0:17:00.840 --> 0:17:04.240 about ten seconds, if you're keeping your eyes still, all 0:17:04.280 --> 0:17:09.639 these surrounding blobs just start to disappear into the background, 0:17:09.880 --> 0:17:13.080 and soon you are looking at what appears to be 0:17:13.560 --> 0:17:17.199 a blank page. Now this is wild, and if you 0:17:17.280 --> 0:17:19.720 move your eyes around just a bit, all these blobs 0:17:19.760 --> 0:17:23.720 come back into your consciousness. This illusion is called the 0:17:23.920 --> 0:17:27.400 Troxlur effect, and what it demonstrates is that if there's 0:17:27.440 --> 0:17:29.920 a blob of something in your peripheral vision that is 0:17:30.000 --> 0:17:35.360 not changing at all, it will evaporate from your perception. Now, 0:17:35.359 --> 0:17:38.560 what is going on here? The answer is that your 0:17:38.640 --> 0:17:44.120 visual system is always seeking motion and change. Something that's 0:17:44.160 --> 0:17:47.280 fixed and none changing doesn't really matter, and it quickly 0:17:47.320 --> 0:17:53.720 becomes invisible. Good information is expected to update, and things 0:17:53.760 --> 0:17:58.320 that don't change get ignored by the system. So what 0:17:58.480 --> 0:18:03.040 prevents your kitchen, your workplace from becoming like a Troxler 0:18:03.160 --> 0:18:07.639 illusion with all the motionless features disappearing. Well, First, most 0:18:07.640 --> 0:18:10.840 of the world is made up of hard edges, not blobs, 0:18:10.880 --> 0:18:13.639 and those are easier for your visual system to hang onto. 0:18:14.119 --> 0:18:18.520 But there's a deeper reason, although you're not generally aware 0:18:18.520 --> 0:18:23.760 of it. Your eyes are constantly jumping and jiggling around. 0:18:24.160 --> 0:18:27.880 So just observe a friend's eyeballs. You'll notice that her 0:18:28.000 --> 0:18:32.520 eyeballs are making about three rapid jumps every second of 0:18:32.560 --> 0:18:36.760 her waking life. These are called thecods. Bang bang bang bang, bang, 0:18:37.080 --> 0:18:40.000 And if you watch even more closely, you'll find that 0:18:40.040 --> 0:18:43.280 in between the big jumps, her eyes are constantly performing 0:18:43.320 --> 0:18:50.240 these little micro jitters. They're called microsucods. Bang. Now, is 0:18:50.280 --> 0:18:55.040 something wrong with your friend's eyes? No, these rapid movements, 0:18:55.119 --> 0:18:57.680 both the big and the small. What they're doing is 0:18:57.720 --> 0:19:03.040 they're keeping the image on the retina fresh totally unconsciously, 0:19:03.119 --> 0:19:08.040 her eyes are working to maintain a constantly changing image. 0:19:08.520 --> 0:19:11.760 Why do they bother. It's because any image that remains 0:19:12.160 --> 0:19:16.840 perfectly fixed on one position on the retina will become invisible. 0:19:17.720 --> 0:19:20.120 Here's how to prove this to yourself. If you wear 0:19:20.480 --> 0:19:24.159 contact lenses, take a marker and draw a small shape 0:19:24.200 --> 0:19:25.879 on the front of your contact right in the middle. 0:19:26.200 --> 0:19:29.040 When you put the contact back in place on your eye, 0:19:29.280 --> 0:19:32.439 you'll see something there, but it won't last long. It 0:19:32.640 --> 0:19:39.360 rapidly fades to invisibility. Now, this phenomenon underscores the fundamental 0:19:39.400 --> 0:19:42.959 fact that brains care about change. Just like with the 0:19:43.200 --> 0:19:49.040 Troxler effect, features that don't change yield little information about 0:19:49.040 --> 0:19:54.960 the world. All the important information comes from things in flux. Now, 0:19:55.000 --> 0:19:58.040 if you don't have contact lenses, don't worry, because you're 0:19:58.160 --> 0:20:03.119 already performing a similar experiment without knowing it. So you 0:20:03.200 --> 0:20:06.720 have blood vessels that sit on top of your retina 0:20:06.720 --> 0:20:09.480 at the back of your eye, and these blood vessels 0:20:10.000 --> 0:20:13.800 should be seen superimposed on top of everything you look 0:20:13.840 --> 0:20:18.320 at because they're smack in front of your photoreceptors. But 0:20:18.359 --> 0:20:23.040 these blood vessels are totally invisible to your perception. Just 0:20:23.080 --> 0:20:26.240 like the drawing on your contact lens. These blood vessels 0:20:26.359 --> 0:20:30.879 are fixed in position with respect to your retina, So 0:20:30.920 --> 0:20:35.359 no matter how much your eyes move, they can't refresh 0:20:35.520 --> 0:20:39.280 the image of these blood vessels. Even though the vessels 0:20:39.280 --> 0:20:44.800 interpose themselves between you and the world, they perceptually disappear. 0:20:44.880 --> 0:20:47.880 It's like a magic trick. Now you might have noticed 0:20:47.920 --> 0:20:52.320 a flash of these vessels when the eye doctor shines 0:20:52.359 --> 0:20:55.520 the pen light in your eyes. Boom, there they are. 0:20:56.119 --> 0:20:59.639 In this situation, the beam of light can cause the 0:20:59.720 --> 0:21:04.399 vessel to cast a shadow at an unusual angle, and 0:21:04.440 --> 0:21:08.080 your visual system suddenly takes notice something unexpected has just 0:21:08.160 --> 0:21:10.760 occurred at the retina, and that's the only time you 0:21:10.880 --> 0:21:15.639 witness this massive network that obstructs your view. If you 0:21:15.680 --> 0:21:18.919 haven't seen this before, pause this podcast. Go into a 0:21:19.000 --> 0:21:21.680 dark room and shine a beam of light in your eye. 0:21:21.920 --> 0:21:25.080 From an angle, you'll see the blood vessels appear in 0:21:25.119 --> 0:21:27.920 front of you. Just note that your visual system will 0:21:27.920 --> 0:21:30.919 adapt fairly quickly, so the trick is to keep moving 0:21:30.960 --> 0:21:35.320 the light to different angles to maintain the image. So 0:21:36.160 --> 0:21:41.679 this strategy of ignoring the unchanging keeps your brain poised 0:21:41.920 --> 0:21:46.040 to detect anything that moves or shifts or transforms at 0:21:46.040 --> 0:21:50.199 the extreme. This is how reptile visual systems work. A 0:21:50.320 --> 0:21:53.800 reptile can't see you if you stand still because they 0:21:54.000 --> 0:21:58.440 only register change. They don't bother with position, and such 0:21:58.520 --> 0:22:02.760 a system is perfectly sufficient. Reptiles have been surviving and 0:22:02.880 --> 0:22:06.199 thriving for tens of millions of years. You probably remember 0:22:06.240 --> 0:22:09.040 the scene in Jurassic Park where they say don't move. 0:22:09.080 --> 0:22:11.800 The dinosaurs can't see you if you don't move. Well, 0:22:12.200 --> 0:22:16.840 this is why reptiles ignore anything that does not change. 0:22:17.640 --> 0:22:22.119 So let's return to the waterfall illusion. Why doesn't your 0:22:22.200 --> 0:22:26.560 visual system shift so much that the waterfall is perceived 0:22:26.720 --> 0:22:30.040 as standing still. Well, first, there might be limits to 0:22:30.080 --> 0:22:35.000 the recalibration. It simply can't recalibrate enough to subtract off 0:22:35.000 --> 0:22:37.800 the massive motion of the falls. But I think there's 0:22:37.840 --> 0:22:41.400 another possibility, which is that you haven't watched the waterfall 0:22:41.800 --> 0:22:45.280 for long enough, and if you did, it would eventually 0:22:45.320 --> 0:22:48.600 recalibrate all the way. How long might that take? Two 0:22:48.640 --> 0:22:52.440 months of steering at the waterfall two years. In theory, 0:22:52.520 --> 0:22:54.959 if you were to watch for a long enough time, 0:22:55.480 --> 0:22:59.200 then the short term changes in your visual system would 0:22:59.200 --> 0:23:04.119 eventually to longer lasting changes, leading eventually to changes at 0:23:04.160 --> 0:23:09.359 the deepest levels of the system. So ever present background 0:23:09.400 --> 0:23:13.439 motion would become invisible to us. And this leads to 0:23:13.480 --> 0:23:16.400 a crazy speculation that I made in my book Live Wire. 0:23:16.560 --> 0:23:21.600 It's crazy, but logically sound. Are parts of the world 0:23:21.760 --> 0:23:26.080 invisible to us that should be obvious. Imagine there was 0:23:26.080 --> 0:23:31.560 something like a cosmic rainfall that had existed your entire life. 0:23:31.640 --> 0:23:35.280 It would be completely invisible to you because you've never 0:23:35.320 --> 0:23:39.080 seen otherwise. So your visual system would have set the 0:23:39.200 --> 0:23:44.120 downward motion as it's zero point. If the cosmic rain 0:23:44.359 --> 0:23:47.399 suddenly stopped, it would seem as though the whole world 0:23:47.400 --> 0:23:51.160 were suddenly moving upward. We would believe that something had 0:23:51.280 --> 0:23:55.800 just appeared ascending rain, even though the real rain had 0:23:55.880 --> 0:24:00.359 just ended. And this situation could happen in any sensory channel. 0:24:00.440 --> 0:24:04.960 Imagine the beat beep of a cosmic alarm clock with 0:24:05.000 --> 0:24:07.879 no snooze button all the time, all throughout the cosmos 0:24:07.960 --> 0:24:12.080 bee beep beep. If it were totally regular, you wouldn't 0:24:12.119 --> 0:24:15.440 hear it because your brain would have adapted to it. 0:24:15.880 --> 0:24:20.520 If the cosmic alarm suddenly ceased, everyone would hear a 0:24:20.680 --> 0:24:23.560 great beat, beat, beat, but we would have no idea 0:24:23.600 --> 0:24:28.119 that we were experiencing the after effect with the external 0:24:28.200 --> 0:24:36.600 sound totally inside our heads. Successful adaptation makes regularities invisible. 0:24:53.480 --> 0:24:57.560 So we've been talking about these illusions as the results 0:24:57.600 --> 0:25:00.720 of adaptation, but there's actually the other way to look 0:25:00.760 --> 0:25:06.280 at this. These illusions result from prediction. So if you 0:25:06.320 --> 0:25:09.560 subtract away the downward motion of the waterfall or the 0:25:09.720 --> 0:25:13.159 rocking of the boat, or the drawing on your contact lens, 0:25:13.560 --> 0:25:19.439 that is equivalent to predicting its continued existence. In other words, 0:25:19.480 --> 0:25:24.159 when brain circuitry adjusts, it's making a guess about what 0:25:24.320 --> 0:25:27.080 the world is likely to be in the next moment. 0:25:27.680 --> 0:25:31.959 It stops talking about news that is expected to continue. 0:25:32.160 --> 0:25:35.720 So think again about your retinal blood vessels. They are 0:25:35.800 --> 0:25:41.439 perceptually invisible because your visual system predicts them away. It 0:25:41.600 --> 0:25:44.960 knows they are going to be there, so it ignores them. 0:25:45.240 --> 0:25:49.760 It's only if those expectations are violated, like when a 0:25:49.840 --> 0:25:52.760 light shines in from a strange angle that your brain 0:25:53.160 --> 0:25:58.440 spends any energy on representing that data. Your brain doesn't 0:25:58.480 --> 0:26:02.240 want to pay the energy car of spiking neurons, so 0:26:02.320 --> 0:26:06.520 the goal is to reconfigure the network to waste as 0:26:06.640 --> 0:26:11.200 little power as possible. If a pattern streams in that's 0:26:11.320 --> 0:26:16.520 predictable or even partially guessable, the system saves energy by 0:26:16.600 --> 0:26:20.119 structuring itself around that input, so it is not to 0:26:20.160 --> 0:26:24.040 be surprised by it. If your brain is quieter, that 0:26:24.119 --> 0:26:28.439 means there are fewer violations of expectations. Things in the 0:26:28.480 --> 0:26:34.040 outside world are going approximately as forecast. In other words, 0:26:34.080 --> 0:26:38.840 an energy conscious brain wants to predict away everything possible 0:26:39.200 --> 0:26:44.560 so it can save its energy for just representing the unexpected. 0:26:45.400 --> 0:26:51.199 Silence is golden. So although many neuroscientists think of activity 0:26:51.200 --> 0:26:54.880 in neurons as the representation of things in the world, 0:26:55.280 --> 0:26:59.800 it may turn out that spikes are the unpredicted, energy 0:27:00.000 --> 0:27:06.399 expensive part. The representation of something totally expected would be 0:27:06.480 --> 0:27:10.960 nothing but a hush falling over the neuronal forest. The 0:27:11.000 --> 0:27:16.080 system makes adjustments only when it gets surprised. If your 0:27:16.080 --> 0:27:18.600