WEBVTT - Re-Release "Why Do Brains Dream" 0:00:04.740 --> 0:00:08.940 Why do brains dream? And what's going on with the 0:00:09.000 --> 0:00:13.720 bizarreness of dreams? Why doesn't your clock work in your dreams? 0:00:14.280 --> 0:00:16.079 And even though you spend a lot of your day 0:00:16.140 --> 0:00:20.020 working on cell phones and computers, why do those almost 0:00:20.320 --> 0:00:23.880 never make appearances in your dream content? There are so 0:00:23.900 --> 0:00:26.140 many cool questions that we're going to address today. Is 0:00:27.200 --> 0:00:31.340 dream content the same across cultures and across time? Are 0:00:31.420 --> 0:00:35.260 dreams experienced in black and white or in color? Why 0:00:35.320 --> 0:00:38.660 do I think about dreams as the strange love child 0:00:38.740 --> 0:00:43.360 between brain plasticity and the rotation of the planet? What 0:00:43.420 --> 0:00:48.020 is the relationship between schizophrenia and dreaming? And in the future, 0:00:48.520 --> 0:00:51.300 will we be able to read out the content of 0:00:51.380 --> 0:00:58.290 somebody's dreams? Welcome to Inner Cosmos with me, David Eagleman. 0:00:58.690 --> 0:01:01.710 I'm a neuroscientist and an author at Stanford. And in 0:01:01.750 --> 0:01:05.890 these episodes, we sail deeply into our three-pound universe to 0:01:05.950 --> 0:01:09.890 understand why and how our lives look the way they do. 0:01:10.790 --> 0:01:15.429 And how we experience our life includes our nocturnal life. 0:01:15.730 --> 0:01:21.140 We spend hours and hours and hours A third of 0:01:21.230 --> 0:01:26.120 our lives with our eyes closed, lying still and horizontal 0:01:26.520 --> 0:01:30.500 like a meat robot that's been switched off. But why? 0:01:31.240 --> 0:01:34.979 In the last episode, we talked all about sleep. Why 0:01:35.140 --> 0:01:38.140 brains do that and why all animals do that and 0:01:38.200 --> 0:01:42.080 what is happening in the brain during sleep. Today, we're 0:01:42.440 --> 0:01:45.680 going to zoom in on one of my favorite subjects, 0:01:45.880 --> 0:01:50.240 a very strange thing the brain does, which we call sleep. dreaming. 0:01:51.080 --> 0:01:54.030 We spend a fraction of our sleep time locked in 0:01:54.090 --> 0:01:59.640 a different reality, swimming around in plots which aren't real, 0:01:59.670 --> 0:02:04.480 and they're often not even realistic, but which nonetheless convince 0:02:04.600 --> 0:02:09.049 us entirely. And today, we're going to see what that's 0:02:09.110 --> 0:02:12.910 all about. Humans have been writing about dreams and pondering 0:02:12.970 --> 0:02:16.029 about them for a long time. And essentially, we have 0:02:16.169 --> 0:02:19.190 evidence of this from the beginning of writing. We see 0:02:19.350 --> 0:02:25.149 detailed dream reports from ancient Egyptians and Romans and Chinese. 0:02:25.660 --> 0:02:28.260 The Bible is full of dreams. Every culture in the 0:02:28.300 --> 0:02:33.020 world has been fascinated by these strange nocturnal journeys that 0:02:33.060 --> 0:02:35.210 we go on. And as we'll see shortly, there's an 0:02:35.350 --> 0:02:39.590 interesting surprise, which is that dream content, in other words, 0:02:39.630 --> 0:02:43.049 the kind of things that people dream about, is essentially 0:02:43.090 --> 0:02:48.250 the same across cultures and across time. And that serves 0:02:48.389 --> 0:02:51.350 as a clue that will allow us to unlock some 0:02:51.389 --> 0:02:56.500 mysteries about what otherwise feels like a deeply mysterious experience 0:02:56.570 --> 0:03:00.260 that we enjoy or suffer every night of our lives. 0:03:00.760 --> 0:03:04.720 So strap in for an episode... full of surprises. So 0:03:04.760 --> 0:03:07.680 let's start with a poem that Lord Byron wrote in 0:03:07.680 --> 0:03:13.169 1816 called The Dream. He wrote,' Sleep hath its own 0:03:13.290 --> 0:03:18.709 world and a wide realm of wild reality, and dreams 0:03:18.889 --> 0:03:23.739 in their development have breath and tears and tortures and 0:03:23.760 --> 0:03:27.380 the touch of joy. They leave a weight upon our 0:03:27.480 --> 0:03:33.859 waking thoughts.' So, dreams are indeed a wild reality. The 0:03:33.880 --> 0:03:37.960 thing we all find so amazing about dreaming is its bizarreness. 0:03:38.940 --> 0:03:43.690 Seeing new things, you hold strange beliefs and you fall 0:03:43.790 --> 0:03:46.790 for it all. Hook, line, and sinker. Whatever your brain 0:03:47.190 --> 0:03:51.010 serves up to you, you believe it entirely. So we'll 0:03:51.050 --> 0:03:53.840 talk about all this, but let's first zoom in on 0:03:53.900 --> 0:03:59.860 what's happening inside the brain. Dreaming occurs during rapid eye 0:03:59.920 --> 0:04:04.410 movement or REM sleep. where your eyes are darting back 0:04:04.450 --> 0:04:07.190 and forth. There's nothing coming in the eyes, of course, 0:04:07.210 --> 0:04:12.100 they're shut. So all the activity is internally generated. For 0:04:12.140 --> 0:04:16.000 good housekeeping, I'll just note that REM sleep isn't necessarily 0:04:16.080 --> 0:04:20.409 equivalent to dream sleep, but that's when most dreaming occurs. Now, 0:04:20.650 --> 0:04:23.390 how do we know that? Well, it's because if you 0:04:23.450 --> 0:04:26.770 wait until someone enters REM sleep, their eyes are going 0:04:26.810 --> 0:04:28.990 back and forth, and then you shake them awake and 0:04:29.029 --> 0:04:34.240 you say, hey, what were you just experiencing? They'll tell you, whoa, 0:04:34.250 --> 0:04:37.960 I was just panicking because I was at work but 0:04:38.020 --> 0:04:40.520 realized I'd forgotten to put on pants and I was 0:04:40.600 --> 0:04:43.820 hiding behind the plant pots and figuring out how to 0:04:43.880 --> 0:04:47.250 escape before the meeting started. But if you wake them 0:04:47.350 --> 0:04:51.170 up during the rest of the 90-minute sleep cycle, during 0:04:51.190 --> 0:04:54.710 the stages that we call slow-wave sleep, and you say, hey, 0:04:54.810 --> 0:04:58.830 what were you just experiencing? They'll generally say, nothing, I 0:04:59.130 --> 0:05:03.580 wasn't there. Now, for completeness, I'll mention that sometimes people 0:05:03.640 --> 0:05:08.960 report some form of mental activity during slow-wave sleep, but 0:05:09.000 --> 0:05:12.669 when they do, this is usually just a thought or 0:05:13.070 --> 0:05:17.270 making a plan, but it lacks the visual vividness and 0:05:17.310 --> 0:05:23.000 the hallucinatory components of sleep. typical dreams. So, dreaming of 0:05:23.040 --> 0:05:28.060 the sort we're interested in today happens during REM sleep. Now, 0:05:28.339 --> 0:05:33.650 although dreams seem so untethered and ethereal, there are specific 0:05:33.710 --> 0:05:37.270 things happening under the hood. Dreaming depends on the normal 0:05:37.350 --> 0:05:41.589 functioning of a particular network of brain areas. This is 0:05:41.630 --> 0:05:47.200 primarily in the Now, we know that from brain imaging, 0:05:47.240 --> 0:05:51.600 but also because damage in this network can produce temporary 0:05:51.640 --> 0:05:57.450 or permanent dream loss or impairment, like loss of visual 0:05:57.610 --> 0:06:01.130 dream imagery. Now, not all of those areas might sound 0:06:01.170 --> 0:06:04.010 familiar to you, but you've probably heard of the limbic system, 0:06:04.529 --> 0:06:08.250 which sits at the heart of your emotions. So presumably, 0:06:08.610 --> 0:06:12.890 this is why dreams are so overloaded with high emotion. 0:06:13.710 --> 0:06:15.640 And I also want to mention a brain area that 0:06:15.740 --> 0:06:19.960 is suppressed during REM sleep, and that is the hippocampus. 0:06:20.300 --> 0:06:24.060 This is an area that's required to convert short-term memory 0:06:24.160 --> 0:06:27.070 into long-term memory. And this is why when you wake 0:06:27.110 --> 0:06:31.210 up from a dream, you can remember it so clearly, 0:06:31.250 --> 0:06:35.180 but after about 15 minutes, it just slips away from you. 0:06:35.240 --> 0:06:38.520 You just can't hold on to what happened. This is 0:06:38.560 --> 0:06:42.740 because you have the short-term memory just fine, but you're 0:06:42.779 --> 0:06:46.650 not converting it into long-term memory. And so it simply 0:06:47.170 --> 0:06:50.289 fades before your eyes. And by the way, I think 0:06:50.310 --> 0:06:54.150 this experience that we have every morning can enhance our 0:06:54.330 --> 0:06:56.690 empathy for about what it would be like to have 0:06:56.750 --> 0:07:01.159 something like Alzheimer's disease. Because a person with Alzheimer's says 0:07:01.279 --> 0:07:03.400 on the phone, okay, I'll be right down there in 0:07:03.440 --> 0:07:06.380 a moment, and they hang up, and then the memory 0:07:06.420 --> 0:07:09.850 of that just slips away. So even though you might 0:07:09.970 --> 0:07:13.430 tell a relative with Alzheimer's, come on, just try harder 0:07:13.450 --> 0:07:17.150 to remember. Just think about somebody telling that to you 0:07:17.630 --> 0:07:20.880 about your dreams in the mornings. So back to the 0:07:20.920 --> 0:07:23.860 Byron poem. The line that I thought was interesting was, 0:07:24.440 --> 0:07:28.960 they leave a weight upon our waking thoughts because they 0:07:29.000 --> 0:07:32.190 don't directly interact with our waking thoughts, but instead they 0:07:32.260 --> 0:07:36.210 can just put a spin on our mood, even if 0:07:36.270 --> 0:07:40.220 we don't remember why. Okay, so what do we see 0:07:40.260 --> 0:07:44.300 in the brain when dreaming begins? Well, just before the 0:07:44.580 --> 0:07:48.250 onset of your eyes darting back and forth, you have 0:07:48.310 --> 0:07:51.670 a group of neurons in the brainstem, specifically in an 0:07:51.810 --> 0:07:55.250 area called the pons, and these crank up. And these 0:07:55.310 --> 0:08:00.500 trigger several consequences. The first is that your major muscle 0:08:00.560 --> 0:08:04.880 groups get paralyzed. This is called atonia. So you've got 0:08:04.900 --> 0:08:10.420 this elaborate neural circuitry that keeps the body paralyzed during dreaming. 0:08:10.900 --> 0:08:14.060 And by the way, the very elaborateness of this circuitry 0:08:14.580 --> 0:08:19.540 emphasizes the biological importance of dream sleep. You don't get 0:08:19.600 --> 0:08:23.880 this kind of complexity by accident. Presumably, dreaming would be 0:08:24.130 --> 0:08:28.980 unlikely to evolve and remain without an important function behind it. 0:08:29.590 --> 0:08:32.010 So why do we get this paralysis of the muscles? 0:08:32.470 --> 0:08:36.410 It allows the possibility for the brain to experience things 0:08:36.490 --> 0:08:41.620 and simulate reality without actually moving the body. The second 0:08:41.679 --> 0:08:44.839 consequence of these neurons in the pons is that they 0:08:45.020 --> 0:08:48.960 trigger these waves of activity known as PGO waves. And 0:08:49.000 --> 0:08:53.990 these are key to the experience of dreaming. These PGO 0:08:54.030 --> 0:08:58.030 waves travel to a tiny area called the geniculate nucleus. 0:08:58.090 --> 0:09:01.230 And from there, these waves of activity slam into the 0:09:01.670 --> 0:09:04.820 occipital cortex at the back of your head. In other words, 0:09:05.200 --> 0:09:09.700 they drive activity into the visual systems. So that's why 0:09:09.740 --> 0:09:13.710 these are called PGO waves, because they start in the pons, P, 0:09:14.110 --> 0:09:18.650 move to the geniculate, G, and go to the occipital cortex, O, PGO. 0:09:18.990 --> 0:09:22.300 I mention all this detail because PGO waves are the 0:09:22.440 --> 0:09:28.820 neural correlate of dreaming. the visual areas become alive with activity, 0:09:28.920 --> 0:09:33.449 allowing us to see our dreams. You blast activity into 0:09:33.470 --> 0:09:38.110 the visual cortex and you enjoy visual experience, even though 0:09:38.270 --> 0:09:43.130 your eyes are closed. Now, interestingly, when you measure electrical 0:09:43.190 --> 0:09:47.200 activity across the brain with EEG, When you do this 0:09:47.240 --> 0:09:50.470 during REM sleep, it looks really similar to that during 0:09:50.710 --> 0:09:54.890 the waking state. You see similar patterns in both states. 0:09:55.250 --> 0:09:58.670 For example, lots of gamma wave frequency, which is generally 0:09:58.730 --> 0:10:03.459 thought to reflect something about cognitive processing. With the exception 0:10:03.679 --> 0:10:08.120 of the paralyzed muscles and the lack of external visual 0:10:08.250 --> 0:10:11.530 input because your eyes are closed, the states of REM 0:10:11.590 --> 0:10:15.050 sleep and of wakefulness, they kind of resemble one another. 0:10:15.470 --> 0:10:17.590 And this is what we might expect from a state 0:10:17.670 --> 0:10:23.979 of consciousness that's altered, but it's some sort of consciousness nonetheless. Okay, 0:10:24.080 --> 0:10:27.160 so that's what's happening in the brain when we dream. 0:10:27.300 --> 0:10:31.180 But why do we dream? Well, there are various hypotheses 0:10:31.240 --> 0:10:33.740 about this. And I mentioned one last week, which is 0:10:33.780 --> 0:10:38.489 that maybe dreams allow us to simulate rare situations. In 0:10:38.550 --> 0:10:44.770 other words, these nocturnal neural simulations might allow us and 0:10:44.850 --> 0:10:49.320 all animals to test out activities without the danger and 0:10:49.390 --> 0:10:51.480 risk of doing so in the real world. So you 0:10:51.520 --> 0:10:56.140 can practice through different scenarios with your muscles all shut down. 0:10:56.850 --> 0:11:01.010 So people who advocate this theory suggest that the periodic 0:11:01.110 --> 0:11:05.990 stimulation of the cortex in this semi-random and nonspecific manner, 0:11:06.490 --> 0:11:10.250 this can maintain circuits that are really important for survival, 0:11:10.730 --> 0:11:13.950 but they rarely get activated because you don't run into 0:11:13.990 --> 0:11:17.210 emergencies that often. So the idea is that you're just 0:11:17.309 --> 0:11:21.329 sending in random activity, keeping the engine oiled in case 0:11:21.390 --> 0:11:25.520 you need one of these programs sometimes. And some researchers 0:11:25.559 --> 0:11:28.470 suggest that it's even more specific than that. A version 0:11:28.490 --> 0:11:33.349 of this framework called threat simulation theory suggests that dreams 0:11:33.429 --> 0:11:37.360 exist to simulate threatening events and to rehearse what to 0:11:37.390 --> 0:11:40.310 do in the face of threat. And that seems on 0:11:40.350 --> 0:11:43.730 its surface, at least consistent with the high prevalence of 0:11:44.150 --> 0:11:48.160 violence and aggression in dreams. But I want to note 0:11:48.220 --> 0:11:51.280 that there's no data that directly supports this idea. And 0:11:51.320 --> 0:11:55.800 I mentioned last week that this theory suggests that people 0:11:55.840 --> 0:11:59.560 exposed to more survival threats in waking life might have 0:11:59.580 --> 0:12:03.330 a more common experience of threat dreams at night. But 0:12:03.370 --> 0:12:07.110 the data do not support that. Also, the threat perception 0:12:07.150 --> 0:12:10.550 theory doesn't explain the fact that most dreams are totally 0:12:10.610 --> 0:12:15.490 meaningless and bizarre instead of involving any meaningful threat perception 0:12:15.530 --> 0:12:18.550 or threat avoidance. So back to the question of why 0:12:18.650 --> 0:12:22.030 we dream. A few years ago, my colleague Don Vaughn 0:12:22.090 --> 0:12:25.469 and I proposed a new theory, which is the only 0:12:25.570 --> 0:12:30.160 one that makes quantitative predictions across species and And if 0:12:30.210 --> 0:12:33.490 you heard episode 11, you may remember about this. That 0:12:33.510 --> 0:12:36.510 theory has to do with brain plasticity. And here's the 0:12:36.570 --> 0:12:39.370 idea in a nutshell. We know that if a person 0:12:39.429 --> 0:12:43.810 goes blind, other senses start to take over what was 0:12:43.850 --> 0:12:47.000 the visual cortex. In other words, the territory of the 0:12:47.030 --> 0:12:51.179 visual system is not fixed permanently, but instead the wiring 0:12:51.220 --> 0:12:53.560 of the brain is quite fluid and there's a takeover 0:12:53.600 --> 0:12:56.760 that happens. Okay. Now, it turns out that if you 0:12:56.780 --> 0:13:00.750 take a normally sighted person and you blindfold him, you 0:13:00.780 --> 0:13:05.310 can start seeing the beginnings of this takeover in neuroimaging 0:13:05.910 --> 0:13:09.670 starting in about one hour. And that really surprised us 0:13:09.730 --> 0:13:14.309 because of how fast the takeover starts to happen. So 0:13:14.490 --> 0:13:17.850 what does this rapidity of the takeover of the cortex 0:13:18.150 --> 0:13:22.160 have to do with dreaming? Well, in the incessant competition 0:13:22.220 --> 0:13:25.740 for territory in the brain, the visual system has a 0:13:25.800 --> 0:13:31.660 special challenge because of the planet's rotation. We experience darkness 0:13:31.700 --> 0:13:34.860 for about 12 out of every 24 hours. And obviously, 0:13:34.900 --> 0:13:40.070 I'm talking about our evolutionary history, not the recently electrically 0:13:40.160 --> 0:13:44.510 blessed times. In other words, our ancestors all spent their 0:13:44.570 --> 0:13:49.550 lives as inadvertent participants in a blindfold experiment every single night. 0:13:50.150 --> 0:13:54.710 So how did their visual cortex defend its territory when 0:13:54.730 --> 0:13:57.530 you've got no data coming in through the eyes? So 0:13:57.590 --> 0:14:01.840 our proposal is that the visual cortex preserves its territory 0:14:02.320 --> 0:14:05.080 by finding a way to remain active at night. And 0:14:05.100 --> 0:14:09.679 we call this the defensive activation theory. And the purpose 0:14:09.920 --> 0:14:13.550 of dreaming here is to keep cells in the visual 0:14:13.610 --> 0:14:18.050 cortex active. And that's how it resists conquest by the 0:14:18.130 --> 0:14:22.210 territories of the Other senses. Vision is the only one 0:14:22.230 --> 0:14:26.010 of the senses that's disadvantaged by darkness. You can still 0:14:26.350 --> 0:14:28.850 touch and smell and taste and hear the dark. And 0:14:28.890 --> 0:14:32.810 so it's the only sense that needs internally generated activity 0:14:33.290 --> 0:14:37.410 to maintain its borders during the long night. So as 0:14:37.450 --> 0:14:41.090 a result, about every 90 minutes, you get this internally 0:14:41.210 --> 0:14:46.270 generated activity that blasts exclusively into the visual cortex. And 0:14:46.460 --> 0:14:51.910 so dreams are experienced as primarily visual. So several predictions 0:14:51.950 --> 0:14:56.960 immediately fall out of the defensive activation theory. First, because 0:14:57.060 --> 0:15:00.950 brain plasticity diminishes with age, This would suggest that the 0:15:00.990 --> 0:15:04.690 amount of time spent in REM sleep should also decrease 0:15:04.810 --> 0:15:07.850 as one moves through the lifespan. And that turns out 0:15:07.890 --> 0:15:11.810 to be exactly what happens. REM sleep in humans accounts 0:15:11.870 --> 0:15:15.170 for half of an infant's sleep time, but as a 0:15:15.210 --> 0:15:20.100 person grows older, the percentage of REM sleep decreases steadily. 0:15:20.170 --> 0:15:24.480 In other words, diminishing plasticity of the brain correlates with 0:15:24.660 --> 0:15:27.860 less REM sleep. But more importantly, we made a more 0:15:28.320 --> 0:15:32.680 quantified prediction. We said, hey, maybe the more flexible an 0:15:32.900 --> 0:15:36.900 animal's brain is, the more REM sleep it requires to 0:15:36.950 --> 0:15:40.290 defend its visual cortex. So Dawn and I studied the 0:15:40.610 --> 0:15:45.070 plasticity in 25 species of primates, and we found that 0:15:45.110 --> 0:15:49.300 the species with more plastic brains spend more time in 0:15:49.420 --> 0:15:53.520 REM sleep every night. So you'd think that these two measures, 0:15:53.680 --> 0:15:57.880 REM sleep and brain plasticity, they would be totally unconnected. 0:15:57.920 --> 0:16:00.380 So it was very surprising to find that they are 0:16:00.500 --> 0:16:05.860 tightly linked. So dreaming defends the visual cortex. Now, the 0:16:05.980 --> 0:16:10.990 circuitry that underpins dreaming, this is so ancient and deeply wired. 0:16:11.520 --> 0:16:14.210 that even people who are blind have it. But note 0:16:14.270 --> 0:16:18.770 that blind people don't have visual dreams. Instead, their nighttime 0:16:18.790 --> 0:16:23.630 hallucinations involve the other senses, like feeling their way around 0:16:23.970 --> 0:16:29.050 an oddly arranged kitchen or hearing strangers speaking. Why? Because 0:16:29.310 --> 0:16:34.510 other sensory areas have annexed the underused visual cortex. So 0:16:34.550 --> 0:16:37.010 think about it this way. People who can see and 0:16:37.030 --> 0:16:40.320 those with visual impairments, they both have activity shooting into 0:16:40.340 --> 0:16:44.960 their occipital lobe during dreaming, but the senses processed by 0:16:45.020 --> 0:16:49.990 that territory are different. The defensive activation theory also makes 0:16:50.090 --> 0:16:53.970 more general predictions. If dream sleep is triggered by the 0:16:54.210 --> 0:16:57.880 absence of visual input during the night, We might presume 0:16:57.920 --> 0:17:02.220 that a person deprived of vision while awake could similarly 0:17:02.320 --> 0:17:06.700 experience dreamlike visual hallucinations. And that is exactly what happens 0:17:06.760 --> 0:17:10.919 when people have degenerating vision or they end up in 0:17:11.090 --> 0:17:14.609 solitary confinement or even just in a dark float tank. 0:17:15.030 --> 0:17:18.770 Any situation in which vision gets deprived, the brain starts 0:17:18.970 --> 0:17:23.510 fighting back and and having hallucinations. So that's the idea. 0:17:24.230 --> 0:17:28.450 Dreams are a screensaver. The brain puts activity into the 0:17:28.510 --> 0:17:33.290 visual cortex to prevent it from getting ruined during the night. Okay, 0:17:33.310 --> 0:17:36.950 so you might say, fine, your hypothesis seems plausible, but 0:17:37.430 --> 0:17:41.500 why do dreams seem to be stories? After all, we 0:17:41.540 --> 0:17:45.000 don't just see random snow like on an old television. 0:17:45.340 --> 0:17:49.700 We experience stories. And in fact, There are various stories 0:17:49.740 --> 0:17:54.119 of someone discovering something in dreams, like Krikuli thinking of 0:17:54.180 --> 0:17:57.050 the structure of the benzene ring based on a dream 0:17:57.090 --> 0:18:00.330 he had of two snakes biting each other's tails. Or 0:18:00.430 --> 0:18:03.930 Paul McCartney dreaming the melody for the song Yesterday. So 0:18:03.970 --> 0:18:08.609 what's up with dream content? The key is that the 0:18:08.710 --> 0:18:14.209 brain is a storytelling machine. Your brain is deeply carved 0:18:14.570 --> 0:18:19.020 with roadways that represent your experience in the world. And 0:18:19.040 --> 0:18:22.619 these are stored in what are called associative neural networks, 0:18:23.080 --> 0:18:26.100 which means that everything connects to other things based on 0:18:26.420 --> 0:18:28.860 how they relate. So if I ask you to think 0:18:28.940 --> 0:18:32.770 of a mouse that's physically connected in your brain's networks 0:18:32.810 --> 0:18:36.770 to your concept of cheese or exterminators or Mickey Mouse 0:18:36.850 --> 0:18:40.850 or mazes or running wheels or mouse traps or pet stores, 0:18:41.250 --> 0:18:45.430 everything else you've ever associated with a mouse. So all 0:18:45.520 --> 0:18:50.920 concepts are related and linked physically. So what happens during 0:18:51.060 --> 0:18:55.600 sleep and dreaming? Well, the synapses that are used during 0:18:55.660 --> 0:18:59.879 the day tend to be slightly more chemically active. I'll 0:18:59.900 --> 0:19:02.640 just say they're more hot from the activity of the day. 0:19:03.119 --> 0:19:06.920 And so your dreams usually involve something from the day 0:19:06.980 --> 0:19:10.000 or something that's on your mind. They often involve the 0:19:10.480 --> 0:19:14.320 memories and desires and fears and hopes that are brewing 0:19:14.380 --> 0:19:19.119 in your inner life. But the associations are much looser. 0:19:19.760 --> 0:19:22.150 So maybe you have something about your boss or something 0:19:22.170 --> 0:19:24.889 that happened in a class or whatever, but the dream 0:19:24.970 --> 0:19:30.720 quickly goes off in some weird direction. The associations are looser. 0:19:30.859 --> 0:19:34.780 And so we end up dreaming stories with looser associations, 0:19:34.880 --> 0:19:39.010 following paths along the associative neural network that we might 0:19:39.050 --> 0:19:42.820 not otherwise take. But things make sense, at least in 0:19:42.859 --> 0:19:47.110 the light of dream logic. So, even if the visual 0:19:47.210 --> 0:19:51.470 cortex is just getting volleys of random attributes, you get 0:19:52.030 --> 0:19:55.630 meaning out of it. You get some sort of story. Now, 0:19:55.750 --> 0:19:59.230 people often wake up and say, no way, that wasn't random. 0:19:59.290 --> 0:20:02.109 That was a metaphor. I'm trying to decide if I 0:20:02.150 --> 0:20:05.240 should quit this job. And I had a dream that 0:20:05.260 --> 0:20:08.280 I went into the office and the floor was tilted 0:20:08.359 --> 0:20:10.600 at 45 degrees, and no matter what I did, I 0:20:10.680 --> 0:20:14.000 kept slipping back down. And that was a metaphor about 0:20:14.020 --> 0:20:17.480 the impossibility of my advancement there. That was a sign 0:20:17.560 --> 0:20:22.520 from my unconscious. Okay, so maybe. The interesting thing about 0:20:22.660 --> 0:20:28.300 interpreting the content of dreams is that they might be 100% random, 0:20:28.500 --> 0:20:31.659 but they act like a Rorschach blot, which is to 0:20:31.720 --> 0:20:34.940 say you can assign any sort of meaning to them. 0:20:35.350 --> 0:20:37.889 You remember a Rorschach plot, which is where you squish 0:20:37.930 --> 0:20:40.570 some ink between two pages and then you pull the 0:20:40.609 --> 0:20:43.229 pages apart and you get some random shape and you 0:20:43.330 --> 0:20:48.110 ask people to interpret what they're seeing, to assign meaning 0:20:48.270 --> 0:20:50.610 to it. And it's easy enough to say, yeah, that 0:20:50.650 --> 0:20:53.090 looks like this thing in my life, maybe this issue 0:20:53.130 --> 0:20:56.010 that's been chewing at me. And that's how Dr. Rorschach 0:20:56.190 --> 0:20:59.639 used this to determine meaning. What had meaning in a 0:20:59.740 --> 0:21:03.200 person's mind? But you would never say the blob of 0:21:03.420 --> 0:21:07.320 ink carried meaning. It's all about the imposition of meaning 0:21:07.460 --> 0:21:11.220 by the viewer. And this, I suggest, could be what 0:21:11.280 --> 0:21:15.869 happens with dreams as well. You interpret the ink blob, 0:21:15.970 --> 0:21:19.669 and that interpretation might have significance to your life, but 0:21:19.690 --> 0:21:23.669 you impose that meaning instead of it being inherent in 0:21:23.710 --> 0:21:28.230 the dreams. Here's an interesting parallel. Pablo Picasso once said, 0:21:28.250 --> 0:21:31.689 And that could be the same thing with dreams. They 0:21:32.090 --> 0:21:42.169 are a lie that makes us realize the truth. So 0:21:42.290 --> 0:21:44.150 in this sense, it's the same thing. If you're thinking 0:21:44.190 --> 0:21:47.310 about some problem and you go to the bookshelf and 0:21:47.330 --> 0:21:49.770 you pull off any random book and turn to a 0:21:49.850 --> 0:21:53.420 random page and you read the sentence, often it will 0:21:53.460 --> 0:21:57.219 very tangentially give you some push to the thing that 0:21:57.240 --> 0:22:00.260 you're trying to solve. But that doesn't mean the author 0:22:00.380 --> 0:22:03.459 of the book cared about you and wrote that sentence 0:22:03.520 --> 0:22:06.520 with that future moment in mind. It simply tells us 0:22:06.560 --> 0:22:09.300 that lots of things in the world are like lots 0:22:09.340 --> 0:22:13.340 of other things and can therefore be interpret it. And 0:22:13.400 --> 0:22:15.340 I do just want to add one more word of 0:22:15.480 --> 0:22:19.040 caution to the game of dream interpretation. By the time 0:22:19.100 --> 0:22:21.199 you turn to the person next to you and say, 0:22:21.280 --> 0:22:24.240 oh my gosh, I just had the strangest dream. And 0:22:24.260 --> 0:22:28.350 you tell them, you are forced to add another layer 0:22:28.590 --> 0:22:31.850 of narrative scaffolding so that it makes sense as a 0:22:31.930 --> 0:22:35.610 story that you're telling. So a narrative that's very thin 0:22:36.070 --> 0:22:39.210 can seem to have more flesh after you've pushed it 0:22:39.250 --> 0:22:42.930 through your storytelling machinery. In other words, we layer on 0:22:43.030 --> 0:22:46.120 narrative structure to give our dreams a shape that they 0:22:46.130 --> 0:22:49.060 didn't actually have. And so especially if you've had a 0:22:49.180 --> 0:22:52.360 dream that you've told over and over, it usually has 0:22:52.420 --> 0:22:56.220 more of a scaffolding than it actually started with. Nonetheless, 0:22:56.640 --> 0:23:01.600 the content of dreams has always fascinated humans, spurring societies 0:23:01.660 --> 0:23:05.939 to invent cultural uses for dreams in religious ceremonies and 0:23:06.310 --> 0:23:12.870 medicinal practices or doomed attempts to predict the future. Sigmund Freud, 0:23:12.910 --> 0:23:15.810 who lived decades before modern brain science, he thought that 0:23:15.869 --> 0:23:19.889 dreams provided an inroad into the underlying functions of the 0:23:19.930 --> 0:23:24.400 brain and especially the subconscious. He suggested, as had many 0:23:24.440 --> 0:23:29.580 before him, that dreams concealed hidden meanings that were just 0:23:29.640 --> 0:23:33.600 at the threshold of breaking through the barrier to consciousness. 0:23:34.040 --> 0:23:38.990 More specifically, he proposed that dreams constitute a disguised attempt 0:23:39.150 --> 0:23:43.930 at wish fulfillment. Similarly, Carl Jung posited that dreams may 0:23:44.390 --> 0:23:49.030 offset features of a character or personality that you might 0:23:49.170 --> 0:23:52.700 ignore while you're awake. In general, both those theories have 0:23:52.740 --> 0:23:56.840 been criticized because they can't be proven wrong. And critics 0:23:56.859 --> 0:24:02.500 have pointed out that the wish-fulfillment interpretation seems improbable in 0:24:02.600 --> 0:24:07.570 view of the recurring nightmares that attend post-traumatic stress disorder. 0:24:07.970 --> 0:24:10.390 So I'm, of course, not the first to propose that 0:24:10.470 --> 0:24:15.109 dreams might have no inherent meaning. The psychiatrists J. Allen 0:24:15.150 --> 0:24:18.670 Hobson and Robert McCarley suggest that dreams may have no 0:24:18.760 --> 0:24:20.960 meaning at all, and they have a model they call 0:24:20.980 --> 0:24:25.690 the activation synthesis model. which proposes that random activity from 0:24:25.710 --> 0:24:28.790 the brainstem goes to the cortex, which tries to turn 0:24:28.830 --> 0:24:32.350 it into motor output. But given the paralysis of the 0:24:32.390 --> 0:24:37.160 major muscle groups, the brain has to explain the paradox 0:24:37.260 --> 0:24:43.280 between outgoing motor signals and a lack of expected sensory feedback. 0:24:43.700 --> 0:24:49.600 So in this situation, the cortex synthesizes an explanation, essentially 0:24:50.000 --> 0:24:54.439 weaving a story from the random inputs. Now, Hobson and 0:24:54.480 --> 0:24:57.940 McCarley faced criticism for this aspect of their theory because 0:24:58.320 --> 0:25:02.270 dreams often do have a relationship to life experience. So 0:25:02.430 --> 0:25:05.790 in that light, they made modifications to allow that dream 0:25:05.850 --> 0:25:09.290 content relates to memories and fears and desires and hopes 0:25:09.330 --> 0:25:13.350 and so on, but that these provide the background context 0:25:13.430 --> 0:25:18.109 and not the exact content of the cortex's interpretation of 0:25:18.170 --> 0:25:21.510 the random activity. So which is it? Does dream content 0:25:21.590 --> 0:25:24.619 tell us anything at all or nothing? Well, here's how 0:25:24.660 --> 0:25:27.880 we can approach the problem. When you look at one 0:25:27.980 --> 0:25:30.919 person's dreams, like you think about your own dreams, you 0:25:30.940 --> 0:25:33.210 may not be able to use the content as a 0:25:33.290 --> 0:25:36.610 key to unlock much about the brain. But as you 0:25:36.730 --> 0:25:41.119 move from considering one dreamer to doing the science and 0:25:41.140 --> 0:25:44.080 looking at thousands of dream reports and hundreds of thousands 0:25:44.380 --> 0:25:47.379 from all over the world, you can start to see 0:25:47.420 --> 0:25:51.690 the fence lines. You see that dreams aren't a wild 0:25:51.810 --> 0:25:56.869 unleashing where anything can happen. Instead, dreams have particular rules 0:25:56.990 --> 0:26:00.369 to the stories, and that gives us hints. So what 0:26:00.390 --> 0:26:02.880 are those rules? Well, I recently read a very cool 0:26:02.920 --> 0:26:06.270 new book by my colleague Rahul Jandayal called This Is 0:26:06.369 --> 0:26:09.210 Why You Dream. And he outlines a series of studies 0:26:09.690 --> 0:26:15.139 that demonstrate a remarkable consistency of themes across cultures and 0:26:15.160 --> 0:26:20.359 across decades. The same kinds of dreams are reported. For example, 0:26:20.880 --> 0:26:24.369 being unable to find one's way or being late for 0:26:24.390 --> 0:26:27.750 an important event. In dreams, both men and women usually 0:26:27.790 --> 0:26:32.270 experience more aggression than friendliness. Although men in almost all 0:26:32.310 --> 0:26:35.590 societies have greater physical aggression in their dreams than women do. 0:26:35.990 --> 0:26:40.390 Everyone experiences more misfortune than good fortune. People experience more 0:26:40.530 --> 0:26:45.300 negative emotions than positive emotions. So the cross-cultural research suggests 0:26:45.380 --> 0:26:49.040 that dreams are similar across a wide range of cultures 0:26:49.080 --> 0:26:51.500 in terms of the subject matter, the level of aggression, 0:26:51.960 --> 0:26:54.899 familiarity with the people in the dreams. For example, let 0:26:54.940 --> 0:26:57.840 me zoom in on this. Jandale points out that in 0:26:57.859 --> 0:27:01.780 the 1950s, there were questionnaires given to college students in 0:27:01.820 --> 0:27:05.350