Why do brains dream? And what's going on with the bizarreness of dreams? Why doesn't your clock work in your dreams? And even though you spend a lot of your day working on cell phones and computers, why do those almost never make appearances in your dream content? There are so many cool questions that we're going to address today. Is dream content the same across cultures and across time? Are dreams experienced in black and white or in color? Why do I think about dreams as this strange love child between brain plasticity and the rotation of the planet. What is the relationship between schizophrenia and dreaming? And in the future will we be able to read out the content of somebody's dream? Welcome to Inner Cosmos with me David Eagleman. I'm a neuroscientist and author at Stanford and in these episodes we sail deeply into our three pound universe to understand why and how our lives look the way they do and how we experience our life includes our nocturnal life. We spend a third of our lives with our eyes closed, lying still and horizontal, like a meat robot that's been switched off. But why in the last episode, we talked all about sleep, why brains do that, and why all animals do that, and what is happening in the brain during sleep. Today we're going to zoom in on one of my favorite subjects, a very strange thing the brain does, which we call dreaming. We spend a fraction of our sleep time locked in a different reality, swimming around in plots which aren't real, and they're not even realistic, but which nonetheless convince us entirely. And today we're going to see what that's all about. Humans have been writing about dreams and pondering about them for a long time, and essentially we have evidence of this from the beginning of writing. We see detailed dream reports from ancient Egyptians and Romans and Chinese. The Bible is full of dreams. Every culture in the world has been fascinated by these strange nocturnal journeys that we go on. And as we'll see shortly, there's an interesting surprise, which is that dream content, in other words, the kind of things that people dream about is essentially the same across cultures and across time, and that serves as a clue that will allow us to unlock some mysteries about what otherwise feels like a deeply mysterious experience that we enjoy or suffer every night of our life lives. So strap in for an episode full of surprises. So let's start with a poem that Lord Byron wrote in eighteen sixteen called The Dream. He wrote, sleep hath its own world and a wide realm of wild reality. And dreams, in their development have breath and tears, and tortures and the touch of joy. They leave a weight upon our waking thoughts. So dreams are indeed a wild reality. The thing we all find so amazing about dreaming is its bizarreness. You're seeing new things, you hold strange beliefs, and you fall for it all hook line and sinker. Whatever your brain serves up to you, you believe it entirely. So we'll talk about all this, But let's first zoom in on what's happening inside the brain. Dreaming occurs during rapid eye moos movement or rem sleep, where your eyes are darting back and forth. There's nothing coming in the eyes, of course, they're shut, so all the activity is internally generated. For good housekeeping, all just note that rem sleep isn't necessarily equivalent to dream sleep, but that's when most dreaming occurs. Now, how do we know that, Well, it's because if you wait until someone enters rems sleep, their eyes are going back and forth, and then you shake them awake and you say, hey, what were you just experiencing? They'll tell you WHOA, I was just panicking because I was at work but realized I had forgotten to put on pants, and I was hiding behind the plant pots and figuring out how to escape before the meeting started. But if you wake them up during the rest of the ninety minute sleep cycle, during the stages that we call slow wave sleep, and you say, hey, what were you just experiencing, they'll generally say nothing, I wasn't there. Now, for completeness, I'll mention that sometimes people report some form of mental activity during slow wave sleep, but when they do, this is usually just a thought or making a plan. But it lacks the visual vividness and the hallucinatory components of typical dreams. So dreaming of the sort we're interested in today happens during rem sleep. Now, although dreams seem so untethered and ethereal, there are specific things happening under the hood. Dreaming depends on the normal functioning of a particular network of brain areas. This is primarily in the limbic and paralymbic and association areas. Now we know that from brain imaging, but also because damage in this network can produce temporary or permanent dream loss or impairment like loss of visual dream imagery. Now, not all of those areas might sound familiar to you, but you've probably heard of the limbic system, which sits at the heart of your emotions. So presumably this is why dreams are so overloaded with high emotion. And I also want to mention a brain era that is suppressed during rem sleep, and that is the hippocampus. This is an area that's required to convert short term memory into long term memory. And this is why when you wake up from a dream, you can remember it so clearly, but after about fifteen minutes it just slips away from you. You just can't hold on to what happened. This is because you have the short term memory just fine, but you're not converting it into long term memory, and so it simply fades before your eyes. And by the way, I think this experience that we have every morning can enhance our empathy about what it would be like to have something like Alzheimer's disease, because a person with Allsheimers says on the phone, Okay, I'll be right down there in a moment, and they hang up, and then the memory of that just slips away. So even though you might tell a relative with Alzheimer's, come on, just try harder to remember. Just think about somebody telling that to you about your dreams in the morning. So back to the Byron poem, the line that I thought was interesting was they leave a weight upon our waking thoughts because they don't directly interact with our waking thoughts, but instead they can just put a spin on our mood, even if we don't remember why. Okay, so what do we see in the brain when dreaming begins, Well, just before the onset of your eyes darting back and forth, you have a group of neurons in the brain stem, specifically in an area called the ponds, and these crank up, and these trigger several consequences. The first is that your major muscle groups get paralyzed. This is called atonia. So you've got this elaborate neural circuitry that keeps the body paralyzed during dreaming. And by the way, the very elaborateness of this circuitry emphasizes the biological importance of dream sleep. You don't get this kind of complexity by accident. Presumably, dreaming would be unlikely to evolve and remain without an important function behind it. So why do we get this paralysis of the muscles. It allows the possibility for the brain to experience things and simulate reality without actually moving the body. The second consequence of these neurons in the ponds is that they trigger these waves of activity known as PGO waves, and these are key to the experience of dreaming. These PGO waves travel to a tiny area called the geniculate nucleus and from there these waves of activity slam into the occipital cortex at the back of your head. In other words, they drive activity into the visual system. So that's why these are called PGO waves. Because they start in the ponds, p move to the geniculate g and go to the occipital cortex. Oh pgoh I mention all this detail. Because PGO waves are the neural correlate of dreaming, the visual areas become alive with activity allowing us to see our dreams. You blast activity into the visual cortex and you enjoy visual experience even though your eyes are closed. Now, interestingly, when you measure electrical activity across the brain with EEG, when you do this during rem sleep, it looks really similar to that during the waking state. You see similar patterns in both states, for example, lots of gamma wave frequency, which is generally thought to reflect something about cognitive processing, with the exception of the paralyzed muscles and the lack of external visual input because your eyes are closed, the states of rem sleep and of wakefulness they kind of resemble one another. And this is what we might expect from a state of consciousness that's altered. But it's some sort of consciousness nonetheless. Okay, so that's what's happening in the brain when we dream. But why do we dream? Well, there are various hypotheses about this, and I mentioned one last week, which is that maybe dreams allow us to simulate rare situations. In other words, these nocturnal neural simulations might allow us and all animals to test out activities without the danger and risk of doing so in the real world. So you can practice through different scenarios with your muscles all shut down. So people who advocate this theory suggest that the periodic stimulation of the cortex in this semi random and non specific manner this can maintain circuits that are really important for survival, but they rarely get activated because you don't run into emergencies that often. So the idea is that you're just sending in random activity, keeping the engine oiled in case you need one of these programs sometime, and some researchers suggest that it's even more specific than that. A version of this framework called threat simulation theory, suggests that dreams exist to simulate threatening events and to rehearse what to do in the face of threat, and that seems on its surface at least consistent with the high prevalence of violence and aggression in dreams. But I want to note that there's no data that directly supports this idea, and I mentioned last week that this theory suggests that people exposed to more survival threats in waking life might have a more common experience of threat dreams at night, but the data do not support that. Also, the threat perception theory doesn't explain the fact that most dreams are totally meaningless and bizarre instead of involving any meaningful threat perception or threat avoidance. So back to the question of why we dream. A few years ago, my colleague Don Vaughan and I proposed a new theory which is the only one that makes quantitative predictions across species. And if you heard episode eleven you may remember about this. That theory has to do with brain plasticity. And here's the idea. In a nutshell, we know that if a person goes blind, other senses start to take over what was the visual cortex. In other words, the territory of the visual system is not fixed permanently, but instead the wiring of the brain is quite fluid, and there's a takeover that happens. Okay, now, it turns out that if you take a normally cited person and you blindfold, you can start seeing the beginnings of this takeover in neuroimaging starting in about one hour. And that really surprised us because of how fast the takeover starts to happen. So what does this rapidity of the takeover of the cortex have to do with dreaming? While in the incessant competition for territory in the brain, the visual system has a special challenge because of the planet's rotation, we experience darkness for about twelve out of every twenty four hours. And obviously I'm talking about our evolutionary history, not the recently electrically blessed times. In other words, our ancestors all spent their lives as inadvertent participants in a blindfold experiment every single night. So how did their visual cortex defend its territory when you've got no data coming in through the eyes. So our proposal is that the visual corret tex preserves its territory by finding a way to remain active at night. And we call this the defensive activation theory. And the purpose of dreaming here is to keep cells in the visual cortex active, and that's how it resists conquest by the territories of the other senses. Vision is the only one of the senses that's disadvantaged by darkness. You can still touch and smell, and taste and hear in the dark, and so it's the only sense that needs internally generated activity to maintain its borders during the long night. So, as a result, about every ninety minutes you get this internally generated activity that blasts exclusively into the visual cortex, and so dreams are experienced as primarily visual. So several predictions immediately fall out of the defensive activation theory. First, because brain plasticity diminishes with age, this would suggest that the amount of time spent in REM sleep should also decrease as one moves through the lifespan, and that turns out to be exactly what happens. REM sleep in humans accounts for half of an infant's sleep time, but as a person grows older, the percentage of remsleep decreases steadily. In other words, diminishing plasticity of the brain correlates with less REM sleep. But more importantly, we made a more quantified prediction. We said, hey, maybe the more flexible and animal's brain is the more REM sleep it requires to defend its visual cortex. So Don and I studied the plasticity in twenty five species of primates and we found that the species with more plastic brains spend more time in REMS sleep every night. So you'd think that these two measures REM sleep and brain plasticity they would be totally unconnected, So it was very surprising to find that they are tightly linked. So dreaming defends the visual cortex. Now, the circuitry that underpins dreaming. This is so ancient and deeply wired that even people who are blind have it. But note that blind people don't have visual dreams. Instead, their nighttime hallucinations involve the other senses, like feeling their way around an oddly arranged kitchen or hearing strangers speaking. Why because other sensory eras have annexed the underused visual cortex. So think about it this way. People who can see and those with visual impairments, they both have activity shooting into their occipital lobe during dreaming, but the senses processed by that territory are different. The defensive activation theory also makes more general predictions. If dream sleep is triggered by the absence of visual input during the night, we might presume that a person deprived of vision while awake could similarly experience dreamlike visual hallucinations, and that is exactly what happens when people have degenerating vision, or they end up in solitary confinement or even just in a dark float tank. Any situation in which vision gets deprived, the brain starts fighting back and having hallucinations. So that's the idea. Dreams are a screensaver. The brain puts activity into the visual cortex to prevent it from getting ruined during the night. Okay, so you might say, fine, your hypothesis seems plausible, But why do dreams seem to be stories? After all, we don't just see random snow like on an old television. We experience stories, And in fact, there are various stories of someone discovering something in dreams, like Kurkule thinking of the structure of the Benzene ring based on a dream he had of two snakes biting each other's tails, or Paul McCartney dreaming the melody for the song Yesterday. So what's up with dream content? The key is that the brain is a storytelling machine. Your brain is deeply carved with roadways that represent your experience in the world, and these are stored in what are called associative neural networks, which means that everything connects to other things based on how they relate. So if I ask you to think of a mouse that's physically connected in your brain's networks to your concept of cheese or exterminators or mickey mouse or mazes or running wheels, or mouse traps or pet stores everything else you've ever associated with a mouse. So all concepts are related and linked physically to what happens during sleep and dreaming. Well, the synapses that are used during the day tend to be slightly more chemically active, just say they're more hot from the activity of the day, and so your dreams usually involve something from the day or something that's on your mind. They often involve the memories and desires and fears and hopes that are brewing in your inner life, but the associations are much looser. So maybe you have something about your boss or something that happened in a class or whatever, but the dream quickly goes off in some weird direction. The associations are looser, and so we end up dreaming stories with looser associations, following paths along the associative neural network that we might not otherwise take. But things make sense, at least in the light of dream logic. So even if the visual cortex is just getting volleys of random activity, you get meaning out of it. You get some sort of story. Now, people often wake up and say, no, way, that wasn't random. That was a metaphor. I'm trying to decide if I should quit this job, and I had a dream that I went into the office and the floor was tilted at forty five degrees and no matter what I did, I kept slipping back down, And that was a metaphor about the impossibility of my advancement. There, that was a sign from my unconscious. Okay, so maybe the interesting thing about interpreting the content of dreams is that they might be one hundred percent random, but they act like a Rorshack blot, which is to say, you can assign any sort of meaning to them. You remember a Rorshack blot, which is where you squish some ink between two pages, and then you pull the pages apart and you get some random shape and you ask people to interpret what they're seeing to assign meaning to it. And it's easy enough to say, yeah, that looks like this thing in my life, maybe this issue that's been chewing at me. And that's how doctor Rhorshak used this to determine what had meaning in a person's mind. But you would never say the blob of ink carried meaning. It's all about the imposition of meaning by the viewer. And this I suggest could be what happens with dreams as well. You interpret the ink blob, and that interpretation might have significance to your life, but you imposed that meaning instead of it being inherent in the dream. Here's an interesting parallel. Pablo Picasso once said, we all know that art is not truth. Art is a lie that makes us realize the truth, and that could be the same thing with dreams. They are a lie that makes us realize the truth. So in this sense, it's the same thing. If you're thinking about some problem and you go to the bookshelf and you pull off any random book and turn to a random page and you read the sentence. Often it will very tangentially give you some push to the thing that you're trying to solve. But that doesn't mean the awe of the book cared about you and wrote that sentence with that future moment in mind. It simply tells us that lots of things in the world are like lots of other things and can therefore be interpreted. And I do just want to add one more word of caution to the game of dream interpretation. By the time you turn to the person next to you and say, oh my gosh, I just had the strangest dream, and you tell them, you are forced to add another layer of narrative scaffolding so that it makes sense as a story that you're telling. So a narrative that's very thin can seem to have more flesh after you've pushed it through your storytelling machinery. In other words, we layer on narrative structure to give our dreams a shape that they didn't actually have. And so especially if you've had a dream that you've told over and over, it usually has more of a scaffolding than it actually started with. Nonetheless, the content of dreams has always fascinated humans, spurring societies to invent cultural uses for dreams in religious ceremonies and medicinal practices, or doomed attempts to predict the future. Sigmund Freud, who lived decades before modern brain science, he thought that dreams provided an inroad into the underlying functions of the brain and especially the subconscious. He suggested, as had many before him, that dreams concealed hidden meanings that were just at the threshold of breaking through the barrier to consciousness. More specifically, he proposed that dreams constitute a disguised attempt at wish fulfillment. Similarly, Carl Jung positive that dreams may offset features of a character personality that you might ignore while you're awake. In general, both those theories have been criticized because they can't be proven wrong, and critics have pointed out that the wish fulfillment interpretation seems improbable in view of the recurring nightmares that attend post traumatic stress disorder. So I am, of course not the first to propose that dreams might have no inherent meaning. The psychiatrists j al And Hobson and Robert McCarley suggest that dreams may have no meaning at all, and they have a model they call the activation synthesis model, which proposes that random activity from the brain stem goes to the cortex, which tries to turn it into motor output. But given the paralysis of the major muscle groups, the brain has to explain the paradox between outgoing motor signals and a lack of expected sensory feedback. So in this situation, the cortex synthesizes an explanation, essentially weaving a story from the random inputs. Now Hobson and McCary faced criticism for this aspect of their theory, because dreams often do have a relation relationship to life experience. So in that light, they made modifications to allow that dream content relates to memories and fears and desires and hopes and so on, but that these provide the background context and not the exact content of the core texas interpretation of the random activity. So which is it, Does dream content tell us anything at all? Or nothing? Well, here's how we can approach the problem. When you look at one person's dreams like you think about your own dreams, you may not be able to use the content as a key to unlock much about the brain. But as you move from considering one dreamer to doing the science and looking at thousands of dream reports and hundreds of thousands from all over the world, you can start to see the fence lines. You see that dreams aren't a wild unleashing where anything can happen. Instead, dreams have particular rules to the stories, and that gives us hints. So what are those rules? Well? I recently read a very cool new book by my colleague Rahul John day Al called This Is Why You Dream, and he outlines a series of studies that demonstrate a remarkable consistency of themes across cultures in across decades. The same kinds of dreams are reported, for example, being unable to find one's way or being late for an important event. In dreams, both men and women usually experience more aggression than friendliness, although men in almost all societies have greater physical aggression in their dreams than women do. Everyone experiences more misfortune than good fortune. People experience more negative emotions than positive emotions. So the cross cultural research suggests that dreams are similar across a wide range of cultures in terms of the subject matter, the level of aggression, familiarity with the people in the dreams. For example, let me zoom in on this. Jandeal points out that in the nineteen fifties there were questions given to college students in America and college students in Japan, and the questionnaire asked them what they had dreamt about, like showing up somewhere with no clothes or flying or trying to do something again and again, or being buried alive, or something involving school and their teachers or whatever. And as it turns out, although American and Japanese students were living pretty different lives, their answers were essentially the same. So for the Americans, the top five dreams were falling, being attacked or pursued, trying again and again to do something, a dream involving school or teachers, or studying, and sexual experiences. And when you compare this to the Japanese students, it was essentially the same. Their dreams were ranked as being attacked or pursued, falling, trying again and again to do something, dreams about school or teachers or studying, and being frozen with fright, and sexual experiences were number six. So halfway across the world, very different cultures, same dream content. Now, fifty years after that study, similar questions were given to students in China and Germany and they came up with the century the same answers. Again, you find people dreaming of being chaste, or pursued, or arriving too late, like missing an airplane or a final exam. Now you might say, fine, but these are all industrialized countries and maybe their lives are sort of similar. But in fact people have also studied indigenous societies in Brazil and Mexico and Australia and they find extremely similar results. The cultures are quite different, but the plots of the dreams are not. So. There have been lots of studies on this sort of thing now, and they all conclude that dream content is a century same across cultures and across time. As one example, wherever you are in the world, men dream more about other men, while women dream equally of men and women, and all people across cultures are more likely to be victims of violence rather than perpetrators. So dreams cover similar themes across the world, irrespective of language and culture and time. It doesn't seem to matter how rich you are, how poor you are. So what does that mean? Well, what's the one thing we all have in common? Our organs on the inside and specifically the brain. As easy as it is to look at different cultures around the world, with different governments and different religions and different costumes, this variability has cropped up in the last millisecond of human history. And if you look just under the skin and the bone, you find the same brain in us all. It's the same engine under the hood. So whenever we find something clear like oh, everyone dreams about the same thing, that we are looking at something fundamental and ancient and universal and we find all sorts of other interesting answers by looking across thousands of dreams. For example, what if dreams are a way to live out fantasies like sexual fantasy. That seems consistent with how we talk about dreams a way of acting out our desires. But in studies across all cultures, it turns out that fewer than ten percent of dreams are sexual, which I find fascinating given the way we talk about dreams. If a young single person says to another single person, oh, I had a dream about you last night, then everyone titters, makes assumptions about that, and we have terms like dream girl or dream guide, But in fact, ninety percent of our dreams have nothing to do with that. So the content is remarkably similar across the world. But where things get really interesting is when we zoom in one more level and really examine what dreamscapes look like like in detail. And I'm not asking about what's in the dream script here, but if you look very closely across thousands of dream reports, you can ask what is not in the script. So I want to return to a question I posed at the beginning, which is that we live in a society where we are constantly hunched over our cell phones and our laptops. How come these essentially never show up in our dreams? Isn't that weird? And even in the era before computers, back when people read a lot more books, how come no one sits around reading books in their dreams. Why can't you even see any text in your dreams? Why do you spend so much time writing stuff down in real life but you never can write stuff down in a dream. How come you never do math in a dream, like calculating a tip for a restaurant bill, Even if that comes up in a dream plot, it feels totally impossible to get the right answer. Well, all of this serves as a big clue into the networks that are active during a dream, and this is pretty well worked out with brain imaging now, although anyone who is being very observant probably could have guessed this all even a century ago. What reading and writing, and cell phones and computers all have in common is that they require the use of a network in the brain that we summarize as the executive network, which involves a lot of activity largely distributed in the frontal lobes and the parietal lobes, and you need this for rule based problem solving and for making decisions when you've got a particular goal in front of you that you're trying to solve. And the key is that activity in the executive network is reduced. I'll just say it's shut down while you are dreaming, so you're not able to read and write and launch an app on your cell phone screen and so on, because that work requires the executive network, and that network is off duty for the night. But what is on is an network we summarize as the default mode network, which is what kindles to life when you shut your eyes and you're not involved in gathering information and acting on some other task. So this is called the default mode network, but many scientists are renaming this the imagination network because this is what lets your storehouse of experiences fly. You can piece ideas together and come up with new hypotheses and thoughts, and this is what's firing on all cylinders when you dream. So the executive network has reduced, the imagination network is enhanced, and this is what gives dreams their very untethered character. There are other clues you can also gather from looking at detailed dream reports, which is that sometimes objects will turn into other objects, but they're always related in some way. What I mean is you might have your bicycle turn into a motorcycle, or your small house becomes a mansion, or the cafeteria becomes a restaurant, or the hammer becomes a screwdriver, but you don't find the bicycle turning into a restaurant or your house turning into a hammer. What does that tell us? Well, under the hood, we have what are known as semantic maps, and more than associations, which I mentioned before, semantic maps refer to the way that all the stuff in your brain is laid out in terms of relationships between concepts. For example, in your semantic map, you have the concept of an animal, which might include categories like mammals and birds and reptiles and so on, and within one of those categories you might have more specific concepts like dog and cat and eagle and salmon and snake and frog and so on. Things are laid out in your brain based on their conceptual meaning. And what we find is that dreams follow the semantic maps. In other words, they stay local in the little clusters of the maps. So this is why you see the bicycle turning into the motorcycle because they're both forms of transportation, but not into the screwdriver, which is a tool and in a totally different category. These things live in different little neighborhoods of the semantic map. So again, a careful look at dream reports tells us important clues about what is going on under the hood, and that tells us that while the themes are universal strife and falling and attempting to do something over and over, the details are filled in with the specifics of your semantic maps. And this all brings me back around to a question I had since I was a kid about whether dreams are experienced in black and white or in color. Now. When I was a child, I remember at some point that my parents told me that we dream in black and white, and I was confused by this because I didn't seem to dream in black and white. As far as I could tell, my dreams were in color, so I thought I must be somehow assessing that incorrectly. Because they were both very smart and educated. My mother was a biology teacher, my father was a medical doctor, and as I grew older, I asked my friends about this, but I couldn't find anyone who thought that their dreams were not in color. Now, because I genuinely knew that my parents were brilliant, I thought maybe there was something they knew that I didn't like. Maybe we were dreaming in black and white but didn't realize it. And then I found several large scale surveys that reported what my parents had said, that most people dream in black and white. One study was from nineteen fifty eight and they said only nine percent of people dream in color, and a paper in nineteen forty two reported that only ten percent claimed to frequently or very frequently dream in color, and a report in nineteen fifty one had the highest percentage. They said that twenty nine percent of dreams are in color. But even this didn't make things clear what was going on, because dream reports from earlier, like Freud's reports of dreams in nineteen hundred, often had explicit color descriptions in them. So somehow it seemed to be only my parents' generation that dreamed in black and white. And it turns out if you run the study now, almost everyone says they dream in color. As you may have guessed by now, the hypothesis on this is that people were influenced by watching black and white television and black and white movies and black and white photographs, and once the technology changed to color, then so did dreams. I'm putting a very nice paper by Eric Schwitzkeebel on my site that compiles all these studies through the years and the change in the reports. Again, this demonstrates that the content of our dreams has to do with the experiences that we have. Now. I'll tell you an interesting side note on this. I happened to live in California, and during COVID, everyone in my neighborhood wore masks, and it was a very weird time, and so we rarely saw other human beings for a couple of years without everyone being in masks. And one of the things I remember from that time was that we would sometimes watch a movie on TV, and it was so shocking that people in the movie would push their way through a dense crowd and sit so close to their friends and the people they were talking with, and they were indoors with such density. And it became in twenty twenty so noticeable and so shocking when we were in the middle of COVID. Interestingly, it's a little hard to remember that now, but I remember it well because I was taking careful notes on what was going on with my dreams during this time, and specifically, since my real life involved the very thinned out crowds and everyone in masks, did that show up in my dreams? And the answer was is no. The thing that struck me the entire time during lockdown is that my dreams were totally normal. In my dreams, I was at a big party and there were lots of people around, and no one ever had masks in my dreams, which struck me as weird when I would wake up. Why didn't my dreams reflect my daily experience? Now, this is just a single person's report, and I didn't think to study this carefully at the time by doing a big online survey of everyone. But if it's generally true that other people were having these very salient daily experiences of lockdown and masks and yet it wasn't appearing in their dreams, the only interpretation I can make is that our dreams reflect pathways that get laid down over our youth when we're young. And if that's the case, then maybe we could still find some black and white dreaming among older people. Now. In other words, maybe you'd still see some of that now even though those people have been I'm watching color television for sixty years now. With all the things on my plate, I'm not going to have time to pull off that study. But I would love it if listeners could ask their oldest living relatives, they a parent, or grandparent or great grandparent, how they dream and determine if anyone still dreams in black and white and what their age is. Okay, so that story of black and white dreaming correlated with black and white television. That seems like a clean story, But there's one thing that still bugs me about it. Although television and movies were in black and white, obviously the world wasn't. When my parents walked outside, they saw green trees and blue skies and red cardinals and yellow sunflowers and so on. So why didn't their dreams reflect that. Well, there are two things to say here. First, not everyone dreamed in black and white. Instead, it was between let's say, seventy to ninety percent. So had I been alive in the nineteen fifties, I would have studied the issue of how many hours of television people watched each day, and perhaps some measure of how many black and white movies they saw, on how many newspapers with black and white photos, and so on, and correlated that to whether or not they had black and white dreams. Second, I have a totally speculative hypothesis. I don't have any evidence for this, but I'm going to say it anyway. What if some part of the brain knows, at least in part, that dreams are like a fake movie, and in that way it takes on the property of fiction, which in those days was black and white. Because note that even when you're in the middle of a movie, some part of your brain still knows you're watching a movie. And this is why when the train comes at the camera, you don't actually duck and scream. So even though we're compelled by fiction, we don't fall for it completely. Could it be that to the brain, dreams are more like movies. I don't know, because I missed that era, so the opportunity seems gone to study that. But I'm wondering if we're going to have another shot at studying this question once virtual reality becomes ubiquitous and pervasive, and whether a big chunk of the population will report dreams that are more like synthesized virtual reality worlds than like our daily life. To me, this would shed light on the mystery of the black and white dreams and the details by which your dreams are determined by your experiences in the world. Now, there's still lots of stuff that I think we don't understand. One is why we can have false, implanted memories inside a dream. This is something I've wondered about for years. For example, I had a dream many years ago about my brother's wedding and he was yelling at me and saying, I can't believe you said that to Jenny and ruined my wedding, and I felt horrible. I felt as guilty as I ever felt. Why did I do that? Why was I so insensitive? And then I woke up. As it turns out, my brother had a lovely wedding that went just perfectly, and I of course never said anything bad to his bride, my lovely sister in law Jenny. Ever, there was never trouble of any sort there. So why did I fall for the false memory? Why couldn't I access my real memories of the wedding while I was inside the dream? In other words, the issue is that not only did I have a false memory, but also I had no access to my real memory of the wedding. One set of memories was replaced by another. And this is what freaks me out, not just about dreams, but about our sense of reality. We buy whatever our brains serve up to us at every moment. We buy the plot of a dream completely. And by the way, this is one interpretation of what happened in schizophrenia. The way psychiatrists used to describe this about a century ago is that psychosis is essentially an intrusion of the dream state into the waking state. So the next time you see a man talking to himself on the street and having an argument with someone who is not there in front of him, interpret it through this lens that he is having an awake dream, and you might understand what's happening in a fresh way. So what we see here, and what we've seen in many of these episodes, is that we accept whatever reality is served up to us. And by the way, I think this is why literature works with our species, because we can so easily adopt a different reality. We can read a book and we laugh and we weep. We are effortlessly deluded, And to understand this, I think we shouldn't use the term suspension of disbeliefs. It's the wrong term because an applies that disbelief is the standard, disbelief is the normal operating mode. In fact, it might be just the opposite. Our brains are perfectly willing to accept whatever they are fed. Now, I want to hit one more question in today's episode, which is where is dreaming headed? Would it be possible to read out the content of somebody's dreams? Well, if you happen to have heard my episode number twenty seven, you'll know that the answer is already yes, at least to some degree. Different research groups have been working since at least twenty eleven to see if they can measure brain activity in the visual cortex to assess what somebody is seeing in front of their eyes. In other words, if you're looking at a building, there's a different activity in your visual cortext than if you're looking at a puppy or a cup of coffee and so on, and so the research question is can you tell what a person is seeing just by measuring the pasterns in their visual cortex, And the answer is that you can pretty well tell what somebody is seeing this way. After that research came out, people realized we might be able to measure dreams because dreams are activity in the primary visual cortex. Dreams are experienced as visual because it's activity in the visual cortex, and your brain understands that as visual experience. So Yuki also comment. Tani and his colleagues published a paper a few years ago in which he took this same approach. He had people look at a bunch of pictures and watch a bunch of videos while they were in the brain scanner, and then he used machine learning to decode how the picture out there corresponds to the activity patterns in the brain. And now you let people go to sleep in the scanner, and you measure their visual cortex, and you use the machine learning model to decode what that activity would correspond to, in other words, what they were presumably seeing in their dream. And once the machine learning model gives an answer, then you can make a little video reconstruction of their dream, and you can watch what presumably was the experience for the dreamer, albeit with very low resolution. Now, when you're decoding pictures or videos, it's easier to know if you're getting it right. When you're decoding dreaming, it's harder to know for sure that you got the right answer. You have to wake up your participants and say, hey, what do you remember, and then you compare that to what you think you might have decoded. But even with those caveats, this gives the rough start to a dream decoder that could tell you what you were dreaming about, and presumably this will only become higher resolution with time. So it maybe that in the future we are able to know what you dreamed even if you can't remember it. You could keep a digitized dream diary that way, or you could share your dreams with a girlfriend or boyfriend, or less likely, a dream could be used in a court of lossome day. Who knows where this will all go, but we can imagine that in two hundred years from now, dreams may be a very different sort of thing than the entirely private experience that we enjoy now. So let's wrap up. To my mind, the biggest issue that freaks me out about dreams is this issue that we believe patently impossible situations, and we accept false memories even when they contradict events that we know to be true. In waking life. This represents a deeper question about why we can be so easily fooled, but many other questions about dreams can be answered. In the meantime, we're in a moment of using the tools of neuroscience to tie specific details of the biology to the experience of dreaming. We still have a lot long way to go, but some promising leads can come from the way that dream content changes with drugs. For example, dreams can be made more vivid and frightening by drugs that affect the dopamine system or by alkaloids. Antidpressants tend to reduce the amount of dream sleep. Medications to reduce epileptic seizures also seem to reduce nightmares. So we can imagine in the near future having systematic studies to work out the effects of different drugs on dream content, and to do this in conjunction with brain imaging, and we might be able to further nail down the relationships between dream content and specific bits of the dream generation network. And beyond drugs, we can also look to disease, because specific problems lead to the loss or impairment of dreaming. People can lose dreaming with injuries to the inferior paride lobe on either side. Patients with certain forms of dementia report bland dreams, involving less aggression and less narrative complexity and less emotional content. So by looking at changes to the brain and corresponding changes in dreaming, we can better map out the landscape of what is involved. And fundamentally, studying dreaming may help us to comprehend consciousness. Why because there are so many similarities between rem sleep and the awake conscious state. Now there are major differences as well, but it's important to note that we find both waking life and dreaming equally compelling and believable, which tells us that all you need to do is strike the right notes on the neural piano and you have your totally believable experience. But what would be cool is if there were something like consciousness during a dream state that was even close to our daily, normal conscious experience. And that's what we're going to talk about next week. We're going to talk about the very rare circumstance when you are dreaming and you realize that you are dreaming, and you take control of your dream and become an active director of the plot, you are essentially conscious, but living inside the walls of your skull. So join me next week as we sail into the very strange and amazing territory of lucid dreaming. Go to Eagleman dot com slash podcast for more information and find further reading. Send me an email at podcast at egleman dot com with questions or discussion, and check out and subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments until next time. I'm David Eagleman, and this is Inner ca Ismus.