Speaker 1: What do your nightly dreams have to do with the
Speaker 1: rotation of the planet. Why are so many musicians blind?
Speaker 1: Can blind people learn how to echolocate like a bat?
Speaker 1: Once we find alien life on other planets, should we
Speaker 1: expect that aliens have dreams at night? Welcome to Inner
Speaker 1: Cosmos with me, David Eagleman. I'm a neuroscientist and an
Speaker 1: author at Stanford University, and I've spent my whole career
Speaker 1: studying the intersection between how the brain works and how
Speaker 1: we experience life. I sort of personally have a love
Speaker 1: hate relationship with dreaming because we take it so seriously.
Speaker 1: We find ourselves in some biziz situation that doesn't make
Speaker 1: any sense, but while you are there, you buy it,
Speaker 1: hook line and sinker. You are emotionally tossed around and
Speaker 1: buffeted in the winds of situations that the moment after
Speaker 1: you wake up you realize weren't real, and more importantly,
Speaker 1: they typically make no sense at all. So why were
Speaker 1: you so caught up in that situation? I've always called
Speaker 1: dreaming sticking my head in the night blender, by which
Speaker 1: I simply mean that I don't necessarily look forward to
Speaker 1: it because I don't know where it's going to take me,
Speaker 1: and I generally feel that all that emotional energy I
Speaker 1: expend isn't that useful. But whatever your take on dreaming is,
Speaker 1: it's one of those absolutely insanely bizarre facts about our
Speaker 1: existence that we totally take for granted. You wake up
Speaker 1: and you say, oh, wow, I was just having a
Speaker 1: dance party with a pack of why monkeys. Or I
Speaker 1: was just at work where I forgot to put on
Speaker 1: pants and I was trying to hide behind my desk.
Speaker 1: Or I was on a river cruise and begging a
Speaker 1: person not to break up with me even though we're
Speaker 1: not even dating. And I find this absolutely stunning that
Speaker 1: it doesn't even bother us that we've just transitioned from
Speaker 1: one reality to another, Like in one second, you went
Speaker 1: from being in some bizarre situation to lying horizontal in
Speaker 1: your bed, and we're so used to it that we
Speaker 1: just say, oh, that was interesting. I just dreamed that
Speaker 1: I was on a hang glider riding over Istanbul and
Speaker 1: I was trying to figure out how not to crash
Speaker 1: because there were a lot of pigeons swirling around me.
Speaker 2: And if there's somebody there, then maybe we tell them.
Speaker 1: Our dream and Otherwise, we just get up and we
Speaker 1: brush our teeth and go about our day without even
Speaker 1: giving a second thought about what just happened. We were
Speaker 1: in one reality and totally emotional invested in it, and
Speaker 1: then a moment later we said, hmm, I guess I'm
Speaker 1: in this other reality now. Now this is a testament
Speaker 1: to the human mind's ability to accept the absolutely amazing
Speaker 1: and bizarre as something not even worth investigating.
Speaker 2: Think about it this way.
Speaker 1: Just imagine if dreams happened right in the middle of
Speaker 1: your day. So you're on a walk, or you're sitting
Speaker 1: at your desk or eating at a restaurant with friends,
Speaker 1: and suddenly your reality morphs to a completely bizarre other reality,
Speaker 1: and now you're on a street you've never seen before,
Speaker 1: or you're flying, or you're falling from a building, or
Speaker 1: you're being chased by a monster. You would be terrified
Speaker 1: by this lack of cohesion in your reality, the fact
Speaker 1: that you just flipped from one to another. And as
Speaker 1: soon as you were back in this reality, you'd presumably
Speaker 1: run to your doctor and say you just had this
Speaker 1: bizarre hallucination and thought you were somewhere else or maybe
Speaker 1: someone else, and you'd be terrified, But because we're so
Speaker 1: used to dreaming, we wake up and we think, oh,
Speaker 1: that was weird, and then we go.
Speaker 2: About our new reality.
Speaker 1: And in fact, one of the evergreen questions among philosophers
Speaker 1: is whether we live in a simulation. And I'm going
Speaker 1: to dive deep into that question in a different episode,
Speaker 1: but for now, I'll just mention the question, which is,
Speaker 1: how would you know if you are a brain in
Speaker 1: a vat who is being stimulated in just the right
Speaker 1: way to think that you're listening to a podcast and
Speaker 1: seeing the world around you and eating.
Speaker 2: Delicious food and so on.
Speaker 1: Or the modern version of that is whether we are
Speaker 1: living in a computer simulation. So I suggest the mere
Speaker 1: existence of dreams is sufficient to prove that all this
Speaker 1: could be a simulation, because dreams demonstrate to us so
Speaker 1: clearly that we completely buy what ever reality we find
Speaker 1: ourselves in, and when we find ourselves in another reality,
Speaker 1: we say, oh, I guess that one wasn't real.
Speaker 2: I was fooled, but this is real.
Speaker 1: But let me leave that as a teaser for the
Speaker 1: episode on whether we're living in a simulation and how
Speaker 1: we'd know. For today, I want to dive into dreaming
Speaker 1: in particular, and from the brain's point.
Speaker 2: Of view, why do we dream?
Speaker 1: Why does our consciousness go on these bizarre flights every
Speaker 1: ninety minutes during the night. So as a neuroscientist, I've
Speaker 1: always been fascinated by dreaming, by what.
Speaker 2: The heck this is all about.
Speaker 1: So in this episode, we're going to talk about the why,
Speaker 1: and I'm going to tell you about some hypotheses that
Speaker 1: people have proposed over the centuries, and then I'm going
Speaker 1: to tell you about a new hypothesis that makes quantitative
Speaker 1: predictions across animal species. So let's start with an interesting fact,
Speaker 1: which is that all animals appear to have dream sleep,
Speaker 1: also as REM sleep. REM is spelled r EM, and
Speaker 1: it stands for rapid eye movement. So in this stage
Speaker 1: of sleep, the eyes are darting back and forth under
Speaker 1: the eyelids. And if you haven't really watched someone while
Speaker 1: they're sleeping, you should in a respectful manner because you'll
Speaker 1: see how amazing this is. Every ninety minutes or so,
Speaker 1: their eyes start jiggling back and forth. Now here's the
Speaker 1: cool part. If you wake them up right when this
Speaker 1: is happening and you say, quick, what were you just
Speaker 1: thinking about? They will tell you that they were just
Speaker 1: riding a camel in a shopping mall, or they were
Speaker 1: running from a pack of leprechauns, or they were flying
Speaker 1: around their house or whatever. And this is how we know,
Speaker 1: from years of experiments of waking people up at different
Speaker 1: stages of sleep, this is how we know that rem
Speaker 1: sleep is when dreams happen. Because if you wake someone
Speaker 1: up during a deeper stage of sleep, what we call
Speaker 1: stage one or two or three, when their eyes are
Speaker 1: not moving, and you say, quick, what were you just
Speaker 1: thinking about, they'll generally say nothing at all, there was
Speaker 1: nothing going on in my consciousness. Now, for completeness, I'll
Speaker 1: just flag that some people will point out there can
Speaker 1: be some sort of dreaming during non rem sleep, but
Speaker 1: this is a very different type of dreaming. When it happens,
Speaker 1: it's just a feeling of something like a simple thought,
Speaker 1: rather than a vivid experience like we typically think of
Speaker 1: with dreams with its whirl of color and activity and
Speaker 1: magnified emotions. Now, just a quick side note, some people
Speaker 1: say I don't dream at night, but in fact you do.
Speaker 1: Everybody does. It's just a matter of waking up at
Speaker 1: the right time. So if I snuck into your house
Speaker 1: and woke you up right when you were having rapid
Speaker 1: eye movement sleep. You'd say, WHOA, I was just dreaming.
Speaker 1: But what often happens is that we enjoy a round
Speaker 1: of dream sleep and then we sink back into a
Speaker 1: deeper sleep, so by the time we wake up, we
Speaker 1: don't remember the dream. So some people get up in
Speaker 1: the morning and they're convinced that they didn't dream at all,
Speaker 1: while in fact they did, they simply don't remember it. Now,
Speaker 1: the question I want to address is why we dream?
Speaker 1: And the first clue comes from a simple observation that
Speaker 1: this is not just a human thing, but something about
Speaker 1: brains in general. You've seen your dog have dreams where
Speaker 1: she kicks her legs around and barks like she's chasing
Speaker 1: a rabbit. But it turns out that all mammals dream,
Speaker 1: and all birds and reptiles they all exhibit rem sleep.
Speaker 1: Even fish have a form of rem sleep. So why
Speaker 1: is this so conserved across the whole animal kingdom. While
Speaker 1: there are various ideas that have been proposed for why
Speaker 1: we dream, some researchers point out that dream sleep seems
Speaker 1: to be important for memory consolidation, which means nailing down
Speaker 1: the memories that you take in during the day. So
Speaker 1: you run around during the day with your eyes open
Speaker 1: and you experience all kinds of new things.
Speaker 2: And the idea is that when you sleep and.
Speaker 1: Dream, you are nailing that down and taking out the
Speaker 1: neural trash in a way that's necessary and useful for
Speaker 1: locking down the memories.
Speaker 2: Another popular idea has been that.
Speaker 1: Dreams help us solve problems because the brain is able
Speaker 1: to process information and make connections that are not possible
Speaker 1: during waking hours. So the idea is that the brain
Speaker 1: is in a more relaxed state during dream sleep. It's
Speaker 1: not as focused on the external world, and this allows
Speaker 1: it to focus on internal thoughts and ideas, which can
Speaker 1: lead to new insights and solutions to problems. And in fact,
Speaker 1: studies have shown that if you are trying to solve
Speaker 1: a problem and you go to sleep, you're more likely
Speaker 1: to solve it when you wake up than if you're
Speaker 1: just thinking about it during the daytime. And if you're
Speaker 1: deprived of dream sleep, you have more difficulty solving problems.
Speaker 1: But of course this might be related to problems of
Speaker 1: sleep in general. Others suggest that rem sleep is involved
Speaker 1: more generally in creativity, and others talk about its role
Speaker 1: in emotional processing. And finally, several thinkers have suggested that
Speaker 1: dreams help us prepare for new situations like fighting or
Speaker 1: escaping from situations, because these are things that we experience
Speaker 1: in real life very rarely, and so the idea is
Speaker 1: that dreams give us practice at these rare situations.
Speaker 2: They keep the wheels greased.
Speaker 1: And I just want to be clear that these are
Speaker 1: all hypotheses for why we dream, and they're not exclusive.
Speaker 1: Dreams might serve multiple roles, so it's not as though
Speaker 1: one of these has to be right at the expense
Speaker 1: of others. But the first thing that I hope is
Speaker 1: clear is that there's a lot of speculation about dreams,
Speaker 1: but we don't really have a single theory that would
Speaker 1: qualify as the answer to why we dream. And certainly
Speaker 1: we don't have a theory that allows us to look
Speaker 1: across the animal kingdom to answer a different question, which
Speaker 1: is why do different animal species, even those who are
Speaker 1: closely related, dream different amounts. For example, if you look
Speaker 1: at the vervet monkey and it's total sleeping time, it
Speaker 1: only spends six percent of that sleep time in rem
Speaker 1: In other words, having dreams, whereas another monkey, the Reesus
Speaker 1: macaque monkey, spends eighteen percent of its time in rem sleep,
Speaker 1: three times as much, even though they're both primate species.
Speaker 1: So it's never been clear to me how any of
Speaker 1: the hypotheses previously proposed would account for any of this.
Speaker 1: But recently, my colleague and I proposed a very new
Speaker 1: kind of theory about dreaming, and it's one that I'm
Speaker 1: very excited about because it gives accurate quantity tative predictions
Speaker 1: across species, something that no other hypothesis even strives to do.
Speaker 1: But before I tell you about that, I need to
Speaker 1: lay some foundation. So let's start with the story of
Speaker 1: a young boy named Ronnie who was born blind. He
Speaker 1: was born in North Carolina, and when he was just
Speaker 1: past year old, his mother abandoned him. She said that
Speaker 1: his blindness was her punishment from God. So he ended
Speaker 1: up being raised in poverty by his grandparents until he
Speaker 1: was five, and then he was sent off to a
Speaker 1: school for the sightless. When he was six years old,
Speaker 1: his mother came by just once and she had another child,
Speaker 1: now a little girl, and his mother said, ron I
Speaker 1: want you to feel her eyes. You know, her eyes
Speaker 1: are so pretty she didn't shame me the way that
Speaker 1: you did. She can see, and that was the last
Speaker 1: time he ever had contact with his mother. So most
Speaker 1: of us can't even imagine a childhood this hard. But
Speaker 1: the silver lining became the fact that Ronnie had a
Speaker 1: gift for music. His instructors spotted this talent and he
Speaker 1: started to formally study classical music with the violin, and
Speaker 1: in no time, this kid was a virtuoso.
Speaker 2: And from there he went.
Speaker 1: On to master guitar and piano and several other string
Speaker 1: and woodwind instruments, and by the time he grew into
Speaker 1: young adulthood, he became one of the most popular performers
Speaker 1: of his day. His name was Ronnie Millsap, and you
Speaker 1: may or may not have heard of him, but in
Speaker 1: the seventies and eighties he dominated pop music and country
Speaker 1: western markets. He released thirty five country music hits at
Speaker 1: the number one slot, and he.
Speaker 2: Earned six Grammy Awards.
Speaker 1: Now you might think it's amazing that Ronnie Millsap could
Speaker 1: be blind and have such an amazing musical career, but
Speaker 1: this is actually not such an uncommon story. Think of
Speaker 1: Andrea about or Ray, Charles Stevie Wonder, or Diane Shore
Speaker 1: or Jose Feliciano or Jeff Healy. All of them are blind.
Speaker 1: For all of them, their brains learned to rely on
Speaker 1: the signals of sound and touch in their environment, and
Speaker 1: they became better at processing those signals then cited people.
Speaker 1: As I write about my book Live Wired, Musical stardom
Speaker 1: is not guaranteed for people who are blind, but brain
Speaker 1: reorganization is guaranteed because if a sense is not getting used,
Speaker 1: like vision, it gets taken over by neighboring senses. There's
Speaker 1: nothing special about the cells, the neurons in the visual
Speaker 1: cortex at the back of your brain. They are simply
Speaker 1: neurons that happened to be involved in processing edges or
Speaker 1: colors for people who have functioning eyes. But if you
Speaker 1: go blind, these exact same neurons can process other types
Speaker 1: of information, So the territory gets redeployed. It gets taken
Speaker 1: over by hearing and by touch, and you get better
Speaker 1: at those other senses. For Ronnie Millsap, his visual cortex
Speaker 1: was not getting used because his eyes were not functioning,
Speaker 1: so these other brain areas took over and he got
Speaker 1: better at those as a result. Perfect musical pitch, for example,
Speaker 1: is much more common in the blind population, and blind
Speaker 1: people are up to ten times better at determining whether
Speaker 1: a musical pitch is subtly wobbling up and down.
Speaker 2: Why, it's just because.
Speaker 1: They have more brain territory devoted to the task of listening.
Speaker 1: There was a recent experiment in which people who were
Speaker 1: sighted or blind had one ear plugged up, and then
Speaker 1: they were asked to point to the locations of sounds
Speaker 1: in the room and pinpointing where a sound is coming
Speaker 1: from normally requires a comparison of the signals at both years,
Speaker 1: So it was expected that everyone would fail miserably at
Speaker 1: this task, and that's what happened with the people who
Speaker 1: could see. But for the blind participants, they were able
Speaker 1: to generally tell where the sounds were positioned. How it's
Speaker 1: because the exact shape of the cartilage of the outer ear,
Speaker 1: even just one ear, it bounces sound around in subtle
Speaker 1: ways that gives clues to location, but only if one
Speaker 1: is highly attuned to pick up on those very subtle signals.
Speaker 1: So people with sight, they have less cortex devoted to sound,
Speaker 1: and so their ability to extract subtle sound information it's underdeveloped,
Speaker 1: But with blind people that skill gets developed. And this
Speaker 1: sort of extreme talent with sound, this is common among
Speaker 1: the blind. Take a young man named Ben Underwood. When
Speaker 1: he was two years old, Ben stopped seeing out of
Speaker 1: his left eye and his mother took him to the
Speaker 1: doctor and they soon discovered that he had retinal cancer
Speaker 1: that was in both eyes. So they tried chemotherapy, they
Speaker 1: tried radiation, but that didn't work, and finally the surgeons
Speaker 1: had to remove both of his eyes. And you can
Speaker 1: imagine the pain that the family went through here. But
Speaker 1: by the time Ben was seven years old, he devised
Speaker 1: a technique that was totally unexpected and unbelievably useful, which
Speaker 1: is that he would click with his mouth and he
Speaker 1: would listen for the returning echoes. And in this way
Speaker 1: he could hear the locations of an open doorway, or
Speaker 1: of a person, or of a parked car, or a
Speaker 1: garbage can and so on. He was echo locating. He
Speaker 1: was bouncing his sound waves off objects in the environment,
Speaker 1: and he was listening to what returned. Now, I saw
Speaker 1: a documentary about Ben a little while ago, and it
Speaker 1: kicked off with the statement that Ben was quote the
Speaker 1: only person in the world who can see with echolocation. Now,
Speaker 1: first of all, we don't really know if he's seeing
Speaker 1: in the same way that you and I might.
Speaker 2: Think of site.
Speaker 1: But much more importantly, Ben was not the only one
Speaker 1: using echolocation. Thousands of blind people do this. In fact,
Speaker 1: the phenomenon has been discussed since at least the nineteen forties,
Speaker 1: when the word echolocation was first coined in an article
Speaker 1: in the journal Science, and this was titled Echolocation by
Speaker 1: Blind Men, bat and Radar. The author wrote, quote, many
Speaker 1: blind persons develop, in the course of time a considerable
Speaker 1: ability to avoid obstacle by means of auditory cues received
Speaker 1: from sounds of their own making. And this included clicking
Speaker 1: or their own footsteps, or cane tapping or finger snapping.
Speaker 1: So the author demonstrated that their ability to echo locate
Speaker 1: was drastically reduced if you put in distracting noises or
Speaker 1: put ear plugs in them. Anyway, the general story is straightforward.
Speaker 1: If a sense is not getting used, it gets taken
Speaker 1: over by neighboring senses. So, for example, if you're blind,
Speaker 1: the territory gets redeployed to hearing and to touch. There's
Speaker 1: nothing special about the neurons and the visual cortex. They
Speaker 1: just happen to be involved in processing vision if you
Speaker 1: have functioning eyes, but if you go blind, these exact
Speaker 1: same neurons can process other types of information. Now, in
Speaker 1: recent decades, there have been thousands of papers demonstrating brain plasticity,
Speaker 1: that is, the brain's ability to reconfigure and adjust its
Speaker 1: own circuitry, And in my book Livewireed, I attempt to
Speaker 1: build up frameworks to surface the big lessons from these papers.
Speaker 1: But to my mind, the biggest surprise about brain plasticity
Speaker 1: is its speed. So some years ago, researchers at McGill
Speaker 1: University took several adults who had recently lost their site,
Speaker 1: and they put them into a brain scanner and the
Speaker 1: participants were asked to listen to sounds. Now, not surprisingly,
Speaker 1: the sounds caused activity in their auditory cortex, but the
Speaker 1: sounds also caused activity in their occipital core.
Speaker 2: Text that's at the back of the brain.
Speaker 1: It's normally what we would think of as visual cortex,
Speaker 1: and that activity would not have been seen there even
Speaker 1: a few weeks earlier when the participants had sight. Now,
Speaker 1: the activity wasn't as strong as that scene in people
Speaker 1: who have been blowing for a long time. But it
Speaker 1: was detectable in the occipital cortex nonetheless, and this demonstrated
Speaker 1: that the brain can implement changes rapidly when vision disappears,
Speaker 1: but how rapidly so. Next, my colleagues at Harvard, led
Speaker 1: by Alvaro pascal Leone, began to wonder about the speed
Speaker 1: at which these major takeovers can happen, and they noted
Speaker 1: that instructors at a school for the blind were required
Speaker 1: to blindfold themselves for seven full days so that they
Speaker 1: could gain a first hand understanding of their students' living experiences.
Speaker 1: So when these cited instructors blindfolded themselves, they became aware
Speaker 1: of enhanced skills with sounds. They could orient to things better,
Speaker 1: and they could judge their distance, and they could identify things.
Speaker 1: Several described identifying people more rapidly and accurately just as
Speaker 1: they started talking, or even just given the cadence of
Speaker 1: their footsteps, And the instructions learned new things like how
Speaker 1: to differentiate cars just by the sounds of their motors.
Speaker 1: So this god pascal Leone and his colleagues, considering what
Speaker 1: would happen if a sighted person were blindfolded in.
Speaker 2: The laboratory for several days.
Speaker 1: So they launched this experiment and what they found was
Speaker 1: nothing short of remarkable. They discovered that when you were
Speaker 1: temporarily blinded, there was neural reorganization, just like we see
Speaker 1: in blind subjects, and it was rapid. In one of
Speaker 1: their studies, people who could see normally were blindfolded for
Speaker 1: five days and they were put through intensive braille training,
Speaker 1: and at the end of the five days, the subjects
Speaker 1: had become quite good at detecting the subtle differences between
Speaker 1: braille characters, much better than a control group of cited
Speaker 1: participants who had the same training without the blindfold. But
Speaker 1: what was especially striking was what happened to their brains
Speaker 1: when you measured them in the scanner. Within fire five days,
Speaker 1: the blindfolded participants had recruited their occipital cortex when they
Speaker 1: were touching objects. So control subjects, not surprisingly used only
Speaker 1: a different part of their brain called the somatisensory cortex,
Speaker 1: and the blindfolded subjects they were also showing these occipital
Speaker 1: responses to sounds and words.
Speaker 2: And by the way, you.
Speaker 1: Could disrupt this new occipital lobe activity by magnetic pulses,
Speaker 1: and then the braille reading advantage of the blindfolded subjects
Speaker 1: went away, so that indicates the recruitment of the brain
Speaker 1: area was not an accidental side effect, but this was
Speaker 1: a critical piece of the improved behavioral performance. And importantly,
Speaker 1: because of the plasticity of the brain, when the blindfold
Speaker 1: got removed, the response of the occipital.
Speaker 2: Cortext to touch or sound.
Speaker 1: That disappeared within a day, and at that point the participants'
Speaker 1: brains returned to looking indistinguishable from every other sighted person
Speaker 1: out there. Now here's the key study that really influenced me.
Speaker 1: These same investigators very carefully mapped out the brain using
Speaker 1: more powerful neuroimaging techniques. So volunteers were blindfolded really tightly,
Speaker 1: and they were put in the scanner and they were
Speaker 1: asked to perform a touching task that required really fine
Speaker 1: discrimination with their fingers. And what these investigators saw was
Speaker 1: activity emerging in the primary visual cortex the occipital lobe
Speaker 1: after an hour. And the shock of these findings was
Speaker 1: their sheer speed. So the reorganization of territory that brains do,
Speaker 1: it's not like the glacial drifting of continental plates, but
Speaker 1: it can be remarkably fast. The brain is always sprung
Speaker 1: tight like a mouse trap to implement rapid change.
Speaker 2: So the key is that the brain's changes.
Speaker 1: Are even fast sure than even the most optimistic neuroscientist
Speaker 1: would have dared to guess at the beginning of this century.
Speaker 1: So let's zoom back out to the bigger picture. So
Speaker 1: for survival you need things like sharp teeth and fast legs.
Speaker 1: You also need neural flexibility. This is what allows brains
Speaker 1: to optimize their performance in a variety of environments. But
Speaker 1: the competition in the brain has a potential downside as well,
Speaker 1: which is this, whenever there's an imbalance of activity in
Speaker 1: the senses, a potential takeover can happen, and that can
Speaker 1: happen really rapidly, So a redistribution of the resources that
Speaker 1: can be really useful when a limb has been lost
Speaker 1: or a sense has been lost. But the rapid conquest
Speaker 1: of territory, you might have to actively counterbalance this in
Speaker 1: other scenarios. And this consideration led me and my former
Speaker 1: student Dawn Vaughan to propose a new theory for what
Speaker 1: happens to brains in the dark of the night. So
Speaker 1: now we're back to the main question of this episode.
Speaker 1: What does dreaming have to do with the rotation of
Speaker 1: the planet. And this is one of the unsolved mysteries
Speaker 1: in neurosciences.
Speaker 2: Why brains dream?
Speaker 1: What these bizarre nighttime hallucinations are about.
Speaker 2: Do they have meaning?
Speaker 1: Are they simply random neural activity in search of a
Speaker 1: coherent narrative? And why are dreams so richly visual igniting
Speaker 1: the occipital cortex every night in this conflagration of activity.
Speaker 2: So here's our idea.
Speaker 1: In the chronic and unforgiving competition for brain real estate,
Speaker 1: the visual system has a unique problem to deal with.
Speaker 1: Because of the rotation of the planet, we are cast
Speaker 1: into darkness for an average of twelve hours every cycle.
Speaker 1: And obviously, I'm referring to ninety nine point nine nine
Speaker 1: nine percent of our species evolutionary history. I'm not talking
Speaker 1: about the current electricity blessed times. So it used to
Speaker 1: be really, really dark at night. And I just told
Speaker 1: you about how sensory deprivation triggers neighboring territories in the
Speaker 1: brain to take over. So how does the visual system
Speaker 1: deal with this unfair disadvantage? And we suggest that it
Speaker 1: does so by keeping the occipital cortex active during the night.
Speaker 1: We suggest that dreaming exists to keep the visual cortex
Speaker 1: from being taken over by neighboring areas. Because the rotation
Speaker 1: of the planet doesn't affect your ability to touch and
Speaker 1: hear and taste and smell, only vision suffers in the dark,
Speaker 1: and as a result, the visual cortex finds itself in danger.
Speaker 1: Every night of takeover by the other sense is just
Speaker 1: like with the blindfolded subjects, and given the amazing speed
Speaker 1: with which these changes in territory can happen, this is
Speaker 1: a real threat. So dreams are the means by which
Speaker 1: the visual cortex prevents takeover. So to dig into this
Speaker 1: idea a little more, let's zoom out. Although a sleeper
Speaker 1: looks relaxed and shut down, the brain is fully electrically active,
Speaker 1: so during most of the night there's no dreaming. But
Speaker 1: during rem sleep there's a lot of things that happen.
Speaker 1: So the heart rate and the breathing speed up, your
Speaker 1: small muscles twitch, and your brain waves become smaller and faster,
Speaker 1: and dreaming happens.
Speaker 2: Now.
Speaker 1: REM sleep is triggered by a particular set of neurons
Speaker 1: in the brain stem in a structure called the ponds,
Speaker 1: and that travels to a small nucleus in the thalamus,
Speaker 1: and from there, these waves of electrical activity come banging
Speaker 1: into the occipital cortex at the back of your head.
Speaker 1: Now that's the area of your brain where your visual
Speaker 1: system is. So when these visual areas become alive with
Speaker 1: activity in their cells, that is experienced as visual we see.
Speaker 1: And that's why dreams are pictorial like a film. If
Speaker 1: the activity we're banging into a part of the cortex
Speaker 1: involved in smell, then dreams would just be a smell story,
Speaker 1: but it hits the visual area, and so we find
Speaker 1: ourselves thrown into a movie. Now, if you're seeing all
Speaker 1: kinds of stuff, you might wonder why you're not reacting
Speaker 1: to that with your body, And that's because the circuitry
Speaker 1: involved in dreaming also paralyzes your major muscle groups so
Speaker 1: that you don't act out your dream. You shut down
Speaker 1: your muscles so that you can simulate world experience without
Speaker 1: actually moving your body around, and that combination crafts the
Speaker 1: experience of dreaming. The electrical waves slamming into the occipital
Speaker 1: cortex make your visual system alive with activity, and the
Speaker 1: muscular paralysis keeps you from acting out the dreams. Now,
Speaker 1: we theorize that the circuitry behind visual dreams is not accidental. Instead,
Speaker 1: to prevent takeover, the visual system is forced to fight
Speaker 1: for its territory by generating these short bursts of activity
Speaker 1: every ninety minutes or so when the planet rotates into darkness.
Speaker 1: It's a self defense system that evolved in the face
Speaker 1: of constant competition for sensory real estate. Dreams are a
Speaker 1: screen saver. So the idea is that vision carries mission
Speaker 1: critical information for the brain, but vision is stolen away
Speaker 1: for half of our hours. It's like we're blindfolded for
Speaker 1: half our time here on Earth. So dreams, we say, suggest,
Speaker 1: are the strange love child of neural plasticity and the
Speaker 1: rotation of the planet. Now, one key point to appreciate
Speaker 1: is that these nighttime volleys of activity are very anatomically precise.
Speaker 1: They start in the brain stem and they end up
Speaker 1: in only one place, the occipital cortex. If the circuitry
Speaker 1: sort of randomly grew its branches, we'd expect it to
Speaker 1: connect with all kinds of areas in the brain, but
Speaker 1: it doesn't. It aims with anatomical precision at one area alone,
Speaker 1: a tiny structure called the lateral geniculate nucleus which broadcasts
Speaker 1: specifically to the occipital cortex and through a neuroanatomous lens.
Speaker 1: This is really specific circuitry and that suggests an important role,
Speaker 1: and we suggest that role is defense of the visual system.
Speaker 1: So we call this the defensive activation theory. Now, I
Speaker 1: want to address a question that might be coming up
Speaker 1: for you, which is what about dream content? Why did
Speaker 1: dreams seem to be about something rather than just random
Speaker 1: dots of light? Well, the important thing to understand is
Speaker 1: that the brain is a natural storyteller. When there's activity
Speaker 1: in there, it shapes that into a story of what
Speaker 1: it's seeing. For those of you who know about latent
Speaker 1: diffusion models in AI, that's exactly the same thing. So
Speaker 1: Dolly two and stable diffusion. These are image generators, and
Speaker 1: they work by starting with random activity and that coheres
Speaker 1: into a picture of something in exactly the same way.
Speaker 1: The brain can't see random activity. It has to wrap
Speaker 1: that into something particular that it is seeing. Now, why
Speaker 1: are dreams a story instead of just a picture? The
Speaker 1: key is that everything in the brain is linked by association.
Speaker 1: So when you think of a rabbit, that's linked with
Speaker 1: everything you've ever associated with. Rabbits carrots and shadow puppets
Speaker 1: and the velveteen rabbit and Easter and Alice in Wonderland,
Speaker 1: and maybe a French restaurant that serves rabbit and Roger Rabbit.
Speaker 2: And so on.
Speaker 1: This is how an associative neural network is structured. Everything
Speaker 1: is linked by association. So what happens during dreams is
Speaker 1: that this random activity gets shot into the visual system
Speaker 1: and synapses that are hot from the day will tend
Speaker 1: to get activated again. But from there the activity will
Speaker 1: tend to drift along these associative pathways, and that's why
Speaker 1: dreams seem to have a unifying thread, but they're also
Speaker 1: characterized by bizarreness. The storyline drifts from thing to thing
Speaker 1: in a way that's not quite like the real world
Speaker 1: because it's activity that's moving through this associative neural network,
Speaker 1: and we experience whatever is getting triggered in whatever world,
Speaker 1: So it's tied to our experience from the day and
Speaker 1: of the world we know. But it's a very loose
Speaker 1: sort of story, and the brain is a natural storyteller,
Speaker 1: so things get tied together as best they can, and
Speaker 1: we shouldn't overlook the fact that we are storytellers.
Speaker 2: Even after we wake up.
Speaker 1: So when you tell your spouse or your friend, wow,
Speaker 1: this happened, and then this happened, we can't help but
Speaker 1: impose a narrative over the images we saw. And so
Speaker 1: sometimes the series of images we experienced gets even a
Speaker 1: stronger storyline put on top of it. And by the way,
Speaker 1: I just want to mention one other thing. You might
Speaker 1: wonder how it makes sense that sometimes you hear sounds
Speaker 1: or feel touch or have a smell in a dream
Speaker 1: if the activity is only going into the visual cortex, well,
Speaker 1: that seems to happen sometimes. And that's because although the
Speaker 1: activity is only going in the visual system, it can
Speaker 1: cascade out and keep going to other parts of the brain.
Speaker 1: Everything is connected to everything else with pretty short pathways.
Speaker 1: But it's important to note that dreams are almost entirely
Speaker 1: visual because that's the only place where the activity is
Speaker 1: getting injected. Now, something you might wonder, given this defensive
Speaker 1: activation theory.
Speaker 2: That I've described so far, what about blind people?
Speaker 1: Do they have dreams or do you think they have
Speaker 1: no dreaming at all because their brains don't care about
Speaker 1: the light in the dark. The answer is that people
Speaker 1: who are blind, of course, they have dreams. But if
Speaker 1: they've been blind from birth or from a very young age,
Speaker 1: they have no visual experience in their dreams because their
Speaker 1: visual system was taken over by other senses like hearing
Speaker 1: in touch, and so they have those sensory experiences in
Speaker 1: their dreams because the activity is still going into the
Speaker 1: occipital lobe. It's just that that's no longer visual. So
Speaker 1: their dream is something like, I was feeling my way
Speaker 1: around my living room, but it was weird because someone
Speaker 1: had rearranged all the furniture.
Speaker 2: And then I felt something strange in the corner.
Speaker 1: And I realized it was a bear, and I ran
Speaker 1: and I could hear it behind me, and so on
Speaker 1: this sort of thing. All their experiences in the dream
Speaker 1: involved sound and touch, but not sight. In those born blind,
Speaker 1: you still have these volleys of spikes blasting into the
Speaker 1: back of the brain because that's where the circuitry is going,
Speaker 1: but that part of the brain is no longer visual,
Speaker 1: and their experiences are not visual. And this tells us
Speaker 1: that the circuitry underlying dreaming it's very basic, low level circuitry.
Speaker 1: It's not dependent on the experiences you have during your lifetime,
Speaker 1: and the fundamental nature of this circuitry is also consistent
Speaker 1: with the fact that we find it conserved across the
Speaker 1: animal kingdom. Now, like any scientific idea, the defensive activation
Speaker 1: theory could be correct or could not be.
Speaker 2: So how would we know.
Speaker 1: Well, we can start looking at the predictions that come
Speaker 1: out of this hypothesis. First is just a general observation,
Speaker 1: which is that you can look at the fall off
Speaker 1: in REM's sleep with age. So the fraction of our
Speaker 1: sleeping time that we spend in REM steadily decreases as
Speaker 1: we get older. So as an infant you spend half
Speaker 1: your sleeping time in REM, and as an adult you
Speaker 1: spend only ten to twenty percent of sleep time in REM,
Speaker 1: and when you're elderly you spend even less. And this
Speaker 1: is consistent with the fact that infant's brains are much
Speaker 1: more plastic, and so the competition for territory is really intense,
Speaker 1: and as you get older, things settle into place and
Speaker 1: cortical takeovers are harder to do. So the fall off
Speaker 1: in plasticity parallels the fall off of time that you
Speaker 1: spend in REM sleep. And by the way, this fall
Speaker 1: off in REM is seen across species, so puppies and
Speaker 1: kittens and every kind of baby has more rem sleep. Now,
Speaker 1: this observation by itself isn't proof of anything, but it's
Speaker 1: an interesting correlation.
Speaker 2: But could we look.
Speaker 1: Across species to see if we can make meaningful predictions
Speaker 1: about which species dreams a lot and which a little.
Speaker 1: In other words, how much time each species spends in
Speaker 1: dream time. So the idea is that for a brain
Speaker 1: that is born with a lot of plasticity, a lot
Speaker 1: of flexibility, you need to keep the visual system well
Speaker 1: protected at night. But some animals are born with a
Speaker 1: lot less plasticity, Their brains are more ready to go,
Speaker 1: and so the need to have this defensive activation at
Speaker 1: night would be less. Take primate like the vervet monkey.
Speaker 1: Within three weeks, it learns how to walk, and it
Speaker 1: stops weaning within four months, and it reaches adolescens in
Speaker 1: four years and it can reproduce. Now, look at a
Speaker 1: baby human. We're primates also, But in contrast to the
Speaker 1: vervet monkey, the human primate doesn't walk for a year,
Speaker 1: and it doesn't wean until three years, and it doesn't
Speaker 1: reach adolescens for thirteen years. Why it's because human brains
Speaker 1: drop into the world half baked, and we're incredibly flexible.
Speaker 1: That's how we absorb the language and culture and the
Speaker 1: knowledge around us. We're super flexible, and the consequence is
Speaker 1: that we have an unusually long childhood. But other animals
Speaker 1: arrive more let's call it pre programmed, and they're just
Speaker 1: following more basic instructions of eat, mate, run, approach and
Speaker 1: so on. There's no vervet monkey culture to absorb. They
Speaker 1: don't go to vervet monkey schools so that they can
Speaker 1: learn about the discoveries of other monkeys before them so
Speaker 1: they can springboard to the next steps. Instead, they live
Speaker 1: essentially the same life as all the generations before them.
Speaker 1: So different species, even closely related primate species, can have
Speaker 1: very different levels of plasticity. And the question is how
Speaker 1: does this translate to the amount of dreaming they do
Speaker 1: each night. And our hypothesis is that the more plastic
Speaker 1: species need more dreaming at night to make sure that
Speaker 1: big changes don't happen and the visual system doesn't get
Speaker 1: taken over.
Speaker 2: If you are a less plastic.
Speaker 1: Species, the brain is essentially more fixed into place and
Speaker 1: there's less risk of takeover of the visual system in
Speaker 1: the darkness. So we studied twenty five primate species and
Speaker 1: we research the plasticity of their brain, or at least
Speaker 1: correlates of plasticity, like how long it takes for them
Speaker 1: to walk or to wean from their mothers, or how
Speaker 1: long until they reach adolescence. And we also research the
Speaker 1: percentage of their sleep time that each species spends in
Speaker 1: REM sleep. Typically, this is measured by setting up infrared
Speaker 1: cameras and watching the animals sleep through the night and
Speaker 1: figuring out what percentage of their sleep time they have
Speaker 1: this rapid eye movement going on underneath their eyelids. And
Speaker 1: what's striking is that this varies pretty widely. So the
Speaker 1: vervet monkey spends six percent of its sleep time in
Speaker 1: REM and then you have a spider monkey spending seven percent,
Speaker 1: and a yellow babboon spending eight percent, and a barbary
Speaker 1: macaque monkey spending nine percent, all the way to a
Speaker 1: bornean orangutan spending twelve percent, to a chimpanzee spending sixteen percent,
Speaker 1: to a reeseus macaque monkey spending eighteen percent, to humans
Speaker 1: spending twenty one percent of their sleeping time in REM. Now,
Speaker 1: we compiled all this data and we found statistically significant
Speaker 1: correlations between plasticity and the amount of remsleep In other words,
Speaker 1: the less plastic an animal is, the less remsleep it
Speaker 1: has during the night, and animals with brains that are
Speaker 1: more plastic, whose brains have more territory shifting around, they
Speaker 1: have more rem sleep. And by the way, as a control,
Speaker 1: we gathered four other variables across these species, like weight
Speaker 1: and length and how many offspring they have and average lifespan.
Speaker 1: And as expected, all of those measures show no significant
Speaker 1: correlations with the amount of remsleep, but the measures of
Speaker 1: how plastic an animal was did correlate, And if you're
Speaker 1: interested in the details, you can read our scientific publication
Speaker 1: linked to the podcast website. Now, there are several ways
Speaker 1: to test this framework further. For example, what happens if
Speaker 1: somebody doesn't get the normal amount of dream sleep. Well,
Speaker 1: as it turns out, sleep can be suppressed by certain antidepressants.
Speaker 1: For the cognianty these are monoamine oxidase inhibitors and tricyclic antidepressants. Anyway,
Speaker 1: the defensive activation theory would predict that if you're not
Speaker 1: getting adequate rem sleep, you're going to have some sort
Speaker 1: of visual consequences, and so it's interesting that patients on
Speaker 1: these medications characteristically get blurry vision. Now this is typically
Speaker 1: marked up to dry eyes, but I want to note
Speaker 1: our alternative hypothesis here, which is that it might be
Speaker 1: related to more takeover of the visual cortex. I don't
Speaker 1: know for sure that this is true yet, but this
Speaker 1: is a direction the research is going to go. And
Speaker 1: also we can test across a huge variety of animal species,
Speaker 1: not just primates. For example, some mammals are born immature,
Speaker 1: meaning that they're unable to walk, or get food, or
Speaker 1: regulate their own temperature, or defend themselves. These are animals
Speaker 1: like humans and ferrets and platypuses. Other mammals are born mature,
Speaker 1: such as guinea pig or sheep or giraffe. They come
Speaker 1: out of the womb with teeth and fur and open
Speaker 1: eyes and ability to regulate their own temperature, and they
Speaker 1: walk within an hour of being born, and they eat
Speaker 1: solid food. So here's the important clue. As a general rule,
Speaker 1: the animals born immature have much more rem sleep, up
Speaker 1: to eight times as much, and this difference is especially
Speaker 1: clear in the first months of life. In our interpretation,
Speaker 1: when a highly plastic brain drops into the world, it
Speaker 1: needs to constantly fight to keep things balanced. But when
Speaker 1: a brain arrives mostly solidify, there's less need for it
Speaker 1: to engage in this nighttime fighting. I just want to
Speaker 1: mention as a caveat that there's likely to be many
Speaker 1: surprises here because an animal's sleeping and dreaming can be
Speaker 1: very different depending on lots of other things. For example,
Speaker 1: take the elephant. They have a really surprisingly small amount
Speaker 1: of rem sleep a few minutes at most, and it
Speaker 1: first blush, I thought this weighed against our hypothesis, But
Speaker 1: it turns out that elephants sleep very little, around two
Speaker 1: hours a night, and they have excellent night vision because
Speaker 1: of specializations in their retinas, and as a result, their
Speaker 1: visual corettix is active during almost all hours of the
Speaker 1: day and the night, and so that doesn't face the
Speaker 1: same threat of encroachment from the other senses. So the
Speaker 1: hypothesis predicts that elephants should have very little rems sleep.
Speaker 1: So stay tuned on the future of this hypothesis. But
Speaker 1: I wanted to take the chance to walk you through
Speaker 1: a few of the details about how you might think
Speaker 1: about a question like dreaming and come up with new
Speaker 1: frameworks and then test those. Now, one last idea to
Speaker 1: close this out, what does our defensive activation theory mean
Speaker 1: for aliens on other planets. In Live, wied I proposed
Speaker 1: a hypothesis that we won't actually be able to test
Speaker 1: until the very distant future when we discover life on
Speaker 1: other planets. Some planet, especially those that are orbiting red
Speaker 1: dwarf stars, become locked into place such that they always
Speaker 1: have the same surface facing their star. They have permanent
Speaker 1: day on one side and permanent night on the other.
Speaker 1: If life forms on that planet were to have plastic
Speaker 1: brains that are even vaguely similar to ours, the prediction
Speaker 1: would be that those on the daylight side of the
Speaker 1: planet might have vision like us, but they would not
Speaker 1: have dreams. They wouldn't need them because they never get
Speaker 1: plunged into darkness, and the same prediction would apply for
Speaker 1: very fast spinning planets. If their nighttime is shorter than
Speaker 1: the time of a sensory takeover in the brain, then
Speaker 1: they also wouldn't need dreams. So thousands of years from
Speaker 1: now we might finally know whether we dreamers are in
Speaker 1: the universal minority.
Speaker 2: That's all for this week.
Speaker 1: To find out more and to share your thoughts, head
Speaker 1: over to Eagleman dot com, Slash Podcasts, and you can
Speaker 1: also watch full episodes of Inner Cosmos on YouTube. Subscribe
Speaker 1: to my channel so you can follow along each week
Speaker 1: for new updates. I'd love to hear your questions, so
Speaker 1: please send those to podcasts at eagleman dot com and
Speaker 1: I will do a special episode where I answer questions.
Speaker 1: Until next time, I'm David Eagleman, signing off to you
Speaker 1: from the Inner Cosmos.