June 5, 2023 • 47 mins
Why are some of the best musicians blind? Can blind people learn how to echolocate, like a bat? What do your nightly dreams have to do with the rotation of the planet? Once we find alien life on other planets, should we expect that aliens have dreams at night? Find out why dreaming might be the strange lovechild of brain plasticity.
Mark as Played
Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:05):
What do your nightly dreams have to do with the
rotation of the planet. Why are so many musicians blind?
Can blind people learn how to echolocate like a bat?
Once we find alien life on other planets, should we
expect that aliens have dreams at night? Welcome to Inner
(00:27):
Cosmos with me, David Eagleman. I'm a neuroscientist and an
author at Stanford University, and I've spent my whole career
studying the intersection between how the brain works and how
we experience life. I sort of personally have a love
(00:52):
hate relationship with dreaming because we take it so seriously.
We find ourselves in some biziz situation that doesn't make
any sense, but while you are there, you buy it,
hook line and sinker. You are emotionally tossed around and
buffeted in the winds of situations that the moment after
(01:16):
you wake up you realize weren't real, and more importantly,
they typically make no sense at all. So why were
you so caught up in that situation? I've always called
dreaming sticking my head in the night blender, by which
I simply mean that I don't necessarily look forward to
it because I don't know where it's going to take me,
(01:37):
and I generally feel that all that emotional energy I
expend isn't that useful. But whatever your take on dreaming is,
it's one of those absolutely insanely bizarre facts about our
existence that we totally take for granted. You wake up
and you say, oh, wow, I was just having a
(01:58):
dance party with a pack of why monkeys. Or I
was just at work where I forgot to put on
pants and I was trying to hide behind my desk.
Or I was on a river cruise and begging a
person not to break up with me even though we're
not even dating. And I find this absolutely stunning that
it doesn't even bother us that we've just transitioned from
(02:21):
one reality to another, Like in one second, you went
from being in some bizarre situation to lying horizontal in
your bed, and we're so used to it that we
just say, oh, that was interesting. I just dreamed that
I was on a hang glider riding over Istanbul and
I was trying to figure out how not to crash
because there were a lot of pigeons swirling around me.
Speaker 2 (02:44):
And if there's somebody there, then maybe we tell them.
Speaker 1 (02:47):
Our dream and Otherwise, we just get up and we
brush our teeth and go about our day without even
giving a second thought about what just happened. We were
in one reality and totally emotional invested in it, and
then a moment later we said, hmm, I guess I'm
in this other reality now. Now this is a testament
(03:08):
to the human mind's ability to accept the absolutely amazing
and bizarre as something not even worth investigating.
Speaker 2 (03:17):
Think about it this way.
Speaker 1 (03:18):
Just imagine if dreams happened right in the middle of
your day. So you're on a walk, or you're sitting
at your desk or eating at a restaurant with friends,
and suddenly your reality morphs to a completely bizarre other reality,
and now you're on a street you've never seen before,
or you're flying, or you're falling from a building, or
(03:41):
you're being chased by a monster. You would be terrified
by this lack of cohesion in your reality, the fact
that you just flipped from one to another. And as
soon as you were back in this reality, you'd presumably
run to your doctor and say you just had this
bizarre hallucination and thought you were somewhere else or maybe
someone else, and you'd be terrified, But because we're so
(04:05):
used to dreaming, we wake up and we think, oh,
that was weird, and then we go.
Speaker 2 (04:09):
About our new reality.
Speaker 1 (04:11):
And in fact, one of the evergreen questions among philosophers
is whether we live in a simulation. And I'm going
to dive deep into that question in a different episode,
but for now, I'll just mention the question, which is,
how would you know if you are a brain in
a vat who is being stimulated in just the right
way to think that you're listening to a podcast and
(04:34):
seeing the world around you and eating.
Speaker 2 (04:36):
Delicious food and so on.
Speaker 1 (04:38):
Or the modern version of that is whether we are
living in a computer simulation. So I suggest the mere
existence of dreams is sufficient to prove that all this
could be a simulation, because dreams demonstrate to us so
clearly that we completely buy what ever reality we find
(05:01):
ourselves in, and when we find ourselves in another reality,
we say, oh, I guess that one wasn't real.
Speaker 2 (05:07):
I was fooled, but this is real.
Speaker 1 (05:09):
But let me leave that as a teaser for the
episode on whether we're living in a simulation and how
we'd know. For today, I want to dive into dreaming
in particular, and from the brain's point.
Speaker 2 (05:21):
Of view, why do we dream?
Speaker 1 (05:23):
Why does our consciousness go on these bizarre flights every
ninety minutes during the night. So as a neuroscientist, I've
always been fascinated by dreaming, by what.
Speaker 2 (05:34):
The heck this is all about.
Speaker 1 (05:35):
So in this episode, we're going to talk about the why,
and I'm going to tell you about some hypotheses that
people have proposed over the centuries, and then I'm going
to tell you about a new hypothesis that makes quantitative
predictions across animal species. So let's start with an interesting fact,
which is that all animals appear to have dream sleep,
(05:59):
also as REM sleep. REM is spelled r EM, and
it stands for rapid eye movement. So in this stage
of sleep, the eyes are darting back and forth under
the eyelids. And if you haven't really watched someone while
they're sleeping, you should in a respectful manner because you'll
(06:20):
see how amazing this is. Every ninety minutes or so,
their eyes start jiggling back and forth. Now here's the
cool part. If you wake them up right when this
is happening and you say, quick, what were you just
thinking about? They will tell you that they were just
riding a camel in a shopping mall, or they were
running from a pack of leprechauns, or they were flying
(06:42):
around their house or whatever. And this is how we know,
from years of experiments of waking people up at different
stages of sleep, this is how we know that rem
sleep is when dreams happen. Because if you wake someone
up during a deeper stage of sleep, what we call
stage one or two or three, when their eyes are
(07:03):
not moving, and you say, quick, what were you just
thinking about, they'll generally say nothing at all, there was
nothing going on in my consciousness. Now, for completeness, I'll
just flag that some people will point out there can
be some sort of dreaming during non rem sleep, but
this is a very different type of dreaming. When it happens,
(07:24):
it's just a feeling of something like a simple thought,
rather than a vivid experience like we typically think of
with dreams with its whirl of color and activity and
magnified emotions. Now, just a quick side note, some people
say I don't dream at night, but in fact you do.
(07:45):
Everybody does. It's just a matter of waking up at
the right time. So if I snuck into your house
and woke you up right when you were having rapid
eye movement sleep. You'd say, WHOA, I was just dreaming.
But what often happens is that we enjoy a round
of dream sleep and then we sink back into a
deeper sleep, so by the time we wake up, we
(08:07):
don't remember the dream. So some people get up in
the morning and they're convinced that they didn't dream at all,
while in fact they did, they simply don't remember it. Now,
the question I want to address is why we dream?
And the first clue comes from a simple observation that
this is not just a human thing, but something about
(08:29):
brains in general. You've seen your dog have dreams where
she kicks her legs around and barks like she's chasing
a rabbit. But it turns out that all mammals dream,
and all birds and reptiles they all exhibit rem sleep.
Even fish have a form of rem sleep. So why
is this so conserved across the whole animal kingdom. While
(08:51):
there are various ideas that have been proposed for why
we dream, some researchers point out that dream sleep seems
to be important for memory consolidation, which means nailing down
the memories that you take in during the day. So
you run around during the day with your eyes open
and you experience all kinds of new things.
Speaker 2 (09:14):
And the idea is that when you sleep and.
Speaker 1 (09:17):
Dream, you are nailing that down and taking out the
neural trash in a way that's necessary and useful for
locking down the memories.
Speaker 2 (09:27):
Another popular idea has been that.
Speaker 1 (09:29):
Dreams help us solve problems because the brain is able
to process information and make connections that are not possible
during waking hours. So the idea is that the brain
is in a more relaxed state during dream sleep. It's
not as focused on the external world, and this allows
it to focus on internal thoughts and ideas, which can
(09:50):
lead to new insights and solutions to problems. And in fact,
studies have shown that if you are trying to solve
a problem and you go to sleep, you're more likely
to solve it when you wake up than if you're
just thinking about it during the daytime. And if you're
deprived of dream sleep, you have more difficulty solving problems.
But of course this might be related to problems of
(10:13):
sleep in general. Others suggest that rem sleep is involved
more generally in creativity, and others talk about its role
in emotional processing. And finally, several thinkers have suggested that
dreams help us prepare for new situations like fighting or
escaping from situations, because these are things that we experience
(10:35):
in real life very rarely, and so the idea is
that dreams give us practice at these rare situations.
Speaker 2 (10:42):
They keep the wheels greased.
Speaker 1 (10:44):
And I just want to be clear that these are
all hypotheses for why we dream, and they're not exclusive.
Dreams might serve multiple roles, so it's not as though
one of these has to be right at the expense
of others. But the first thing that I hope is
clear is that there's a lot of speculation about dreams,
but we don't really have a single theory that would
qualify as the answer to why we dream. And certainly
(11:07):
we don't have a theory that allows us to look
across the animal kingdom to answer a different question, which
is why do different animal species, even those who are
closely related, dream different amounts. For example, if you look
at the vervet monkey and it's total sleeping time, it
(11:27):
only spends six percent of that sleep time in rem
In other words, having dreams, whereas another monkey, the Reesus
macaque monkey, spends eighteen percent of its time in rem sleep,
three times as much, even though they're both primate species.
So it's never been clear to me how any of
the hypotheses previously proposed would account for any of this.
(11:49):
But recently, my colleague and I proposed a very new
kind of theory about dreaming, and it's one that I'm
very excited about because it gives accurate quantity tative predictions
across species, something that no other hypothesis even strives to do.
But before I tell you about that, I need to
lay some foundation. So let's start with the story of
(12:11):
a young boy named Ronnie who was born blind. He
was born in North Carolina, and when he was just
past year old, his mother abandoned him. She said that
his blindness was her punishment from God. So he ended
up being raised in poverty by his grandparents until he
was five, and then he was sent off to a
(12:33):
school for the sightless. When he was six years old,
his mother came by just once and she had another child,
now a little girl, and his mother said, ron I
want you to feel her eyes. You know, her eyes
are so pretty she didn't shame me the way that
you did. She can see, and that was the last
(12:53):
time he ever had contact with his mother. So most
of us can't even imagine a childhood this hard. But
the silver lining became the fact that Ronnie had a
gift for music. His instructors spotted this talent and he
started to formally study classical music with the violin, and
in no time, this kid was a virtuoso.
Speaker 2 (13:15):
And from there he went.
Speaker 1 (13:16):
On to master guitar and piano and several other string
and woodwind instruments, and by the time he grew into
young adulthood, he became one of the most popular performers
of his day. His name was Ronnie Millsap, and you
may or may not have heard of him, but in
the seventies and eighties he dominated pop music and country
(13:38):
western markets. He released thirty five country music hits at
the number one slot, and he.
Speaker 2 (13:45):
Earned six Grammy Awards.
Speaker 1 (13:48):
Now you might think it's amazing that Ronnie Millsap could
be blind and have such an amazing musical career, but
this is actually not such an uncommon story. Think of
Andrea about or Ray, Charles Stevie Wonder, or Diane Shore
or Jose Feliciano or Jeff Healy. All of them are blind.
(14:11):
For all of them, their brains learned to rely on
the signals of sound and touch in their environment, and
they became better at processing those signals then cited people.
As I write about my book Live Wired, Musical stardom
is not guaranteed for people who are blind, but brain
reorganization is guaranteed because if a sense is not getting used,
(14:36):
like vision, it gets taken over by neighboring senses. There's
nothing special about the cells, the neurons in the visual
cortex at the back of your brain. They are simply
neurons that happened to be involved in processing edges or
colors for people who have functioning eyes. But if you
(14:57):
go blind, these exact same neurons can process other types
of information, So the territory gets redeployed. It gets taken
over by hearing and by touch, and you get better
at those other senses. For Ronnie Millsap, his visual cortex
was not getting used because his eyes were not functioning,
(15:19):
so these other brain areas took over and he got
better at those as a result. Perfect musical pitch, for example,
is much more common in the blind population, and blind
people are up to ten times better at determining whether
a musical pitch is subtly wobbling up and down.
Speaker 2 (15:40):
Why, it's just because.
Speaker 1 (15:41):
They have more brain territory devoted to the task of listening.
There was a recent experiment in which people who were
sighted or blind had one ear plugged up, and then
they were asked to point to the locations of sounds
in the room and pinpointing where a sound is coming
from normally requires a comparison of the signals at both years,
(16:05):
So it was expected that everyone would fail miserably at
this task, and that's what happened with the people who
could see. But for the blind participants, they were able
to generally tell where the sounds were positioned. How it's
because the exact shape of the cartilage of the outer ear,
even just one ear, it bounces sound around in subtle
(16:29):
ways that gives clues to location, but only if one
is highly attuned to pick up on those very subtle signals.
So people with sight, they have less cortex devoted to sound,
and so their ability to extract subtle sound information it's underdeveloped,
But with blind people that skill gets developed. And this
(16:53):
sort of extreme talent with sound, this is common among
the blind. Take a young man named Ben Underwood. When
he was two years old, Ben stopped seeing out of
his left eye and his mother took him to the
doctor and they soon discovered that he had retinal cancer
that was in both eyes. So they tried chemotherapy, they
(17:14):
tried radiation, but that didn't work, and finally the surgeons
had to remove both of his eyes. And you can
imagine the pain that the family went through here. But
by the time Ben was seven years old, he devised
a technique that was totally unexpected and unbelievably useful, which
is that he would click with his mouth and he
(17:37):
would listen for the returning echoes. And in this way
he could hear the locations of an open doorway, or
of a person, or of a parked car, or a
garbage can and so on. He was echo locating. He
was bouncing his sound waves off objects in the environment,
(17:57):
and he was listening to what returned. Now, I saw
a documentary about Ben a little while ago, and it
(18:20):
kicked off with the statement that Ben was quote the
only person in the world who can see with echolocation. Now,
first of all, we don't really know if he's seeing
in the same way that you and I might.
Speaker 2 (18:32):
Think of site.
Speaker 1 (18:33):
But much more importantly, Ben was not the only one
using echolocation. Thousands of blind people do this. In fact,
the phenomenon has been discussed since at least the nineteen forties,
when the word echolocation was first coined in an article
in the journal Science, and this was titled Echolocation by
(18:54):
Blind Men, bat and Radar. The author wrote, quote, many
blind persons develop, in the course of time a considerable
ability to avoid obstacle by means of auditory cues received
from sounds of their own making. And this included clicking
(19:15):
or their own footsteps, or cane tapping or finger snapping.
So the author demonstrated that their ability to echo locate
was drastically reduced if you put in distracting noises or
put ear plugs in them. Anyway, the general story is straightforward.
If a sense is not getting used, it gets taken
(19:36):
over by neighboring senses. So, for example, if you're blind,
the territory gets redeployed to hearing and to touch. There's
nothing special about the neurons and the visual cortex. They
just happen to be involved in processing vision if you
have functioning eyes, but if you go blind, these exact
same neurons can process other types of information. Now, in
(20:00):
recent decades, there have been thousands of papers demonstrating brain plasticity,
that is, the brain's ability to reconfigure and adjust its
own circuitry, And in my book Livewireed, I attempt to
build up frameworks to surface the big lessons from these papers.
But to my mind, the biggest surprise about brain plasticity
(20:20):
is its speed. So some years ago, researchers at McGill
University took several adults who had recently lost their site,
and they put them into a brain scanner and the
participants were asked to listen to sounds. Now, not surprisingly,
the sounds caused activity in their auditory cortex, but the
(20:40):
sounds also caused activity in their occipital core.
Speaker 2 (20:44):
Text that's at the back of the brain.
Speaker 1 (20:46):
It's normally what we would think of as visual cortex,
and that activity would not have been seen there even
a few weeks earlier when the participants had sight. Now,
the activity wasn't as strong as that scene in people
who have been blowing for a long time. But it
was detectable in the occipital cortex nonetheless, and this demonstrated
(21:07):
that the brain can implement changes rapidly when vision disappears,
but how rapidly so. Next, my colleagues at Harvard, led
by Alvaro pascal Leone, began to wonder about the speed
at which these major takeovers can happen, and they noted
that instructors at a school for the blind were required
(21:28):
to blindfold themselves for seven full days so that they
could gain a first hand understanding of their students' living experiences.
So when these cited instructors blindfolded themselves, they became aware
of enhanced skills with sounds. They could orient to things better,
and they could judge their distance, and they could identify things.
(21:50):
Several described identifying people more rapidly and accurately just as
they started talking, or even just given the cadence of
their footsteps, And the instructions learned new things like how
to differentiate cars just by the sounds of their motors.
So this god pascal Leone and his colleagues, considering what
(22:11):
would happen if a sighted person were blindfolded in.
Speaker 2 (22:14):
The laboratory for several days.
Speaker 1 (22:17):
So they launched this experiment and what they found was
nothing short of remarkable. They discovered that when you were
temporarily blinded, there was neural reorganization, just like we see
in blind subjects, and it was rapid. In one of
their studies, people who could see normally were blindfolded for
five days and they were put through intensive braille training,
(22:40):
and at the end of the five days, the subjects
had become quite good at detecting the subtle differences between
braille characters, much better than a control group of cited
participants who had the same training without the blindfold. But
what was especially striking was what happened to their brains
when you measured them in the scanner. Within fire five days,
(23:01):
the blindfolded participants had recruited their occipital cortex when they
were touching objects. So control subjects, not surprisingly used only
a different part of their brain called the somatisensory cortex,
and the blindfolded subjects they were also showing these occipital
responses to sounds and words.
Speaker 2 (23:22):
And by the way, you.
Speaker 1 (23:23):
Could disrupt this new occipital lobe activity by magnetic pulses,
and then the braille reading advantage of the blindfolded subjects
went away, so that indicates the recruitment of the brain
area was not an accidental side effect, but this was
a critical piece of the improved behavioral performance. And importantly,
(23:44):
because of the plasticity of the brain, when the blindfold
got removed, the response of the occipital.
Speaker 2 (23:51):
Cortext to touch or sound.
Speaker 1 (23:53):
That disappeared within a day, and at that point the participants'
brains returned to looking indistinguishable from every other sighted person
out there. Now here's the key study that really influenced me.
These same investigators very carefully mapped out the brain using
more powerful neuroimaging techniques. So volunteers were blindfolded really tightly,
(24:16):
and they were put in the scanner and they were
asked to perform a touching task that required really fine
discrimination with their fingers. And what these investigators saw was
activity emerging in the primary visual cortex the occipital lobe
after an hour. And the shock of these findings was
their sheer speed. So the reorganization of territory that brains do,
(24:41):
it's not like the glacial drifting of continental plates, but
it can be remarkably fast. The brain is always sprung
tight like a mouse trap to implement rapid change.
Speaker 2 (24:55):
So the key is that the brain's changes.
Speaker 1 (24:58):
Are even fast sure than even the most optimistic neuroscientist
would have dared to guess at the beginning of this century.
So let's zoom back out to the bigger picture. So
for survival you need things like sharp teeth and fast legs.
You also need neural flexibility. This is what allows brains
(25:19):
to optimize their performance in a variety of environments. But
the competition in the brain has a potential downside as well,
which is this, whenever there's an imbalance of activity in
the senses, a potential takeover can happen, and that can
happen really rapidly, So a redistribution of the resources that
(25:41):
can be really useful when a limb has been lost
or a sense has been lost. But the rapid conquest
of territory, you might have to actively counterbalance this in
other scenarios. And this consideration led me and my former
student Dawn Vaughan to propose a new theory for what
(26:04):
happens to brains in the dark of the night. So
now we're back to the main question of this episode.
What does dreaming have to do with the rotation of
the planet. And this is one of the unsolved mysteries
in neurosciences.
Speaker 2 (26:19):
Why brains dream?
Speaker 1 (26:21):
What these bizarre nighttime hallucinations are about.
Speaker 2 (26:25):
Do they have meaning?
Speaker 1 (26:26):
Are they simply random neural activity in search of a
coherent narrative? And why are dreams so richly visual igniting
the occipital cortex every night in this conflagration of activity.
Speaker 2 (26:40):
So here's our idea.
Speaker 1 (26:42):
In the chronic and unforgiving competition for brain real estate,
the visual system has a unique problem to deal with.
Because of the rotation of the planet, we are cast
into darkness for an average of twelve hours every cycle.
And obviously, I'm referring to ninety nine point nine nine
(27:03):
nine percent of our species evolutionary history. I'm not talking
about the current electricity blessed times. So it used to
be really, really dark at night. And I just told
you about how sensory deprivation triggers neighboring territories in the
brain to take over. So how does the visual system
(27:23):
deal with this unfair disadvantage? And we suggest that it
does so by keeping the occipital cortex active during the night.
We suggest that dreaming exists to keep the visual cortex
from being taken over by neighboring areas. Because the rotation
(27:44):
of the planet doesn't affect your ability to touch and
hear and taste and smell, only vision suffers in the dark,
and as a result, the visual cortex finds itself in danger.
Every night of takeover by the other sense is just
like with the blindfolded subjects, and given the amazing speed
with which these changes in territory can happen, this is
(28:09):
a real threat. So dreams are the means by which
the visual cortex prevents takeover. So to dig into this
idea a little more, let's zoom out. Although a sleeper
looks relaxed and shut down, the brain is fully electrically active,
(28:31):
so during most of the night there's no dreaming. But
during rem sleep there's a lot of things that happen.
So the heart rate and the breathing speed up, your
small muscles twitch, and your brain waves become smaller and faster,
and dreaming happens.
Speaker 2 (28:48):
Now.
Speaker 1 (28:49):
REM sleep is triggered by a particular set of neurons
in the brain stem in a structure called the ponds,
and that travels to a small nucleus in the thalamus,
and from there, these waves of electrical activity come banging
into the occipital cortex at the back of your head.
Now that's the area of your brain where your visual
system is. So when these visual areas become alive with
(29:12):
activity in their cells, that is experienced as visual we see.
And that's why dreams are pictorial like a film. If
the activity we're banging into a part of the cortex
involved in smell, then dreams would just be a smell story,
but it hits the visual area, and so we find
ourselves thrown into a movie. Now, if you're seeing all
(29:36):
kinds of stuff, you might wonder why you're not reacting
to that with your body, And that's because the circuitry
involved in dreaming also paralyzes your major muscle groups so
that you don't act out your dream. You shut down
your muscles so that you can simulate world experience without
actually moving your body around, and that combination crafts the
(29:58):
experience of dreaming. The electrical waves slamming into the occipital
cortex make your visual system alive with activity, and the
muscular paralysis keeps you from acting out the dreams. Now,
we theorize that the circuitry behind visual dreams is not accidental. Instead,
(30:19):
to prevent takeover, the visual system is forced to fight
for its territory by generating these short bursts of activity
every ninety minutes or so when the planet rotates into darkness.
It's a self defense system that evolved in the face
of constant competition for sensory real estate. Dreams are a
(30:43):
screen saver. So the idea is that vision carries mission
critical information for the brain, but vision is stolen away
for half of our hours. It's like we're blindfolded for
half our time here on Earth. So dreams, we say, suggest,
are the strange love child of neural plasticity and the
(31:06):
rotation of the planet. Now, one key point to appreciate
is that these nighttime volleys of activity are very anatomically precise.
They start in the brain stem and they end up
in only one place, the occipital cortex. If the circuitry
sort of randomly grew its branches, we'd expect it to
(31:26):
connect with all kinds of areas in the brain, but
it doesn't. It aims with anatomical precision at one area alone,
a tiny structure called the lateral geniculate nucleus which broadcasts
specifically to the occipital cortex and through a neuroanatomous lens.
This is really specific circuitry and that suggests an important role,
(31:50):
and we suggest that role is defense of the visual system.
So we call this the defensive activation theory. Now, I
want to address a question that might be coming up
for you, which is what about dream content? Why did
dreams seem to be about something rather than just random
dots of light? Well, the important thing to understand is
(32:12):
that the brain is a natural storyteller. When there's activity
in there, it shapes that into a story of what
it's seeing. For those of you who know about latent
diffusion models in AI, that's exactly the same thing. So
Dolly two and stable diffusion. These are image generators, and
(32:32):
they work by starting with random activity and that coheres
into a picture of something in exactly the same way.
The brain can't see random activity. It has to wrap
that into something particular that it is seeing. Now, why
are dreams a story instead of just a picture? The
(32:54):
key is that everything in the brain is linked by association.
So when you think of a rabbit, that's linked with
everything you've ever associated with. Rabbits carrots and shadow puppets
and the velveteen rabbit and Easter and Alice in Wonderland,
and maybe a French restaurant that serves rabbit and Roger Rabbit.
Speaker 2 (33:14):
And so on.
Speaker 1 (33:15):
This is how an associative neural network is structured. Everything
is linked by association. So what happens during dreams is
that this random activity gets shot into the visual system
and synapses that are hot from the day will tend
to get activated again. But from there the activity will
tend to drift along these associative pathways, and that's why
(33:39):
dreams seem to have a unifying thread, but they're also
characterized by bizarreness. The storyline drifts from thing to thing
in a way that's not quite like the real world
because it's activity that's moving through this associative neural network,
and we experience whatever is getting triggered in whatever world,
(34:00):
So it's tied to our experience from the day and
of the world we know. But it's a very loose
sort of story, and the brain is a natural storyteller,
so things get tied together as best they can, and
we shouldn't overlook the fact that we are storytellers.
Speaker 2 (34:17):
Even after we wake up.
Speaker 1 (34:19):
So when you tell your spouse or your friend, wow,
this happened, and then this happened, we can't help but
impose a narrative over the images we saw. And so
sometimes the series of images we experienced gets even a
stronger storyline put on top of it. And by the way,
(34:39):
I just want to mention one other thing. You might
wonder how it makes sense that sometimes you hear sounds
or feel touch or have a smell in a dream
if the activity is only going into the visual cortex, well,
that seems to happen sometimes. And that's because although the
activity is only going in the visual system, it can
cascade out and keep going to other parts of the brain.
(35:01):
Everything is connected to everything else with pretty short pathways.
But it's important to note that dreams are almost entirely
visual because that's the only place where the activity is
getting injected. Now, something you might wonder, given this defensive
(35:35):
activation theory.
Speaker 2 (35:36):
That I've described so far, what about blind people?
Speaker 1 (35:39):
Do they have dreams or do you think they have
no dreaming at all because their brains don't care about
the light in the dark. The answer is that people
who are blind, of course, they have dreams. But if
they've been blind from birth or from a very young age,
they have no visual experience in their dreams because their
(36:00):
visual system was taken over by other senses like hearing
in touch, and so they have those sensory experiences in
their dreams because the activity is still going into the
occipital lobe. It's just that that's no longer visual. So
their dream is something like, I was feeling my way
around my living room, but it was weird because someone
had rearranged all the furniture.
Speaker 2 (36:21):
And then I felt something strange in the corner.
Speaker 1 (36:23):
And I realized it was a bear, and I ran
and I could hear it behind me, and so on
this sort of thing. All their experiences in the dream
involved sound and touch, but not sight. In those born blind,
you still have these volleys of spikes blasting into the
back of the brain because that's where the circuitry is going,
but that part of the brain is no longer visual,
(36:45):
and their experiences are not visual. And this tells us
that the circuitry underlying dreaming it's very basic, low level circuitry.
It's not dependent on the experiences you have during your lifetime,
and the fundamental nature of this circuitry is also consistent
with the fact that we find it conserved across the
(37:06):
animal kingdom. Now, like any scientific idea, the defensive activation
theory could be correct or could not be.
Speaker 2 (37:13):
So how would we know.
Speaker 1 (37:15):
Well, we can start looking at the predictions that come
out of this hypothesis. First is just a general observation,
which is that you can look at the fall off
in REM's sleep with age. So the fraction of our
sleeping time that we spend in REM steadily decreases as
we get older. So as an infant you spend half
(37:37):
your sleeping time in REM, and as an adult you
spend only ten to twenty percent of sleep time in REM,
and when you're elderly you spend even less. And this
is consistent with the fact that infant's brains are much
more plastic, and so the competition for territory is really intense,
and as you get older, things settle into place and
(37:58):
cortical takeovers are harder to do. So the fall off
in plasticity parallels the fall off of time that you
spend in REM sleep. And by the way, this fall
off in REM is seen across species, so puppies and
kittens and every kind of baby has more rem sleep. Now,
(38:18):
this observation by itself isn't proof of anything, but it's
an interesting correlation.
Speaker 2 (38:23):
But could we look.
Speaker 1 (38:24):
Across species to see if we can make meaningful predictions
about which species dreams a lot and which a little.
In other words, how much time each species spends in
dream time. So the idea is that for a brain
that is born with a lot of plasticity, a lot
of flexibility, you need to keep the visual system well
(38:46):
protected at night. But some animals are born with a
lot less plasticity, Their brains are more ready to go,
and so the need to have this defensive activation at
night would be less. Take primate like the vervet monkey.
Within three weeks, it learns how to walk, and it
stops weaning within four months, and it reaches adolescens in
(39:10):
four years and it can reproduce. Now, look at a
baby human. We're primates also, But in contrast to the
vervet monkey, the human primate doesn't walk for a year,
and it doesn't wean until three years, and it doesn't
reach adolescens for thirteen years. Why it's because human brains
(39:31):
drop into the world half baked, and we're incredibly flexible.
That's how we absorb the language and culture and the
knowledge around us. We're super flexible, and the consequence is
that we have an unusually long childhood. But other animals
arrive more let's call it pre programmed, and they're just
(39:53):
following more basic instructions of eat, mate, run, approach and
so on. There's no vervet monkey culture to absorb. They
don't go to vervet monkey schools so that they can
learn about the discoveries of other monkeys before them so
they can springboard to the next steps. Instead, they live
essentially the same life as all the generations before them.
(40:15):
So different species, even closely related primate species, can have
very different levels of plasticity. And the question is how
does this translate to the amount of dreaming they do
each night. And our hypothesis is that the more plastic
species need more dreaming at night to make sure that
(40:36):
big changes don't happen and the visual system doesn't get
taken over.
Speaker 2 (40:40):
If you are a less plastic.
Speaker 1 (40:41):
Species, the brain is essentially more fixed into place and
there's less risk of takeover of the visual system in
the darkness. So we studied twenty five primate species and
we research the plasticity of their brain, or at least
correlates of plasticity, like how long it takes for them
to walk or to wean from their mothers, or how
(41:04):
long until they reach adolescence. And we also research the
percentage of their sleep time that each species spends in
REM sleep. Typically, this is measured by setting up infrared
cameras and watching the animals sleep through the night and
figuring out what percentage of their sleep time they have
this rapid eye movement going on underneath their eyelids. And
(41:26):
what's striking is that this varies pretty widely. So the
vervet monkey spends six percent of its sleep time in
REM and then you have a spider monkey spending seven percent,
and a yellow babboon spending eight percent, and a barbary
macaque monkey spending nine percent, all the way to a
bornean orangutan spending twelve percent, to a chimpanzee spending sixteen percent,
(41:50):
to a reeseus macaque monkey spending eighteen percent, to humans
spending twenty one percent of their sleeping time in REM. Now,
we compiled all this data and we found statistically significant
correlations between plasticity and the amount of remsleep In other words,
(42:11):
the less plastic an animal is, the less remsleep it
has during the night, and animals with brains that are
more plastic, whose brains have more territory shifting around, they
have more rem sleep. And by the way, as a control,
we gathered four other variables across these species, like weight
and length and how many offspring they have and average lifespan.
(42:34):
And as expected, all of those measures show no significant
correlations with the amount of remsleep, but the measures of
how plastic an animal was did correlate, And if you're
interested in the details, you can read our scientific publication
linked to the podcast website. Now, there are several ways
to test this framework further. For example, what happens if
(42:54):
somebody doesn't get the normal amount of dream sleep. Well,
as it turns out, sleep can be suppressed by certain antidepressants.
For the cognianty these are monoamine oxidase inhibitors and tricyclic antidepressants. Anyway,
the defensive activation theory would predict that if you're not
getting adequate rem sleep, you're going to have some sort
(43:16):
of visual consequences, and so it's interesting that patients on
these medications characteristically get blurry vision. Now this is typically
marked up to dry eyes, but I want to note
our alternative hypothesis here, which is that it might be
related to more takeover of the visual cortex. I don't
know for sure that this is true yet, but this
(43:38):
is a direction the research is going to go. And
also we can test across a huge variety of animal species,
not just primates. For example, some mammals are born immature,
meaning that they're unable to walk, or get food, or
regulate their own temperature, or defend themselves. These are animals
like humans and ferrets and platypuses. Other mammals are born mature,
(44:00):
such as guinea pig or sheep or giraffe. They come
out of the womb with teeth and fur and open
eyes and ability to regulate their own temperature, and they
walk within an hour of being born, and they eat
solid food. So here's the important clue. As a general rule,
the animals born immature have much more rem sleep, up
(44:20):
to eight times as much, and this difference is especially
clear in the first months of life. In our interpretation,
when a highly plastic brain drops into the world, it
needs to constantly fight to keep things balanced. But when
a brain arrives mostly solidify, there's less need for it
to engage in this nighttime fighting. I just want to
(44:41):
mention as a caveat that there's likely to be many
surprises here because an animal's sleeping and dreaming can be
very different depending on lots of other things. For example,
take the elephant. They have a really surprisingly small amount
of rem sleep a few minutes at most, and it
first blush, I thought this weighed against our hypothesis, But
(45:03):
it turns out that elephants sleep very little, around two
hours a night, and they have excellent night vision because
of specializations in their retinas, and as a result, their
visual corettix is active during almost all hours of the
day and the night, and so that doesn't face the
same threat of encroachment from the other senses. So the
(45:25):
hypothesis predicts that elephants should have very little rems sleep.
So stay tuned on the future of this hypothesis. But
I wanted to take the chance to walk you through
a few of the details about how you might think
about a question like dreaming and come up with new
frameworks and then test those. Now, one last idea to
close this out, what does our defensive activation theory mean
(45:48):
for aliens on other planets. In Live, wied I proposed
a hypothesis that we won't actually be able to test
until the very distant future when we discover life on
other planets. Some planet, especially those that are orbiting red
dwarf stars, become locked into place such that they always
have the same surface facing their star. They have permanent
(46:10):
day on one side and permanent night on the other.
If life forms on that planet were to have plastic
brains that are even vaguely similar to ours, the prediction
would be that those on the daylight side of the
planet might have vision like us, but they would not
have dreams. They wouldn't need them because they never get
(46:31):
plunged into darkness, and the same prediction would apply for
very fast spinning planets. If their nighttime is shorter than
the time of a sensory takeover in the brain, then
they also wouldn't need dreams. So thousands of years from
now we might finally know whether we dreamers are in
(46:51):
the universal minority.
Speaker 2 (46:57):
That's all for this week.
Speaker 1 (46:59):
To find out more and to share your thoughts, head
over to Eagleman dot com, Slash Podcasts, and you can
also watch full episodes of Inner Cosmos on YouTube. Subscribe
to my channel so you can follow along each week
for new updates. I'd love to hear your questions, so
please send those to podcasts at eagleman dot com and
(47:20):
I will do a special episode where I answer questions.
Until next time, I'm David Eagleman, signing off to you
from the Inner Cosmos.
What do your nightly dreams have to do with the
rotation of the planet. Why are so many musicians blind?
Can blind people learn how to echolocate like a bat?
Once we find alien life on other planets, should we
expect that aliens have dreams at night? Welcome to Inner
(00:27):
Cosmos with me, David Eagleman. I'm a neuroscientist and an
author at Stanford University, and I've spent my whole career
studying the intersection between how the brain works and how
we experience life. I sort of personally have a love
(00:52):
hate relationship with dreaming because we take it so seriously.
We find ourselves in some biziz situation that doesn't make
any sense, but while you are there, you buy it,
hook line and sinker. You are emotionally tossed around and
buffeted in the winds of situations that the moment after
(01:16):
you wake up you realize weren't real, and more importantly,
they typically make no sense at all. So why were
you so caught up in that situation? I've always called
dreaming sticking my head in the night blender, by which
I simply mean that I don't necessarily look forward to
it because I don't know where it's going to take me,
(01:37):
and I generally feel that all that emotional energy I
expend isn't that useful. But whatever your take on dreaming is,
it's one of those absolutely insanely bizarre facts about our
existence that we totally take for granted. You wake up
and you say, oh, wow, I was just having a
(01:58):
dance party with a pack of why monkeys. Or I
was just at work where I forgot to put on
pants and I was trying to hide behind my desk.
Or I was on a river cruise and begging a
person not to break up with me even though we're
not even dating. And I find this absolutely stunning that
it doesn't even bother us that we've just transitioned from
(02:21):
one reality to another, Like in one second, you went
from being in some bizarre situation to lying horizontal in
your bed, and we're so used to it that we
just say, oh, that was interesting. I just dreamed that
I was on a hang glider riding over Istanbul and
I was trying to figure out how not to crash
because there were a lot of pigeons swirling around me.
Speaker 2 (02:44):
And if there's somebody there, then maybe we tell them.
Speaker 1 (02:47):
Our dream and Otherwise, we just get up and we
brush our teeth and go about our day without even
giving a second thought about what just happened. We were
in one reality and totally emotional invested in it, and
then a moment later we said, hmm, I guess I'm
in this other reality now. Now this is a testament
(03:08):
to the human mind's ability to accept the absolutely amazing
and bizarre as something not even worth investigating.
Speaker 2 (03:17):
Think about it this way.
Speaker 1 (03:18):
Just imagine if dreams happened right in the middle of
your day. So you're on a walk, or you're sitting
at your desk or eating at a restaurant with friends,
and suddenly your reality morphs to a completely bizarre other reality,
and now you're on a street you've never seen before,
or you're flying, or you're falling from a building, or
(03:41):
you're being chased by a monster. You would be terrified
by this lack of cohesion in your reality, the fact
that you just flipped from one to another. And as
soon as you were back in this reality, you'd presumably
run to your doctor and say you just had this
bizarre hallucination and thought you were somewhere else or maybe
someone else, and you'd be terrified, But because we're so
(04:05):
used to dreaming, we wake up and we think, oh,
that was weird, and then we go.
Speaker 2 (04:09):
About our new reality.
Speaker 1 (04:11):
And in fact, one of the evergreen questions among philosophers
is whether we live in a simulation. And I'm going
to dive deep into that question in a different episode,
but for now, I'll just mention the question, which is,
how would you know if you are a brain in
a vat who is being stimulated in just the right
way to think that you're listening to a podcast and
(04:34):
seeing the world around you and eating.
Speaker 2 (04:36):
Delicious food and so on.
Speaker 1 (04:38):
Or the modern version of that is whether we are
living in a computer simulation. So I suggest the mere
existence of dreams is sufficient to prove that all this
could be a simulation, because dreams demonstrate to us so
clearly that we completely buy what ever reality we find
(05:01):
ourselves in, and when we find ourselves in another reality,
we say, oh, I guess that one wasn't real.
Speaker 2 (05:07):
I was fooled, but this is real.
Speaker 1 (05:09):
But let me leave that as a teaser for the
episode on whether we're living in a simulation and how
we'd know. For today, I want to dive into dreaming
in particular, and from the brain's point.
Speaker 2 (05:21):
Of view, why do we dream?
Speaker 1 (05:23):
Why does our consciousness go on these bizarre flights every
ninety minutes during the night. So as a neuroscientist, I've
always been fascinated by dreaming, by what.
Speaker 2 (05:34):
The heck this is all about.
Speaker 1 (05:35):
So in this episode, we're going to talk about the why,
and I'm going to tell you about some hypotheses that
people have proposed over the centuries, and then I'm going
to tell you about a new hypothesis that makes quantitative
predictions across animal species. So let's start with an interesting fact,
which is that all animals appear to have dream sleep,
(05:59):
also as REM sleep. REM is spelled r EM, and
it stands for rapid eye movement. So in this stage
of sleep, the eyes are darting back and forth under
the eyelids. And if you haven't really watched someone while
they're sleeping, you should in a respectful manner because you'll
(06:20):
see how amazing this is. Every ninety minutes or so,
their eyes start jiggling back and forth. Now here's the
cool part. If you wake them up right when this
is happening and you say, quick, what were you just
thinking about? They will tell you that they were just
riding a camel in a shopping mall, or they were
running from a pack of leprechauns, or they were flying
(06:42):
around their house or whatever. And this is how we know,
from years of experiments of waking people up at different
stages of sleep, this is how we know that rem
sleep is when dreams happen. Because if you wake someone
up during a deeper stage of sleep, what we call
stage one or two or three, when their eyes are
(07:03):
not moving, and you say, quick, what were you just
thinking about, they'll generally say nothing at all, there was
nothing going on in my consciousness. Now, for completeness, I'll
just flag that some people will point out there can
be some sort of dreaming during non rem sleep, but
this is a very different type of dreaming. When it happens,
(07:24):
it's just a feeling of something like a simple thought,
rather than a vivid experience like we typically think of
with dreams with its whirl of color and activity and
magnified emotions. Now, just a quick side note, some people
say I don't dream at night, but in fact you do.
(07:45):
Everybody does. It's just a matter of waking up at
the right time. So if I snuck into your house
and woke you up right when you were having rapid
eye movement sleep. You'd say, WHOA, I was just dreaming.
But what often happens is that we enjoy a round
of dream sleep and then we sink back into a
deeper sleep, so by the time we wake up, we
(08:07):
don't remember the dream. So some people get up in
the morning and they're convinced that they didn't dream at all,
while in fact they did, they simply don't remember it. Now,
the question I want to address is why we dream?
And the first clue comes from a simple observation that
this is not just a human thing, but something about
(08:29):
brains in general. You've seen your dog have dreams where
she kicks her legs around and barks like she's chasing
a rabbit. But it turns out that all mammals dream,
and all birds and reptiles they all exhibit rem sleep.
Even fish have a form of rem sleep. So why
is this so conserved across the whole animal kingdom. While
(08:51):
there are various ideas that have been proposed for why
we dream, some researchers point out that dream sleep seems
to be important for memory consolidation, which means nailing down
the memories that you take in during the day. So
you run around during the day with your eyes open
and you experience all kinds of new things.
Speaker 2 (09:14):
And the idea is that when you sleep and.
Speaker 1 (09:17):
Dream, you are nailing that down and taking out the
neural trash in a way that's necessary and useful for
locking down the memories.
Speaker 2 (09:27):
Another popular idea has been that.
Speaker 1 (09:29):
Dreams help us solve problems because the brain is able
to process information and make connections that are not possible
during waking hours. So the idea is that the brain
is in a more relaxed state during dream sleep. It's
not as focused on the external world, and this allows
it to focus on internal thoughts and ideas, which can
(09:50):
lead to new insights and solutions to problems. And in fact,
studies have shown that if you are trying to solve
a problem and you go to sleep, you're more likely
to solve it when you wake up than if you're
just thinking about it during the daytime. And if you're
deprived of dream sleep, you have more difficulty solving problems.
But of course this might be related to problems of
(10:13):
sleep in general. Others suggest that rem sleep is involved
more generally in creativity, and others talk about its role
in emotional processing. And finally, several thinkers have suggested that
dreams help us prepare for new situations like fighting or
escaping from situations, because these are things that we experience
(10:35):
in real life very rarely, and so the idea is
that dreams give us practice at these rare situations.
Speaker 2 (10:42):
They keep the wheels greased.
Speaker 1 (10:44):
And I just want to be clear that these are
all hypotheses for why we dream, and they're not exclusive.
Dreams might serve multiple roles, so it's not as though
one of these has to be right at the expense
of others. But the first thing that I hope is
clear is that there's a lot of speculation about dreams,
but we don't really have a single theory that would
qualify as the answer to why we dream. And certainly
(11:07):
we don't have a theory that allows us to look
across the animal kingdom to answer a different question, which
is why do different animal species, even those who are
closely related, dream different amounts. For example, if you look
at the vervet monkey and it's total sleeping time, it
(11:27):
only spends six percent of that sleep time in rem
In other words, having dreams, whereas another monkey, the Reesus
macaque monkey, spends eighteen percent of its time in rem sleep,
three times as much, even though they're both primate species.
So it's never been clear to me how any of
the hypotheses previously proposed would account for any of this.
(11:49):
But recently, my colleague and I proposed a very new
kind of theory about dreaming, and it's one that I'm
very excited about because it gives accurate quantity tative predictions
across species, something that no other hypothesis even strives to do.
But before I tell you about that, I need to
lay some foundation. So let's start with the story of
(12:11):
a young boy named Ronnie who was born blind. He
was born in North Carolina, and when he was just
past year old, his mother abandoned him. She said that
his blindness was her punishment from God. So he ended
up being raised in poverty by his grandparents until he
was five, and then he was sent off to a
(12:33):
school for the sightless. When he was six years old,
his mother came by just once and she had another child,
now a little girl, and his mother said, ron I
want you to feel her eyes. You know, her eyes
are so pretty she didn't shame me the way that
you did. She can see, and that was the last
(12:53):
time he ever had contact with his mother. So most
of us can't even imagine a childhood this hard. But
the silver lining became the fact that Ronnie had a
gift for music. His instructors spotted this talent and he
started to formally study classical music with the violin, and
in no time, this kid was a virtuoso.
Speaker 2 (13:15):
And from there he went.
Speaker 1 (13:16):
On to master guitar and piano and several other string
and woodwind instruments, and by the time he grew into
young adulthood, he became one of the most popular performers
of his day. His name was Ronnie Millsap, and you
may or may not have heard of him, but in
the seventies and eighties he dominated pop music and country
(13:38):
western markets. He released thirty five country music hits at
the number one slot, and he.
Speaker 2 (13:45):
Earned six Grammy Awards.
Speaker 1 (13:48):
Now you might think it's amazing that Ronnie Millsap could
be blind and have such an amazing musical career, but
this is actually not such an uncommon story. Think of
Andrea about or Ray, Charles Stevie Wonder, or Diane Shore
or Jose Feliciano or Jeff Healy. All of them are blind.
(14:11):
For all of them, their brains learned to rely on
the signals of sound and touch in their environment, and
they became better at processing those signals then cited people.
As I write about my book Live Wired, Musical stardom
is not guaranteed for people who are blind, but brain
reorganization is guaranteed because if a sense is not getting used,
(14:36):
like vision, it gets taken over by neighboring senses. There's
nothing special about the cells, the neurons in the visual
cortex at the back of your brain. They are simply
neurons that happened to be involved in processing edges or
colors for people who have functioning eyes. But if you
(14:57):
go blind, these exact same neurons can process other types
of information, So the territory gets redeployed. It gets taken
over by hearing and by touch, and you get better
at those other senses. For Ronnie Millsap, his visual cortex
was not getting used because his eyes were not functioning,
(15:19):
so these other brain areas took over and he got
better at those as a result. Perfect musical pitch, for example,
is much more common in the blind population, and blind
people are up to ten times better at determining whether
a musical pitch is subtly wobbling up and down.
Speaker 2 (15:40):
Why, it's just because.
Speaker 1 (15:41):
They have more brain territory devoted to the task of listening.
There was a recent experiment in which people who were
sighted or blind had one ear plugged up, and then
they were asked to point to the locations of sounds
in the room and pinpointing where a sound is coming
from normally requires a comparison of the signals at both years,
(16:05):
So it was expected that everyone would fail miserably at
this task, and that's what happened with the people who
could see. But for the blind participants, they were able
to generally tell where the sounds were positioned. How it's
because the exact shape of the cartilage of the outer ear,
even just one ear, it bounces sound around in subtle
(16:29):
ways that gives clues to location, but only if one
is highly attuned to pick up on those very subtle signals.
So people with sight, they have less cortex devoted to sound,
and so their ability to extract subtle sound information it's underdeveloped,
But with blind people that skill gets developed. And this
(16:53):
sort of extreme talent with sound, this is common among
the blind. Take a young man named Ben Underwood. When
he was two years old, Ben stopped seeing out of
his left eye and his mother took him to the
doctor and they soon discovered that he had retinal cancer
that was in both eyes. So they tried chemotherapy, they
(17:14):
tried radiation, but that didn't work, and finally the surgeons
had to remove both of his eyes. And you can
imagine the pain that the family went through here. But
by the time Ben was seven years old, he devised
a technique that was totally unexpected and unbelievably useful, which
is that he would click with his mouth and he
(17:37):
would listen for the returning echoes. And in this way
he could hear the locations of an open doorway, or
of a person, or of a parked car, or a
garbage can and so on. He was echo locating. He
was bouncing his sound waves off objects in the environment,
(17:57):
and he was listening to what returned. Now, I saw
a documentary about Ben a little while ago, and it
(18:20):
kicked off with the statement that Ben was quote the
only person in the world who can see with echolocation. Now,
first of all, we don't really know if he's seeing
in the same way that you and I might.
Speaker 2 (18:32):
Think of site.
Speaker 1 (18:33):
But much more importantly, Ben was not the only one
using echolocation. Thousands of blind people do this. In fact,
the phenomenon has been discussed since at least the nineteen forties,
when the word echolocation was first coined in an article
in the journal Science, and this was titled Echolocation by
(18:54):
Blind Men, bat and Radar. The author wrote, quote, many
blind persons develop, in the course of time a considerable
ability to avoid obstacle by means of auditory cues received
from sounds of their own making. And this included clicking
(19:15):
or their own footsteps, or cane tapping or finger snapping.
So the author demonstrated that their ability to echo locate
was drastically reduced if you put in distracting noises or
put ear plugs in them. Anyway, the general story is straightforward.
If a sense is not getting used, it gets taken
(19:36):
over by neighboring senses. So, for example, if you're blind,
the territory gets redeployed to hearing and to touch. There's
nothing special about the neurons and the visual cortex. They
just happen to be involved in processing vision if you
have functioning eyes, but if you go blind, these exact
same neurons can process other types of information. Now, in
(20:00):
recent decades, there have been thousands of papers demonstrating brain plasticity,
that is, the brain's ability to reconfigure and adjust its
own circuitry, And in my book Livewireed, I attempt to
build up frameworks to surface the big lessons from these papers.
But to my mind, the biggest surprise about brain plasticity
(20:20):
is its speed. So some years ago, researchers at McGill
University took several adults who had recently lost their site,
and they put them into a brain scanner and the
participants were asked to listen to sounds. Now, not surprisingly,
the sounds caused activity in their auditory cortex, but the
(20:40):
sounds also caused activity in their occipital core.
Speaker 2 (20:44):
Text that's at the back of the brain.
Speaker 1 (20:46):
It's normally what we would think of as visual cortex,
and that activity would not have been seen there even
a few weeks earlier when the participants had sight. Now,
the activity wasn't as strong as that scene in people
who have been blowing for a long time. But it
was detectable in the occipital cortex nonetheless, and this demonstrated
(21:07):
that the brain can implement changes rapidly when vision disappears,
but how rapidly so. Next, my colleagues at Harvard, led
by Alvaro pascal Leone, began to wonder about the speed
at which these major takeovers can happen, and they noted
that instructors at a school for the blind were required
(21:28):
to blindfold themselves for seven full days so that they
could gain a first hand understanding of their students' living experiences.
So when these cited instructors blindfolded themselves, they became aware
of enhanced skills with sounds. They could orient to things better,
and they could judge their distance, and they could identify things.
(21:50):
Several described identifying people more rapidly and accurately just as
they started talking, or even just given the cadence of
their footsteps, And the instructions learned new things like how
to differentiate cars just by the sounds of their motors.
So this god pascal Leone and his colleagues, considering what
(22:11):
would happen if a sighted person were blindfolded in.
Speaker 2 (22:14):
The laboratory for several days.
Speaker 1 (22:17):
So they launched this experiment and what they found was
nothing short of remarkable. They discovered that when you were
temporarily blinded, there was neural reorganization, just like we see
in blind subjects, and it was rapid. In one of
their studies, people who could see normally were blindfolded for
five days and they were put through intensive braille training,
(22:40):
and at the end of the five days, the subjects
had become quite good at detecting the subtle differences between
braille characters, much better than a control group of cited
participants who had the same training without the blindfold. But
what was especially striking was what happened to their brains
when you measured them in the scanner. Within fire five days,
(23:01):
the blindfolded participants had recruited their occipital cortex when they
were touching objects. So control subjects, not surprisingly used only
a different part of their brain called the somatisensory cortex,
and the blindfolded subjects they were also showing these occipital
responses to sounds and words.
Speaker 2 (23:22):
And by the way, you.
Speaker 1 (23:23):
Could disrupt this new occipital lobe activity by magnetic pulses,
and then the braille reading advantage of the blindfolded subjects
went away, so that indicates the recruitment of the brain
area was not an accidental side effect, but this was
a critical piece of the improved behavioral performance. And importantly,
(23:44):
because of the plasticity of the brain, when the blindfold
got removed, the response of the occipital.
Speaker 2 (23:51):
Cortext to touch or sound.
Speaker 1 (23:53):
That disappeared within a day, and at that point the participants'
brains returned to looking indistinguishable from every other sighted person
out there. Now here's the key study that really influenced me.
These same investigators very carefully mapped out the brain using
more powerful neuroimaging techniques. So volunteers were blindfolded really tightly,
(24:16):
and they were put in the scanner and they were
asked to perform a touching task that required really fine
discrimination with their fingers. And what these investigators saw was
activity emerging in the primary visual cortex the occipital lobe
after an hour. And the shock of these findings was
their sheer speed. So the reorganization of territory that brains do,
(24:41):
it's not like the glacial drifting of continental plates, but
it can be remarkably fast. The brain is always sprung
tight like a mouse trap to implement rapid change.
Speaker 2 (24:55):
So the key is that the brain's changes.
Speaker 1 (24:58):
Are even fast sure than even the most optimistic neuroscientist
would have dared to guess at the beginning of this century.
So let's zoom back out to the bigger picture. So
for survival you need things like sharp teeth and fast legs.
You also need neural flexibility. This is what allows brains
(25:19):
to optimize their performance in a variety of environments. But
the competition in the brain has a potential downside as well,
which is this, whenever there's an imbalance of activity in
the senses, a potential takeover can happen, and that can
happen really rapidly, So a redistribution of the resources that
(25:41):
can be really useful when a limb has been lost
or a sense has been lost. But the rapid conquest
of territory, you might have to actively counterbalance this in
other scenarios. And this consideration led me and my former
student Dawn Vaughan to propose a new theory for what
(26:04):
happens to brains in the dark of the night. So
now we're back to the main question of this episode.
What does dreaming have to do with the rotation of
the planet. And this is one of the unsolved mysteries
in neurosciences.
Speaker 2 (26:19):
Why brains dream?
Speaker 1 (26:21):
What these bizarre nighttime hallucinations are about.
Speaker 2 (26:25):
Do they have meaning?
Speaker 1 (26:26):
Are they simply random neural activity in search of a
coherent narrative? And why are dreams so richly visual igniting
the occipital cortex every night in this conflagration of activity.
Speaker 2 (26:40):
So here's our idea.
Speaker 1 (26:42):
In the chronic and unforgiving competition for brain real estate,
the visual system has a unique problem to deal with.
Because of the rotation of the planet, we are cast
into darkness for an average of twelve hours every cycle.
And obviously, I'm referring to ninety nine point nine nine
(27:03):
nine percent of our species evolutionary history. I'm not talking
about the current electricity blessed times. So it used to
be really, really dark at night. And I just told
you about how sensory deprivation triggers neighboring territories in the
brain to take over. So how does the visual system
(27:23):
deal with this unfair disadvantage? And we suggest that it
does so by keeping the occipital cortex active during the night.
We suggest that dreaming exists to keep the visual cortex
from being taken over by neighboring areas. Because the rotation
(27:44):
of the planet doesn't affect your ability to touch and
hear and taste and smell, only vision suffers in the dark,
and as a result, the visual cortex finds itself in danger.
Every night of takeover by the other sense is just
like with the blindfolded subjects, and given the amazing speed
with which these changes in territory can happen, this is
(28:09):
a real threat. So dreams are the means by which
the visual cortex prevents takeover. So to dig into this
idea a little more, let's zoom out. Although a sleeper
looks relaxed and shut down, the brain is fully electrically active,
(28:31):
so during most of the night there's no dreaming. But
during rem sleep there's a lot of things that happen.
So the heart rate and the breathing speed up, your
small muscles twitch, and your brain waves become smaller and faster,
and dreaming happens.
Speaker 2 (28:48):
Now.
Speaker 1 (28:49):
REM sleep is triggered by a particular set of neurons
in the brain stem in a structure called the ponds,
and that travels to a small nucleus in the thalamus,
and from there, these waves of electrical activity come banging
into the occipital cortex at the back of your head.
Now that's the area of your brain where your visual
system is. So when these visual areas become alive with
(29:12):
activity in their cells, that is experienced as visual we see.
And that's why dreams are pictorial like a film. If
the activity we're banging into a part of the cortex
involved in smell, then dreams would just be a smell story,
but it hits the visual area, and so we find
ourselves thrown into a movie. Now, if you're seeing all
(29:36):
kinds of stuff, you might wonder why you're not reacting
to that with your body, And that's because the circuitry
involved in dreaming also paralyzes your major muscle groups so
that you don't act out your dream. You shut down
your muscles so that you can simulate world experience without
actually moving your body around, and that combination crafts the
(29:58):
experience of dreaming. The electrical waves slamming into the occipital
cortex make your visual system alive with activity, and the
muscular paralysis keeps you from acting out the dreams. Now,
we theorize that the circuitry behind visual dreams is not accidental. Instead,
(30:19):
to prevent takeover, the visual system is forced to fight
for its territory by generating these short bursts of activity
every ninety minutes or so when the planet rotates into darkness.
It's a self defense system that evolved in the face
of constant competition for sensory real estate. Dreams are a
(30:43):
screen saver. So the idea is that vision carries mission
critical information for the brain, but vision is stolen away
for half of our hours. It's like we're blindfolded for
half our time here on Earth. So dreams, we say, suggest,
are the strange love child of neural plasticity and the
(31:06):
rotation of the planet. Now, one key point to appreciate
is that these nighttime volleys of activity are very anatomically precise.
They start in the brain stem and they end up
in only one place, the occipital cortex. If the circuitry
sort of randomly grew its branches, we'd expect it to
(31:26):
connect with all kinds of areas in the brain, but
it doesn't. It aims with anatomical precision at one area alone,
a tiny structure called the lateral geniculate nucleus which broadcasts
specifically to the occipital cortex and through a neuroanatomous lens.
This is really specific circuitry and that suggests an important role,
(31:50):
and we suggest that role is defense of the visual system.
So we call this the defensive activation theory. Now, I
want to address a question that might be coming up
for you, which is what about dream content? Why did
dreams seem to be about something rather than just random
dots of light? Well, the important thing to understand is
(32:12):
that the brain is a natural storyteller. When there's activity
in there, it shapes that into a story of what
it's seeing. For those of you who know about latent
diffusion models in AI, that's exactly the same thing. So
Dolly two and stable diffusion. These are image generators, and
(32:32):
they work by starting with random activity and that coheres
into a picture of something in exactly the same way.
The brain can't see random activity. It has to wrap
that into something particular that it is seeing. Now, why
are dreams a story instead of just a picture? The
(32:54):
key is that everything in the brain is linked by association.
So when you think of a rabbit, that's linked with
everything you've ever associated with. Rabbits carrots and shadow puppets
and the velveteen rabbit and Easter and Alice in Wonderland,
and maybe a French restaurant that serves rabbit and Roger Rabbit.
Speaker 2 (33:14):
And so on.
Speaker 1 (33:15):
This is how an associative neural network is structured. Everything
is linked by association. So what happens during dreams is
that this random activity gets shot into the visual system
and synapses that are hot from the day will tend
to get activated again. But from there the activity will
tend to drift along these associative pathways, and that's why
(33:39):
dreams seem to have a unifying thread, but they're also
characterized by bizarreness. The storyline drifts from thing to thing
in a way that's not quite like the real world
because it's activity that's moving through this associative neural network,
and we experience whatever is getting triggered in whatever world,
(34:00):
So it's tied to our experience from the day and
of the world we know. But it's a very loose
sort of story, and the brain is a natural storyteller,
so things get tied together as best they can, and
we shouldn't overlook the fact that we are storytellers.
Speaker 2 (34:17):
Even after we wake up.
Speaker 1 (34:19):
So when you tell your spouse or your friend, wow,
this happened, and then this happened, we can't help but
impose a narrative over the images we saw. And so
sometimes the series of images we experienced gets even a
stronger storyline put on top of it. And by the way,
(34:39):
I just want to mention one other thing. You might
wonder how it makes sense that sometimes you hear sounds
or feel touch or have a smell in a dream
if the activity is only going into the visual cortex, well,
that seems to happen sometimes. And that's because although the
activity is only going in the visual system, it can
cascade out and keep going to other parts of the brain.
(35:01):
Everything is connected to everything else with pretty short pathways.
But it's important to note that dreams are almost entirely
visual because that's the only place where the activity is
getting injected. Now, something you might wonder, given this defensive
(35:35):
activation theory.
Speaker 2 (35:36):
That I've described so far, what about blind people?
Speaker 1 (35:39):
Do they have dreams or do you think they have
no dreaming at all because their brains don't care about
the light in the dark. The answer is that people
who are blind, of course, they have dreams. But if
they've been blind from birth or from a very young age,
they have no visual experience in their dreams because their
(36:00):
visual system was taken over by other senses like hearing
in touch, and so they have those sensory experiences in
their dreams because the activity is still going into the
occipital lobe. It's just that that's no longer visual. So
their dream is something like, I was feeling my way
around my living room, but it was weird because someone
had rearranged all the furniture.
Speaker 2 (36:21):
And then I felt something strange in the corner.
Speaker 1 (36:23):
And I realized it was a bear, and I ran
and I could hear it behind me, and so on
this sort of thing. All their experiences in the dream
involved sound and touch, but not sight. In those born blind,
you still have these volleys of spikes blasting into the
back of the brain because that's where the circuitry is going,
but that part of the brain is no longer visual,
(36:45):
and their experiences are not visual. And this tells us
that the circuitry underlying dreaming it's very basic, low level circuitry.
It's not dependent on the experiences you have during your lifetime,
and the fundamental nature of this circuitry is also consistent
with the fact that we find it conserved across the
(37:06):
animal kingdom. Now, like any scientific idea, the defensive activation
theory could be correct or could not be.
Speaker 2 (37:13):
So how would we know.
Speaker 1 (37:15):
Well, we can start looking at the predictions that come
out of this hypothesis. First is just a general observation,
which is that you can look at the fall off
in REM's sleep with age. So the fraction of our
sleeping time that we spend in REM steadily decreases as
we get older. So as an infant you spend half
(37:37):
your sleeping time in REM, and as an adult you
spend only ten to twenty percent of sleep time in REM,
and when you're elderly you spend even less. And this
is consistent with the fact that infant's brains are much
more plastic, and so the competition for territory is really intense,
and as you get older, things settle into place and
(37:58):
cortical takeovers are harder to do. So the fall off
in plasticity parallels the fall off of time that you
spend in REM sleep. And by the way, this fall
off in REM is seen across species, so puppies and
kittens and every kind of baby has more rem sleep. Now,
(38:18):
this observation by itself isn't proof of anything, but it's
an interesting correlation.
Speaker 2 (38:23):
But could we look.
Speaker 1 (38:24):
Across species to see if we can make meaningful predictions
about which species dreams a lot and which a little.
In other words, how much time each species spends in
dream time. So the idea is that for a brain
that is born with a lot of plasticity, a lot
of flexibility, you need to keep the visual system well
(38:46):
protected at night. But some animals are born with a
lot less plasticity, Their brains are more ready to go,
and so the need to have this defensive activation at
night would be less. Take primate like the vervet monkey.
Within three weeks, it learns how to walk, and it
stops weaning within four months, and it reaches adolescens in
(39:10):
four years and it can reproduce. Now, look at a
baby human. We're primates also, But in contrast to the
vervet monkey, the human primate doesn't walk for a year,
and it doesn't wean until three years, and it doesn't
reach adolescens for thirteen years. Why it's because human brains
(39:31):
drop into the world half baked, and we're incredibly flexible.
That's how we absorb the language and culture and the
knowledge around us. We're super flexible, and the consequence is
that we have an unusually long childhood. But other animals
arrive more let's call it pre programmed, and they're just
(39:53):
following more basic instructions of eat, mate, run, approach and
so on. There's no vervet monkey culture to absorb. They
don't go to vervet monkey schools so that they can
learn about the discoveries of other monkeys before them so
they can springboard to the next steps. Instead, they live
essentially the same life as all the generations before them.
(40:15):
So different species, even closely related primate species, can have
very different levels of plasticity. And the question is how
does this translate to the amount of dreaming they do
each night. And our hypothesis is that the more plastic
species need more dreaming at night to make sure that
(40:36):
big changes don't happen and the visual system doesn't get
taken over.
Speaker 2 (40:40):
If you are a less plastic.
Speaker 1 (40:41):
Species, the brain is essentially more fixed into place and
there's less risk of takeover of the visual system in
the darkness. So we studied twenty five primate species and
we research the plasticity of their brain, or at least
correlates of plasticity, like how long it takes for them
to walk or to wean from their mothers, or how
(41:04):
long until they reach adolescence. And we also research the
percentage of their sleep time that each species spends in
REM sleep. Typically, this is measured by setting up infrared
cameras and watching the animals sleep through the night and
figuring out what percentage of their sleep time they have
this rapid eye movement going on underneath their eyelids. And
(41:26):
what's striking is that this varies pretty widely. So the
vervet monkey spends six percent of its sleep time in
REM and then you have a spider monkey spending seven percent,
and a yellow babboon spending eight percent, and a barbary
macaque monkey spending nine percent, all the way to a
bornean orangutan spending twelve percent, to a chimpanzee spending sixteen percent,
(41:50):
to a reeseus macaque monkey spending eighteen percent, to humans
spending twenty one percent of their sleeping time in REM. Now,
we compiled all this data and we found statistically significant
correlations between plasticity and the amount of remsleep In other words,
(42:11):
the less plastic an animal is, the less remsleep it
has during the night, and animals with brains that are
more plastic, whose brains have more territory shifting around, they
have more rem sleep. And by the way, as a control,
we gathered four other variables across these species, like weight
and length and how many offspring they have and average lifespan.
(42:34):
And as expected, all of those measures show no significant
correlations with the amount of remsleep, but the measures of
how plastic an animal was did correlate, And if you're
interested in the details, you can read our scientific publication
linked to the podcast website. Now, there are several ways
to test this framework further. For example, what happens if
(42:54):
somebody doesn't get the normal amount of dream sleep. Well,
as it turns out, sleep can be suppressed by certain antidepressants.
For the cognianty these are monoamine oxidase inhibitors and tricyclic antidepressants. Anyway,
the defensive activation theory would predict that if you're not
getting adequate rem sleep, you're going to have some sort
(43:16):
of visual consequences, and so it's interesting that patients on
these medications characteristically get blurry vision. Now this is typically
marked up to dry eyes, but I want to note
our alternative hypothesis here, which is that it might be
related to more takeover of the visual cortex. I don't
know for sure that this is true yet, but this
(43:38):
is a direction the research is going to go. And
also we can test across a huge variety of animal species,
not just primates. For example, some mammals are born immature,
meaning that they're unable to walk, or get food, or
regulate their own temperature, or defend themselves. These are animals
like humans and ferrets and platypuses. Other mammals are born mature,
(44:00):
such as guinea pig or sheep or giraffe. They come
out of the womb with teeth and fur and open
eyes and ability to regulate their own temperature, and they
walk within an hour of being born, and they eat
solid food. So here's the important clue. As a general rule,
the animals born immature have much more rem sleep, up
(44:20):
to eight times as much, and this difference is especially
clear in the first months of life. In our interpretation,
when a highly plastic brain drops into the world, it
needs to constantly fight to keep things balanced. But when
a brain arrives mostly solidify, there's less need for it
to engage in this nighttime fighting. I just want to
(44:41):
mention as a caveat that there's likely to be many
surprises here because an animal's sleeping and dreaming can be
very different depending on lots of other things. For example,
take the elephant. They have a really surprisingly small amount
of rem sleep a few minutes at most, and it
first blush, I thought this weighed against our hypothesis, But
(45:03):
it turns out that elephants sleep very little, around two
hours a night, and they have excellent night vision because
of specializations in their retinas, and as a result, their
visual corettix is active during almost all hours of the
day and the night, and so that doesn't face the
same threat of encroachment from the other senses. So the
(45:25):
hypothesis predicts that elephants should have very little rems sleep.
So stay tuned on the future of this hypothesis. But
I wanted to take the chance to walk you through
a few of the details about how you might think
about a question like dreaming and come up with new
frameworks and then test those. Now, one last idea to
close this out, what does our defensive activation theory mean
(45:48):
for aliens on other planets. In Live, wied I proposed
a hypothesis that we won't actually be able to test
until the very distant future when we discover life on
other planets. Some planet, especially those that are orbiting red
dwarf stars, become locked into place such that they always
have the same surface facing their star. They have permanent
(46:10):
day on one side and permanent night on the other.
If life forms on that planet were to have plastic
brains that are even vaguely similar to ours, the prediction
would be that those on the daylight side of the
planet might have vision like us, but they would not
have dreams. They wouldn't need them because they never get
(46:31):
plunged into darkness, and the same prediction would apply for
very fast spinning planets. If their nighttime is shorter than
the time of a sensory takeover in the brain, then
they also wouldn't need dreams. So thousands of years from
now we might finally know whether we dreamers are in
(46:51):
the universal minority.
Speaker 2 (46:57):
That's all for this week.
Speaker 1 (46:59):
To find out more and to share your thoughts, head
over to Eagleman dot com, Slash Podcasts, and you can
also watch full episodes of Inner Cosmos on YouTube. Subscribe
to my channel so you can follow along each week
for new updates. I'd love to hear your questions, so
please send those to podcasts at eagleman dot com and
(47:20):
I will do a special episode where I answer questions.
Until next time, I'm David Eagleman, signing off to you
from the Inner Cosmos.
Host
David Eagleman
Popular Podcasts
Dateline NBC
Current and classic episodes, featuring compelling true-crime mysteries, powerful documentaries and in-depth investigations. Follow now to get the latest episodes of Dateline NBC completely free, or subscribe to Dateline Premium for ad-free listening and exclusive bonus content: DatelinePremium.com
24/7 News: The Latest
The latest news in 4 minutes updated every hour, every day.
Therapy Gecko
An unlicensed lizard psychologist travels the universe talking to strangers about absolutely nothing. TO CALL THE GECKO: follow me on https://www.twitch.tv/lyleforever to get a notification for when I am taking calls. I am usually live Mondays, Wednesdays, and Fridays but lately a lot of other times too. I am a gecko.