February 19, 2024 • 31 mins
Do our visual systems see in frames like a movie camera or instead analyze the world continuously? Why do you see multiple hands when you clap under yellow street lamps? How did Hollywood launch from the question of whether all four legs of a galloping horse come off the ground at once? And what is the very surprising thing that happens if you stare at your ceiling fan for a long time while it turns? This week’s episode is about visual perception -- and a series of eye-opening revelations about how the brain takes in information from the world.
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Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:05):
When you watch a car commercial, have you ever noticed
that sometimes the wheels seem to turn backwards? Or sometimes
you see a video of a helicopter and it seems
like the blades are barely turning, But you never see
that in real life. So what's going on there? And
what does this have to do with yellow street lamps?
Or whether all four legs of a horse come off
(00:27):
the ground when it runs, or the very surprising thing
that happens if you stare and stare at your ceiling
fan while it turns. Welcome to Inner Cosmos with me
and David Eagleman. I'm a neuroscientist and author at Stanford,
and in these episodes we sail deeply into our three
(00:49):
pound universe to understand why and how our lives look
the way they do, and in this case, why the
world looks the way it does. Today's episode is about
visual perception and a series of really strange surprises about
(01:12):
whether our visual systems analyze the world continuously or instead
whether we see in frames like a movie camera. So
have you ever noticed what happens when you film a
car going by on your cell phone camera and then
you watch the video When you look at the hubcaps
(01:32):
on the car, say with some spokes on it or
some pattern. When you look at the hubcaps, it looks
like they're spinning the wrong way, or maybe they occasionally
look like they're not spinning at all, even though the
car is moving. So the first question is why do
we so rarely notice this, Like, why doesn't it blow
(01:52):
our minds and we say, oh my god, that's not
consistent with what I just saw with my own eyes
and what I filmed. Why is it that we're so
nonchalant about that. I'll be addressing that in some future episodes,
but today the main question I want to ask is
why does it happen? Why does the wheel look on
your video like it's not spinning correctly even though that's
(02:17):
not what you just witnessed in real life. So to
understand that, let's step back to the eighteen seventies here
in Palo Alto, California, where I am so Leland Stanford,
who was a wealthy industrialist and the governor of California
and started Stanford University. Leland Stanford had some horses that
(02:40):
he loved, and so he hired a guy named Edward
Moybridge to take pictures of his horses running. Now, Moybridge
was this really talented guy with a collection of cameras
that could achieve shutter speeds of about one to one
thousandth of a second, and this was a really big
(03:01):
deal in eighteen seventy eight. So Stanford and Moydbridge wanted
to take these really fast photographs of the running horses
because no one had ever done that. And it turns
out there was a debate about how horses arranged their
legs when they galloped, and the question was whether all
(03:21):
four legs ever come off the ground at the same time.
And Stanford realized he could put these things together, his
horses and this new photographic technology to finally answer this question,
do all the legs come off the ground? Now, if
you've ever watched a galloping horse, you know that everything
(03:42):
is moving just slightly too fast for you to confidently
be able to answer this question, and so they needed
a new way to address this. As an interesting side note,
there were earlier paintings that showed a horse with all
four of its legs off the ground when it was
in the middle of a gallop. And in these paintings,
(04:03):
the two front legs were extended in the air, and
the two back legs were kicked out behind the horse.
But nobody really knew if this was possible, and right
in eighteen seventy eight, this was the birth of chronophotography,
which meant taking these fast pictures of complicated movements to
(04:23):
really get what was happening there. So Moybridge set up
a series of cameras on Leland Stanford's track in Palo Alto,
and he took these photographs as the horse went by,
and these photographs became very famous because of their clarity
and because they answered the question. It turns out that
(04:46):
all four of a horse's legs do come off the ground,
but this happens when the legs are gathered underneath the
horse rather than extended front and back. But what happened
next is the important part. Weybridge. He took his very
clear photographs and he put them in what's called a zootrope,
which is like an upright cylinder, like a big jar
(05:10):
with vertical slits in it, and you put the photographs
inside the cylinder, and then you spin the cylinder and
as it rotates, you see one of the photos through
one of the slits, and then when the next slit
rotates around, you see the second photo in that same spot,
and then as the cylinder continues to spin, you then
(05:32):
see the third photo through the third slit, and so on.
And Moidbridge was able to make the first prototype of
a motion picture this way. And there's very rich history
to all the pieces that came together for movie technology
over the next decades. But this is the main idea.
(05:52):
Your brain sees picture one, and then picture two, and
then picture three, and it interprets that as smooth motion. Now,
this is exactly how modern movies work. On a video
you watch on your cell phone, you look at a
snapshot of Tom Cruise with his left foot in the air,
(06:12):
and then another still shot of him with his foot
a little lower, and another with his foot now touching
the ground. And in the next photograph his right foot
is a few inches off the floor. And as long
as you flash these snapshots quickly, then it looks like
he's racing down the sidewalk after the bad guy. And
you're totally caught up in the emotion of the scene
(06:35):
and not even considering that your visual cortex is being
fooled into believing something motion that is not actually there. Now,
what strikes me. Is interesting is that we are so
used to this that it's difficult to recreate for ourselves
the absolute shock that people must have had when they
(06:57):
saw this phenomenon for the first time. I mean, how
stunning would that be to witness a series of still
shots looking like they were moving. Keep in mind that
in the entire history of the world before this moment,
no one had ever had a chance to capture photons
from the scene, make a photograph, and then swap those
(07:19):
out so rapidly that it looks like smooth motion. It wasn't,
in fact, even until eighteen sixty eight that somebody made
a flip book. You remember those little books where you
hold your thumb to zip through all the pages rapidly,
and it looks like smooth motion of some drawing. And
although the zootrope was around with little hand drawings since
(07:42):
eighteen sixty six, no one had done this with photographs
until Moybridge did. Before that, no one had ever seen
motion pictures. I mean, just imagine the next time you're
watching a TikTok video that a smart person like Benjamin
Franklin went his whole life without ever seeing a moving picture.
(08:05):
And just imagine how gobsmacked he would be to see that.
And remember that this technology was so stunning that it
came to be called a move e, as in something
that seems to move, and we still call it that. Okay, now,
how does this work? Exactly? So, whether you're talking about
a zootrope or a flip book, or a movie projector
(08:28):
or what happens on your cell phone screen when you're
watching a YouTube video, it's all the same thing. It
relies on the phenomenon of apparent motion. So what is
a parent motion. It's a visual illusion that occurs when
a series of still images are shown in rapid succession,
and you get this perception of continuous, smooth movement. So
(08:52):
that is the basis for the zootropes and the flip
books and modern films and videos. Now, the reason this
works is because the brain retains the impression of the
previous image for just a brief moment after it's disappeared,
and that creates a smooth transition to the next image.
And this phenomenon of apparent motion is closely related to
(09:15):
a different phenomenon called the persistence of vision, which is
that your brain holds onto an image for a split
second after it disappears, and that allows you to perceive
smooth motion even when there are gaps between the individual images,
like when you're spinning the cylinder the zootrope. You see
(09:37):
a flash of image one, and then you don't see
anything while the cylinder spins a bit more, and then
you see a flash of image two, and then you
don't see anything, and so on. But your brain clocks
the image and retains it long enough that it bridges
the gap between So it's because of apparent motion and
(10:12):
persistence of vision that we can make and enjoy motion
pictures or TV, or video games or virtual reality. Okay,
now there's one more concept that's important here. Imagine that
I held up the first photograph of Moybridge's horse, and
then I picked up the second picture, and I held
that in front of it, and then I took the
(10:32):
third picture and held that up and so on. I'd
be going way too slow to fool you into thinking
there's motion. You might cognitively get it that there's a
sequence being shown, but you wouldn't have the direct perceptual
experience of motion. But if I flashed the photos quickly,
then it works. So how quickly do I have to
(10:56):
flash these. So to get at this answer, you can
do a simple experiment where you just flash an LED
light on and off and on and off, and if
you're doing it slowly, like flash flash, then you see
a flashing light. If you do it more quickly like
flash flash, flash flash, then you still see that it's flashing.
(11:17):
But of course it's a bit faster now. But now
you would just a little bit faster than that, and
suddenly the flashing light looks solid to you. You can't
distinguish this from a light that's just on. So the
speed of flashing at which the light suddenly looks like
a continuous light is called the flicker fusion threshold. Because
(11:41):
suddenly the flicker fuses to look like a solid your
brain just can't see the fact that the light is
turning on and off. So movies only work when the
successive images are flashed faster than the flicker fusion threshold,
so that you don't see one photo and then the
next and the next, but instead you see a smooth transition. Now,
(12:05):
how fast do you need to flash it? Well, the
exact threshold that you can measure in people that changes
a bit. When you're talking about the size of the
thing flashing and the intensity, and whether it's black or
white or color. But generally you need to flash something
like forty times per second and then it looks smooth. Now,
as a quick side note, you may know that old
(12:27):
movies were shot at twenty four frames per second, so
why didn't people see a flickering between the frames. The
answer is that movie projectors would flash each frame two
or sometimes three times before going to the next frame,
so you had a flicker of forty eight or sometimes
(12:47):
seventy two flashes per second, so the whole thing looked smooth.
And similarly, television is traditionally shot at thirty frames per second,
and in the same way, all the frames are doubled. Phones,
by the way, typically use sixty refreshes per second, although
a lot of high end phones have an even higher
(13:08):
refresh rate. So the point is, when you look at
any of these technologies, movies or television or phone, they're
all flickering above the flicker fusion threshold, and so everything
looks wonderfully smooth. So when you're walking around in the world,
a lot of lighting can look like it's continuous, but
(13:29):
you can demonstrate to yourself that it must be flickering
because of strange effects that you can get. For example,
many cars have moved to led headlights or blinkers that flicker,
and you can't tell that when you're looking right at
the light, But if you move your eyes to the side,
you'll see a series of images of the headlight or
(13:50):
the blinker because it hits your eye in different spots
when it comes on. Or take those yellow street lights
that you see. They're called sodium vapor lamps, and even
though it looks like a solid light, they're actually flickering
on and off with the alternating current, which is sixty
times a second in America and fifty times a second
(14:12):
in Europe. Now, the light looks solid because it's flickering
faster than the critical flicker fusion frequency. But if you
swing your hand around, you'll notice a strobing effect. You'll
see multiple locations of your hand that are spaced apart,
and the amount there spaced has to do with how
(14:34):
fast you're moving your hand. So when you see these
sorts of effects, that's how you know something is actually
flickering even though it looks solid to you. Okay, so
we see how movies work by flashing a bunch of images,
and even though it looks like continuous motion, it's actually
discrete frames. And this trick has revolutionized the way that
(14:56):
we communicate information. Compared to two hundred years ago, we
have YouTube and TikTok and television and video conferencing and
so on. Cool. But this particular trick of stringing together
discrete frames can yield strange illusions, and people started noticing
these things during the first days of movies. So originally
(15:21):
the movies were westerns with wagons, and the wagon would
be rolling forward along the dirt road, but it often
looked in the movie like the spokes were turning the
other way, or sometimes the spokes would look like they
were turning the right way but not at the right speed,
or sometimes not turning at all. So why does that happen? Well,
(15:44):
imagine the wagon wheel turning clockwise. The camera captures a
frame where the wheel is in this position, and then
when the next frame is captured some tens of milliseconds later,
the wheel has turned a bit and the spokes are
in a slightly position. But here's the tricky part. All
the spokes look alike, and so your brain's job is
(16:06):
to figure out how the spokes in the first frame
match up to the spokes in the second frame. And
generally it can only do this by looking for the
shortest distance that things must have changed. So, let's say
the spoke has rotated ninety percent of the way to
the position of the next spoke. The brain will erroneously
(16:29):
think it has rotated the other way, because from frame
one to frame two, your brain sees that the shortest
distance is not a rotation this way, but a rotation
the other way. And so this effect came to be
known as the wagon wheel illusion. And now that you're
going to keep an eye out for it, you're gonna
see this everywhere. In car commercials, the wheel doesn't appear
(16:52):
to be turning correctly, but instead the turning of the
hubcap seems to run the wrong way sometimes or slow
down in its rotation to a halt, even though the
car is zooming down the highway. And you can see
this with helicopters or drones on your television or on
your phone. It always looks like something is wrong. The
rotor blades are hardly turning it all, or maybe they
(17:15):
turn the other way and the helicopter lifts up even
though it doesn't make any sense. There's no adequate spinning
to make that happen. This is all the wagon wheel effect. Okay,
so now we are all set up for an observation
that came as a big surprise in the neuroscience world.
Some colleagues of Mind published a paper in nineteen ninety
(17:38):
six pointing out that you could get these kind of
motion illusions not just in movies, but in real life
under a continuous light. Now this was a big claim
because it suggested the possibility that our brains are actually
seeing in snapshots like the frames of a video camera.
(17:59):
Is that true or not true? Well, I'm gonna come
to that in a second, But first I want to
tell you how to experience the illusion. So, if you
have a ceiling fan that's turning, lie back on your
bed and stare at the fan. Do this in the
middle of the day with no lights on, so there's
no flickering lights. There's only sunlight. Now, let's say your
(18:22):
fan is turning clockwise and you stare at it. You'll
see that it's turning clockwise. But if you stare and
stare long enough, occasionally you will see it turn the
other way. For just a second or two, the fan
seems to reverse direction. So please try this, although it
might take a few minutes of just staring and staring
(18:44):
at it before you see it, but you'll know when
you see it the fan suddenly runs backward for just
a moment. Now, I just want to be clear that
you can see this illusion in the day under sunlight,
so this is not explained by something like a subtle
flickering of the lights because of the electrical current. Now,
(19:19):
this illusion with the fan seems like a subtle thing
that no one would care about, but it caused a
lot of discussion in the neuroscience community when this was noticed.
Even one of my mentors, Francis Crick, the co discoverer
of the structure of DNA, he wrote about this too,
And I'll tell you why it was such a big
deal to neuroscientists because it wasn't clear why this would
(19:42):
happen in the visual system, which seems to be taking
in information continuously. So the hypothesis that took hold was
that maybe the brain is taking in the world in
frames like a movie camera. And this is what the
researchers who found this suggested. They suggested that vision might
(20:03):
be like the filming of a spoked wheel with a
video camera, and that perhaps this illusion with the fan
was evidence of discrete perception, in other words, that we
see in frames like a video camera. Now, if the
claim were true, that's a big deal. But I started
(20:24):
to suspect that something wasn't right here, because, first of all,
there are some important differences between watching the fan in
the daylight and watching the wagon wheel effect in movies. First,
in the movie, if you have the wagon rolling at
a particular speed, the wheel seems to be going backwards
at a fixed speed the whole time. But that doesn't
(20:46):
happen with the fan. It only happens for just a
moment after you've been staring at it for a while. Second,
when you see the fan reverse, it seems to happen
at a faster than normal speed, even though in the
movies the reverse spinning of the wheel is always slower.
And Third, in the movies, sometimes the wagon wheel effect
(21:07):
can look like the wheel is stopped because the camera
keeps catching the wheel when the spokes are in the
same position, so it looks like nothing's turning. But that
never happens with the fan in daylight. You never see
it look like it stopped, And little problems like that
started making me suspicious that maybe the wagon wheel affect
in movies and this issue about the fan reversing under
(21:31):
daylight had only a superficial similarity, and the two effects
had totally different reasons for actually happening. And I really
wanted to understand this very fundamental point about the visual system.
So what I did is I drove to the pawn
shop and I bought an old record player for eight dollars,
(21:51):
and then I spent two dollars at the art store
to get a circle of styrofoam picture this like a
big hockey puck. And then with my student Keith Klein
and my colleague Alex Holcomb, we put evenly spaced black
dots all around the outside of the styrofoam puck, and
we put that on the record player. So when you
(22:12):
flick on the record player and look at it from
the side, you see these little black dots moving from
left to right. Now, why did I use a record
player instead of a computer screen, Because the computer screen
inherently has a frame rate, and I wanted to make
sure we were really doing smooth motion here, continuous motion,
(22:34):
and the way this experiment goes when you watch this
record player in the daylight next to a big window
with no lights on. That's how we made sure there
was no flicker that influenced anything. When you stare at
these smoothly moving dots going from left to right, and
you keep your eyes in one place and you stare
and stare for a few minutes, it eventually works. You
(22:54):
see the stream of dots suddenly reverse direction, just for
a few seconds. It looks like they're zipping from right
to left, and then you see things going back to
normal again. So we were able to reproduce the illusion.
But now here was the important trick. We now placed
a mirror right next to the record player. So now
(23:17):
what you see are two pucks rotating side by side
in opposite directions. So you're seeing one stream of dots
moving left to right and the other reflected stream of
dots moving right to left. Now, the question is do
you see both streams of dots reverse direction at the
(23:38):
same time, or do you see one puck reverse and
then maybe later the other puck reverses, and then the
first puck reverses again. If your visual system is snapping frames,
and here we have to assume that the length of
the frames are changing for some reason. If your visual
system is snapping frames, then both pucks should reverse at
(24:00):
the same time, because that's what would happen if you
were filming with a video camera whose frame rate was changing.
But if the two pucks reverse independently, that suggests something
very different is going on. So you keep doing gets
fixed right in the middle between these two streams of dots,
(24:20):
and what happens. What happens is you see one puck reverse,
and then the other, and then the first one again.
And this result seems to rule out snapshots, because again,
if our brains processed visual information like a camera in
discrete frames, then you would expect both sets of dots
to always switch directions at the same time. But that's
(24:43):
not what happens. Now. You might argue a point here,
which is that because I had people stare right in
between the dots, they were seeing one puck in the
left hemisphere and the reflected puck in the right hemisphere.
And what if perhaps the two hemispheres of the brain
both do snapshots, but with independent frame rates. So I
(25:04):
address that frame in fifteen seconds by turning the whole
contraption on its side, so that you could see both
pucks in the same hemisphere and you get the same result.
The pucks reverse direction independently at different times from one another.
So it appears that the visual system is not taking snapshots.
But what is the explanation here, Well, you have some
(25:28):
populations of neurons in your visual cortex that detect right
word motion, and when those are active, you say, ah,
there's clearly right word motion in the world. But you
also have populations of cells that pick up on leftward motion.
And these populations are always balanced in a competition. But
here's the key, for technical reasons that you can read
(25:50):
about in my papers on this. It turns out that
those leftward populations can sometimes be fooled by a lot
of right word motion. They get a little bit activated
by the wrong direction of motion. And so even though
your brain is ninety eight percent sure that the motion
is to the right and the right word population is
(26:11):
screaming with activity, there's a little bit of activity in
the leftward population as well. And as I said, these
left and right word populations are always in a rivalrous
relationship with one another, and so every once in a while,
the leftward story wins for just a little bit. In
other words, because of this battle going on under the
(26:33):
hood between different explanations, the leftward motion detectors are intermittently
able to drive perception for just a moment. Now. If
you listen to my episode about the dress and other illusions.
This is very similar to other things that we've seen,
Like with a cube that's drawn as just the wire
(26:55):
frame of the cube, you can see it coming out
of the page one way, or you can see it
coming out the other way, and those perceptions will switch
back and forth. Now, which orientation the cube is in
that happens to have a fifty to fifty chance that
it might be one way or the other. But here
what happens with the ceiling fan reversing. That's more like
(27:16):
your brain saying, okay, ninety eight percent chance it's moving
this way and only two percent chance is moving the
other way. So almost all of the time you see
it correctly, and only once in a great while will
the underdog neural population win, and then you'll see it
the other way. You'll see this illusory reversed motion for
(27:38):
just a moment now, because this is much more rare
for the underdog to win. You have to stare at
the fan for a while to see the illusion. So
what we've seen here and in other episodes is that
you have populations of cells in your brain that are
fighting for different interpretations of the world, and it's always
(27:59):
just a of which population dominates the hill at any
given moment. That's what you perceive. And it's all because
your brain is locked in silence and darkness and it
has to put together a story of what's going on
in the outside world based just on little trickles of
(28:19):
data that it can pick up on. That's why there's
so many different types of illusions. So let's wrap up.
This kind of thing happens a lot in science where
two different phenomenas sort of look alike, like the wagon
wheel effect in movies and the illusory motion reversal of
the fan, and so we don't know if there's one
(28:41):
explanation underlying two things, or they're actually underpinned by very
different things. As Carl Sagan said, science is always alternating
between lumping and splitting, meaning that sometimes you realize that
two disparate phenomenon are actually the same thing, and then
you can lump them together and what happens equally often
(29:03):
is that two things you assumed were the same are
actually different phenomenon. So illusory reversal of the fan under
sunlight appears to happen not because the visual cortex is
snapping snapshots, but instead even weirder because of different political
parties in your brain engaging in their parliamentary debates. So
(29:26):
this simple set of experiments gave us real insight into
what was going on and led to four publications and
a lot of neuroscience is very pricey to run, and
so I was very pleased that the total amount I
spent on these experiments to address a very fundamental point
about the visual system was ten dollars. One of the
(29:48):
joys of life is careful observation of what is actually
in front of us, figuring out how we're actually seeing
the world, because we make lots of assumptions about what
we're seeing. But if you observe carefully, you'll start to
notice all the very weird things that your visual system
serves up to you. And after this careful observation, you
(30:10):
can sometimes set up simple experiments to understand what's actually
happening under the hood. And as we practice that, we
get deeper insight about how our brains are actively constructing
the reality that we typically take for granted. Now, if
(30:32):
you want more detail on any of these experiments, please
find my papers at eagleman dot com slash podcast. You'll
always find references there for further reading. Send me an
email at podcasts at eagleman dot com with questions or discussion,
and I'll be making an episode soon in which I
address those. And check out and subscribe to Inner Cosmos
on YouTube for videos of each episode and to leave
(30:55):
comments until next time. I'm David Eagleman, and this is
in our cosmos.
When you watch a car commercial, have you ever noticed
that sometimes the wheels seem to turn backwards? Or sometimes
you see a video of a helicopter and it seems
like the blades are barely turning, But you never see
that in real life. So what's going on there? And
what does this have to do with yellow street lamps?
Or whether all four legs of a horse come off
(00:27):
the ground when it runs, or the very surprising thing
that happens if you stare and stare at your ceiling
fan while it turns. Welcome to Inner Cosmos with me
and David Eagleman. I'm a neuroscientist and author at Stanford,
and in these episodes we sail deeply into our three
(00:49):
pound universe to understand why and how our lives look
the way they do, and in this case, why the
world looks the way it does. Today's episode is about
visual perception and a series of really strange surprises about
(01:12):
whether our visual systems analyze the world continuously or instead
whether we see in frames like a movie camera. So
have you ever noticed what happens when you film a
car going by on your cell phone camera and then
you watch the video When you look at the hubcaps
(01:32):
on the car, say with some spokes on it or
some pattern. When you look at the hubcaps, it looks
like they're spinning the wrong way, or maybe they occasionally
look like they're not spinning at all, even though the
car is moving. So the first question is why do
we so rarely notice this, Like, why doesn't it blow
(01:52):
our minds and we say, oh my god, that's not
consistent with what I just saw with my own eyes
and what I filmed. Why is it that we're so
nonchalant about that. I'll be addressing that in some future episodes,
but today the main question I want to ask is
why does it happen? Why does the wheel look on
your video like it's not spinning correctly even though that's
(02:17):
not what you just witnessed in real life. So to
understand that, let's step back to the eighteen seventies here
in Palo Alto, California, where I am so Leland Stanford,
who was a wealthy industrialist and the governor of California
and started Stanford University. Leland Stanford had some horses that
(02:40):
he loved, and so he hired a guy named Edward
Moybridge to take pictures of his horses running. Now, Moybridge
was this really talented guy with a collection of cameras
that could achieve shutter speeds of about one to one
thousandth of a second, and this was a really big
(03:01):
deal in eighteen seventy eight. So Stanford and Moydbridge wanted
to take these really fast photographs of the running horses
because no one had ever done that. And it turns
out there was a debate about how horses arranged their
legs when they galloped, and the question was whether all
(03:21):
four legs ever come off the ground at the same time.
And Stanford realized he could put these things together, his
horses and this new photographic technology to finally answer this question,
do all the legs come off the ground? Now, if
you've ever watched a galloping horse, you know that everything
(03:42):
is moving just slightly too fast for you to confidently
be able to answer this question, and so they needed
a new way to address this. As an interesting side note,
there were earlier paintings that showed a horse with all
four of its legs off the ground when it was
in the middle of a gallop. And in these paintings,
(04:03):
the two front legs were extended in the air, and
the two back legs were kicked out behind the horse.
But nobody really knew if this was possible, and right
in eighteen seventy eight, this was the birth of chronophotography,
which meant taking these fast pictures of complicated movements to
(04:23):
really get what was happening there. So Moybridge set up
a series of cameras on Leland Stanford's track in Palo Alto,
and he took these photographs as the horse went by,
and these photographs became very famous because of their clarity
and because they answered the question. It turns out that
(04:46):
all four of a horse's legs do come off the ground,
but this happens when the legs are gathered underneath the
horse rather than extended front and back. But what happened
next is the important part. Weybridge. He took his very
clear photographs and he put them in what's called a zootrope,
which is like an upright cylinder, like a big jar
(05:10):
with vertical slits in it, and you put the photographs
inside the cylinder, and then you spin the cylinder and
as it rotates, you see one of the photos through
one of the slits, and then when the next slit
rotates around, you see the second photo in that same spot,
and then as the cylinder continues to spin, you then
(05:32):
see the third photo through the third slit, and so on.
And Moidbridge was able to make the first prototype of
a motion picture this way. And there's very rich history
to all the pieces that came together for movie technology
over the next decades. But this is the main idea.
(05:52):
Your brain sees picture one, and then picture two, and
then picture three, and it interprets that as smooth motion. Now,
this is exactly how modern movies work. On a video
you watch on your cell phone, you look at a
snapshot of Tom Cruise with his left foot in the air,
(06:12):
and then another still shot of him with his foot
a little lower, and another with his foot now touching
the ground. And in the next photograph his right foot
is a few inches off the floor. And as long
as you flash these snapshots quickly, then it looks like
he's racing down the sidewalk after the bad guy. And
you're totally caught up in the emotion of the scene
(06:35):
and not even considering that your visual cortex is being
fooled into believing something motion that is not actually there. Now,
what strikes me. Is interesting is that we are so
used to this that it's difficult to recreate for ourselves
the absolute shock that people must have had when they
(06:57):
saw this phenomenon for the first time. I mean, how
stunning would that be to witness a series of still
shots looking like they were moving. Keep in mind that
in the entire history of the world before this moment,
no one had ever had a chance to capture photons
from the scene, make a photograph, and then swap those
(07:19):
out so rapidly that it looks like smooth motion. It wasn't,
in fact, even until eighteen sixty eight that somebody made
a flip book. You remember those little books where you
hold your thumb to zip through all the pages rapidly,
and it looks like smooth motion of some drawing. And
although the zootrope was around with little hand drawings since
(07:42):
eighteen sixty six, no one had done this with photographs
until Moybridge did. Before that, no one had ever seen
motion pictures. I mean, just imagine the next time you're
watching a TikTok video that a smart person like Benjamin
Franklin went his whole life without ever seeing a moving picture.
(08:05):
And just imagine how gobsmacked he would be to see that.
And remember that this technology was so stunning that it
came to be called a move e, as in something
that seems to move, and we still call it that. Okay, now,
how does this work? Exactly? So, whether you're talking about
a zootrope or a flip book, or a movie projector
(08:28):
or what happens on your cell phone screen when you're
watching a YouTube video, it's all the same thing. It
relies on the phenomenon of apparent motion. So what is
a parent motion. It's a visual illusion that occurs when
a series of still images are shown in rapid succession,
and you get this perception of continuous, smooth movement. So
(08:52):
that is the basis for the zootropes and the flip
books and modern films and videos. Now, the reason this
works is because the brain retains the impression of the
previous image for just a brief moment after it's disappeared,
and that creates a smooth transition to the next image.
And this phenomenon of apparent motion is closely related to
(09:15):
a different phenomenon called the persistence of vision, which is
that your brain holds onto an image for a split
second after it disappears, and that allows you to perceive
smooth motion even when there are gaps between the individual images,
like when you're spinning the cylinder the zootrope. You see
(09:37):
a flash of image one, and then you don't see
anything while the cylinder spins a bit more, and then
you see a flash of image two, and then you
don't see anything, and so on. But your brain clocks
the image and retains it long enough that it bridges
the gap between So it's because of apparent motion and
(10:12):
persistence of vision that we can make and enjoy motion
pictures or TV, or video games or virtual reality. Okay,
now there's one more concept that's important here. Imagine that
I held up the first photograph of Moybridge's horse, and
then I picked up the second picture, and I held
that in front of it, and then I took the
(10:32):
third picture and held that up and so on. I'd
be going way too slow to fool you into thinking
there's motion. You might cognitively get it that there's a
sequence being shown, but you wouldn't have the direct perceptual
experience of motion. But if I flashed the photos quickly,
then it works. So how quickly do I have to
(10:56):
flash these. So to get at this answer, you can
do a simple experiment where you just flash an LED
light on and off and on and off, and if
you're doing it slowly, like flash flash, then you see
a flashing light. If you do it more quickly like
flash flash, flash flash, then you still see that it's flashing.
(11:17):
But of course it's a bit faster now. But now
you would just a little bit faster than that, and
suddenly the flashing light looks solid to you. You can't
distinguish this from a light that's just on. So the
speed of flashing at which the light suddenly looks like
a continuous light is called the flicker fusion threshold. Because
(11:41):
suddenly the flicker fuses to look like a solid your
brain just can't see the fact that the light is
turning on and off. So movies only work when the
successive images are flashed faster than the flicker fusion threshold,
so that you don't see one photo and then the
next and the next, but instead you see a smooth transition. Now,
(12:05):
how fast do you need to flash it? Well, the
exact threshold that you can measure in people that changes
a bit. When you're talking about the size of the
thing flashing and the intensity, and whether it's black or
white or color. But generally you need to flash something
like forty times per second and then it looks smooth. Now,
as a quick side note, you may know that old
(12:27):
movies were shot at twenty four frames per second, so
why didn't people see a flickering between the frames. The
answer is that movie projectors would flash each frame two
or sometimes three times before going to the next frame,
so you had a flicker of forty eight or sometimes
(12:47):
seventy two flashes per second, so the whole thing looked smooth.
And similarly, television is traditionally shot at thirty frames per second,
and in the same way, all the frames are doubled. Phones,
by the way, typically use sixty refreshes per second, although
a lot of high end phones have an even higher
(13:08):
refresh rate. So the point is, when you look at
any of these technologies, movies or television or phone, they're
all flickering above the flicker fusion threshold, and so everything
looks wonderfully smooth. So when you're walking around in the world,
a lot of lighting can look like it's continuous, but
(13:29):
you can demonstrate to yourself that it must be flickering
because of strange effects that you can get. For example,
many cars have moved to led headlights or blinkers that flicker,
and you can't tell that when you're looking right at
the light, But if you move your eyes to the side,
you'll see a series of images of the headlight or
(13:50):
the blinker because it hits your eye in different spots
when it comes on. Or take those yellow street lights
that you see. They're called sodium vapor lamps, and even
though it looks like a solid light, they're actually flickering
on and off with the alternating current, which is sixty
times a second in America and fifty times a second
(14:12):
in Europe. Now, the light looks solid because it's flickering
faster than the critical flicker fusion frequency. But if you
swing your hand around, you'll notice a strobing effect. You'll
see multiple locations of your hand that are spaced apart,
and the amount there spaced has to do with how
(14:34):
fast you're moving your hand. So when you see these
sorts of effects, that's how you know something is actually
flickering even though it looks solid to you. Okay, so
we see how movies work by flashing a bunch of images,
and even though it looks like continuous motion, it's actually
discrete frames. And this trick has revolutionized the way that
(14:56):
we communicate information. Compared to two hundred years ago, we
have YouTube and TikTok and television and video conferencing and
so on. Cool. But this particular trick of stringing together
discrete frames can yield strange illusions, and people started noticing
these things during the first days of movies. So originally
(15:21):
the movies were westerns with wagons, and the wagon would
be rolling forward along the dirt road, but it often
looked in the movie like the spokes were turning the
other way, or sometimes the spokes would look like they
were turning the right way but not at the right speed,
or sometimes not turning at all. So why does that happen? Well,
(15:44):
imagine the wagon wheel turning clockwise. The camera captures a
frame where the wheel is in this position, and then
when the next frame is captured some tens of milliseconds later,
the wheel has turned a bit and the spokes are
in a slightly position. But here's the tricky part. All
the spokes look alike, and so your brain's job is
(16:06):
to figure out how the spokes in the first frame
match up to the spokes in the second frame. And
generally it can only do this by looking for the
shortest distance that things must have changed. So, let's say
the spoke has rotated ninety percent of the way to
the position of the next spoke. The brain will erroneously
(16:29):
think it has rotated the other way, because from frame
one to frame two, your brain sees that the shortest
distance is not a rotation this way, but a rotation
the other way. And so this effect came to be
known as the wagon wheel illusion. And now that you're
going to keep an eye out for it, you're gonna
see this everywhere. In car commercials, the wheel doesn't appear
(16:52):
to be turning correctly, but instead the turning of the
hubcap seems to run the wrong way sometimes or slow
down in its rotation to a halt, even though the
car is zooming down the highway. And you can see
this with helicopters or drones on your television or on
your phone. It always looks like something is wrong. The
rotor blades are hardly turning it all, or maybe they
(17:15):
turn the other way and the helicopter lifts up even
though it doesn't make any sense. There's no adequate spinning
to make that happen. This is all the wagon wheel effect. Okay,
so now we are all set up for an observation
that came as a big surprise in the neuroscience world.
Some colleagues of Mind published a paper in nineteen ninety
(17:38):
six pointing out that you could get these kind of
motion illusions not just in movies, but in real life
under a continuous light. Now this was a big claim
because it suggested the possibility that our brains are actually
seeing in snapshots like the frames of a video camera.
(17:59):
Is that true or not true? Well, I'm gonna come
to that in a second, But first I want to
tell you how to experience the illusion. So, if you
have a ceiling fan that's turning, lie back on your
bed and stare at the fan. Do this in the
middle of the day with no lights on, so there's
no flickering lights. There's only sunlight. Now, let's say your
(18:22):
fan is turning clockwise and you stare at it. You'll
see that it's turning clockwise. But if you stare and
stare long enough, occasionally you will see it turn the
other way. For just a second or two, the fan
seems to reverse direction. So please try this, although it
might take a few minutes of just staring and staring
(18:44):
at it before you see it, but you'll know when
you see it the fan suddenly runs backward for just
a moment. Now, I just want to be clear that
you can see this illusion in the day under sunlight,
so this is not explained by something like a subtle
flickering of the lights because of the electrical current. Now,
(19:19):
this illusion with the fan seems like a subtle thing
that no one would care about, but it caused a
lot of discussion in the neuroscience community when this was noticed.
Even one of my mentors, Francis Crick, the co discoverer
of the structure of DNA, he wrote about this too,
And I'll tell you why it was such a big
deal to neuroscientists because it wasn't clear why this would
(19:42):
happen in the visual system, which seems to be taking
in information continuously. So the hypothesis that took hold was
that maybe the brain is taking in the world in
frames like a movie camera. And this is what the
researchers who found this suggested. They suggested that vision might
(20:03):
be like the filming of a spoked wheel with a
video camera, and that perhaps this illusion with the fan
was evidence of discrete perception, in other words, that we
see in frames like a video camera. Now, if the
claim were true, that's a big deal. But I started
(20:24):
to suspect that something wasn't right here, because, first of all,
there are some important differences between watching the fan in
the daylight and watching the wagon wheel effect in movies. First,
in the movie, if you have the wagon rolling at
a particular speed, the wheel seems to be going backwards
at a fixed speed the whole time. But that doesn't
(20:46):
happen with the fan. It only happens for just a
moment after you've been staring at it for a while. Second,
when you see the fan reverse, it seems to happen
at a faster than normal speed, even though in the
movies the reverse spinning of the wheel is always slower.
And Third, in the movies, sometimes the wagon wheel effect
(21:07):
can look like the wheel is stopped because the camera
keeps catching the wheel when the spokes are in the
same position, so it looks like nothing's turning. But that
never happens with the fan in daylight. You never see
it look like it stopped, And little problems like that
started making me suspicious that maybe the wagon wheel affect
in movies and this issue about the fan reversing under
(21:31):
daylight had only a superficial similarity, and the two effects
had totally different reasons for actually happening. And I really
wanted to understand this very fundamental point about the visual system.
So what I did is I drove to the pawn
shop and I bought an old record player for eight dollars,
(21:51):
and then I spent two dollars at the art store
to get a circle of styrofoam picture this like a
big hockey puck. And then with my student Keith Klein
and my colleague Alex Holcomb, we put evenly spaced black
dots all around the outside of the styrofoam puck, and
we put that on the record player. So when you
(22:12):
flick on the record player and look at it from
the side, you see these little black dots moving from
left to right. Now, why did I use a record
player instead of a computer screen, Because the computer screen
inherently has a frame rate, and I wanted to make
sure we were really doing smooth motion here, continuous motion,
(22:34):
and the way this experiment goes when you watch this
record player in the daylight next to a big window
with no lights on. That's how we made sure there
was no flicker that influenced anything. When you stare at
these smoothly moving dots going from left to right, and
you keep your eyes in one place and you stare
and stare for a few minutes, it eventually works. You
(22:54):
see the stream of dots suddenly reverse direction, just for
a few seconds. It looks like they're zipping from right
to left, and then you see things going back to
normal again. So we were able to reproduce the illusion.
But now here was the important trick. We now placed
a mirror right next to the record player. So now
(23:17):
what you see are two pucks rotating side by side
in opposite directions. So you're seeing one stream of dots
moving left to right and the other reflected stream of
dots moving right to left. Now, the question is do
you see both streams of dots reverse direction at the
(23:38):
same time, or do you see one puck reverse and
then maybe later the other puck reverses, and then the
first puck reverses again. If your visual system is snapping frames,
and here we have to assume that the length of
the frames are changing for some reason. If your visual
system is snapping frames, then both pucks should reverse at
(24:00):
the same time, because that's what would happen if you
were filming with a video camera whose frame rate was changing.
But if the two pucks reverse independently, that suggests something
very different is going on. So you keep doing gets
fixed right in the middle between these two streams of dots,
(24:20):
and what happens. What happens is you see one puck reverse,
and then the other, and then the first one again.
And this result seems to rule out snapshots, because again,
if our brains processed visual information like a camera in
discrete frames, then you would expect both sets of dots
to always switch directions at the same time. But that's
(24:43):
not what happens. Now. You might argue a point here,
which is that because I had people stare right in
between the dots, they were seeing one puck in the
left hemisphere and the reflected puck in the right hemisphere.
And what if perhaps the two hemispheres of the brain
both do snapshots, but with independent frame rates. So I
(25:04):
address that frame in fifteen seconds by turning the whole
contraption on its side, so that you could see both
pucks in the same hemisphere and you get the same result.
The pucks reverse direction independently at different times from one another.
So it appears that the visual system is not taking snapshots.
But what is the explanation here, Well, you have some
(25:28):
populations of neurons in your visual cortex that detect right
word motion, and when those are active, you say, ah,
there's clearly right word motion in the world. But you
also have populations of cells that pick up on leftward motion.
And these populations are always balanced in a competition. But
here's the key, for technical reasons that you can read
(25:50):
about in my papers on this. It turns out that
those leftward populations can sometimes be fooled by a lot
of right word motion. They get a little bit activated
by the wrong direction of motion. And so even though
your brain is ninety eight percent sure that the motion
is to the right and the right word population is
(26:11):
screaming with activity, there's a little bit of activity in
the leftward population as well. And as I said, these
left and right word populations are always in a rivalrous
relationship with one another, and so every once in a while,
the leftward story wins for just a little bit. In
other words, because of this battle going on under the
(26:33):
hood between different explanations, the leftward motion detectors are intermittently
able to drive perception for just a moment. Now. If
you listen to my episode about the dress and other illusions.
This is very similar to other things that we've seen,
Like with a cube that's drawn as just the wire
(26:55):
frame of the cube, you can see it coming out
of the page one way, or you can see it
coming out the other way, and those perceptions will switch
back and forth. Now, which orientation the cube is in
that happens to have a fifty to fifty chance that
it might be one way or the other. But here
what happens with the ceiling fan reversing. That's more like
(27:16):
your brain saying, okay, ninety eight percent chance it's moving
this way and only two percent chance is moving the
other way. So almost all of the time you see
it correctly, and only once in a great while will
the underdog neural population win, and then you'll see it
the other way. You'll see this illusory reversed motion for
(27:38):
just a moment now, because this is much more rare
for the underdog to win. You have to stare at
the fan for a while to see the illusion. So
what we've seen here and in other episodes is that
you have populations of cells in your brain that are
fighting for different interpretations of the world, and it's always
(27:59):
just a of which population dominates the hill at any
given moment. That's what you perceive. And it's all because
your brain is locked in silence and darkness and it
has to put together a story of what's going on
in the outside world based just on little trickles of
(28:19):
data that it can pick up on. That's why there's
so many different types of illusions. So let's wrap up.
This kind of thing happens a lot in science where
two different phenomenas sort of look alike, like the wagon
wheel effect in movies and the illusory motion reversal of
the fan, and so we don't know if there's one
(28:41):
explanation underlying two things, or they're actually underpinned by very
different things. As Carl Sagan said, science is always alternating
between lumping and splitting, meaning that sometimes you realize that
two disparate phenomenon are actually the same thing, and then
you can lump them together and what happens equally often
(29:03):
is that two things you assumed were the same are
actually different phenomenon. So illusory reversal of the fan under
sunlight appears to happen not because the visual cortex is
snapping snapshots, but instead even weirder because of different political
parties in your brain engaging in their parliamentary debates. So
(29:26):
this simple set of experiments gave us real insight into
what was going on and led to four publications and
a lot of neuroscience is very pricey to run, and
so I was very pleased that the total amount I
spent on these experiments to address a very fundamental point
about the visual system was ten dollars. One of the
(29:48):
joys of life is careful observation of what is actually
in front of us, figuring out how we're actually seeing
the world, because we make lots of assumptions about what
we're seeing. But if you observe carefully, you'll start to
notice all the very weird things that your visual system
serves up to you. And after this careful observation, you
(30:10):
can sometimes set up simple experiments to understand what's actually
happening under the hood. And as we practice that, we
get deeper insight about how our brains are actively constructing
the reality that we typically take for granted. Now, if
(30:32):
you want more detail on any of these experiments, please
find my papers at eagleman dot com slash podcast. You'll
always find references there for further reading. Send me an
email at podcasts at eagleman dot com with questions or discussion,
and I'll be making an episode soon in which I
address those. And check out and subscribe to Inner Cosmos
on YouTube for videos of each episode and to leave
(30:55):
comments until next time. I'm David Eagleman, and this is
in our cosmos.
Host
David Eagleman
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