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