Speaker 1: Why do you still feel the waves after you get
Speaker 1: off a boat and when you get off the treadmill
Speaker 1: at the gym. Why does everything seem to be flowing
Speaker 1: past you faster? What does heartbreak have in common with
Speaker 1: drug withdrawal? And why after you stare at a waterfall
Speaker 1: for a while, do the rocks on the side seem
Speaker 1: to be crawling upward? Why did people in the nineteen
Speaker 1: eighties think their book pages had some color red in them,
Speaker 1: but no one thought that before or after the eighties?
Speaker 1: And what does any of this have to do with
Speaker 1: the great philosopher Aristotle watching a horse stuck in the river?
Speaker 1: Welcome to Inner Cosmos with me David Eagleman. I'm a
Speaker 1: neuroscientist and author at Stanford and in these episodes we
Speaker 1: sail deeply into our three pound universe to understand why
Speaker 1: and how our lives look the way they do. Today's
Speaker 1: episode is about the way the brain readjusts its circuitry
Speaker 1: on really fast timescales, and why your brain is constantly
Speaker 1: doing this. In the nineteen eighties, tens of thousands of
Speaker 1: people began to notice something really weird when they looked
Speaker 1: at a floppy disk envelope with the black and white
Speaker 1: IBM logo on the front, the letters IBM seemed to
Speaker 1: have a red tint, And the same thing happened when
Speaker 1: people looked at pages in a book, the text seemed
Speaker 1: like it was shaded red. Now, if that's not weird enough,
Speaker 1: check this out. This only happened in the nineteen eighties.
Speaker 1: People didn't perceive a red tint before or after this decade.
Speaker 1: So what was changing about brains during that window. I'll
Speaker 1: tell you the answer. But to understand this, we're first
Speaker 1: going to step back even further in history, twenty four
Speaker 1: hundred years further to Aristotle. Now, Aristotle, as you know,
Speaker 1: is a philosopher and a polymath whose intellectual interests were boundless.
Speaker 1: He wrote about essentially every subject you could imagine. But
Speaker 1: we're coming to Aristotle not because of something he wrote,
Speaker 1: but instead because he was insightful enough to pause and
Speaker 1: notice when something didn't make sense. What happened is that
Speaker 1: someone was trying to cross a fast flowing river on
Speaker 1: his horse, and the horse got stuck halfway in the river.
Speaker 1: So Aristotle, along with some other people, ended up watching
Speaker 1: this rescue operation. The way you might have a bunch
Speaker 1: of people looking at a car crash nowadays. So you
Speaker 1: have all these Greeks all standing there in their togas,
Speaker 1: talking and chatting and watching the horse in the fast
Speaker 1: flowing river. Now most of the people were presumably just
Speaker 1: talking about stuff that made no difference to history. But
Speaker 1: Aristotle stood at the side of the river intently watching
Speaker 1: this horse. Now Here was the key about Aristotle, a
Speaker 1: general characteristic that made him so famous to history. He
Speaker 1: was a good observer. He paid attention. He noticed when
Speaker 1: things were unusual, and when he found something, he would
Speaker 1: sink his teeth into the problem and wouldn't let go.
Speaker 1: And what he noticed here was something that possibly the
Speaker 1: others did too, but they ignored it, and they didn't
Speaker 1: think any further on it. What he noticed is that
Speaker 1: once the horse got unstuck and the man made it
Speaker 1: to the shore, and Aristotle looked away, he noticed that
Speaker 1: everything on the land, the rocks, the huts, the trees,
Speaker 1: everything seemed to be drifting, and they all seemed to
Speaker 1: be moving in the opposite direction of the river. Now
Speaker 1: you may have seen something like this too. The easiest
Speaker 1: way to experience Aristotle's confusion and delight is to stare
Speaker 1: at a waterfall and after you keep your eyes locked
Speaker 1: on it for a while, look over to the rocks
Speaker 1: on the side of the waterfall, and the rocks appear
Speaker 1: to be moving upward. Now, this illusion that he first
Speaker 1: made note of twenty four hundred years ago has come
Speaker 1: to be known as the motion after effect. Why does
Speaker 1: it happen. The activity of particular neurons in your visual
Speaker 1: cortex represents downward motion, and the activity of other neurons
Speaker 1: represents upward motion. And they're always locked in battle, and
Speaker 1: most of the time the competition is evenly pitched, and
Speaker 1: they evenly inhibit each other, and as a result, the
Speaker 1: world appears to you to be moving neither up nor down. So,
Speaker 1: given this, a popular explanation for the motion after effect
Speaker 1: is fatigue. By staring at the downward motion of the waterfall,
Speaker 1: you burn up a good deal of energy in your
Speaker 1: downward coating neurons, and now their vigor is temporarily depleted.
Speaker 1: So the battle tips in favor of the upward encoding neurons,
Speaker 1: and as a result of this unbalanced activity, you perceive
Speaker 1: that the rocks are climbing upward. You have this net
Speaker 1: movement upward. Now, this fatigue hypothesis is very appealing in
Speaker 1: its simplicity, but it's wrong. Why because it can't explain
Speaker 1: some critical facts about the illusion. So imagine you watch
Speaker 1: the downward waterfall for a while and then you close
Speaker 1: your eyes tightly, say, for three hours you close your eyes.
Speaker 1: When you reopen your eyes, you'll see the rocks crawling upward.
Speaker 1: And what that tells us is it's not about a
Speaker 1: temporary energy depletion in neurons. There's something deeper going on here.
Speaker 1: And here's what that deeper thing is is the illusion
Speaker 1: comes about not because of passive fatigue, but instead because
Speaker 1: of an active recalibration in your brain. Your visual system
Speaker 1: is exposed to continuous downward motion, and after a while
Speaker 1: it comes to assume this is the new normal. At first,
Speaker 1: downward motion is dramatic information to your brain, but after
Speaker 1: a while of staring, you receive no new information from
Speaker 1: downwardness here. So, as far as your brain is concerned,
Speaker 1: this is the new reality, a world that flows more
Speaker 1: down than up. So your visual system carefully rebalances its
Speaker 1: expectations to mirror the world to expect more downward than
Speaker 1: upward activity. Now, when you look away from the waterfall
Speaker 1: and toward the cliff side. This recalibrated set point becomes
Speaker 1: obvious because now rocks and trees are flowing upward toward
Speaker 1: the sky. Your set point has shifted. In other words,
Speaker 1: what counts as standing still has shifted. Why well, your
Speaker 1: brain always wants to set up a ground truth so
Speaker 1: that it can be better at detecting change. In this case,
Speaker 1: when your visual field is filled with the site of
Speaker 1: the waterfall, your brain is working to subtract off all
Speaker 1: that downward motion. All that downward motion is no longer
Speaker 1: informative in the way that it was, and so the
Speaker 1: circuitry it adjusts itself so that it can be maximumly
Speaker 1: sensitive to new information. Now, if you watch for it
Speaker 1: the way Aristotle would, you'll see that this kind of
Speaker 1: recalibration happens to you all the time. Think about when
Speaker 1: you're on a small boat on the ocean. You are
Speaker 1: rocking with the waves for a while, and then when
Speaker 1: you get off the boat, the land seems to be
Speaker 1: rocking for a while. It feels as though you're still
Speaker 1: on the water. Now you may have thought about it
Speaker 1: this way, Hey, my body thinks it's still on the water.
Speaker 1: What you're actually feeling is a negative image of the
Speaker 1: water's motion. And you may have noticed this sort of
Speaker 1: illusion if you're a runner. So here's what normally happens.
Speaker 1: Your body sends motor commands to the legs. It says, okay,
Speaker 1: move fast, and correlated with your running is a visual
Speaker 1: sensation where the visual world is flowing by your eyes
Speaker 1: as a result. But when you run on a treadmill
Speaker 1: at the gym, your brain is sending the same signals
Speaker 1: to your legs. Run fast, but now your visual system
Speaker 1: doesn't get the world flowing past. Instead, you're looking at
Speaker 1: the gym wall in front of you the whole time.
Speaker 1: So now when you step off, you experience the treadmill illusion.
Speaker 1: With each step you take toward the locker room, the
Speaker 1: world seems to stream by at a faster pace. It
Speaker 1: looks as though you're moving forward more quickly than you
Speaker 1: really are. Why does it happen Because your brain has
Speaker 1: an expectation about how the act of moving your legs
Speaker 1: should translate to the flow of the visual scene past
Speaker 1: your eyes, and now after the treadmill, it's had to
Speaker 1: adjust that relationship. So now there's an after effect. This
Speaker 1: is just like with Aristotle's horse or the waterfall or
Speaker 1: the feeling that you're still rocking after you get off
Speaker 1: the boat. Your brain is readjusting its expectations about the world.
Speaker 1: In the case of the treadmill, it's how the active
Speaker 1: moving your legs should translate to the flow of the
Speaker 1: visual scene past your eyes, and when you go back
Speaker 1: to the normal world, there's an after effect. As another example,
Speaker 1: you've surely noticed color after effects. If you stare at
Speaker 1: a red dot for a little while and then you
Speaker 1: look somewhere else and blink, you'll see the dot, but
Speaker 1: now it looks green. Or if you stare at something
Speaker 1: bright yellow for a little while and then you look away,
Speaker 1: you'll see the opposite color blue. And these types of
Speaker 1: color after effects can be surprisingly sophisticated. So consider something
Speaker 1: called the McCullough after effect, named after the researcher Celeste McCulloch,
Speaker 1: who discovered this in the nineteen sixties. So really get this.
Speaker 1: I want you to imagine a bunch of black and
Speaker 1: white lines like black white, black white, black white. Now
Speaker 1: imagine a group of these that are horizontal and a
Speaker 1: separate group of these that are vertical. Now, this can't
Speaker 1: just be imagined as an illusion, but has to be
Speaker 1: looked at to work. So please find a picture of
Speaker 1: these lines on eagleman dot com Slash podcast to go
Speaker 1: over there and take a look at this. Okay, So
Speaker 1: look farther down the page and you'll see lines that
Speaker 1: look the same as these black and white lines, but
Speaker 1: now they're colored. The horizontal lines are green and the
Speaker 1: vertical lines are red. And what I want you to
Speaker 1: do is stare at these colored lines for a bit,
Speaker 1: like a few minutes. The green lines go side side,
Speaker 1: the red lines go up and down. Now go back
Speaker 1: to the original black and white lines and you'll see
Speaker 1: something amazing. You'll see that the spaces between the black
Speaker 1: and white horizontal lines now look reddish, and the spaces
Speaker 1: between the black and white vertical lines look greenish. This
Speaker 1: is the McCullock effect. You gotta try this for yourself.
Speaker 1: It's really worthwhile. Now why does it happen. It's because
Speaker 1: when you stared at the colored lines, your brain said, hey, wait,
Speaker 1: for some strange reason, everything horizontal is appearing green to
Speaker 1: me and everything vertical is looking red to me. But
Speaker 1: those features shouldn't really be correlated, and so it adjusts
Speaker 1: itself to cancel out this strange relationship. Going on, and
Speaker 1: when you look at the black and white lines again,
Speaker 1: you're seeing the after effect. The horizontal lines were being
Speaker 1: internally shifted in your brain towards the opposite color red,
Speaker 1: and the vertical lines toward green. And again, I just
Speaker 1: want to be clear, this has nothing to do with fatigue,
Speaker 1: because in nineteen seventy five, two researchers showed that if
Speaker 1: you stare at these red and green lines for fifteen minutes,
Speaker 1: the after effect can last three and a half months.
Speaker 1: So your brain is always doing an active recalibration of
Speaker 1: the world out there. And this active recalibration is why
Speaker 1: in the nineteen eighties many people began to see the
Speaker 1: text in the books appearing to have a red tint
Speaker 1: because at that time the population had just begun to
Speaker 1: use computer monitors to do word processing. And as some
Speaker 1: of you remember, unlike modern monitors, these early monitors only
Speaker 1: displayed one color green, and so you typed a bunch
Speaker 1: of lines of text on the screen and it looked
Speaker 1: like horizontal lines of green on a black background. So
Speaker 1: people would say there at these horizontal green rows for
Speaker 1: hours at a time, all day while they were doing
Speaker 1: their computer work. And so when they picked up a book,
Speaker 1: the horizontal lines of black and white text were shaded
Speaker 1: with the complementary color red. It was the McCullock effect.
Speaker 1: Their brains were adjusting to a world of horizontal green lines,
Speaker 1: and their reality changed accordingly. Now I mentioned that the
Speaker 1: computer users in the nineteen eighties also experienced this illusion
Speaker 1: when they looked at the IBM logo emblazoned on the
Speaker 1: front of the floppy disk sleeve. If any of you
Speaker 1: remember the old IBM logo, it's the three letters spelled
Speaker 1: out of horizontal lines, as though you're looking at IBM
Speaker 1: put through a egg slicer, And so the logo, just
Speaker 1: like the horizontal lines in the book, looked tinged with red,
Speaker 1: and designers at IBM were flummixed about this because they
Speaker 1: had definitely not printed they're black and white design with
Speaker 1: any red in it, and yet everyone was insisting that
Speaker 1: they had. So all these after effects teach us something wild.
Speaker 1: Even though you just open your eyes and look around
Speaker 1: the world and assume you're just seeing reality out there,
Speaker 1: in fact, your brain is doing massive work behind the scenes.
Speaker 1: It's always recalibrating itself on the fly to try to
Speaker 1: improve what it's seeing, to try to get rid of
Speaker 1: information that doesn't matter, so it can surface the information
Speaker 1: that does matter. And this active recalibration of the world
Speaker 1: that's happening behind the scenes. This applies to everything. How
Speaker 1: much motion is in the world, how stable the ground is,
Speaker 1: whether vision flows past us when we move our legs,
Speaker 1: whether lions are infused with color. None of the stuff
Speaker 1: is decided in our genetics. It's all calibrated by our experience.
Speaker 1: Now I want to take this concept of adaptation one
Speaker 1: level deeper. I want to introduce the concept that your
Speaker 1: brain is making things invisible if they are expected. So
Speaker 1: take this as an example. Let's say you take a
Speaker 1: yellow ping pong ball and you cut it exactly in half,
Speaker 1: and you lay one hemisphere over each eye, and what
Speaker 1: you'll see is the whole world is an even blanket
Speaker 1: of yellow color. But within a few moments you don't
Speaker 1: see any color at all. It's as though you're blind.
Speaker 1: Your visual system just assumes that the world has become yellower,
Speaker 1: and so it adapts so that you'll be sensitive to
Speaker 1: other changes. So you should try this. You can find
Speaker 1: any sort of totally featureless scene that's entirely one color
Speaker 1: and fill your eyes with it. Like walk up to
Speaker 1: a wall that's entirely painted red. Walk up real close.
Speaker 1: Your eyes are filled with red, and you'll see the
Speaker 1: color will quickly drain away to neutral. Your brain says, hey,
Speaker 1: there's no more information here. I expect this color, So
Speaker 1: I'm going to adjust things so that becomes invisible and
Speaker 1: I'll tell you something else. The scene doesn't even have
Speaker 1: to be featureless for things to fade away. So in
Speaker 1: eighteen oh four, a Swiss physician named Ignos Troxler noticed
Speaker 1: something really stunning, which is, if you stare at a
Speaker 1: central point in the middle of a bunch of blobs
Speaker 1: of color, all the busyness in the periphery, all those
Speaker 1: blobs will eventually disappear. I have an example of this
Speaker 1: on eagleman dot com slash podcast. Check it out. It's
Speaker 1: really amazing. What you do is you keep your gaze
Speaker 1: fixed right in the middle of the picture, and you'll
Speaker 1: see there's all these smooth blobs of color, and within
Speaker 1: about ten seconds, if you're keeping your eyes still, all
Speaker 1: these surrounding blobs just start to disappear into the background,
Speaker 1: and soon you are looking at what appears to be
Speaker 1: a blank page. Now this is wild, and if you
Speaker 1: move your eyes around just a bit, all these blobs
Speaker 1: come back into your consciousness. This illusion is called the
Speaker 1: Troxlur effect, and what it demonstrates is that if there's
Speaker 1: a blob of something in your peripheral vision that is
Speaker 1: not changing at all, it will evaporate from your perception. Now,
Speaker 1: what is going on here? The answer is that your
Speaker 1: visual system is always seeking motion and change. Something that's
Speaker 1: fixed and none changing doesn't really matter, and it quickly
Speaker 1: becomes invisible. Good information is expected to update, and things
Speaker 1: that don't change get ignored by the system. So what
Speaker 1: prevents your kitchen, your workplace from becoming like a Troxler
Speaker 1: illusion with all the motionless features disappearing. Well, First, most
Speaker 1: of the world is made up of hard edges, not blobs,
Speaker 1: and those are easier for your visual system to hang onto.
Speaker 1: But there's a deeper reason, although you're not generally aware
Speaker 1: of it. Your eyes are constantly jumping and jiggling around.
Speaker 1: So just observe a friend's eyeballs. You'll notice that her
Speaker 1: eyeballs are making about three rapid jumps every second of
Speaker 1: her waking life. These are called thecods. Bang bang bang bang, bang,
Speaker 1: And if you watch even more closely, you'll find that
Speaker 1: in between the big jumps, her eyes are constantly performing
Speaker 1: these little micro jitters. They're called microsucods. Bang. Now, is
Speaker 1: something wrong with your friend's eyes? No, these rapid movements,
Speaker 1: both the big and the small. What they're doing is
Speaker 1: they're keeping the image on the retina fresh totally unconsciously,
Speaker 1: her eyes are working to maintain a constantly changing image.
Speaker 1: Why do they bother. It's because any image that remains
Speaker 1: perfectly fixed on one position on the retina will become invisible.
Speaker 1: Here's how to prove this to yourself. If you wear
Speaker 1: contact lenses, take a marker and draw a small shape
Speaker 1: on the front of your contact right in the middle.
Speaker 1: When you put the contact back in place on your eye,
Speaker 1: you'll see something there, but it won't last long. It
Speaker 1: rapidly fades to invisibility. Now, this phenomenon underscores the fundamental
Speaker 1: fact that brains care about change. Just like with the
Speaker 1: Troxler effect, features that don't change yield little information about
Speaker 1: the world. All the important information comes from things in flux. Now,
Speaker 1: if you don't have contact lenses, don't worry, because you're
Speaker 1: already performing a similar experiment without knowing it. So you
Speaker 1: have blood vessels that sit on top of your retina
Speaker 1: at the back of your eye, and these blood vessels
Speaker 1: should be seen superimposed on top of everything you look
Speaker 1: at because they're smack in front of your photoreceptors. But
Speaker 1: these blood vessels are totally invisible to your perception. Just
Speaker 1: like the drawing on your contact lens. These blood vessels
Speaker 1: are fixed in position with respect to your retina, So
Speaker 1: no matter how much your eyes move, they can't refresh
Speaker 1: the image of these blood vessels. Even though the vessels
Speaker 1: interpose themselves between you and the world, they perceptually disappear.
Speaker 1: It's like a magic trick. Now you might have noticed
Speaker 1: a flash of these vessels when the eye doctor shines
Speaker 1: the pen light in your eyes. Boom, there they are.
Speaker 1: In this situation, the beam of light can cause the
Speaker 1: vessel to cast a shadow at an unusual angle, and
Speaker 1: your visual system suddenly takes notice something unexpected has just
Speaker 1: occurred at the retina, and that's the only time you
Speaker 1: witness this massive network that obstructs your view. If you
Speaker 1: haven't seen this before, pause this podcast. Go into a
Speaker 1: dark room and shine a beam of light in your eye.
Speaker 1: From an angle, you'll see the blood vessels appear in
Speaker 1: front of you. Just note that your visual system will
Speaker 1: adapt fairly quickly, so the trick is to keep moving
Speaker 1: the light to different angles to maintain the image. So
Speaker 1: this strategy of ignoring the unchanging keeps your brain poised
Speaker 1: to detect anything that moves or shifts or transforms at
Speaker 1: the extreme. This is how reptile visual systems work. A
Speaker 1: reptile can't see you if you stand still because they
Speaker 1: only register change. They don't bother with position, and such
Speaker 1: a system is perfectly sufficient. Reptiles have been surviving and
Speaker 1: thriving for tens of millions of years. You probably remember
Speaker 1: the scene in Jurassic Park where they say don't move.
Speaker 1: The dinosaurs can't see you if you don't move. Well,
Speaker 1: this is why reptiles ignore anything that does not change.
Speaker 1: So let's return to the waterfall illusion. Why doesn't your
Speaker 1: visual system shift so much that the waterfall is perceived
Speaker 1: as standing still. Well, first, there might be limits to
Speaker 1: the recalibration. It simply can't recalibrate enough to subtract off
Speaker 1: the massive motion of the falls. But I think there's
Speaker 1: another possibility, which is that you haven't watched the waterfall
Speaker 1: for long enough, and if you did, it would eventually
Speaker 1: recalibrate all the way. How long might that take? Two
Speaker 1: months of steering at the waterfall two years. In theory,
Speaker 1: if you were to watch for a long enough time,
Speaker 1: then the short term changes in your visual system would
Speaker 1: eventually to longer lasting changes, leading eventually to changes at
Speaker 1: the deepest levels of the system. So ever present background
Speaker 1: motion would become invisible to us. And this leads to
Speaker 1: a crazy speculation that I made in my book Live Wire.
Speaker 1: It's crazy, but logically sound. Are parts of the world
Speaker 1: invisible to us that should be obvious. Imagine there was
Speaker 1: something like a cosmic rainfall that had existed your entire life.
Speaker 1: It would be completely invisible to you because you've never
Speaker 1: seen otherwise. So your visual system would have set the
Speaker 1: downward motion as it's zero point. If the cosmic rain
Speaker 1: suddenly stopped, it would seem as though the whole world
Speaker 1: were suddenly moving upward. We would believe that something had
Speaker 1: just appeared ascending rain, even though the real rain had
Speaker 1: just ended. And this situation could happen in any sensory channel.
Speaker 1: Imagine the beat beep of a cosmic alarm clock with
Speaker 1: no snooze button all the time, all throughout the cosmos
Speaker 1: bee beep beep. If it were totally regular, you wouldn't
Speaker 1: hear it because your brain would have adapted to it.
Speaker 1: If the cosmic alarm suddenly ceased, everyone would hear a
Speaker 1: great beat, beat, beat, but we would have no idea
Speaker 1: that we were experiencing the after effect with the external
Speaker 1: sound totally inside our heads. Successful adaptation makes regularities invisible.
Speaker 1: So we've been talking about these illusions as the results
Speaker 1: of adaptation, but there's actually the other way to look
Speaker 1: at this. These illusions result from prediction. So if you
Speaker 1: subtract away the downward motion of the waterfall or the
Speaker 1: rocking of the boat, or the drawing on your contact lens,
Speaker 1: that is equivalent to predicting its continued existence. In other words,
Speaker 1: when brain circuitry adjusts, it's making a guess about what
Speaker 1: the world is likely to be in the next moment.
Speaker 1: It stops talking about news that is expected to continue.
Speaker 1: So think again about your retinal blood vessels. They are
Speaker 1: perceptually invisible because your visual system predicts them away. It
Speaker 1: knows they are going to be there, so it ignores them.
Speaker 1: It's only if those expectations are violated, like when a
Speaker 1: light shines in from a strange angle that your brain
Speaker 1: spends any energy on representing that data. Your brain doesn't
Speaker 1: want to pay the energy car of spiking neurons, so
Speaker 1: the goal is to reconfigure the network to waste as
Speaker 1: little power as possible. If a pattern streams in that's
Speaker 1: predictable or even partially guessable, the system saves energy by
Speaker 1: structuring itself around that input, so it is not to
Speaker 1: be surprised by it. If your brain is quieter, that
Speaker 1: means there are fewer violations of expectations. Things in the
Speaker 1: outside world are going approximately as forecast. In other words,
Speaker 1: an energy conscious brain wants to predict away everything possible
Speaker 1: so it can save its energy for just representing the unexpected.
Speaker 1: Silence is golden. So although many neuroscientists think of activity
Speaker 1: in neurons as the representation of things in the world,
Speaker 1: it may turn out that spikes are the unpredicted, energy
Speaker 1: expensive part. The representation of something totally expected would be
Speaker 1: nothing but a hush falling over the neuronal forest. The
Speaker 1: system makes adjustments only when it gets surprised. If your
Speaker 1: brain thinks that all bricks weigh the same amount, and
Speaker 1: then you attempt to pick up a brick made of lead.
Speaker 1: The violation of your expectation causes cascades of changes to
Speaker 1: deal with this new turn of events. But in contrast,
Speaker 1: if everything is successfully predicted, there's no need to change anything.
Speaker 1: For these reasons. When you first look at the Troxler picture,
Speaker 1: you notice the blobs, and when you first put in
Speaker 1: the contact lens you detect the drawn on shape. But
Speaker 1: after a short time, your brain adjusts itself. It's no
Speaker 1: longer surprised. Let me give you another example of the
Speaker 1: brain predicting things away. So I talked in a previous
Speaker 1: episode about the neosensory wristband, which converts sounds into patterns
Speaker 1: of vibration on the skin, and deaf people wear the wristband,
Speaker 1: and with time they can start to hear the world
Speaker 1: through the signals that get to their brain via the skin.
Speaker 1: If you're interested in learning more, that was episode twelve.
Speaker 1: But the point I want to make here is what
Speaker 1: happens when a hearing person first puts on the wristband.
Speaker 1: They always react with surprise when they're feeling all the
Speaker 1: sounds of the world, and they say WHOA, this thing
Speaker 1: is picking up on my own voice. They're always startled
Speaker 1: by that because it seems like they shouldn't be registering
Speaker 1: their own speech. But of course, your ears pick up
Speaker 1: on your voice all the time. It's typically the loudest
Speaker 1: voice in your conversations because your own mouth is the
Speaker 1: closest one to your ears. But because you can perfectly
Speaker 1: predict your own vocalizations, you hardly hear your own voice.
Speaker 1: You don't notice it your brain is predicting it away.
Speaker 1: Or take this example, when people wear the wristband, they're
Speaker 1: struck by the volume of other predictable sounds that they
Speaker 1: normally pay no attention to because they're the ones who
Speaker 1: create the sounds, like flushing the toilet or closing the
Speaker 1: door behind them, or their own footsteps. It's not that
Speaker 1: your auditory system doesn't register these sounds, but instead it's
Speaker 1: that you actively predict them away. So you flush the
Speaker 1: toilet or close the door or walk and these things
Speaker 1: don't sound particularly loud to you, but it becomes obvious
Speaker 1: when you're wearing the wristband. You can't believe how loud
Speaker 1: these events are because your brain has not yet learned
Speaker 1: to predict the signals coming up your arm, so your
Speaker 1: brain actively recalibrates because that allows it to burn less energy.
Speaker 1: But there's an even deeper principle at work here in
Speaker 1: the darkness of your skull. You your brain is striving
Speaker 1: to build an internal model of the outside world. When
Speaker 1: you walk around your house, you don't pay any attention
Speaker 1: to the environment because you already have a good model
Speaker 1: of it. In contrast, when you're driving in a foreign
Speaker 1: city you're trying to find a way to a particular hotel,
Speaker 1: you're forced to look around at everything, the street signs,
Speaker 1: the store naims, the building numbers because you don't already
Speaker 1: have a good model of what to expect. So how
Speaker 1: do you build up a good internal model? What is
Speaker 1: the neural technology that allows you to zoom in on
Speaker 1: those data points that don't match your expectation while ignoring
Speaker 1: everything else that's already counted for. We call this attention.
Speaker 1: You pay attention to the unexpected bang, or the unforeseen
Speaker 1: brush on your skin, or the surprising movement in your periphery.
Speaker 1: Attending allows you to put your high resolution sensors on
Speaker 1: the problem and figure out how to incorporate it into
Speaker 1: your model. Ah that's just the lawnmower. Ah, that's the kitten. Ah,
Speaker 1: that's the housefly. Your model is now updated. In contrast,
Speaker 1: you don't pay any attention to the feeling of the
Speaker 1: shoe on your left foot, because you already have an
Speaker 1: internal model of it, and that model is consistently predicting
Speaker 1: what you're receiving, at least until you get a pebble
Speaker 1: in your shoe that draws your attention because suddenly the
Speaker 1: model calls for an update. The difference between predictions and
Speaker 1: outcomes is the key to understanding a strange property of learning,
Speaker 1: which is that if you're predicting something perfectly, your brain
Speaker 1: doesn't need a change. Further so, say you learn that
Speaker 1: the ding of your phone predicts that you just got
Speaker 1: a text message. Your brain will quickly learn the relationship
Speaker 1: between those two, in large part because of the relevance
Speaker 1: of text messages to your social life. Then let's say
Speaker 1: your phone gets a software update, and as a consequence,
Speaker 1: the arrival of the text message is now signaled by
Speaker 1: a ding plus a vibration. It turns out that your
Speaker 1: brain won't train up on the vibration. This is an
Speaker 1: effect known as blocking. Your brain already knows that the
Speaker 1: ding predicts the text, so it has no need to
Speaker 1: learn about something new. If your phone merely vibrates without
Speaker 1: the ding, your brain won't know the meaning of that queue.
Speaker 1: It has learned nothing about that. This phenomenon of blocking
Speaker 1: makes sense when we understand that changes in the brain
Speaker 1: happen only when there's a difference between what was expected
Speaker 1: and what actually happened. So having an internal model of
Speaker 1: the world allows us to make predictions and quickly detect
Speaker 1: when we're wrong, which tells us where to attend and
Speaker 1: how to update things. And this sort of system, by
Speaker 1: the way, is becoming interesting to engineers thinking about the
Speaker 1: future of machinery. Several companies are starting to work on
Speaker 1: devices that operate this way, from tractors to airplanes. An
Speaker 1: internal model of the world allows a machine to make
Speaker 1: its best guesses about the events that are expected to unfold,
Speaker 1: and when events are consistent with the predictions of the
Speaker 1: machine's algorithm, nothing has to change, and it's only when
Speaker 1: the inputs go off script that the software needs to
Speaker 1: pay attention to update the machines model. Okay, so now
Speaker 1: that we've talked about all the ways that your system adapts,
Speaker 1: it will now be easy to understand how drugs modify
Speaker 1: nervous systems. When you consume a drug, that changes the
Speaker 1: number of receptors for that drug in the brain to
Speaker 1: such an extent that you can look at a brain
Speaker 1: after a person has died and determine his addictions by
Speaker 1: gauging his molecular changes. This is why people become desensitized
Speaker 1: or tolerant to a drug. The brain comes to predict
Speaker 1: the presence of the drug. It adapts its receptor expression
Speaker 1: so it can maintain a stable equilibrium when it receives
Speaker 1: the next hit. In a physical literal way, the brain
Speaker 1: comes to expect the drug to be there. The biological
Speaker 1: details have calibrated themselves accordingly, and because the system now
Speaker 1: predicts a certain amount to be present, more of the
Speaker 1: drug is needed to achieve the original high. This recalibration
Speaker 1: that's the basis of the ugly symptoms of drug withdrawal.
Speaker 1: The more the brain is adapted to the drug, the
Speaker 1: harder the fall is when the drug is taken away
Speaker 1: with draw All symptoms vary by a drug, from sweating
Speaker 1: to shakes to depression, but what they all have in
Speaker 1: common is a powerful absence of something that was anticipated.
Speaker 1: I suggested in my book Live Wire that this understanding
Speaker 1: of neural predictions also gives us an understanding of heartbreak.
Speaker 1: And this is because the people that you love become
Speaker 1: part of you, not just metaphorically but physically. You absorb
Speaker 1: people into your internal model of the world. Your brain
Speaker 1: refashions itself around the expectation of their presence. So after
Speaker 1: the breakup with a lover, or the death of a friend,
Speaker 1: or the loss of a parent, the sudden absence represents
Speaker 1: a major departure from homeostasis. As Khalil Jabraun put it
Speaker 1: in the prophet quote and ever has a bin that
Speaker 1: love knows not its own depth until the hour of separation.
Speaker 1: In this way, your brain becomes like a negative image
Speaker 1: of everyone you've ever come in contact with. Your lovers,
Speaker 1: your friends, your parents. They fill in their expected shapes,
Speaker 1: and just like feeling the waves after you've departed the
Speaker 1: boat or craving the drug when it's not there, so
Speaker 1: your brain calls for the people in your life to
Speaker 1: be there. And when someone moves away or rejects you,
Speaker 1: or dies, your brain struggles with its thwarted expectations. Slowly
Speaker 1: through time, it has to readjust to a world without
Speaker 1: that person. Now I want to cover one more thing
Speaker 1: in today's episode, which is how your brain knows how
Speaker 1: to adjust. And I suggest it in Live Wire that
Speaker 1: there's a very simple but surprise strategy underlying all this.
Speaker 1: And to understand this, consider something like phototropism in plants.
Speaker 1: This is where plants capture maximum light by constantly adjusting
Speaker 1: their position. So if you watch a plant growing in
Speaker 1: fast motion, you'll see that it doesn't grow straight towards
Speaker 1: the light source. Instead, it overshoots its trajectory by a
Speaker 1: little bit, then it undershoots by a bit, and back
Speaker 1: and forth. It's not a pre planned mission. It's a
Speaker 1: spastic dance with constant correction. And by the way, you
Speaker 1: find something similar in the strategy that bacteria use to
Speaker 1: find food. When they're searching for the center of a
Speaker 1: food source, like a bit of sugar that's fallen on
Speaker 1: the kitchen counter, they make their way to the sugar
Speaker 1: by employing three elegantly simple rules. One randomly selected direction
Speaker 1: and move in a straight line. Two if things are
Speaker 1: getting better, keep going. Three if things are getting worse,
Speaker 1: randomly change directions by tumbling. In other words, the strategy
Speaker 1: is to lock down the approach when conditions are improving
Speaker 1: and dump it when it's not working. By this simple policy,
Speaker 1: a bacterium can quickly and efficiently work its way to
Speaker 1: the densest point of the food source. So I've proposed
Speaker 1: that there's a similar principle at work in the brain.
Speaker 1: Instead of working its way towards maximizing sunlight or food,
Speaker 1: it works towards maximizing information. And I call this strategy infotropism.
Speaker 1: The hypothesis suggests that neural circuitry constantly shifts to maximize
Speaker 1: the amount of information it can extract from the world.
Speaker 1: So consider what we've heard in some of the previous episodes.
Speaker 1: We saw the way the brain comes to figure out
Speaker 1: its sensory organs, its eyes and ears and nose and fingertips.
Speaker 1: When you're a baby, you don't know what those data
Speaker 1: streams mean, and your brain figures it out with experience.
Speaker 1: And we saw the way a brain comes to learn
Speaker 1: how to control a body, no matter if that body
Speaker 1: possesses fins or legs or robotic arms, whatever the case.
Speaker 1: The brain fine tunes its circuitry to maximize data it
Speaker 1: streams from the world. So this fine tuning is helped
Speaker 1: along by rewards, which cause broadcasts throughout the circuitry to
Speaker 1: announce that something worked. In this way, with a minimum
Speaker 1: of preprogramming, the system works out how to optimize its
Speaker 1: interaction with the world. Or Another example is that your
Speaker 1: brain will adjust to whatever language you're exposed to as
Speaker 1: a baby, such that you can hear those sounds, but
Speaker 1: you lose the ability to distinguish other sounds and other languages.
Speaker 1: This is all infotropism at work. Baby's brains adjust to
Speaker 1: maximize the data that matters around them. On a longer
Speaker 1: time scale, when a person goes blind, other senses take
Speaker 1: over the visual cortex, and in an upcoming episode we'll
Speaker 1: learn how neurons actually pull this off, but for now,
Speaker 1: note that we can interpret this takeover as in photropism.
Speaker 1: The brain is maximizing its resources so that it can
Speaker 1: best interpret whatever data flows in. And just think about
Speaker 1: the Mecullic effect with the horizontal and vertical colored lines.
Speaker 1: Your visual system works to separate the dimensions of color
Speaker 1: and orientation because it's trying to maximize information from the world,
Speaker 1: and so accordingly, it doesn't want to mix together these
Speaker 1: separable measures. So although the effect is typically viewed as
Speaker 1: simply a fun visual illusion. The work happens under the
Speaker 1: hood for a deeper reason. If something were causing a
Speaker 1: tinge to appear on lines, like some strange overhead lighting
Speaker 1: or something wrong with your optics, your brain would reorganize
Speaker 1: itself to take care of this, canceling out this weird relationship,
Speaker 1: and by doing so, it would maximize your capacity to
Speaker 1: extract information about colors and orientations separately. By separating two
Speaker 1: dimensions that statistically should be unyoked, your brain can best
Speaker 1: gather information from the world. Here's a cool example of
Speaker 1: infotropism at the level of neurons, the retina in your eye,
Speaker 1: the set of photoreceptors the back of your eye captures
Speaker 1: the light of the world and tells the brain what
Speaker 1: it's seeing out there. But it reads the world differently
Speaker 1: in the day and in the night. So in bright
Speaker 1: daylight there are plenty of photons to capture, and so
Speaker 1: each photoreceptor just takes care of its own tiny dot
Speaker 1: of the scene, and this gives nice and high resolution.
Speaker 1: But at night it's a different story. There are few
Speaker 1: photons to be captured, and so now the important job
Speaker 1: of the retina is to detect that something was there,
Speaker 1: even if it can't tell that with high resolution. So
Speaker 1: at night, the photoreceptors do their work very differently. Essentially,
Speaker 1: they join forces with their neighbors. They change the details
Speaker 1: of their internal molecular cascades so they can link arms
Speaker 1: with one another. Now they're operating as a team, they
Speaker 1: can detect much lower levels of light, they can detect
Speaker 1: fewer photons out there. So what we have here is
Speaker 1: this beautiful and sophisticated strategy that has the retina operate
Speaker 1: differently as the light levels go up or down. When
Speaker 1: it's bright out, the system achieves high spatial resolution, and
Speaker 1: when it's dark, photoreceptors pool together to have a better
Speaker 1: chance to catch photons, which results in vision that's more
Speaker 1: sensitive to dim light but blurrier in resolution. You've surely
Speaker 1: noticed this at nighttime when you can't quite make out
Speaker 1: what something is out there, whereas it would be easy
Speaker 1: in the daytime. So the point here is that your eyes,
Speaker 1: your retinas, put enormous work into shifting themselves around to
Speaker 1: maximize information that matters whether the photons are plentiful or
Speaker 1: they're rare. The retina optimizes itself to get useful data.
Speaker 1: In the day, it captures the most detail so that
Speaker 1: it can spot the rabbit at the distance. In dim light,
Speaker 1: it shifts to higher sensitivity to capture whatever's out there
Speaker 1: with lower detail, capturing the shadowy essence of the jaguar
Speaker 1: lurking in the gloom. Mother Nature figured out not only
Speaker 1: how to build an eye, but also how to adjust
Speaker 1: its circuitry on the fly, so it can operate well
Speaker 1: in different contexts, all to make the best use of
Speaker 1: what's available. It is infotropic. Just like the plant seeks light,
Speaker 1: the retina seeks data, and more generally, as we've seen
Speaker 1: in this episode, the brain is always seeking information. It
Speaker 1: constantly adjusts its circuitry to maximize the data it can
Speaker 1: draw from the world. So if you're staring at a waterfall,
Speaker 1: it discounts the downward motion so it can be sensitive
Speaker 1: to other things. If you put on yellow sunglasses, your
Speaker 1: brain discounts that shift in color so that it can
Speaker 1: be more sensitive to other information. And you can detect
Speaker 1: this shift when you take off the sunglasses and you
Speaker 1: have a big blue color after effect. So to wrap up,
Speaker 1: this episode plugs into the big picture we've been talking
Speaker 1: about in many episodes, and we'll see in many more,
Speaker 1: which is that the brain is locked in silence and darkness,
Speaker 1: and its job is to build an internal model of
Speaker 1: the outside world. And what we're seeing a little more
Speaker 1: clearly now is that its internal model is really there
Speaker 1: to make predictions. If the world proceeds as expected, the
Speaker 1: brain saves energy. So when the world changes, let's say,
Speaker 1: becoming a place with lots of downward motion or yellow
Speaker 1: light or something, the brain adjusts to match it so
Speaker 1: that it can be maximumly positioned to gather new information
Speaker 1: that's unexpected. So these after effect illusions seem funny and
Speaker 1: may be trivial, but they're actually a powerful way for
Speaker 1: us to reveal this constant adaptation that your brain is doing.
Speaker 1: Over the course of many episodes coming up, we're going
Speaker 1: to see how fundamentally the brain is a prediction machine,
Speaker 1: and that is the driving engine behind its constant self reconfiguration.
Speaker 1: By modeling the state of the world, the brain reshapes
Speaker 1: itself to have good expectation and therefore to be maximally
Speaker 1: sensitive to the unexpected. Go to Eagleman dot com slash
Speaker 1: podcast for more information and to find further reading. Send
Speaker 1: me an email at podcasts at eagleman dot com with
Speaker 1: questions or discussions, and I'll be making an episode soon
Speaker 1: in which I address those. Until next time, I'm David Eagleman,
Speaker 1: and this is Inner Cosmos.