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  • Why do we see illusions?

  • I'm going to tell you about some of my research

  • where I provided evidence

  • for a different kind of hypothesis

  • than the one might be on the book

  • on your coffee stand.

  • Alright, so let's look at one of the illusions here.

  • And this is a stand-in for many, many kinds of illusions

  • that are explained by this hypothesis.

  • I'm just going to walk through it for this particular one.

  • As usual in these things,

  • these two lines are in fact parallel,

  • but you perceive them to bow outwards at their centers.

  • At the center where those radial lines are,

  • it's wider in your visual field

  • than the parts above and below.

  • And this is remarkable

  • because it's a remarkably simple stimulus.

  • It's just a bunch of straight lines.

  • Why should one of the most complicated objects in the universe

  • be unable to render this incredibly simple image?

  • When you want to answer questions like this,

  • you need to ask,

  • "Well, what might this mean to your brain?"

  • And what your brain is going to think this is

  • is not some lines on a page,

  • your brain has evolved to handle

  • the kinds of natural stimuli

  • that it encounters in real life.

  • So when does the brain encounter stimuli like this?

  • Well, it seems a little bit odd,

  • but in fact, you've been encountering this stimulus all day long.

  • Whenever you move,

  • whenever you move forward in particular.

  • When you move forward, you get optic flow,

  • flowing outwards in an individual field

  • like when the Enterprise goes into Warp.

  • All of these objects flow outwards

  • and they leave trails or blur lines on your retina.

  • They're activating mini-neurons all in a row.

  • So, this is a version of what happens in real life

  • and this another version of what happens in real life all the time.

  • In fact, cartoonists know about this.

  • They put these blur lines in their cartoons

  • and it means to your brain, motion.

  • Now it's not that in real life you see blur lines,

  • the point is that it's the stimulus at the back of your eye

  • that has these optic blurs in them

  • and that's what tells your brain that you're moving.

  • When you move forward, your eyes fixate like cameras,

  • like snap-shot cameras,

  • it fixates,

  • it fixates,

  • little camera shots,

  • and each time it fixates,

  • when you're moving forward,

  • you get all this flowing outwards.

  • So when you take a fixation,

  • you end up with this weird optic blur stuff

  • and it tells you the direction you're moving.

  • Alright, that's half the story.

  • That's what this stimulus means.

  • It means that your brain thinks

  • when it's looking at the first image

  • that you're actually on your way,

  • moving towards the center.

  • Still doesn't explain why you should perceive

  • these straight lines as bowed outwards.

  • To understand the rest of the story,

  • you have to understand that our brains are slow.

  • What you would like is that when light hits your eye,

  • then, ping!

  • Immediately you have a perception

  • of what the world is like.

  • But it doesn't work that way.

  • It takes about a tenth of a second

  • for your perception to be created.

  • And a tenth of a second doesn't sound very long,

  • but it's a long time in normal behaviors.

  • If you're moving just at one meter per second,

  • which is fairly slow,

  • then a tenth of second, you've moved ten centimeters.

  • So if you didn't correct for this delay,

  • then anything that you perceived

  • to be within ten centimeters of you,

  • then by the time you perceived it,

  • you would have bumped into it or just passed it.

  • And of course, this is going to be much worse,

  • it's going to be much worse

  • in a situation like this.

  • Your perception is behind.

  • What you want is that your perception should look like this.

  • You want your perceptions at any time, "t",

  • to be of the world at time "t".

  • But the only way that your brain can do that

  • is that it has to instead of generating

  • a perception of the way that the world was

  • when light hit your retina,

  • it has to do something fancier.

  • It can't passively respond and create a best guess,

  • it has to create a best guess about the next moment.

  • What will the world look like in a tenth of a second?

  • Build a perception of that

  • because by the time your perception

  • of the near future occurs in your brain,

  • the near future will have arrived

  • and you'll have a perception of the present,

  • which is what you want.

  • In my research, I provided a lot of evidence that,

  • and there's other research areas that have provided evidence,

  • that the brain is filled with these sorts of mechanisms

  • that try to compensate for its slowness.

  • And I've shown that huge swaths of illusions are explained by this,

  • this just being one example.

  • But let me finish it by saying

  • how exactly does this explain this particular example?

  • So, the question really we have to ask is

  • how do those two vertical lines in that first stimulus,

  • how do they change in the next moment

  • were I moving towards the center

  • that all those optical lines are suggesting that I'm moving,

  • what happens to them?

  • Well, let's just imagine.

  • Imagine you've got a doorway.

  • You got a doorway,

  • and imagine it's a cathedral doorway

  • just to make it more concrete,

  • it's going to be helpful in second.

  • When you are very far away from it,

  • the sides are perfectly parallel.

  • But now imagine what happens when you get closer.

  • When you get really,

  • it all flows outwards in your visual field,

  • flowing outwards,

  • but when you're really close,

  • imagine the sides of the doorway are here and here,

  • but if you look up at this cathedral doorway

  • and you did your fingers like this,

  • the sides of the doorways are going up

  • like railroad tracks in the sky.

  • What started off as two parallel lines

  • in fact bows outwards at eye-level

  • and doesn't go outwards nearly as much above.

  • So in the next moment,

  • you have a shape that's more like this next picture.

  • It ends up in fact, the projective geometry,

  • there's the way the things project

  • in fact change in this way in the next moment.

  • So when you have a stimulus like this,

  • well, your brain has no problem,

  • there's just two vertical lines

  • and there's no cues

  • that there's going to be a change in the next moment,

  • so just render it as it is.

  • But if you add cues,

  • and this is just one of many kinds of cues

  • that can lead to these kinds of illusions,

  • this very strong optic blur cue,

  • then you're going to perceive instead

  • exactly how it will appear in the next moment.

  • All of our perceptions are always trying to be about the present,

  • but you have to perceive the future

  • to in fact perceive the present.

  • And these illusions are failed perceptions of the future

  • because they are just static images on a page,

  • they're not changing like in real life.

  • And let me just end by showing one illusion here,

  • and if I could, I'll quickly show two.

  • This one's fun.

  • If you just fixate at the middle, there,

  • and make stabbing motions with your head,

  • looming towards it like this.

  • Everybody do that.

  • Make short, stabbing motions.

  • Because I've added blur to these optic flow lines,

  • your brain says, "they're probably already moving,

  • that's why they're blurry."

  • When you do it, they should be bursting out

  • in your visual field faster than they should.

  • They shouldn't be moving that much.

  • And a final one I'll just leave in the background is this.

  • Here are the cues of motion,

  • the kinds of cues that you get on your retina

  • when things are moving.

  • You don't have to do anything here,

  • just look at it.

  • And many of you, raise your hand,

  • if things are moving when they shouldn't be.

  • It's weird, right?

  • But what you have now are the cues,

  • that from your brain's point of view,

  • your eye,

  • you have the stimulus on your eyes saying,

  • "Oh, these things are moving."

  • Render a perception of what they'll do in the next moment.

  • In the next moment, they should be moving

  • and they should have shifted.

  • Alright, thank you very much.

Why do we see illusions?

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B1 TED-Ed outwards brain perception stimulus moving

【TED-Ed】Why do we see illusions? - Mark Changizi

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    emkulu posted on 2013/09/28
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