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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.


【TED-Ed】なぜ幻覚は起きるのか? (Why do we see illusions? - Mark Changizi)

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emkulu 2013 年 9 月 28 日 に公開
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