My name is Pascal Wallisch, a professor of psychology

and data science at NYU.

I'm an expert in visual perception.

I'm here to answer your questions from Twitter.

This is Optical Illusion Support.

[upbeat music]

Ann O'Malley, oh, nice username, at anomaly, asks,

have you seen the optical illusion where you stare

at a spiral, then look away,

and things appear to be crawling?

Yes, this is a very striking example.

It's called the motion after effect.

When you take a look at this video here,

if you look at the dot in the center, it's 10 seconds or so,

and then look away, I recommend looking at your hand,

the hand will appear like it is crawling.

And what's going on here?

We actually understand this very well.

There's been thousands of studies on this over the decades.

We know the neurons in your brain

that are underlying the perception of motion,

once they get active for a while, they get tired,

they fire at lower rate than they did before,

and this is not, you know, anthropomorphizing that.

You can measure this with voltage

in the brain, and we have.

User Espernyan asks,

I was staring at the spinning dance illusion thing,

and she started spinning backwards.

This goes viral on social media every so often,

and let me be very clear early on.

Every time this does go viral, people say,

Oh, if you see it spinning clockwise or counterclockwise,

this will tell you whether you're

like a left brain or a right brain person,

and I'm here to tell you no.

That has nothing to do with that.

That is literally fake news, that is not true.

Reasons you can tell that is not true is that if you look

at this video, you might notice that midway looking at it,

the dancer changes the direction of motion.

So if I was clockwise before,

now it goes counterclockwise, and vice versa.

Obviously, nothing has changed.

And similarly, if you watch this again,

you will have a different experience maybe.

The image itself is devoid of critical information,

so the contour is just black, so it's unclear

if this hand is in front or this hand is in front.

So whether the dancer will spin this way

or that way will depend on your assumption

if you see the front hand or the back hand, yes?

And the way you can switch this,

if you deliberately force yourself to perceive

that one arm is the front and one is the back.

This highlights two important things.

One, your perception of the world is an interpretation

of the world, and that interpretation depends

on the assumptions that you make about the world

if there's not enough information about the world.

This is a great one.

Spyros Langkos asks, what is motion-induced blindness?

Motion-induced blindness is a great example

of like how important motion is.

So what you see in the video is

if you focus on the dot in the middle,

and there's three yellow dots that are always there,

if you keep focusing it on the middle,

those dots in the periphery start to disappear.

Now, I'm claiming that they're always there.

They never go away.

You can confirm that, and you can rewind the video

by just by keeping looking at that.

So what's going on here?

Motion is very, very strong.

So this is another example

of how your perception is a best guess of the world.

So there's two competing models now.

One is that there's three dots and they're always there.

And one is a rotating plane

that's kind of occluding those dots,

and when in doubt, your brain will go

to move, to motion, and I'm actually now gonna ruin magic

for you I guess, because most magic tricks rely on this.

The idea is if I create some distraction by, you know,

making a lot of motion here,

I can do literally anything else

in plain sight and you will not see it.

Sass the Seabass asks can someone please explain

the difference between the phi phenomenon

and beta motion to me?

And I'm happy to help.

Beta motion is happening right now.

If you're seeing this on a device,

and this is a video of me talking, there is no motion.

So you see I'm moving right now, but there's no motion.

You see still images, something like 30 or 60,

or whatever the frame rate of your monitor is, a second.

Your brain is inferring the motion between that.

So basically if there's two objects,

first one is an object here and then object is here,

the brain is filling in that something is moving.

If something is here and then disappears,

and then something is here, the brain concludes, infers,

there's an unconscious inference, conclusion, it's a guess,

it didn't just was here and then it disappeared

and then was here.

No, it moved, that's the inference.

And if you make this fast enough, 20 hertz or 30 hertz,

it will appear like smooth motion.

That's the beta motion.

What phi is, is you have the objects static,

sometimes in a ring, and you have then

some shadowy thing occluding, it will appear

like a object is moving around them.

That's phi.

That's what phi motion is.

Coli Annn asks, how do mirages happen?

So weird.

That is a example of a genuine optical illusion.

Most things that are called optic illusions are

not optical illusions.

What do I mean by that?

Optics means light, it's Greek for light.

So a mirage is an example of an actual optical illusion.

In contrast, most of the other things we'll see

today are not optical illusions, they're visual illusions.

The difference is those are not in the light.

Those are created by your brain.

It usually happens if there is, you know,

air that's being heated by a surface, like for instance,

a road, and that is then hitting a layer of air above it

that's colder, and at that transition between the cold air

which is denser, and the hot air which is less dense,

light will bend.

That's what creates mirage.

Next question by Andres Gomez Emilsson, does it feel

like the dark parts are pushing brightness

onto the center and the white is pushing out?

First of all, this is called the Asahi illusion by Kitaoka.

What I find is most striking

about this is that the center part here, right?

Does to me, and probably to you,

although it depends on your screen, look brighter

than the background white in the background, right?

Can you see that?

And it's not, it's the exact same whiteness

in the center than in the periphery, right?

So what is going on here is that the gradients of luminance

that are pushing into the center,

as the poster asks, make you assume

that you're looking at something like the sun.

The size of your pupil is a reflex.

It dilates and constricts,

depends on how much light is coming in.

So if you look at a very bright light source

like I'm now looking at the studio lights,

my pupils will constrict.

Research shows that if you look at this illusion of an image

that's not brighter, you pupils will also constrict,

which is quite striking, 'cause this is a reflex.

It's not under conscious control.

John Teegarden asks, this is a pretty cool illusion.

Does the black hole look like it's expanding to you?

And this was discovered this year in 2022,

and what was discovered was two things, number one, seven

out of eight people report that yes, the black hole look

like it's growing, it's expanding into the periphery.

Second of all, it's also been shown

that as if it was actually getting darker,

your pupil is dilating as if it was actually getting darker,

so this is again stunning,

'cause this is not under your conscious control.

This is a reflex, and it's responding

to your perception of the image, not to the actual light.

Lisa Gatto2 asks, is there an explanation for the dress?

I was late.

Now Lisa, you are in luck.

I have the actual dress.

The dress itself is unambiguously

in bright light, black and blue.

Yes, we can all agree on that.

It looks black and blue.

But that's not what we were asking.

We were asking was, is the image of the dress

black and blue or white and gold?

If you ask that, most people will say, two thirds,

according to our research, will see this image

as white and gold, so what's going on here?

The lighting information in this image is ambiguous,

because it was taken inside, but it was overexposed

on a cell phones picture of a Samsung camera.

So in other words, it's unclear if this was

artificial lighting or outside lighting,

natural lighting, yes?

If there's not enough information,

our brains generally don't say I don't know.

They're very immodest.

They will fill in the missing information

by making assumptions about the world.

What assumptions you make about the lighting depends

on what experience of light you have.

So what we could show in our research is that

people who are morning people are more likely to see

the dress as white and gold,

whereas night owls are more like see it as black and blue.

And the reason for that is they're assuming

that illumination that they have seen more

of in their life, and mentally subtract that from the image.

Originally in 2015 when this came out,

two thirds of the people saw it

as white and gold and one third saw as black blue,

even though in real life,

it's unambiguously 100% black and blue.

Elizabeth Kropp asks

how did they make Tupac in hologram form?

Well, there's two questions here.

First question is, how do holograms work,

and how did they do this specific one?

Holograms work by having lasers,

two lasers interfere with each other, and lasers by the way,

don't have, just have one wavelength.

They also have one phase, they're phase locked,

so from that interference patterns,

you can then reconstruct the entire image.

In other words, a genuine hologram will look 3D

from any perspective you look at it,

but it will be somewhat monochromatic.

So what it did in a Tupac case,

it's actually not a hologram, just wanna be very clear.

What they did is something else that's called,

it's a version of Pepper's ghost where you have

a transparent screen at 45 degrees where you project

previous images or previous footage of Tupac onto it.

That's an illusion.

If you look out of that plane

from a different perspective, from a different angle,

it would not look 3D like a genuine hologram would.

So the Tupac hologram was not a hologram.

BS Jeffrey asks,

has anyone experienced the wagon wheel effect

when not watching TV or movies?

So first of all, what is the wagon wheel illusion?

Let's say you watch a car, and their tire is spinning

at a certain rate.

That interacts with the sampling rate of the camera.

If the frame rate of the wheels, of the spokes

of the wheels of the car, rims I guess, is faster than that,

what I call aliasing, it will actually look

like it's moving backwards.

So basically that's from the interaction between the frame

in the video and in reality.

Some people get that in reality without the camera,

and this is, to be honest with you,

a very controversial subject, because the implication is

that your mind has a certain frame rate,

and certain people might have a different frame rate

than other people,

which is why some people might see it more than others.

It's as related to how tired you are,

or how well rested you are.

Might be also be an individual difference.

But the implications are profound, 'cause that means

your mind is running at a certain frame rate.

And yeah, I think that is true.

Different people's brains probably run

at different frequencies.

Jarell Ebuka asks, in a world full of confusion,

can we really trust our eyes?

And that's a wonderful question,

and I'll try to give you a nuanced answer.

Your brain does not live in the outside world.

The brain is encased fully in a bony shell.

In other words, it's extremely, completely dark.

What your perception is

of the world is ultimately just a guess.

Now to be fair, that guess is usually a very good guess.

All of us got here without accidents,

usually navigate the world fine, so in other words,

usually your eyes are not lying to you.

And the reason for that is that billion years

of evolution made sure that if you're wrong by this case,

the punishment could be death.

So this has been debugged

in a very relentless process over a billion years.

Only a stoner asks, how real is reality?

Our eyes deceive us, our brains trick us into seeing faces.

Our thoughts create themselves.

And you think any drugs to make shirt trippy?

Username checks out.

That is a really good point.

It is the very existence of illusions that illustrates

the difference between what you perceive

and what is out there, which makes it clear that

what looks to us like reality is ultimately just a guess,

even if the guess is pretty good.

What illusions are is a, you know, a way

for psychologists to show people that there is a difference

between what's out there and what your best guess is.

I don't wanna freak anyone out.

What this means is, and most people don't notice is that

The Matrix actually is a documentary.

It is your brain that creates this matrix for you.

Again, this can be very feel very, feel very claustrophobic,

because you're like, Okay, what is really out there?

You will see motion when there's no motion.

You will see color and there's no color.

You will be confused about the spatial layout of things,

and things like that.

So the question is, what actually is out there?

Chris Lintott asks,

I've written often about the moon illusion,

which makes the moon larger and close to the horizon.

Does anyone know if the Apollo astronauts experience

an Earth illusion?

First of all, great question.

So what the moon illusion is, as the poster asks is

if you look at the moon on the horizon,

it will look considerably larger

than if you look at it high up in the sky.

It's the same moon, so how is that possible?

When I was in grad school a long time ago,

I read a very thick book about 100 theories

of why the moon illusion happens,

so this is still unresolved.

What I find most compelling is once the moon is

on the horizon, you have a local context.

In other words, if there is a hill, or a mountain,

or a house even, it will make the moon look

really, really big, because you know that the house

or the mountain is really, really big, yes?

Whereas if it's high up in the sky,

then you have no reference point.

So it'll look like it actually is visually,

actually very small.

The moon is only one degree of visual angle

or something like that.

Would you get that on the moon?

And the answer is, to my knowledge,

nobody saw the Earth that low.

But my prediction would be, yeah, if you go

to a moon crater and look at the Earth

from that vantage point, you would get an Earth illusion.

Daniel Barker asks, Adelson's checker-shadow illusion hurts

my brain every time I look at it.

Why must you lie to me, eyes?

Okay, as you can see here,

I think we can all agree that the shade

of gray in A looks different than the shade of gray B,

even though I tell you

that they are the exact same shade of gray.

But even if you know that, you cannot see them like that.

You still will see B as lighter than A.

What's going on is that your brain is taking

into account the shadow that's coming

off the cylinder here, and is mentally subtracting that

from the gray in B, which makes it look lighter.

Why must eyes lie?

Well, the eyes must lie to you because the image

that the eye gets is actually a very poor image.

The optics are bad.

The brain has to like, kind of like enhance the image

for you to make sense of the world.

So appearances can be deceiving.

Rich Sweet asks is this Animal Collective album cover moving

or my tripping out?

And here it is.

A good proportion of people will see motion here,

but there is no motion,

and that's what makes it so trippy.

You know that there's no motion, and you can actually see

that if you look at something closely,

but you see things moving in the periphery.

This disconnect between your perception of motion

and you know there's no motion creates this trippy feeling.

Tweetyaca asks does everyone else get

the Hermann grid illusion when using their Mac keyboard,

or is it just me?

The Hermann grid is, people see dark spots,

illusionary dark spots, where bright lines meet.

And you can see that we have a Mac here.

I see that right now.

There's bright lines, and in this intersection

of those bright lines is a dark spot that's not there.

That's illusory, that's a Hermann grid.

This is an example of what's called ladder inhibition.

The idea is in the brain neurons that are next to each other

that process a certain region of space inhibit each other,

and the more they're active,

the more strongly they inhibit their neighbor.

Now, if you're very strongly inhibited

by four strong active neighbors,

you're gonna be strongly inhibited.

It will appear darker,

'cause it can inhibit the peripheral less.

Jessica Pin asks does anyone know what it means that I see

the lines as the same in the Muller Lyar illusion?

Here is the Mullar Lyar illusion.

All of these lines in the center are the same length,

but most people will perceive this one

in the middle as longer than this one,

most people, but not everybody sees that.

This actually happens to be what I'm interested in in my lab

where we study individual differences in perception.

What's going on here is that people take the context

into account when interpreting the scene.

I would bet that you probably have experience

with like maybe you're like an artist

or something like that.

Maybe you can see through what the appearance is,

and you can recover, like you can ignore the context.

You know what the length of lines is, so artists,

for instance, don't see the Muller Lyar illusion.

So people who have a lot of experience with actual drawings,

they will not be fooled by this.

Breakdown Paradise asks, how do magic eye pictures work?

So this is an example of where people are inferring,

or minds are inferring the third dimension

from a two-dimensional image.

This is called stereopsis.

The two eyes see the image slightly differently,

from slightly different perspectives,

but your brain knows that, so basically it can compute

if you see the same image slightly differently,

that there's actually a three-dimensional depth to that.

This distance has to be reconstructed in your mind.

Elizasstuff asks how do 3D movies work?

The idea is that two images are presented.

So if you have like colored lenses,

so let's say one lens is orange and one lens is blue.

So the red will get 3D orange lens,

but not the blue, that creates an offset,

and this offset is the same idea.

It will then created a 3D effect.

Enna Cooper asks, if you have a really long Ames room,

is there any limit to the extent of illusion?

Could you have to crawl on your hands and knees at one end

of the room, yet slip through a gap

in the skirting board at the other?

So here's the Ames room.

So you look at that through a peephole,

and looks like a normal square room, but it's not.

One of those ends will be much farther than the other one,

but it looks like they're on the same distance away, right?

By inference, and if you think you know

they're the same distance way, the one

that is actually farther in reality will then look larger.

So the user asks a good question, how far can you push that?

And I'll put that right back at the user.

Let's find out, because in reality,

we haven't tried that big ones.

The biggest one was made in India in 2015

that's in the Guinness Book World Records,

but that was not that big, so why don't we make

a really big one to see when that breaks down?

Eventually, of course, the curvature

of the Earth will swallow one of the people.

Lupin Chevalsier asks what is forced perspective?

Okay, great, so look at this.

This is is Ames window, not Ames room.

This appears to be a square window, yes?

But it's not.

It's a trapezoid in reality, so this actually is larger

than, this part is larger than this part.

But because you know how perspective works,

you will assume that this is far away, even though it's not.

All of this comes from the fact

that you do not have this third dimension available.

You have to reconstruct it in your mind,

so it's ambiguous if this is far away, if this is far away,

so you use your familiarity to resolve that.

And you are familiar with perspective.

Every painting you look at

since the Renaissance uses this effect

where you basically have a perspective.

It's a flat image which just looks far away

if it's deeper in.

Jim Rose Circus asks, my niece asked me

why is his body so little?

And now I can't unsee it.

Yeah, that is a very good point.

Most people, of course, don't look at logos that closely,

but I'm here to tell you

that there is a bear in the Toblerone logo.

There's an arrow in the FedEx logo.

Once you saw that, you cannot unsee that.

So until your dying day, you will see this little man here

on the body of the KFC logo, and it's not just KFC.

I'm gonna ruin ducks for you forever.

Look at this duck.

All ducks are wearing dog masks.

Now that you saw that, you cannot unsee that.

What this shows is that experience factors

into your perception, just like in a dress

where like your prior experience

with a stimulus helps you resolve what you see.

Noah Gerard asks I need to see the Penrose stairs in person.

I can look at it a million times,

and still not fully understand the physics of it.

Well, there's a reason for that.

This was popularized by MC Escher,

and this 3D reality, this does not exist.

The Penrose stairs are just one example

of what Tara Aders asks, which is what is a blivet?

A blivet is an impossible object,

and they're quite a few of those.

And the whole point of it is you can, so for instance,

if someone asks, Hey, how many spikes are there?

Three, there are three if you focus on this side,

or four if you focus on this side.

Who's right?

Well, you're both wrong.

The same question you can ask is

if I'm asking is this Slinky, by the way,

it's always good you have a Slinky on hand.

Is this a circle or is it a sign wave, yes?

And the answer is the 3D reality is, Slinky is both,

depends on your perspective.

So your perspective also determines what you see,

and in this case, the 3D reality transcends

both the circle or the spiral.

Eat Right Arizona asks have you ever heard

of the age-old diet trick based on the Delboeuf illusion

to use a smaller plate in order to trick the brain

into thinking you are eating more than you actually are?

There's a lot to unpack here.

First of all, what is the Delboeuf illusion?

The answer is a circle surrounded by a larger circle,

as you can see here in the image, will appear larger

than it actually is.

The answer is yes, you can use that,

and yes, this does work.

And as research shows, weight control is all about,

it's not all, but it's in large part about portion control.

Again, when we go back to whether these illusions are good

or bad for you, you can use these illusions

for good, absolutely.

Susan Wardle asks why do we mistakenly see faces

when there are none?

This is a condition that's called pareidolia.

Yes, it is true that most people see faces everywhere.

Many people see that.

Perception's a guess,

and the guess was debugged by evolution,

so if you're wrong, you die.

In other words, if there's actually someone out there

who's trying to get you, and you think there's nobody

out to get you, you might die.

If there's no one, and you think there's someone,

you may get a little scared, but that's fine.

The payoff matrix is vastly in favor of false positives,

meaning thousands of false positives is okay,

meaning seeing face when there are no faces,

but if you miss one actual face, an enemy, you might die.

So the system is built to accept thousands

of false positives for one false negative.

All right, that's it.

That's all the illusion we have time for.

I hope you learned something useful.

See you next time.