What color are the strawberries in this photograph?

They're red, right?

Wrong.

Those strawberries are gray.

If you don't believe me, we look for one of the reddest

looking patches on this image, cut it out.

Now what color is that?

[upbeat music]

It's gray, right?

But when you put it back on the image,

it's red again.

It's weird, right?

This illusion was created by a Japanese researcher

named Akiyoshi Kitaoka,

and it hinges on something called color constancy.

It's an incredible visual phenomenon by which the color

of an object appears to stay more or less the same

regardless of the lighting conditions

under which you see it,

or the lighting conditions under which your brain

thinks you're seeing it.

To explain how color constancy works,

we're gonna be looking at a whole bunch of visual illusions

that mess with the way you perceive color.

Illusions like this one.

Can you tell which of the squares on the left

is the same color as a square on the right?

It's probably not the ones you think.

To help us out, we called up David Eagleman.

He's a neuroscientist at Stanford

and an expert in visual illusions.

You might remember him from our previous episode.

That looks so cool!

[David laughs]

We invited him to WIRED's offices in San Francisco

to spend the day running some experiments.

What is color constancy?

- The brain wants to see on object

as a particular color all the time,

irrespective of what the lighting condition is.

- [Host] Light has a lot to do with how you perceive color,

in large part because light itself can be color.

Tungsten light, named for the filament inside

of incandescent bulbs is orange.

And the color of daylight can vary dramatically

from blueish white at midday

to vibrant reds and yellows and oranges at sunset.

- So for example, we were just outside,

and I was holding white a coffee cup,

and it looks white to you.

And now inside with totally different lighting conditions,

it still looks white to you

even though what's actually hitting your eyes is different

in the lighting.

Because of what's called the illuminate

that comes and reflects off of this,

what exactly hits you is very different in these cases.

This would look white to you if we were under tungsten

or fluorescent or an incandescent bulb.

That's what color constancy is.

- So what's going on with

that picture of the strawberries?

To find out, we worked with an artist named Reina Takahashi

to create some paper strawberries

and put them under different lights.

This looks kind of like midday sun.

It's this very, very bright, clear, kind of white color.

- Yep.

- And this is a much more yellow.

This is clearly like a tungsten light.

So what happens if we move these into the lights?

- Let's try it.

Okay.

They do look slightly different,

but they remain red even though

what's called the illuminate is quite different on them.

- Now there's something interesting going on here.

You can rationally recognize that these objects

are different colors, but your brain still classifies

both grouping of strawberries as red,

which brings us back to Kitaoka's photo.

You might've noticed it has a kind of blue-green

overlay to it.

Researchers think that your visual system

perceives that overlay as the color of the light

that's hitting the strawberries.

And it corrects for that light by subtracting it

from the actual physical gray color of the pixels

in the image.

This causes you to perceive the berries as red.

So check this out.

We have some filters here that like the overlay

on the strawberry photo is blue-green,

which is opposite red on the color spectrum.

And that means if you use these filters to cover up

our camera lens, it actually blocks red light.

So if we now point our camera at these red objects,

the pixels on your computer screen are technically gray.

But to you, these objects probably still look red.

- That's what color constancy is,

is the brain always trying to say

what is that object actually in the world?

- So with that said, you might think that the reason

these strawberries look red to you is because you know

strawberries are supposed to be red.

And while researchers think that might be part

of why this illusions is so compelling,

it's not the whole story.

And here's how we know that.

These objects we just showed you,

unlike with strawberries, you have no prior memory

as to what color this kind of object should or shouldn't be.

And yet when we filter our lens to block the red light,

they still look red to you,

which when you think about it

is a pretty amazing feature of human vision and the brain.

Except there are also illusions that can leverage

that very feature against you.

Okay.

So this is the painting.

- Ah. - It's by an artist

named James Gurney.

You've seen this. - Yeah.

Not that painting, but this sort of illusion, yeah.

- [Host] Okay, so you're familiar with the conceit.

- My guess is that even though it looks like

this is under green light and that's under red light,

that the physical paint in one of these squares,

one of these squares is the same and yet they look

totally different these two conditions.

- Right, exactly.

And it's interesting you say light

'cause that's the condition we're just coming from.

Identical object under slightly different light.

And that is by appearances what looks like

what's going on here.

You've got an identical cube under what looks like

a kind of greenish light and under a kind of reddish light.

But in fact, it's actually entirely different

colors of paint.

But so the brain, maybe it doesn't matter entirely?

- That's exactly, it doesn't matter.

Because it's just what hits your retina.

And usually what hits your retina,

your brain tries to figure out

what is the illuminate that's hitting that and reflecting.

But it doesn't matter.

You can just cheat it.

So at the end it's all coming off

and hitting your retina this way.

- Right.

To help us illustrate what's going on here,

we wanted to bring the painting into the real world

and make it human sized.

So our team built this giant version out of paper.

Okay, so we've done our best to reproduce

Gurney's painting in the real world.

How's that, Juno?

- It's good. - Good?

- Instead of using paint,

what we've used is construction paper.

And interestingly one of the pieces

of construction paper there is exactly the same color

as one of the pieces here.

- Okay, and so when I look at this from where I'm standing,

this upper right cube looks the same color

as that upper right cube.

And this lower right cube looks the same color

as that lower right cube.

But that's not the match.

- Yeah, so the matching squares are actually

this one on the lower right.

- And this one on the top here.

So let's go ahead and prove that,

which we can't do in the painting,

but we can do physically like this.

Okay.

So you can see

that this is exactly the same color

of construction paper here.

- [laughs] And that is,

yeah, that is 100% the same piece of paper.

- [laughs] So if we put it back over there,

the key is that because this side of the world

appears to be bathed in a green illuminate

and this is in a red illuminate,

they end up looking quite different

because the way your brain judges the color

has to do with all the surrounding colors

as well as the illuminate around.

And so what your brain serves up to you

can be completely different that what is actually

physically hitting your retina.

- Okay, so here is my question.

Is this happening in my eye?

Or is it happening in my brain?

- Ah, good question.

That has been debated in the literature

since the beginning of these sorts of illusions.

And the answer is it's both.

There's lots of stuff happening at the level of the eye

all the way back to your brain making what are called

unconscious inferences about the world.

In other words, it's guesses about the world

based on its prior assumptions.

And so there's things happening

at all these different levels.

Your brain takes into account the context all around

and then serves up some story about what it thinks

the color is based on what is most useful.

- Right, 'cause if your brain was a complete literalist,

it would have no problem telling that that square

and that square are the exact same color.

- Yeah, that's right.

- Okay, so for me, this raises the inevitable question

that every college student has, you know, postulated

in their dorm room, which is like, how do I know

that the color I see is the same color you see?

- Right.

Actually you can't know that.

We don't know that.

Your mother taught you call this green,

and my mother taught me to call this green.

So we can transact and negotiate in the outside world,

and I can say pass the green thing.

And you can do it.

But our internal experience, we don't know if it's

the same thing on the inside or not.

And in fact, one of the things that's been surfacing lately

on the internet a lot are illusions where we can actually

demonstrate that people are having slightly different

perceptions of what's going on.

- Like the dress.

- Like the dress!

- [Host] You remember the dress, right?

Of course, you remember the dress.

Everybody remembers the dress, the viral internet sensation

that divided the internet back in 2015.

Some people saw it as blue and black.

Other people saw it as white and gold.

Still drives people up the wall.

So which camp is right?

Well, the short answer is neither.

The actual pixels in the image are blue and brown.

But the full answer is a bit more complicated.

Scientists still aren't sure why two people

can see the dress so differently.

But a popular hypothesis is that the colors you see

depend on how your brain interprets

the light hitting the dress.

If your brain thinks the light falling on the dress

is blue, it subtracts that color from the pixels