The science behind that absurd color-changing dress, explained

By now, everyone on the internet has seen — and fought bitterly over — this stupid dress. Some people will swear up and down that the dress is blue and black. Others insist, without any doubt, that it's white and gold.

As it turns out, the dress is blue and black in reality. But then why do so many people see it so differently in the photo?

Visual scientists can mostly explain what's going on here: We interpret the color of objects based on how we interpret the light around those objects. Different people are interpreting the ambiguous lighting in the dress photo in different ways, so their brains are essentially tricking them into seeing the dress as different colors. (A more detailed explanation is below.)

The problem is that this only partly solves the mystery. Why are different people's brains tricking them in such radically different ways? One visual scientist calls this "one of the biggest individual differences I've ever seen" in color perception. Is it purely random? Or is there some variation in how our brains work? Here's a rundown of everything we know so far about the dress question.

How we see color is a function of how we interpret light

Preparations for a hot air balloon ride near Ottawa: Recognizing that the colors in the sunlight and in the shade are the same is an example of color constancy. (Shanta/Flickr)

First, scientists who study vision agree there's a very basic phenomenon at play here. The colors that we think we see are always determined by how we interpret the light around us. That is, our eyes and brains are always trying to adjust for different types of light in order to make judgments about what color things are.

Normally, our brains are very good at correcting for light

Normally this is a very, very useful skill to have. If you look at a gray building during a sunset, it will be bathed in a much redder light than it is during midday. But our brains are very good at correcting for the effects of that light and seeing the building as the exact same color at all times — gray.

"The brain always faces the problem of figuring out how much of the light arriving at the eye from an object is due to how brightly illuminated the object is and how much is due to how highly reflective the object is," says David Williams, Allyn Professor of Medical Optics and director of the Center for Visual Science. "We are usually extremely good at making this judgment, a perceptual skill known as lightness constancy."

But optical illusions can trick the brain

Occasionally, though, this system breaks down. Vision scientists have exploited this feature of our brains to create optical illusions that fool us about colors. Here's a famous one:

The squares labeled "A" and "B" are the exact same shade of gray. But our eyes see them as different colors because we're making assumptions about the light around the squares. We assume that square "B" is being bathed in shadow, so our brains tell us it must be a lighter shade of gray in reality.

Indeed, if we connect the two squares, it becomes clear that the squares are identical shades:

Below is another optical illusion based on color, as pointed out by New Scientist (which has an excellent dress explainer). Here are two squares on the Rubik's cube. Both are actually the exact same shade of gray. But we see them as wildly different colors because we're adjusting for the light around them:

(Dale Purves/New Scientist)

The dress is an accidental example of an illusion

So that brings us to the photo of the famous dress, which Williams says may be "an accidental example of such an illusion." Let's look at it again:

(Swiked)

The key feature of the photo is that it's very hard to tell how the dress is being lit. Is the light in the room bright or dim? Blue or yellow? There aren't many unambiguous visual clues. So people's brains are fumbling around, making assumptions.

"The photograph does not have a lot of information about the nature of the illumination, and some people may see it as brightly illuminated and others less so," Williams says. "Hence the large variation in their reports of the colors of the dress."

"For example, if your brain decides that the dress is brightly illuminated, then it will also conclude that it must be made of darker fabrics such as blue and black. But if it decides the dress is only dimly illuminated, then it will also reach the conclusion that the dress reflects back a lot of the light falling on it. In that case, the dress must be made of more reflective fabrics light white and gold."

Randall Munroe of XKCD had a vivid illustration of this. If you change the color of the background, the dress looks entirely different (even though it's the exact same dress):

(XKCD)

Williams notes that the best way to tell the true color of the dress would be to photograph it against "a rich background of objects illuminated in the same way as the dress."

But this dress explanation still leaves a huge mystery

The problem is that we still haven't explained a key mystery of this dress business. Why are different people's brains making vastly different assumptions about what's going on? After all, with most optical illusions, we're all fooled in the exact same way. But not here.

Early on in the dress controversy, a number of writers suggested that it's because different people have different numbers of color-sensitive cones in their eyes. But Williams is skeptical that this is the reason.

'People have remarkably similar visual systems'

"Unlikely," he says. "Aside from some folks, mostly male, who are color blind, people have remarkably similar visual systems." Indeed, his research has found that, while people do have wildly varying numbers of cones in their eyes, they still tend to see color in the same way.

Instead, he argues it has to do with differences in how our brains our working. "My guess," Williams says, "is that the dress photo changes in color and lightness from person to person because their brains are making different unconscious assumptions about how the dress is illuminated."

But that just moves the mystery back a step. Why are different people's brains making different assumptions about how the dress is illuminated?

One possibility is that it's all random. Upon first encountering the photo, our eyes happen to randomly fall on a particular contextual clue in the photo — we look at a small part of the photo suggesting that the dress is dimly lit or bathed in blue light and then interpret the color of the dress based on that clue. The clues we ended up detecting at first are almost pure happenstance.

But then, once we see the dress as one color, it's hard to "unsee" it and view it the other way. That is, it's a bit like the famous duck-rabbit illusion — although people can easily flip back and forth between the duck and the rabbit, whereas far fewer people have reported seeing the dress as both colors.

Alternatively, perhaps there are actually subtle differences in how different people's brains are working — and that's why we have two radically opposed factions of dress viewers. Whatever the cause, the dress is proving endlessly fascinating to color researchers. Here's what Jay Neitz, a neuroscientist at the University of Washington who'd fielded endless phone calls about this, told Wired: "I've studied individual differences in color vision for 30 years, and this is one of the biggest individual differences I've ever seen."

Further reading: What the weird dress tells us about the metaphysics of consciousness

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