Last week, a poorly lit picture of a dress sparked one of the fiercest — and perhaps most meaningless — debates in Internet history: Was it white and gold or blue and black? Now that the dust has settled, and the definitive answer established it’s blue and black, it’s time to move onto a more important question: Why did people see the dress differently? ReAction asked biology professor Stephen Van Hooser to break down the science of the dress.

We tend to think we are able to understand the world perfectly through our eyes. In reality, our visual system makes best guesses with limited information. Take, for example, the fact that the world is three-dimensional but the images on our retina are two-dimensional. Our visual system has to make guesses as to the locations of objects based on this two-dimensional image.

Visual illusions force us to confront the fact that the brain still makes guesses, or especially when the visual system doesn’t have all the information its needs.

Consider the “Cornsweet effect” (Purves et al., 1999). If we look at the following scene, we perceive that the gray patch above the white rectangle has the same luminance as the gray patch below the white rectangle.

However, if we fill in the white rectangle with a rounded contour and transition in luminance, our perception changes. We believe the light source must be coming from above, and that it is shining more directly on the upper surface than the lower surface. Yet, because the amount of light that hits our retina from the upper surface and lower surface is the same, we assume that the upper surface is inherently darker than the lower surface. That is, we imagine that if we were to shine a light directly on the two surfaces, the upper surface would appear darker than the lower one.

Our perception is influenced by assumptions that our visual system makes about the light source and reflectance of the materials.

This is what is going on in “the dress,” except that our uncertainty is in the chromatic nature of the illuminating light. Is the illuminating light daylight, which contains a broad range of component colors ranging from blue to red? Or is it an artificial light source, which usually contains much higher percentages of green, yellow and red light?

An object that looks white when illuminated by daylight will reflect more yellow light when it is illuminated by a yellowish light source, simply because of the color of the light that’s being shone on it. However, if we know that the light source is yellow-dominated, and we have some objects in the room that allow us to guess that the light source is yellow-dominated, then our brain will “correct” the raw data it is receiving and we will still perceive the object as white.

Now let’s consider an object that looks blue when illuminated by daylight. When it is illuminated by a yellowish light source, it will still reflect the weak blue light that is present in the yellowish light source, and it will also reflect some weak yellow light simply because the object is getting hit with a lot of yellow light. That is, in terms of “raw data,” our eyes will collect light more broadly across the spectrum — blues, greens, yellows and reds. If we have cues that tell us that the light source is yellow-dominated, we will still see the object as blue.

If we see a blue object illuminated by yellow-dominated light but for some reason our brain thinks that it is actually illuminated by daylight, our brains will perceive the object as white.

The dress illusion is strong because the light source is ambiguous. Some people might imagine that the dress is being illuminated by daylight, and they will perceive the material as white. Others may think it is under strong artificial illumination, and the material will appear blue.

From moment to moment, our impression can change, but we always have a single impression.

It looks pink to me.

This story was written by Stephen Van Hooser.

Update:  Rosa Lafer-Sousa at MIT has combined the dress image with images from Beau Lotto that evoke strong sensations of yellow-dominated or blue-dominated illuminating light. Separately zoom in on both images. See if you can see the dress as blue or white in the different figures! http://web.mit.edu/bcs/nklab/what_color_is_the_dress.shtml