These skulls look purple and orange, but they’re both red?!

“The Dress” controversy had the internet furious. Half of the social media population was very sure it was a blue and black dress, while the other side’s (rightfully) confidence defended the dress being white and gold. Neither were right.

Here comes round two of a similar optical illusion test. Is the left skull purple and the right one orange? Seems to be an easily closed case...

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However, when you isolate the stripes that make up the skulls, you’ll find neither skull has purple bones. In fact, all of the bones are the same color. Take a step back to include the stripes, and they shift to purple and orange.

The pigments morph because of the Munker-​White illusion, which “shifts the perception of two identical color tones when they’re placed against different surrounding hues.”

The illusion most likely results from what David Novick, a computer scientist at the University of Texas at El Paso, calls the “color-completion effect.”

“The phenomenon causes an image to skew toward the color of the objects that surround it.” In a black-and-white image, a gray element would appear lighter when it’s striped with white, and darker when with black.

Many neuroscientists think that our brain does this, because of the neural signals in charge of relaying information about the pigments. When that information about the pigments in our visual field get mixed together, our neural signals create a color somewhere in the middle.

Here, the left skull is striped with blue in the foreground, and the other with yellow stripes. When the original skulls take on the characteristics of the separate surroundings, they look like different colors entirely.