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Your eyes are liars: These strawberries aren't red

Sure, this pie looks like it's decorated with red strawberries, but it's all an illusion. They're actually blue-grey.

Perception is a funny thing. Visual signals need to be interpreted by the brain, which fills in the gaps of what we're seeing with our eyes so that we can make sense of the world around us. The Dress, which had the internet in a tizzy two years ago, was a perfect example of this. Because of the lighting in the background of the image, many people interpreted its colours as white and gold instead of blue and black.

Something similar is going on with this picture of strawberries on a pie, created by Akiyoshi Kitaoka, professor of psychology at Ritsumeikan University in Japan, and optical illusion artist. Believe it or not, there is not a single red pixel in it.

This is because of a phenomenon known as "colour constancy". You may have seen optical illusions that task you with determining which of two areas in an image is darker, but it turns out they are both exactly the same shade.

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This is because our eyes "correct" colours for various conditions. In the case of The Dress, some people corrected the colours based on the source of illumination. In the case of the strawberry pie, it's because of what we know about strawberries.

"The brain corrects colour when objects are filtered through different lights," explained David Atchison, professor in the School of Optometry and Vision Science at the Queensland University of Technology. "If you didn't know that strawberries are supposed to be red, they would indeed look grey."

In other words, because we know strawberries are supposed to be red, our brain perceives a blue filter over the image and corrects for it, making the strawberries appear red.

Need proof? Check out this handy GIF. And if you still don't believe us, Atchison has a hot tip so you can try it at home: "Putting a piece of paper with a hole in it to isolate part of the strawberries will give you the same effect."

strawberrynored-1.gif

Moving the cursor over the "red" parts of the image in Photoshop's colour picker.

Dave Cheng/CNET