What color is this dot? New illusion demonstrates strange vision quirk

An optical illusion made up of nine simple dots reveals surprising things about the eye and the brain

By Nora Bradford edited by Allison Parshall

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Last August, Hinnerk Schulz-Hildebrandt noticed something strange while looking at a flight map on his phone. As his eye followed the plane’s flight path, he noticed that the line appeared purple, then blue, then purple again. The hue kept changing depending on where he looked on the screen; the line looked purple when he looked at it, but blue when he saw it with his peripheral vision.

Schulz-Hildebrandt, a biomedical optics engineer at Harvard Medical School, took his observation and developed an illusion that allows us all to see how malleable our perception of color is. The illusion, recently published in the journal Perception, contains nine purple dots on a blue background. When those of us with color vision focus on one spot, it appears more purple while the rest appears to fade to blue.

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Why does disorienting illusion work? Part of this depends on color-detecting cells, called cones, in our retinas. Different cones detect different wavelengths of light. There are relatively few blue-detecting cells in the center of our retina, called the central fovea, which is the part of the eye where light from the center of our field of vision hits. Having less perception of blue could trick our brain into seeing a more purple dot when we look directly at it.

And there’s another anatomical quirk in our eyeballs that diminishes our perception of blue, specifically in the center of our vision. In front of the fovea is a protective layer of yellow pigments that acts like internal sunglasses, absorbing some of the blue and near-ultraviolet light before it even reaches the cones in our retina. In turn, we do not perceive as much blue in the center of our vision as we do at the periphery. “We usually don’t notice it because our brains have learned to ‘calibrate’ the difference,” says Jenny Bosten, a visual neuroscientist at the University of Sussex in England.

These blue-absorbing pigments can sometimes cause people to see a red dot called a Maxwell’s spot in the center of their vision. Last year, experimental psychologist Akiyoshi Kitaoka of Ritsumeikan University in Japan independently developed the Maxwell point illusion, which is a very similar illusion with the nine blue points of Schulz-Hildebrandt. It uses combinations of blue, green or red dots on a solid green or blue background to highlight the lack of blue perception in central vision.

The blue background of Schulz-Hildebrant’s nine-point illusion probably makes the effect stronger, Bosten says. In a process called simultaneous contrast, our brains perceives colored objects in relation to the color of the background against which they are perceived. When a gray circle is surrounded by a red background, we perceive the gray as “less red”, making it appear slightly green. Likewise, when a blue-violet dot has a blue background, we perceive it as more purple. Combined with the fact that a blue dot will appear less blue when viewed in the center of our vision, these phenomena create a striking illusion.

The illusion doesn’t reveal a new process, Bosten says, “but it exploits several processes we already know to create a beautiful effect.”