Science Finally Explains This Classic Optical Illusion

Caroline Delbert

From Popular Mechanics

  • A new paper explains how a classic optical illusion is formed in the eye, not the brain.
  • Optical illusions often play on how our eyes work together and how our brains interpret visual data.
  • These researchers were able to study a group of children with recently restored vision.

With a new series of experiments, scientists from the Massachusetts Institute of Technology say they have a better idea why we fall for a very old optical illusion called simultaneous brightness contrast.

The question is whether the “illusion” part is generated in our brains or in the visual data our eyes are sending to our brains. Over the course of three experiments, the scientists say they believe the phenomenon occurs in each eye, even before the information from both eyes is merged together.

The classic image above shows two shapes superimposed on a gradient and prompts viewers to guess which shape is darker. Or maybe the image is of a house in sunshine and shadow with two spots highlighted. Which is darker?

Photo credit: Sinha et al., Vision Research, 2020

The answer is that they’re all the same color, and scientists have wondered for a century or more why we see the same color differently based on context.

In their new paper, the MIT researchers went through three studies. The first involved asking a group of American graduate students to decide relative brightness of images. The depictions of contextual brightness were iterated and shuffled—some light on dark surfaces, some dark on light, and so forth—and the students were asked to identify which were light and dark.

In the second study, researchers presented the same images and information, but had subjects wear glasses that isolated each eye. By controlling which images each eye could see separately, they concluded the optical illusion comes from something innate within the eye, not something that comes from the way normal vision acts in stereo. (Many illusions do work by playing on the way our eyes must work together, like Magic Eye pictures and the shallow 3D of movies or the Nintendo 3DS.)

The third study is something totally different. Researchers had the opportunity to study a group of young children in India who were congenitally blind, but could have their sight restored by surgery. By studying this group, the researchers say, they can try to extrapolate how much of our experience with this optical illusion is learned and accumulated from how we’ve seen the world all our lives.

Photo credit: MIT/Vision Research

Indeed, the children with restored vision responded to the optical illusion the same way people with years of vision do.

MIT professor Pawan Sinha, who participated in the experiment, “runs an effort in India called Project Prakash, whose mission is to treat children suffering from preventable forms of blindness such as congenital cataracts,” MIT wrote in a statement. Cataracts are one of the leading causes of blindness in the world, but are often reparable with a very low-cost surgery that inserts a new lens.

All three studies point to one theory, or at least one substantial way to help limit future explanations. People using just one eye still saw the illusion in full. People with brand new vision experienced the illusion as well.

It seems our perception of the relative brightness of these different shapes is formed in each eye and, the researchers say, possibly the retina alone. Future researchers are encouraged to, well, look on the bright side.

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