Science Finally Explains This Classic Optical Illusion
From the popular mechanics
A new paper explains how a classic optical illusion arises in the eye and not in the brain.
Optical illusions often play with how our eyes work together and how our brain interprets visual data.
These researchers were able to examine a group of children with recently restored eyesight.
With a new set of experiments, scientists at the Massachusetts Institute of Technology say they have a better idea of why we're falling for a very old optical illusion called simultaneous brightness contrast.
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The question is whether the “illusion” part is generated in our brain or in the visual data that our eyes send to our brain. In the course of three experiments, the scientists believe that the phenomenon occurs in each eye before the information from both eyes is brought together.
The classic image above shows two shapes overlaid on a gradient, and asks the viewer to guess which shape is darker. Or maybe the picture is of a house in sunshine and shade with two highlighted points. Which is darker?
Photo credit: Sinha et al., Vision Research, 2020
The answer is that they are all the same color, and scientists have been wondering for a century or more why we see the same color differently depending on the context.
In their new work, the MIT researchers went through three studies. The first was to ask a group of American doctoral students to determine the relative brightness of the images. The contextual brightness representations were repeated and mixed - some light on dark surfaces, some dark on light, etc. - and students were asked to identify which were light and dark.
In the second study, the researchers presented the same images and information, but had the subjects wear glasses that isolated each eye. By controlling which images each eye could see separately, they came to the conclusion that the optical illusion comes from something that is innate in the eye, and not something that comes from the way normal vision in stereo works. (Many illusions work by playing the way our eyes have to work together, like magic eye images and the flat 3D of movies or the Nintendo 3DS.)
The third study is completely different. The researchers had the opportunity to examine a group of young children in India who were congenitally blind, but whose eyesight could be restored by surgery. By studying this group, researchers can try to extrapolate how much of our experience with this optical illusion is learned and collected from it, how we have seen the world all our lives.
Photo credits: MIT / Vision Research
Indeed, children with restored vision responded to the optical illusion just as did people with years of vision.
MIT professor Pawan Sinha, who participated in the experiment, "is running an initiative in India called Project Prakash, whose mission is to treat children suffering from avoidable forms of blindness such as congenital cataracts," MIT wrote in a statement . Cataracts are one of the leading causes of blindness worldwide, but can often be remedied by a very inexpensive operation that uses a new lens.
All three studies suggest a theory or at least an essential way to limit future explanations. People who used only one eye still saw the illusion completely. People with brand new visions also experienced the illusion.
It seems that our perception of the relative brightness of these different shapes is formed in each eye and, as the researchers say, may only be formed in the retina. Future researchers are encouraged to look at the good side.
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