You have no doubt been tricked by the classic contrast illusion before: one gray shape is surrounded by a dark background, while another gray shape is surrounded by a lighter background. Although both gray shapes appear starkly different, they are exactly the same shade. 

A new study has helped to unravel how this trick of perception works by investigating how brightness estimation is interpreted by our visual system, including those of children who have recently had their eyesight restored through surgery. Reported in the journal Vision Research, their findings suggest the phenomenon occurs even before the information reaches the brain.

Researchers led by MIT studied nine recently sighted children living in India and found they were tricked by the illusion almost immediately after their sight was restored after cataract surgery. The finding indicates that brightness estimation and the illusion occurs using simple neural circuitry that doesn’t require any previous visual experience. This, according to the researchers, suggests that it most likely occurs before the information reaches the visual cortex of the brain as it does not require high-level processing. 

A second experiment attempts to explain the phenomenon using our understanding of how the visual system constructs a single picture of the world by combining images from our eyes. This combination occurs in a part of the brain’s cortex called V1, so name because it’s the first stage of visual processing in the cortex. Using a set of specially designed images and glasses, the researchers found that brightness estimation did not need to wait until information from the two eyes was merged in the V1. Instead, it appears to have already occurred by that point, perhaps as early as the eye's retina.  

“Our experiments point to the conclusion that this is a low-level phenomenon,” Pawan Sinha, senior study author and a professor of vision & computational neuroscience at MIT’s, said in a statement.

“The implication of results from the first two sets of studies was that if brightness estimation is really a low-level process, and the circuitry is located as early as the retina, then perhaps this is an innate dispensation,” Sinha says. “This is something that the visual system comes prepared to do, right from birth.”

But if you’re still wondering why the illusion manages to trick your perception so effectively, here is Sinha's explanation: “You could have a really dark piece of cloth under a bright spotlight, and the amount of light that you get from it could be the same as, or even more than, the amount of light from a white piece of paper under dim light. The brain is presented with the challenge of figuring out how light or dark a surface is based on just the amount of energy it's receiving. In essence, the brain has to figure out the two numbers that were multiplied (illumination level and surface darkness) to produce the one number it is receiving (incoming energy) – a seemingly impossible task since infinitely many pairs of numbers can all yield the same product.”