Researchers at MIT wanted to know if people are born with the ‘distorted’ vision of an optical illusion or if they develop it later.
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Two gray dots on a background consisting of a gradient from light gray to dark.
The two points are identical but look very different from each other, depending on where they are positioned in relation to the bottom.
For more than 100 years, scientists have tried to decipher the mechanism behind this classic visual illusion called “simultaneous brightness contrast”.
And now they seem to have found the answer.
Until now, scientists thought that this illusion was something that happened in the brain.
However, a study by scientists at the Massachusetts Institute of Technology (MIT) in the United States suggests that it is based on an estimate of the brightness that happens before the information reaches the brain’s visual cortex, possibly inside the retina.
“All of our experiments point to the conclusion that this is a low-level phenomenon,” says Pawan Sinha, professor of computer vision and neuroscience in the Department of Cognitive and Brain Sciences at MIT.
“The results help answer the question of what is the underlying mechanism in this fundamental process of estimating brightness, which is a basic component of many other types of visual analysis.”
This effect has attracted the attention of artists for centuries and several researches on how we perceive tones in contrast to each other have also been conducted since the 19th century, says the scientific publication Science Alert.
But not all questions had been answered.
The MIT researchers carried out a series of experiments to prove their hypothesis.
In one, they created an image of a cube that looked lit from one side, with a face that looked a little brighter than the other.
When identical gray dots are placed on each face of the cube, the dot on the face that appears to be in the shadow appears to be darker than the identical dot placed on the most “illuminated” face.
“This is the opposite of what happens with standard simultaneous contrast screens, where a dot on a dark background looks brighter than a dot on a light background,” says Sinha.
In this second example of simultaneous brightness contrast, two cubes look similar, but they have different effects on the circles on their faces. The upper cube makes the right circle brighter, while the lower cube makes the left circle look brighter
Image: MIT / Pawan Sinha
Although we are not always aware, luminance (a magnitude that expresses the luminous flux in a given direction) contributes to our brightness estimates, suggesting that high-level thinking processes are not required to make this contrast judgment.
This finding suggests that brightness estimation occurs very early, before information from each eye is combined in visual processing and reaches the brain.
The researchers evaluate the hypothesis that the calculation of brightness probably happens in the retina.
“This is something that the visual system is already prepared to do, since birth,” says the researcher.
To prove their findings, the researchers studied blind children who recently had their sight restored, showing them optical illusions.
“The prediction is that if the brightness estimate was really an innate mechanism, soon after children with congenital blindness started to see, they should be victims of the illusion of simultaneous contrast.”
That was exactly the researchers’ finding.
In a study of children and adolescents between eight and 17 years who underwent cataract surgery, they were all susceptible to this illusion. The tests were performed between 24 and 48 hours after removing the surgical bandages after surgery.
Sinha says the conclusions are consistent with other research, but there are still some doubts.
This may mean that other brain processes are also involved in later stages.
“Many of the phenomena that we quickly assign to high-level processes in reality may be instances in some very simple circuit mechanisms in the brain that are innately available.”