Scientists from the University of Exeter have provided new evidence explaining the phenomenon behind simultaneous contrast illusions, which are often associated with trippy optical illusions. Contrary to previous debates about whether the illusions stem from neural processing, the eyes, or psychological phenomena, the researchers now suggest that these illusions arise due to the limitations of our nervous system. The human nervous system relies on the speed at which neurons fire messages to the brain, which affects how we perceive color. However, there is a limit to how quickly neurons can fire, and previous research did not take this limitation into account when studying color perception.
Simultaneous contrast occurs when the same color appears differently when placed alongside a different color. This can be the root cause of optical illusions and is also used in artwork, such as Monet’s paintings that feature different colored brushstrokes without blending them in. The study explored various illusions that appeared to change the color of objects. For example, in one picture, a gray bar was shown against a black-to-white gradient background. Despite being a consistent shade of gray, the bar appeared lighter on the left and darker on the right due to the gradient background. Dark surroundings can make targets appear lighter, and vice versa, leading to the phenomenon of simultaneous contrast.
Another image showed two rectangles overlaying a background of black and white stripes. Although both rectangles were the same shade of gray, they appeared different, purely because of their position in the image. These illusions demonstrate “color constancy,” which refers to our ability to perceive and interpret colors. Scientists believe that our limited perceptual bandwidth can cause color confusion when overwhelmed by multiple colors at once.
To account for these limitations, the researchers developed a model called the Spatiochromatic Bandwidth Limited (SBL) model, which successfully predicted how humans perceive illusions in 35 different cases. The SBL model suggests that neurons with limited contrast bandwidth can combine their signals to allow us to see significant contrasts, but the information is “compressed,” resulting in visual illusions. The model highlights how neurons have evolved to utilize every bit of their capacity, with different neurons being sensitive to various levels of contrasts at different scales.
The lead researcher, Dr. Jolyon Troscianko, believes that these findings also provide insights into the popularity of high-definition televisions. Such televisions can replicate the high contrasts found in natural scenes, but this poses a puzzle for our eyes and brains since our visual system’s highest contrast sensitivity is limited to around 200:1 at a single spatial scale. The SBL model could also be used to further explore how humans and animals perceive color, especially considering that different species have varying bandwidths of color perception.
Moreover, a separate study in 2021 found that individuals with depression experienced less pronounced effects from certain optical illusions. Researchers conducted experiments where individuals with and without depression viewed small squares of the same colors on different backgrounds. The participants with depression perceived the visual illusion presented on a computer screen as significantly weaker compared to those without depression. This suggests altered cortical processing of visual contrast during a major depressive episode. The alteration is likely present in different types of depression and could partially recover with improvement in patients’ condition.
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