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Brain System That Creates Optical Illusions Identified

An optical illusion with grey hues.
The bar in the middle is all one grey level, but it appears lighter on the left and darker on the right due to the background. Credit: Jolyon Troscianko
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In an illuminating study, scientists have unveiled that a number of visual illusions are predominantly attributed to constraints in the operations of our eyes and visual neurons, as opposed to more intricate cognitive functions. This revelation could offer a fresh perspective to the ongoing debate regarding the origin of these illusions. Academics have questioned whether they stem from initial neural processing in the eye and primitive visual centers in the brain, or whether they entail sophisticated cognitive activities such as context comprehension and previous experience.

“Limited bandwidth” in the brain           

Dr. Jolyon Troscianko, a researcher from the University of Exeter, collaborated with peers to develop a model that supports the idea that these visual illusions can be interpreted by considering physiological limitations in how our neurons function and fire, bypassing the need to consider deep-seated psychological processes.

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The way our eyes communicate with our brain, by accelerating or decelerating the firing rate of neurons, has built-in limitations, according to Troscianko, who is based in the Centre for Ecology and Conservation at Exeter's Penryn Campus. Previous research into how we see color may have overlooked these limitations’ impact.

This novel model merges the concept of "limited bandwidth" with other information about how humans process patterns at different scales. The model also bakes in the concept that our vision performs best when perceiving natural scenes.

The model was originally utilized by scientists at the Universities of Exeter and Sussex to explore color perception in animals. Unexpectedly, the model also proved to be useful at predicting how humans see visual illusions, shaking up several long-standing presumptions regarding the workings of these illusions.

High-contrast cognition

Troscianko believes that the results could provide insight into areas unrelated to visual illusions – for example, explaining the popularity of high-definition TVs.

“Modern high dynamic range televisions create bright white regions that are over 10,000 times brighter than their darkest black, approaching the contrast levels of natural scenes,” Troscianko explains.

“How our eyes and brains can handle this contrast is a puzzle because tests show that the highest contrasts we humans can see at a single spatial scale is around 200:1. Even more confusingly, the neurons connecting our eyes to our brains can only handle contrasts of about 10:1.

“Our model shows how neurons with such limited contrast bandwidth can combine their signals to allow us to see these enormous contrasts, but the information is ‘compressed’ – resulting in visual illusions.”

The model is another example of the brain’s incredible adaptability and flexibility,  showing how they have carefully evolved to maximize the potential of their physiologically limited system. Troscianko points to certain neurons that are sensitive to very minute differences in gray levels at medium scales but are rapidly overwhelmed by high contrasts. But this restriction is made up for by other neurons that are less sensitive but can work over a much bigger range of contrasts – allowing us to perceive the deepest blacks and brightest whites.

“Ultimately this shows how a system with a severely limited neural bandwidth and sensitivity can perceive contrasts larger than 10,000:1,” Troscianko concludes.


Reference: Troscianko J, Osorio D. A model of colour appearance based on efficient coding of natural images. PLOS Comput. Biol. 2023;19(6):e1011117. doi:10.1371/journal.pcbi.1011117

This article is a rework of a press release issued by the University of Exeter. Material has been edited for length and content.