Are optical illusions in the eye or in the brain?

06/20/2023

Editorial health diary

The bar in the middle of this figure has one level of gray, but it appears lighter on the left and darker on the right because of the gradient in the background. This is called simultaneous contrast, whereby dark environments make subjects appear brighter and vice versa.
[Imagem: Jolyon Troscianko]

Optical error and illusion

It’s common to hear talk of optical illusions as ways to “trick the brain,” which would be “where seeing really happens.”

But this may not be the whole “scientific truth”. For example, recent research has already shown that optical illusions produce real responses in the eyes.

Now, Joleon Trocianco and Daniel Osorio of the University of Exeter (UK) have concluded that optical illusions result from limitations in the way our eyes and visual neurons function — rather than relying on more complex psychological processes going on in the brain.

“Our eyes send messages to the brain, causing neurons to fire faster or slower,” explained Professor Trociaenko. “However, there is a limit to how quickly they can be triggered, and previous research did not take into account how that limit might affect how we see colour.”

The two cubes have what appear to be yellow and blue tiles on their top surface. However, they are all grey. The model explains how objects appear to be the same color even when the light changes and why this gray appears colored in illusions.
[Imagem: Jolyon Troscianko]

narrow band

The pair of neuroscientists started from a model that explains how we see colors, then added this information that neurons that connect our eyes to our brains have a “bottleneck” in transmitting data.

The result is a model that explains optical illusions by combining this “limited bandwidth” with information about how humans perceive patterns at different scales, along with the assumption that our vision works better when looking at natural scenes.

More specifically, these neurons can handle contrasts between the brightest and darkest areas of an image at a ratio of roughly 10:1. Since there is not enough bandwidth to transmit all the information, our visual system ends up choosing the information it deems most important, and performs “lossy compression”. It is these losses that generate optical illusions.

Our model shows how neurons with limited bandwidth can combine their signals to allow us to see these huge discrepancies, but the information is ‘compressed’ – leading to optical illusions,” the researcher explained.

The two bars in the middle are the same gray, but the one on the left (surrounded by more black bars) looks darker. This is the opposite of the simultaneous contrast example above, because the darker outlines make the subject look darker now.
[Imagem: Jolyon Troscianko]

See contrasts

“For example, some neurons are sensitive to very small differences in gray levels on medium-scale scales, but are easily overwhelmed by high contrasts. Meanwhile, neurons that encode contrasts on larger or smaller scales are much less sensitive, but they can It works on a much wider range of contrasts, giving deep contrasts between black and white.

“Ultimately, this shows how a system with very limited sensitivity and neural bandwidth can perceive contrasts greater than 10,000:1. It upends many long-standing assumptions about how optical illusions work,” Trocianco concluded.