Donders Wonders

Your brain on visual illusions

This post is also available in Dutch.

Sometimes we see things differently from how they really are. Here is what our brain activity looks like when we’re looking at a visual illusion.

As explained in a previous blog post, there is often a difference between how we perceive the world and how things really are. Numerous visual illusions demonstrate this, and you can see one of them below:

(A) Three of the circles form an illusory triangle. (B) The circles don’t form a triangle.

Image source: Peter Kok & Floris de Lange (2014) Current Biology.

In the figure on the right (B), the circles are arranged haphazardly and don’t form any shape. In contrast, in the figure on the left (A), three of the circles form a triangle. The shape seems to pop out against the background. Some people even say they see the triangle as brighter or lighter in color than the background. This illusory triangle is also called a “Kanizsa” shape after the man who popularized them.

How the brain sees illusory shapes

In a recent study, Peter Kok and Floris de Lange from the Donders Institute looked at people’s brain activity while they saw such illusory triangles. While participants were in the fMRI scanner, they saw some sets of circles that made up triangles (like in (A)), and others that didn’t make up any illusory shape (like in (B)).

Then, the researchers compared the brain activity for an illusory triangle to the activity for no illusory triangle. The red color means more activity, and the blue color means less activity. The white dashed triangle shows approximately where people perceived an illusory triangle. What they saw was the following:

Red shows areas of more neural activity, and blue refers to less activity.

Image source: Peter Kok & Floris de Lange (2014) Current Biology.

When people perceived an illusory triangle compared to when they didn’t, two things happened in the visual cortex, the part of the brain that processes visual information. On the one hand, there was more activity in areas corresponding to the middle of the image where the triangle was seen.

This result is very interesting because it links our subjective experience of the visual illusion to brain activity. We can all see the triangle popping out from the three circles even though there is no actual outline of a triangle there. From this studyWhen we see an illusory triangle, the increase in activity is focused in the middle of the image where we perceive the triangle. When we just see the circles without an illusory shape, the increase in activity emphasizes each of the circles.

The main take-away message is that when we see a visual illusion, our brain activity changes accordingly. We perceive the illusion because our brain is “seeing” something else from what is physically present in front of us. This may sound simple, but it is also profound.

Written by Marisha. Edited by Roselyne.

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