Illusions: Deceptions of the Brain, Not the Eyes (Usually)

Back when I used to teach undergraduate Sensation & Perception, I emphasized to students that, despite our belief to the contrary, we most certainly do not perceive the world as it really is.

Although our perception of the world around us is undoubtedly heavily influenced by what’s actually “out there” in the environment, perception is actually a highly constructive process that unfolds inside the brain (and mostly outside of conscious awareness).

We say that perception is a constructive process because your brain almost never gets a complete picture of the world around you. Due to limited cognitive resources, such as attention and memory, and the inherent ambiguity of sensory signals (see the inverse projection problem), your brain often needs to makes guesses and inferences about what it “thinks” is going on in the physical environment.

The constructive nature of human perception is most easily demonstrated using illusions as an example.

Aside from just being fun to look at, illusions are fascinating to perceptual scientists because they reveal important information about how the brain actively constructs a meaningful perception of the world.

Despite the common claim that illusions occur because “your eyes play tricks on you,” most illusions actually have very little to do with the eyes (although there are some exceptions to this). Rather, many of the most fun and interesting illusions occur because your brain encounters something in the environment that is ambiguous, meaning it can be perceived one way or another. Not knowing which perception is “correct,” your brain draws upon prior knowledge, memories, expectations, and other information to make its best guess as to what it thinks it is looking at (or listening to, or touching, etc.). When your brain guesses incorrectly, voila you experience an illusion.

A great example of this is the famous Ames Room Illusion.

If you’ve never heard of the Ames Room Illusion, take a look at the unaltered image below and see if you can figure out what’s going on.


Image Credit: Edinburgh’s Camera Obscura and World of Illusions

The two women are standing in the same physical room, yet the one on the right appears considerably larger – impossibly larger, actually.

If you’ve heard of this illusion before, then you know the explanation has to do with the shape of the room. When viewed head-on, the room appears normal and cube-shaped, with corners that make right angles and a back wall that runs parallel to the presumed front wall. However, that is not at all the case.

The room is actually trapezoidal and shaped as shown below:


Notice that the back wall is slanted so that the person standing on the right is actually much closer to you (the viewer) than the person standing on the left.

How does this explain the illusion?

When viewed head-on, your brain makes the incorrect assumption that the person standing on the left and the person standing on the right are the same distance from you. Why? Because the room is specifically arranged and decorated to mask all depth and distance cues when viewed head-on. And because accurate perception of distance is necessary for accurate perception of size, you end up incorrectly perceiving the relative size of the two women.

So with that brief introduction to perception and perceptual illusions, I thought it would be fun to start a new series of posts featuring some of the most interesting and creative illusions available on the internet (and there are a LOT of them).

The first illusion I’m featuring was created by Mark Vergeer at the University of Leuven in Belgium, and it is called, “Splitting Colors.”

This illusion demonstrates the role that surrounding context plays in shaping our perception of both motion and color (Interesting Side Note: Color itself is a perceptual experience that is entirely an illusion, as color does not exist in the physical environment. What we call color is merely our brain’s interpretation of specific wavelengths of light reflected off surfaces in our surroundings).

Incidentally, Splitting Colors won first place in the Best Illusion of the Year Contest for 2015. The Best Illusion of the Year Contest is hosted each year by the Neural Correlate Society, and draws submissions from visual scientists, ophthalmologists, neurologists, and artists from around the world.

Below is the official description of this illusion, as provided on the contest website:

– Mark Vergeer: “Splitting Colors”. KU Leuven (Belgium)


The Splitting Color illusion is all about how we perceive colors. We start off with two identical, flickering colored stripes that remain unchanged throughout the demonstration. However, different surroundings will make these stripes appear completely different. When the stripe is flanked by a yellow/blue pattern, drifting to the left, it changes appearance, and looks red and cyan, drifting to the right, while the same stripe, flanked by a red/cyan pattern drifting to the right, suddenly looks yellow and blue, drifting to the left. This illusion shows that one and the same object can look completely different depending on its surroundings.

Do you have a favorite visual illusion that you’d like to share? Feel free to leave a comment below.


Brian Kurilla is a psychological scientist with a Ph.D. in cognitive psychology. You can follow Brian on Twitter @briankurilla 

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