Why do optical illusions work?

Optical illusions work because your brain is wired to see what is essential; not what is real.

Here is an amazing optical illusion to present to your brain. I am always astounded when I see it. Look at the picture below. Which square is darker….A or B?

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If you are like me then the answer seems simple and obvious. Square A. But not so fast. In reality both squares are exactly the same shade! Don’t believe it? Neither did I. Take a look at the next picture.

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What you are viewing is Adelson’s checker shadow illusion. It is an optical illusion published by Edward H. Adelson, Professor of Vision Science at MIT in 1995.

What is especially astonishing about this illusion is that even after your brain knows the squares are the same shade it still experiences them as being different!!

So what’s going on here?

I always tell people in my live seminars the best definition of learning they will ever hear is that “learning is connecting new information to what you already know”. That’s how your brain works. It connects new information and stimuli to what you have already stored in your brain. Memories, experiences, emotions, etc.

In the illusion above we have a prior stored memory of how checker boards are “supposed to look”. They are “supposed” to alternate light and dark. This checker board is different though…..but your brain can’t see it because there is nothing like it in your brain to connect it to.

Also, your brain has stored memories of shadows and what they are “supposed to look like”. This shadow is different though; it isn’t there. The shadow just seems to be there; but it’s not. Your brain fills it in based on prior stored memories.

So all in all we have a recipe for getting things wrong!

Remember….what you think, what you say, what you do it always based upon what you know. What you know might be wrong; a wise person is always mindful of this.

For those of you who are inclined to want a technical explanation regarding why the illusion works, here it is:

“The visual system needs to determine the colour of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighbouring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbours. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the colour of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface colour rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.”

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Terry Small


Terry Small, B.Ed., M.A., is a master teacher and learning skills specialist. He has presented on the brain for over 30 years to schools and organisations around the world. His knowledge, warmth, humour and dynamic presentation style have made him a much sought-after speaker at workshops and conferences. Terry is often on television, the radio and in the press. He has presented his ideas to over 200,000 people. Terry believes, “Anyone can learn how to learn easier, better, faster, and that learning to learn is the most important skill a person can acquire.” Terry’s wealth of teaching experience and extensive involvement in applied neuroscience and make him an outstanding resource of the business and educational communities. He resides in Vancouver, Canada. Terry Small’s Philosophy is simple: “Success is a skill anyone can learn.”