#4. A Fun Illusion

I came across this remarkable optical illusion while writing an upcoming post about judgment errors. The illusion is so powerful that I thought it deserved its own little discussion.

Which square is darker, A or B?

If you’re a fan of optical illusions, then you’re probably suspicious of questions like this. If so, your skepticism has led you to guess that, despite what your eyes are telling you, the counterintuitive answer is the correct one. You’re right: Neither square is darker.

You can verify this by zooming in on the two labeled squares and masking out their surroundings. Then, once you’ve seen for yourself that A and B are shaded identically, you can zoom back out and …. Damn! A still looks much darker! Why is that?

A couple of factors combine to make this image deceptive. First, it portrays simple, familiar objects. There’s a checkerboard, a cylinder, and an off-screen light source casting the cylinder’s shadow onto the checkerboard. Familiarity tricks our brains into seeing what we expect to see—the aptly named expectancy effect. And what we expect is for all the dark squares on a checkerboard to be equally dark, and all the light squares to be equally light.

The expectancy effect also comes into play when our brain unconsciously tries to correct for the shadow. We expect the shadow to make B look darker than it really is, and so our brains step in and lighten it for us.

The second factor making the A and B squares appear as different shades is a contrast effect.

B is surrounded by darker squares, making it appear lighter than it really is. A looks darker because it abuts lighter squares.

A related type of contrast effect comes into play in judging sizes. That’s what makes the orange circles appear to be different sizes in the image below.

Going back to the expectancy effect, this drawing of a rabbit demonstrates it quite nicely.

The image is actually ambiguous by design. If you see the bunny, it may be because I told you that’s what you were going to see. People are also more likely to see the bunny around Easter time. But if you haven’t seen it already, you’ll find it’s also a duck facing to the left, with the bunny’s ears morphing into its bill. 

I’ve looked at hundreds of optical illusions, and they affect me. Sure, they’re rather entertaining. But they also impact me more deeply. They teach me not to put complete faith in my own judgments. That means learning to be comfortable leaving the door open to being wrong. And it means, whenever possible, verifying my judgments when they’re likely to lead to decisions with important consequences.

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