Reality vs. Perception? In Adelson’s Illusion Perception is the Easy Victor

All optical illusions fascinate but some astonish. They also teach us things about how human vision operates and how well adapted it is to helping us survive.

For my fiftieth blog post I’ve chosen to look at the celebrated Adleson checkerboard illusion. It’s been published many times and you may have seen it before. But even if you already know it, the illusion retains the power to surprise. It appears almost paradoxical.

Which square is the darker shade of grey, A or B?

A grey and white checkerboard. Some of the squares appear to be in the shadow of a cylinder standing on the top right of the board. The squares in shadow include ones that are perceived as grey and ones perceived as white. On grey square not in the shadow is labelled A. One apparently white square in the shade is labelled B. A copy of A has been cut out and is seen off the board, in the top left of the photograph.

Of course it’s a trick question. Although they look quite different in fact they are the same. It seems almost incredible. But if you don’t believe me, I printed two copies of the picture and cut square A out. Watch while I slide the cut-out square from A to B.

The copy of A is slid over the original A and we see they are the same grey.

The copy of A is slid towards B.

Now the copy of A is almost on top of B. They seem to be the same shade.

Now square A is on top of square B and is the same shade

These are genuine photographs. Although the cut-out is the same throughout we see it miraculously changing colour.

What’s going on and what does this show? Our visual perception is very good at taking the information that comes from our eyes and using it to construct a view of the world.

It processes the information using systems that have evolved over many millions of years and which it is very hard for computer vision to come anywhere near. For example smartphone apps that attempt to recognise colours, such as those designed to assist the blind or VI, only have limited success. But at the same time the human ability to interpret what actually falls on the retina means we can be misled sometimes.

You can find the original image of the illusion and read Adelson’s explanation here. If you are like me then however many times you look at the illusion A and B still look quite different. So do what I did – print off two copies, cut A out and do some experiments. You will be as astonished as I was!


About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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