The Café Wall Illusion

By Frederic Friedel

Take a look at this image. Is there any way you can imagine that the four horizontal blue bars are perfectly parallel, not slanting? Try to see them that way, with a conscious effort.

Impossible, right? It is an example of the famous café wall illusion, a geometrical image which makes parallel straight lines appear to be sloped. It was first discovered in 1898, and then and re-discovered by Professor Richard Gregory in 1973 — on a wall outside a café in St Michael’s Hill, Bristol. The wall has alternate rows of black and white tiles, which seem tilted and not perfectly horizontal. He and Priscilla Heard described the illusion of slanting lines in Perception, 1979, volume 8, pages 365–380, Brain and Perception Laboratory, University of Bristol. You can read their paper in PDF here.

Professor Richard Gregory in 2010, outside the original Café on St Michael’s Hill, where he described the illusion around forty years ago.

The New World Encyclopedia explains how the illusion works: diagonal lines interact in the brain, where different types of neurons react to the perception of dark and light colors. Because of the placement of the dark and light tiles, different parts of the lines are dimmed or brightened in the retina. Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges. These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.

An interesting feature displayed in the Café Wall illusion (and described in the Gregory/Heard paper) is the role the luminescence of the mortar lines plays. It needs to lie between the luminescence of the dark and light tiles. The image in the middle above works best, although they are all geometrically alike.

The (totally impossible) image at the top of the page, one that has currently gone viral, is by Viktoria Skye, who is a children’s and family magician and balloon twister from Atlanta, Georgia. Victoria likes to create optical illusions and impossible objects using art, science and math. Many of them are published in magic, math, science and puzzle articles, magazines and books. Here’s an earlier café wall illusion. But let us look at the current version:

In the two images above the one on the left is identical to the one at the top of the page. In the second image I have simply cut out the sections between the horizontal blue bars. It is really exactly the same image — I swear!

Here is a larger version by Victoria Skye for you to play around with.

One more? Take a look at the above image. Can you imagine that the circles are quite perfect? And how about this image:

Of course they are perfect circles. I can prove it by changing the background colour of the images to black:

This is the same image as the previous one — the only difference is that the background and the defused bits round the dots is black.

The same for the first image of this series: just the background for the outer circles is now black. All of them, even the ones in the middle, are perfect circles.

And while we are at it, here’s one more:

Will you believe that these are two absolutely identical pictures placed next to each other?

In a previous article, Tricking the brain, on my bio-blog I wrote:

When my son Martin saw these kinds of illusion he said: “My eyes and brain have a bug, I’m sending them in for an upgrade!” But seriously: it is not a defect in our visual system, but rather the result of very sophisticated algorithms used to process data. The system is not very good as a precise physical instrument, its purpose is to interpret the information and help us understand the nature of the objects it perceives.

Also read: Tricking the brain

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The Friedel Chronicles

Frederic Alois Friedel, born in 1945, science journalist, co-founder of ChessBase, studied Philosophy and Linguistics at the University of Hamburg and Oxford.