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Zollner Illusion explanation

This illusion is similar to the Herring Illusion, the Poggendorff Illusion and the Muller-Lyer Illusion.

All these illusions demonstrate how lines can seem to be distorted by their background.

A number of different explanations for how these geometrical illusions might work have been suggested. Some of the explanations refer to the apparent depth created by the illusion (as in the Herring Illusion) while others relate to the way angles can seem distorted (such as with the Poggendorff Illusion). The Zollner illusion has been important in helping understand which of these two explanations is correct.

In the Zollner illusion, the shorter lines are on an angle to the longer lines. This angle helps to create the impression that one end of the longer lines is nearer to us than the other end. This is very similar to the way the Wundt illusion appears. It may be that the Zollner illusion is caused by this impression of depth.

Another way to look at the illusion is that the brain tries to increase the angle between the long line and the shorter lines that cross it. As a result, the brain bends the longer lines towards or away from each other.

Interesting things happen when you change the colours in this illusion. If the illusion is printed in green on a red background and the red and green are equally bright, the illusion disappears. Scientists can use this fact to help them work out how the images we see are simplified by the brain to cause this illusion to work.