In this figure the black lines seem to be unparallel, but in reality they are. 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.
It is interesting to see what happens when the colours in this illusion are changed. If the illusion is printed in green on a red background and the red and green are equally bright, the illusion disappears
This illusion is similar to the Hering illusion, the Poggendorf illusion and the Muller-Lyer distortion illusion. All these illusions demonstrate how lines can seem to be distorted by their background.
