The Muller-Lyer Illusion

The Muller-Lyer Illusion

In chapter 10 we talked about the Muller-Lyer illusion. I thought it was very interesting how most people would think that the line on the left was longer than the line on the right, but they are actually equal in size. The Muller-Lyer illusion is one of the most famous of illusions. It was created by German psychiatrist Franz Muller-Lyer in 1889.

The Explanation

According to the Rochester Institute of Technology, in the three-dimensional world, depth perception concerns judging distance. The closer an object is to the retina, the larger it is on the retina. However, in the two-dimensional world of the Muller-Lyer illusion, our brain makes assumptions about the relative depths of the two shafts based on monocular (pictorial) cues. We are used to seeing outside corners of buildings as near to us with the top and bottom of the corner sloping out and away (like the outward slanting fins of the Muller-Lyer illusion). We are used to seeing inside corners of buildings as farther from us with the top and bottom of the corner sloping in somewhat towards us (like the inward slanting fins of the Muller-Lyer illusion).

The retina is saying that the two shafts are the same length but the brain is interpreting the Muller-Lyer as a depth issue, with the shaft that looks like an outside corner being closer and the shaft that looks like an inside corner being farther away. In other words, the retina is saying "two shafts equal" and the brain is saying "outside shaft shorter than inside shaft".

Psychologists have attempted to support this theory that the Muller-Lyer illusion is caused by our experiences with outside and inside corners, by showing the illusion to an African tribe that lived in circular huts and therefore had no perceptual experiences with corners. People in this tribe didn't seem to be fooled by the illusion thus supporting the "experience with corners" explanation of the illusion.

A counter-study concerned a man who was completely blind (except for light sensitivity) from the of age 3. Recently this man received a successful corneal transplant. Studies have shown that he is impressively free from geometrical illusions that are associated with a suggestion of depth (such as the Shepard Tables illusion shown below -- the two table tops are the same size).

However, he shows roughly normal susceptibility to the Muller-Lyer illusion. This finding suggests that the Muller-Lyer illusion does not depend on processes associated with depth perception.

The Eye Movement Explanation

This explanation suggests that the shaft ending in the inward slanting fins causes people to perceive it as shorter because the perception of the shaft is pulled back by the "turning back" of the fins. In other words, our eyes go out toward the point and then come back as they follow the fin shafts back. This turning back of our eyes (or perception) makes the shaft seem shorter. Conversely, the outward slanting fins draw our perception on farther making that shaft seem longer.