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 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