The Muller-Lyer illusion is an optical illusion consisting of nothing more than an arrow. It consists of two arrow like figures, one with both ends pointing in, and the other with both ends pointing out. When asked to judge the lengths of the two lines, which are equal, viewers will typically claim that the line with inward pointing arrows is longer.

One possible explanation is that one sees the lines as three dimensional, such as the outgoing and ingoing corners of a room. Another possible explanation is that the line with arrows pointing inwards may simply appear longer because the arrows themselves extend past the line.

The illusion is not cross cultural. Non western subjects, and particularly subjects whose day to day surroundings are usually not rectangular, as with few buildings, doors, walls, are much less likely to be affected by it. Researchers discovered that the Zulu people, whose typical dwellings are circular thatched huts with no angular walls, were rarely susceptible to the illusion.

One possible explanation states that the Muller-Lyer illusion occurs because the visual system processes that judge depth and distance assume in general that the angles in configuration corresponds to an object which is closer, and the angles out configuration corresponds to an object which is far away. Basically, there seems to be a simple heuristic that takes those configurations as 90 degree angles. This heuristic speeds up the interpretation process, but gives rise to many optical illusions in unusual scenes.

Neural nets in the visual system of human beings learn how to make a very efficient interpretation of 3D scenes. That is why, when somebody goes away from us, we do not see him getting shorter. And when we stretch one arm and look at the two hands we do not see one hand smaller than the other. We should not forget that, as visual illusions show us quite clearly, what we see is an image created in our brain. Our brain projects the image of the smaller hand to its correct distance in our internal 3D model. This is what is called the size constancy mechanism.

In the Muller-Lyer illusion, the visual system detects the depth cues, which are usually associated with 3D scenes, and incorrectly decides it is a 3D drawing. Then the size constancy mechanism makes us see an erroneous length of the object which, for a true perspective drawing, would be further away.


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