Illusions Com
Muller-Lyer illusion
The Muller-Lyer illusion is an optical illusion consisting of nothing more than an arrow. When viewers are asked to place a mark on the figure at the mid-point, they invariably place it more towards the "tail" end. Another variation 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 (few buildings, doors, walls) are much less likely to be affected by it (Segall, et al., 1963). Researchers discovered that the Zulu people, whose typical dwellings are circular thatched huts with no angular walls, were much less susceptible to the illusion.
On the other hand, experiments have been reported, suggesting that pigeons perceive the standard Muller-Lyer illusion, but not the reversed. Experiments on parrots have also been reported with similar results.
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