The Müller-Lyer Illusion

The Müller-Lyer illusion is one of the most well-known and widely studied visual illusions in psychology. First described by Franz Carl Müller-Lyer in 1889, this illusion demonstrates that our perception of size and length can be systematically distorted, providing important evidence for theories of perception.

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What is the Müller-Lyer Illusion?

The illusion consists of two lines (or shafts) of equal length. One line has outward-pointing fins (arrows pointing outward, like the tails of a fish) and the other has inward-pointing fins (arrows pointing inward, like arrowheads).

Despite the fact that both lines are exactly the same length, the line with outward-pointing fins appears longer than the line with inward-pointing fins.

This effect is robust — even when people know the lines are the same length, they still perceive them as different. This suggests that the illusion operates at a fundamental level of visual processing.

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Explaining the Müller-Lyer Illusion

Gregory's Explanation: Misapplied Size Constancy

Richard Gregory argued that the Müller-Lyer illusion occurs because the brain misapplies size constancy — a mechanism that normally helps us perceive objects accurately.