The Müller-Lyer illusion in Psychology

The Muller-Lyer illusion is a well-known optical illusion in which two lines of the same length appear to be of different lengths. The illusion was first created by a German psychologist named Franz Carl Muller-Lyer in 1889.

What Do You See?

In the top half of image above, which line appears the longest? For most people, the line with the fins of the arrow protruding outward (the center line) appears to be the longest, while the line with the arrow fins pointing inwards appears shorter. While your eyes might tell you that line in the middle is the longest, the shafts of both lines are exactly the same length, as shown in the bottom half of the image.

Like other optical illusions, the Muller-Lyer illusion has become the subject of considerable interest in psychology over the years. Different theories have emerged to explain the phenomenon.

The Müller-Lyer illusion, which was first given as an illustration by Franz Müller-Lyer in 1889 works on optical illusions.

Illusion in psychology

You have just reviewed a number of processes that your perceptual system uses to provide you with an accurate perception of the world. Even so, occasions remain on which your perceptual systems deceive you: When you experience a stimulus pattern in a manner that is demonstrably incorrect, you are experiencing an illusion. 

The word illusion shares the same root as ludicrous—both stem from the Latin illudere, which means “to mock at.” Illusions are shared by most people in the same perceptual situation because of shared physiology in sensory systems and overlapping experiences of the world. ( Hallucinations are nonshared perceptual distortions that individuals experience as a result of unusual physical or mental states.) 

The Müller-Lyer illusion definition psychology

Richard Gregory (1966) suggested that people experience the standard arrow as the exterior corner of a building bulging toward them; people experience the open arrow as an interior corner, farther away. Because of the relationship between size and distance, people experience the arrow that looks like an interior corner as farther away. On this explanation, the Müller-Lyer illusion provides an example of ordinary processes of depth perception leading to an incorrect percept. 

The Size Constancy relation 

Size constancy, like other processes of perceptual organization, can sometimes produce perceptual illusions. This is likely the case with the Müller–Lyer illusion, in which two lines of equal length appear to differ in size. According to one theory, the angled lines provide linear perspective cues that make the vertical line appear closer or farther away (Gregory, 1978). The brain then adjusts for distance, interpreting the fact that the retinal images of the two vertical lines are the same size as evidence that the line on the right is longer.

Culture and Illusion   

If the Müller–Lyer illusion relies on depth cues such as linear perspective that are not recognized in all cultures, are people in some cultures more susceptible to the illusion than others? That is, does vulnerability to an illusion depend on culture and experience, or is it rooted entirely in the structure of the brain? In the 1960s, a team of psychologists and anthropologists set out to answer these questions in what has become a classic study (Segall et al., 1966).

Study

Two hypotheses that guided the investigators are especially relevant. The first, called the carpentered world hypothesis, holds that the nature of architecture in culture influences the tendency to experience particular illusions. People reared in cultures without roads that join at angles, rectangular buildings, and houses with angled roofs lack experience with the kinds of cues that give rise to the Müller–Lyer illusion and hence should be less susceptible to it. The second hypothesis posits that individuals from cultures that do not use sophisticated two-dimensional cues (such as linear perspective) to represent three dimensions in pictures should also be less vulnerable to perceptual illusions of this sort.

The western and non-western difference in experience 

The researchers presented individuals from 14 non-Western and 3 Western societies with several stimuli designed to elicit perceptual illusions. They found that Westerners were consistently more likely to experience the Müller–Lyer illusion than non-Westerners, but they were no more likely to experience other illusions unrelated to angles and sophisticated depth cues. Subsequent studies have replicated these findings with the Müller–Lyer illusion (Pedersen & Wheeler, 1983; Segall et al., 1990). Teasing apart the relative impact of architecture and simple exposure to pictures is difficult, but the available data support both hypotheses (Berry et al., 1992).