Müller-Lyer illusion: what it is and why it occurs

Optical illusions deceive our visual perception system, making us believe that we see a reality that is not what it seems.

The Müller-Lyer illusion is one of the best known and most studied optical illusions, and has served to for scientists to test numerous hypotheses about how human perception works.

In this article we explain what is the Müller-Lyer illusion and what are the main theories that try to explain its operation.

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What is the Müller-Lyer illusion?

The Müller-Lyer illusion is one of the best known geometric optical illusions consisting of a set of lines ending in arrowheads. The orientation of the tips of each arrow determines how we accurately perceive the length of the lines.

As with most visual and perceptual illusions, Müller-Lyer's has served to enable neuroscientists to study the functioning of the brain and the visual system, as well as the way in which we perceive and interpret images and stimuli visuals.

This optical illusion

Named after German psychiatrist and sociologist Franz Carl Müller-Lyer, who published up to 15 versions of this illusion in a well-known German magazine, at the end of the 19th century.

One of the best known versions is the one consisting of two parallel lines: one of them ends in arrows pointing inwards; and the other ends with arrows pointing outwards. When looking at the two lines, the one with the arrows pointing inward is perceived to be significantly longer than the other.

In other alternate versions of the Müller-Lyer illusion, each arrow is placed at the end of a single line, and the observer tends to perceive the midpoint of the line, just to make sure the arrows constantly stay to one side of it.

Explanation of this phenomenon of perception

Although it is not yet known exactly what causes the Müller-Lyer illusion, various authors have contributed different theories, the most popular being the theory of perspective.

In the three-dimensional world, we often tend to use angles to estimate depth and distance. Our brain is used to perceive these angles as closer or further corners, at a greater or lesser distance; and this information is also used to make judgments about size.

When perceiving the arrows in the Müller-Lyer illusion, the brain interprets them as far and near corners, canceling out the information from the retina that tells us that both lines are the same length.

This explanation was supported by a study that compared the response to this optical illusion in children in the United States, and in Zambian children who came from urban and rural settings. Americans, more exposed to rectangular structures, were more susceptible to optical illusion; followed by Zambian children from urban areas; and, lastly, Zambian children in rural areas (less exposed to such structures because they live in natural environments).

With everything, it seems the Müller-Lyer illusion also persists when arrows are replaced by circles, which have nothing to do with perspective or angle and corner theory, which seems to call perspective theory into question.

Another of the theories that have tried to explain this perceptual illusion is the theory of saccadic eye movements. (rapid eye movements when scrolling to extract visual information), which states that we perceive a longer line because we need more saccades to see a line with arrows pointing inward, compared to the line with arrows pointing outwards.

However, this last explanation seems to have little foundation, since the illusion seems to persist when there is no saccadic eye movement.

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What happens in our brain in optical illusions?

We have known for a long time that our brain does not perceive reality as it is, but tends to interpret it in its own way, filling in the missing gaps and generating hypotheses and patterns that allow us to give coherence and meaning to what we see. Our brain resorts to cognitive and perceptual shortcuts to save time and resources.

Optical illusions, such as the Müller-Lyer illusion, generate doubts in our perceptual system, and by not finding a known pattern and congruently, the brain decides to reinterpret what it sees (in this case, the arrows and lines) through its storehouse of previous experiences and statistics; and after having extracted the available information, he reaches a conclusion: the lines with the arrows facing out are longer. An erroneous, but coherent conclusion.

On the one hand, from a physiological point of view, optical illusions (the most frequent, ahead of auditory, tactile and gustatory-olfactory) can be explained as a phenomenon of refraction of light, as when we put a pencil in a glass of water and this, apparently, it twists.

These illusions can also be explained as a perspective effect, in which the observer is forced to use a certain preset point of view, as with anamorphoses, deformed drawings that recover their image without deformation when viewed from a certain angle or cylindrical mirror. In a similar way, certain contrasts between colors and shades, in combination with the movement of the eyes, can generate illusions of a false sensation of movement.

On the other hand, from the point of view of the psychology of perception (or Gestalt psychology), it has been tried to explain that we perceive the information that we comes from the outside, not as isolated data, but as packets of different elements in meaningful contexts, according to some coherence rules interpretive. For example, we tend to group items that are similar, and we also tend to interpret multiple items moving in the same direction as a single item.

In short, what we have learned over the years, thanks to the work of researchers and neuroscientists with optical illusions such as Müller-Lyer's, is to distrust what our eyes see, since many times our brain deceives us, perceiving what is real but does not exist. To paraphrase the French psychologist, Alfred Binet: "Experience and reasoning prove to us that in all perception there is work."

Bibliographic references:

• Bach, M., & Poloschek, C. M. (2006). Optical illusions. Adv Clin Neurosci Rehabil, 6 (2), 20-21.
• Festinger, L., White, C. W., & Allyn, M. R. (1968). Eye movements and decrement in the Müller-Lyer illusion. Perception & psychophysics, 3 (5), 376-382.
• Merleau-Ponty. 2002. Phenomenology of perception. Routledge.