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 it has been used by scientists to test many hypotheses about the functioning of human perception.

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

What is the Müller-Lyer illusion?

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

As with most visual and perceptual illusions, Müller-Lyer’s has enabled neuroscientists to study the functioning of the brain and visual system, as well as how we perceive and interpret images and visual stimuli.

This optical illusion is named after the 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 that consists of two parallel lines: one of them ends in arrows pointing inwards; and the other one ends in arrows pointing outwards. When looking at the two lines, the one with the arrows pointing inwards is perceived as significantly longer than the other.

In other alternative 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 that the arrows remain constantly on one side of the line.

Explanation of this phenomenon of perception

Although it is not yet known exactly what causes Müller-Lyer’s 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 farther corners, at greater or lesser distances; and it also uses this information to make judgments about size.

When perceiving the arrows in the Müller-Lyer illusion, the brain interprets them as far and near corners , cancelling 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 from urban and rural settings. The Americans, who were more exposed to rectangular structures, were more susceptible to the delusion, followed by Zambian children in urban areas, and finally Zambian children in rural areas (who were less exposed to such structures because they lived in natural environments).

However, it seems that Müller-Lyer’s illusion also persists when arrows are replaced by circles , which have no relation to perspective or the theory of angles and corners, which seems to question the theory of perspective.

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

However, this last explanation seems to be unfounded, since the illusion seems to persist when there is no saccadic eye movement.

What happens in our brain in optical illusions?

We have known for some 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 perceptive shortcuts to save time and resources.

Optical illusions, like the Müller-Lyer illusion, generate doubts in our perceptive system, and not finding a known and congruent pattern, the brain decides to reinterpret what it sees (in this case, the arrows and lines) through its store of previous experiences and statistics; and after having extracted the available information, it reaches a conclusion: the lines with the arrows outwards are longer. An erroneous, but coherent conclusion .

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

These illusions can also be explained as an effect of perspective, in which the observer is forced to use a certain pre-established point of view , as happens with anamorphosis, deformed drawings that recover their image without deformation when looked at from a certain angle or cylindrical mirror. Similarly, certain contrasts between colors and shades, in combination with eye movement, can generate illusions of false sensation of movement.

On the other hand, from the point of view of the psychology of perception (or Gestalt psychology), we have tried to explain that we perceive the information that comes to us from the outside, not as isolated data, but as packages of different elements in significant contexts, according to some rules of interpretative coherence. For example, we tend to group elements that are similar, and we also tend to interpret several elements moving in the same direction as a single element.

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