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

Optical illusions trick our visual perception system into believing that we are seeing 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 been used to test scientists in many hypotheses about how human perception works.

In this article we tell you what is the Müller-Lyer illusion and what are the main theories that attempt to explain how it works.

    What is the Müller-Lyer illusion?

    The Müller-Lyer illusion is one of the most famous geometric optical illusions composed of a set of lines ending with arrowheads. The orientation of the tips of each arrow determines how accurately we perceive the length of the lines.

    As with most visual and perceptual illusions, Müller-Lyer has helped neuroscientists study how the brain and visual system work, as well as how we perceive and interpret images and stimuli.

    This optical illusion it is named in honor of 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 that which consists of two parallel lines: one of them ends with arrows pointing inwards; and the other, ends with arrows pointing outwards. Looking at the two lines, the one containing the arrows pointing inward is seen to be much 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 middle of the line, Just to make sure that the arrows constantly stay on one side of it.

    Explanation of this perception phenomenon

    Although it is not yet known exactly what causes the Müller-Lyer illusion, several 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 perceiving these angles as more or less distant, more or less distant corners; and this information is also used to make judgments about size.

    Seeing the arrows of the Müller-Lyer illusion, the brain interprets them as far and near corners, Canceling information in the retina that tells us that the two lines are the same length.

    This explanation was supported by a study comparing the response to this optical illusion among children in the United States and among children in Zambia who came from urban and rural areas. Americans, more exposed to rectangular structures, were more sensitive to optical illusion; monitoring of Zambian children in urban areas; and, finally, Zambian children from rural areas (less exposed to such structures to live in a natural environment).

    However, in the opinion The Müller-Lyer illusion also persists when arrows are replaced by circles, Which have nothing to do with perspective or the theory of angles and corners, which seems to call into question the theory of perspective.

    Another theory that has attempted to explain this perceptual illusion is the theory of saccadic eye movements (rapid movements of the eye as it moves to extract visual information), which states that we perceive a longer line because we need more jerky movements to see a line with arrows pointing inward, Relative to the line with arrows pointing outward.

    However, this latter explanation does not seem to have much basis, as the illusion seems to persist when there is no jerky eye movement.

      What happens to our brain in optical illusions?

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

      Optical illusions, such as the Müller-Lyer illusion, raise doubts in our perceptual system, and by not finding a known and congruent pattern, the brain decides to reinterpret what it sees (in this case, arrows and lines) through its stock of previous experiences and statistics; and after extracting the available information, he comes to a conclusion: the lines with the pointed arrows are longer. A mistaken but consistent conclusion.

      On the one hand, from a physiological point of view, optical illusions (the most common, before auditory, tactile and taste-olfactory) can be explained as a phenomenon of refraction of light., As 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 obliged to use a certain pre-established point of view, As with anamorphoses, distorted drawings that recover their image without distortions when viewed from a certain angle or cylindrical mirror. Likewise, 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 perceptual psychology (or Gestalt psychology), an attempt has been made to explain that we perceive information that comes to us from the outside, not as information. isolated data, but as sets of different elements in significant contexts, according to certain rules of interpretative consistency. For example, we tend to group similar items together, and we also tend to interpret multiple items moving in the same direction as one.

      In short, what we have learned over the years, through the work of optical illusion researchers and neuroscientists like Müller-Lyer, is to beware of what our eyes see, Because many times our brain deceives us, perceiving what is real but which does not exist. Paraphrasing the French psychologist Alfred Binet: “Experience and reasoning prove to us that in all perception there is work.”

      Bibliographical references:

      • Bach, M. and Poloschek, CM (2006). Optical illusions. Adv Clin Neurosci Rehabil, 6 (2), 20-21.
      • Festinger, L., White, CW and Allyn, MR (1968). Eye movements and reduction of the Müller-Lyer illusion. Perception and Psychophysics, 3 (5), 376-382.
      • Merleau-Ponty. 2002. Phenomenology of perception. Routledge.

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