Optical illusions! By Marion Nairabeze, Maëlle Rivalier & Camille Simonet

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First, look at these optical illusions:

 Who do you see? Monroe or Einstein?

Is it Gala or Lincoln?

An old couple or a guitarist and a woman?

What do you see?

In what direction is the train going? Difficult to guess, right?

Second, a short explanation of how vision works for curious people:

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Optical illusions are misperceptions of form, color, size or movement of objects. They are, for the eye, an altered perception of reality.

The eyes behave more or less like a camera. The eye has a diaphragm, an iris which lets in more or less light as it opens or closes. Behind the iris, a deformable lens, called the crystalline, allows the images to be formed on a sensitive membrane: the retina

The retina consists of rods and cones. Cones allow diurnal vision (color). There are three types of cones, receptive to red, green, blue. Rods allow night vision.

Under the influence of the light that enters through the iris, the rods and cones produce small amounts of energy and nerve impulses which are passed on up to the brain by the optical nerve. The brain then analyzes the information.

Optical illusions are errors of interpretation of our brain which tries to give meaning to all the information which is received even if there is none. This results in amplification of contrasts, creating contours, colors, perspectives, reliefs, etc.

Third, an explanation of an illusion:

1) Primary or geometric illusions

These illusions occur when the eye stops only on certain parts of the image (angles, symmetry axes, breaks lines). Then, these areas become more important than others, such as in the Titchener illusion.

2) Effect of angles

We underestimate obtuse angles and overestimate sharp corners because the eye has a tendency to bring the angles to a right angle. In a Zöllner illusion, the lines appear to us deformed because of small lines which are on the big ones. The oblique lines do not seem parallel though they actually are.

The same goes for the Müller-Lyer illusion. The eye does not pay attention to segments but sees first the angles at their end. It appears that one is larger than the other.

 3) Movement  illusion

It turns! It will make us dizzy. Don’t be afraid, you are not hallucinating, you are suffering from an movement illusion. The human eye gets tired fast when it stares at an object for a long time. If, however, we look next to the object, it avoids staring too intensely and the image strikes other segments of the retina which have their full capacity.

4) Artistic illusions

There are three types of artistic illusions. A group that shows illusion drawings gives rise to visual interpretations that are very different from the properties of the elements shown.

Our brain is used to seeing a real head. But here, the head consists of many animals: an elephant, a kangaroo, a bear... So the brain has trouble analyzing this image.

In the following picture we see a tree and the sea, but aren’t they something else? A baby, no?

Each drawing may result in at least two visual mutually exclusive interpretations. The observer can normally voluntarily move from one interpretation to another, once the different interpretations have been identified and given us some clues about the different interpretations.

This cartoon, created by Willian Hill in 1915, is entitled "My mother / My young woman." The image has two possible interpretations: you see the young woman or the old witch. It is sometimes difficult to switch from one to the other!

Impossible groups: different parts of each of the drawings give rise to conflicting interpretations. All items in this category cannot exist, or it would be highly unlikely that they exist in reality.

The Penrose triangle is an impossible object designed by the mathematician Roger Penrose in the 1950s. By the paradox between logic and reality, which extends to other spheres (cultural impossibility), impossible objects thus give rise to mathematical curiosities and artistic inspirations.

Optical illusions and cultures

"The influence of the culture and  of the environment on our visual perception ", this theory was explored for the first time by Robert Laws, a Scottish missionary, working at Malawi in Africa during the 19th century.

Look at this picture:

What do you see? What is the woman carrying on her head? When researchers put this question to African people, they answered “a box”. Then they asked “where does the scene take place?” This person with a culture with few “corners” naturally said that the family was sitting under a tree. We westerners, accustomed to comfort, see a scene which takes place in a room with a window with a view outside.

Finally, optical illusions show that our vision is always a matter of personal interpretation!

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