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> Optical Illusions Issue: 2008-1 Section: Biology







Introduction and Historical Elements

Do we see really correctly, what we see? To which extent do our perceptions alter reality?

Look at the above picture for about one minute. After a while you will realize that you see either faces or cups. From the moment you perceive both images, you will realize that your perception changes from one version to the other and it is quite difficult to say if you see both images simultaneously or successively.

Hence, we conclude that an ambiguous image, as the above, shows that our perception is not an uninfluenced reflection of the reality, but rather a construction of our brain, which is partially based on factual stimulus.


The history of optical illusion goes well back in the past. The philosophers Epicharmus and Protagoras tried to interpret optical illusion. The former claimed that our senses do not pay much attention and so they can easily be fooled. On the contrary the latter, suggested that the environment brings us to a mess, while our senses are always reliable. Many years later, Aristotle said that our senses are reliable but they can easily be cheated as well. Hence, on a very hot day in a road, heat waves rise upwards and we can see them. In this case, our senses are right. But if we try to look at a tree through the heat waves, we will see the tree wiggling. Here our senses have been cheated, since the tree does not move.


Another example comes from the Parthenon, Acropolis. Its horizontal surfaces are curved; its columns are getting narrower going upwards and lean to the interior. The corner columns have longer diameter. These details have been taken into account by the constructors, in order to have a result with perfect proportions, correcting in this way the optical illusion existing before.


Illusions Report

Sea waves seem to move as the wind pushes them, whereas the water goes up and down

You are sitting in a still train and suddenly you feel the train moving, while it is, actually, the nearby train moving.

The illusion that the sun goes round the earth, whereas the opposite actually happens.

After a long ride on a bicycle, the feeling that we walk on unstable ground.

To be tickled before we are actually touched.

The gaze of an expressive portrait which follow us everywhere. For example, the gaze of Mona Lisa follows us everywhere. (fig. 2)

A room emptied of all the furniture seems smaller.

Sight and Senses

Sight starts from the eyes but it doesn't stop there. In order to be somebody able to see, the optical signal must first be transmitted to the brain. There it is processed, analyzed, sometimes corrected and only then we can see.

Quite often what we see depends on what we are interested in seeing. Let us imagine that we are on the pavement and we want to cross the street. We see a car driving near. At that moment we are not interested in the colour or make of the car but in how we can cross the street safely, so our brain does not pay any attention to these elements. On the other hand when we are waiting to the picked up then it is probable that we shall watch for the colour and make of the car without paying attention to its speed.

Perception chooses the important signals and descarts the rest to its untidy depths.

Quite often based on the information provided by the sense organs, we perceive something which our logic refuses. Look at these two pictures. Of the two pictures one is upside down. The real picture is the second one. The first is the same picture reversed, but we regard them as normal. (fig.3)

Let us focus on the illusion which our perception creates during the processing of the optical stimuli-optical illusion.

Optical illusion: phenomenon during which the optical sense creates, under certain circumstances, a false perception about the dimensions, the shape or the colours of the pictures or items.


The Optical Perception

A few optical illusions are due to some failings of our sensors. They have excellent resolution, so as to compare sizes which differ slightly, and remarkable sensitivity, so as to detect even the faintest signals.


The most serious defect we can attribute to our eyes is their sluggishness, which is the reason for many phenomena known since the antiquity. Aristotle would mention the case of the burning torch which when twisted with stretched hands gives the impression of a fully illuminated circle.

Ptolemeus had described the merging of the colours of a spinning top.


Another problem of our eyes is their mechanic instability. This problem is revealed in the Burton figure, in which it is impossible for us to count the dots. The instability of the stare is enough to create deceptive motion.


The most known picture is that of Mackay's, in which a surface is divided into alternating black & white pieces which converge to the centre. When we stare at it firmly for about seconds, we have a sense of wavy movements. And if we look closely we can see a circular motion organized.


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