Ophthalmology

Optical illusion

Synonyms in a broader sense

optical illusions, visual illusion

English: visual trick, optical illusion

definition

Optical illusions or optical illusions are perceptual illusions of the sense of sight, i.e. of sight. These can occur in almost all areas of vision, for example there are:

Deep illusions
Color illusions
geometric illusions
and many more.

Optical illusions come about when the visual system misinterprets a visual stimulus. An image that we are ultimately aware of does not arise solely from objective information from eye and Neurons, but only arises in interaction with ours brain.
So what we finally perceive is subjective and arise from processing a visual stimulus with existing experiences and memories. With the help of additional information from the other senses or by removing triggering factors, optical illusions can often be shown and proven.

In perceptual psychology, optical illusions are examined, as these allow conclusions to be drawn about the processing of optical stimuli in the brain. The Gestalt psychology uses optical illusions by systematically producing and analyzing them.

Types of Optical Illusions

There are practically an infinite number of different ones optical illusionwhich, however, can be divided into different groups based on their origin.

1. Relativity of brightness

The perception of differences in brightness is very subjective. The same shade appears much brighter in twilight than in strong sunlight. For this reason, a gray bar, which has the same gray value everywhere, appears brighter in a dark environment than in a light environment. The brain can also interpret the relationship between light and shadow. The brain has found that an object looks darker in the shadow. It consequently ascribes a lighter color to the same hue if it suspects it to be in a shadow effect, since the color “only became darker through the shadow”.

2. Relativity of colors

If you fix your eyes on a green square for about half a minute and then look directly at an adjacent white surface, a reddish square appears here. This is because we see what is known as an afterimage on the retina in the complementary color of the previously viewed object (complementary colors: Red Green; blue-orange; purple-yellow). The negative afterimage arises from the fact that the color receptors in the retina practically “tire”. After sustained excitement lasting at least 30 seconds, these receptors temporarily “go blind”, which means that they no longer send signals to the brain. During the time it takes to regenerate, the signals of the complementary colors predominate, so the actually white surface appears red.

3. Relativity of size

Everything is relative when it comes to optics. Our brain does not evaluate a character on its own, but always in context. A circle surrounded by many small circles therefore appears larger to us than a circle of the same size surrounded by many large circles. The impression of "relative“Bigger or smaller is transmitted.
In addition, an image is always assessed as part of the three-dimensional world. This means that when processing images, the brain assumes from experience that objects become smaller as the distance from the eye increases. In pictures that give us the impression of spatial depth, objects or people of the same size appear to be smaller at the bottom of the picture than at the back. This type of optical illusion can therefore be used in architecture, photography and film in order to make certain objects appear larger or smaller or closer or further away to the eye of the beholder.

4. Illusions of movement

There are many optical illusions that make the viewer believe that parts of an image are moving. To create this illusion, in some cases the head itself needs to be moved, but sometimes not. The movement can usually be noticed in those places that are not focused by the eye at this moment. An illusion of movement always arises when one looks at an (often small) object that is in front of an environment that does not provide any clue for a spatial location.

5. Relativity of parallel / straight

Lines that are actually straight can appear crooked to the viewer if the overall impression of the picture is confused, for example by different interplay of colors or other disruptive elements. As a result, straight lines often appear curved.

Two parallels can also appear crooked if other lines in the vicinity have a disruptive effect on the overall picture.

This phenomenon of optical illusion was first described by Hugo Münsterberg in 1874 and for this reason is also known as the "Münsterberg illusion".

6. Enhancement of contrasts

When processing visual information, existing contrasts in images are enhanced by the brain. In the case of a white grid on a black background, a viewer thinks that they see gray dots of color at the intersections of the white lines because the contrasts are overemphasized. The gray spots can only be seen, however, as long as one does not concentrate on them. Because this observation was first made by Ludimar Hermann, the grid is also referred to as the Hermann grid.

7. Perception of nonexistent objects

When processing visual impressions, the brain is very much oriented towards lines and edges, as these represent an orientation for it. In addition, when recognizing patterns, it tends to try to rediscover familiar things. As a result, lines and edges are added to the perception, which then serve to create a known object. This leads to the fact that, for example, when looking at an image with circles that have gaps in certain places, one imagines that one recognizes a white triangle.

8. Different perceptible objects

Some objects can be perceived in multiple ways from different perspectives. These include the so-called tilting figures such as the Necker cube. Here our individual experience determines the position in which the figure (the cube) is preferably perceived, although one is nevertheless able to understand both perspectives. The term "tilt figure" comes from the fact that the cube appears to tilt when you focus on one position of the cube while looking at it for a long time.

Optical illusions in everyday life

Optical illusions can be used in a targeted manner in various areas of daily life in order to achieve certain effects. In film, for example, the illusion of movement is exploited, whereby the rapid succession of individual images creates the illusion of movement. There are also some in painting optical illusion used as a stylistic device, for example to achieve optical enlargements.

On the other hand, unwanted optical illusions also occur in everyday life, which trick our perception, which can lead to confusion. Under certain circumstances, for example, roads seem to lead downhill when in reality they go uphill and vice versa. The phenomenon of the illusion of movement can, for example, always be observed when there is a single star in the dark sky, which then appears to be moving.

Summary

Optical illusion are an integral part of our daily life. They are based on the fact that our perception is subjective and objective external stimuli are strong through the brain and already made knowledge and experiences are influenced. Often we take optical illusion only unconsciously true or not even realizing that we are succumbing to an illusion, until switching off the triggering factors or using information from the other sensory organs proves the opposite. This is used in various areas such as film, painting or architecture.