Optical illusions – confused senses

How we perceive things depends on our brain’s ability to process the information gathered by the eyes. Experience plays an important role here: the brain remembers similar objects and associates them with each other. It tries to make connections and construct a spatial image from it. However, our senses can also mislead us: In "optical illusions" the same objects appear to be of different sizes, Same colors lighter or darker, straight lines crooked – or we see things that aren’t there at all. How is it possible?

Optical illusions - confused senses

Objects that are farther away are smaller in the spatial image than those that are close by. That is why the women in the foreground appear smaller – in fact they are the same size everywhere.

Through our eyes we perceive things in our environment. Light waves reflected back from objects are picked up by the retina of the eye. But this does not explain our ability to see things. How we perceive our environment depends on the interaction between the visual apparatus and the brain. Objects that are perceived by the eye must first be processed to form the image that we finally comprehend.

Memory and experience play a major role here. We learn to associate similar objects with each other and thereby recognize what we actually see. The brain compares objects and assigns them to each other. In doing so, it tries to transform information passed on by the eye into a three-dimensional image – after all, we live in a spatial world. For this reason, objects that are in the distance are logically smaller – or rather, we perceive them that way. How we perceive images also depends on the viewer’s perspective.

Visual illusion: Deception of the senses

Optical illusions - confused senses

Image on the left: "Missing lines are completed by our senses and we see a cube with white edges. Image on the right: the parallel lines appear crooked because they are crossed by differently oriented smaller strokes.

This interaction between the eye and the brain is crucial for the ability to understand things and to classify them correctly. But our brain can also be deceived – and our senses lead us astray. The so-called "visual illusion comes about because the brain, with the help of experience, tries to recognize a perceived image. This is actually useful – but leads to confusion in special cases. A two-dimensional image is perceived in three dimensions, and we draw false conclusions about objects we see.

Our brain then strives to recognize a connection between the depicted objects. One figure, for example, appears smaller than the other due to perspective – although both are the same size are. Or straight lines are perceived crooked. Colors are also perceived differently. Depending on the background they are on, they can appear lighter or darker. In optical illusions, we also see things that do not exist at all. How exactly do such phenomena come about?

Larger, smaller – or the same size?

Optical illusions - confused senses

The perception of size is relative: The two blue spheres are the same size. Because the left sphere is surrounded by larger spheres and the right sphere by smaller spheres, the left sphere appears smaller.

When we look at images with different objects, our brain establishes a relationship between them. We recognize a "path, that runs from the front to the back of the image, and therefore the impression of spatial depth is created. Objects at the bottom of the image seem to be "close by" to be located. With greater distance, objects appear smaller. Due to the effort to capture a three-dimensional image, bodies of the same size therefore appear to be different sizes.

We perceive objects of the same size differently even if there are other – smaller or larger – objects in their vicinity. We evaluate things "relatively – thus in dependence or in comparison to others. Something is not just big, small, light or dark – it depends on the respective scale. Someone who is actually tall A dwarf, which is a giant, could still look like a dwarf next to a giant. In relation to the size of other objects, the same body can therefore be relatively large at one time and then appear relatively small again.

How straight lines become crooked

Optical illusions - confused senses

The dividing lines between the large squares seem curved by the much smaller white ones in the corners of the black squares.

Due to different colors, lines that are actually straight can look crooked. In the illustration on the right, for example, the tiny bright squares in the corners of the dark ones are responsible for that. They disturb the overall impression of the dividing lines between the light and dark squares – and the actually straight lines appear curved.

The alignment of straight lines can also have a completely different effect if there are other lines in the environment that cause confusion in the overall image. For example, two parallel lines can appear crooked when they are crossed by other lines, each going in a different direction (see small image above right).

Same colors appear lighter or darker

Optical illusions - confused senses

Area A is much darker than B? – an optical illusion. It only appears that way because we are oriented to the checkerboard pattern and the green body provides a shadow effect. The areas are of the same color (see also image gallery, image 2).

Colors and their brightness can be perceived very differently. In strong sunlight, the same hue appears different than in dim light. On a dark background a color looks much brighter than on a light one. Depending on the environment, the same colors appear different.

Our brain also classifies the relationship between light and shadow. We know from experience that the same object looks much darker when it is in shadow. If we think we see a shadow effect, we assume that the same color is lighter and the shadow makes it look darker.

In the picture on the left, both the shading shown and our experience that a checkerboard pattern must be two-colored play a role – that’s why we perceive the same-colored squares A and B completely differently. In our picture gallery (picture 2) in the second part of the article, you can see for yourself that A and B have the same shade of gray.

Seeing things that do not exist at all?

Optical illusions - confused senses

Enhancement of contrasts: In this image, there seem to be gray spots where the white lines cross.

Some images deceive our senses in such a way that we see things that are not actually there. For example, our brain tries to grasp the shape of a certain object through experience. It then adds lines or edges that are crucial for the overall impression of this object, but are missing in the actual picture (see "Cube with white edges"), small image above left).

The human brain is always trying to grasp what the eye provides it with in terms of impressions. For the processing of visual information, it is strongly oriented towards lines and edges – since these enable orientation. For example, we recognize line drawings with clear contours more quickly than pictures with spongy color shading. Contrasts are enhanced when objects are processed. In the illustration on the right, there is a white grid on a black colored area. The contrast is overemphasized – and we see gray spots of color in the spaces that are not there.

Illusion of movement

Optical illusions - confused senses

The illusion of moving figures: if we look at the black dot and move our head back and forth, the two circles appear to rotate in opposite directions.

In some optical illusions, the viewer believes that parts of the image are moving. This happens, for example, when an object is viewed against a background whose spatial position cannot be assigned. With some images, the head or the graphic itself must be moved in order to see the supposed "movement" to perceive.

This works best if you do not focus on the objects that are perceived to be moving. In the illustration on the left, if you look at the black dot in the center while moving your head back and forth, the two circles appear to rotate in opposite directions. This effect is caused by the diagonally aligned squares that form the respective circle: In the outer circle they are tilted to the left, in the inner circle they are tilted to the right.

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