Types of optical illusions

Optical illusions are phenomena that show how we subjectively perceive objects lying in front of us, often in a way that differs from reality. They are the result of both higher cortical processes and physiological properties of the eye. The brightness of a perceived object depends not only on its actual colour, but also on its surroundings, giving rise to optical illusions such as illusions of brightness and colour. We perceive oblique lines as longer or shorter depending on the surrounding elements, which changes the size of the perceived image. Similarly, horizontal lines that intersect segments that run obliquely produce the illusion of a change in length. Ambiguous figures allow two variants to be seen at the same time, demonstrating the brain's ability to make alternative interpretations. These phenomena also depend on the observer's point of view, highlighting the complexity of cognitive processes. In the following sections we will discuss different examples of illusions.

What is an optical illusion?

An optical illusion is a phenomenon in which the perception of an image differs from reality, which is often due to the limitations of the human eye and the way the brain interprets visual stimuli. An example is the optical illusion of misjudging the size of objects, such as the Ebbinghaus illusion. In this illusion, two identical circles appear different sizes depending on the surrounding elements. This is because the retina is left with a constant image, but the brain takes the context into account by applying the law of constancy of size judgement.

Optical illusions are also caused by looking at patterns or structures for long periods of time. For example, the perception of the shape of buildings can be distorted as a result of the specific viewpoint of the observer. This is why these illusions are often used in architecture, where facades or layouts of spaces are designed to give the impression of being larger or smaller than they actually are. These phenomena show that visual interpretation is dynamic and susceptible to contextual influences.¹

Causes of optical illusions

Optical illusions result from various factors that affect the way the brain interprets visual stimuli. The observer's point of view also plays an important role in their formation. In some cases, the brain is forced to simplify complex scenes, leading to misinterpretation. For example, the two parts of a line, although identical in reality, may be perceived as different due to the surrounding visual context or the use of colours.

Representations of three-dimensional figures on flat surfaces are also the cause of some illusions. Their three-dimensional counterparts in reality can look completely different, which shows how strongly our perception depends on an internal attitude. Another example is illusions of contrast, such as the effect in the area of the intersection of white stripes in the Hermann grid. Here, part of the contour will be recognised as a shadow, even though it is not actually there.²

Optical illusions result from the mechanism of our brain, which tries to fill in missing information based on previous experience. This makes visual perception fast and efficient, but sometimes leads to unexpected errors. These phenomena highlight how multidimensional and complex the process of vision is.

What are optical illusions?

Optical illusions consist of differences between reality and how a given observer's brain interprets the image seen. In particular circumstances, the brain undergoes strong illusions as it tries to simplify and interpret scenes in a way that is consistent with previous experience. In such situations, the misinterpretation of the image is the result of higher cortical processes that are responsible for analysing visual stimuli.

Examples include the sensation of a glow in contrasting images or the disappearance of objects when looking at uniform patterns for long periods of time. These phenomena reveal that the brain often relies on patterns that do not always reflect reality. In the context of objects of similar shape, such as repeating rectangles in architecture - known as the civilisation of rectangles - unambiguous perception is sometimes hampered. Lines intersecting at an angle can change the way we interpret perspective or the size of elements.

Of particular interest are illusions related to architecture, such as the effect of the "wall of a certain café", where lines appear crooked even though they are parallel. These kinds of illusions show that the observer's brain is forced to compromise between what it sees and what it tries to imagine. The process of interpreting a scene depends on many factors, including our prior knowledge and spatial perception.

Muller-lyer illusion

The Müller-Lyer illusion is a classic example of an optical illusion in which two lines of the same length appear to be of different lengths depending on the shape of their ends. The illusion is caused by an erroneous train of thought, resulting from a spatial interpretation that the brain automatically applies. Specifically, the physiological irritation field of the retina remains unchanged, but as a result of prolonged gazing at such figures, the brain becomes delusional. This phenomenon demonstrates the ambiguity of the figure, where parallel realities - such as lines on a plane - are mistakenly read as different in length. This illusion perfectly illustrates how perception based on previous experience can mislead us.

The ponzo illusion

The Ponzo illusion is one of the classic optical illusions that illustrates how the brain interprets depth and perspective. The illusion results from a faulty analysis of the proportions and distances of objects of similar shape. Two horizontal lines of identical length, placed against a background of diagonal lines resembling railway tracks, appear to be of different lengths - the upper line appears longer.

This phenomenon is caused by the involvement of higher cortical processes, which automatically try to interpret the perspective in the image. The brain tries to perceive elements unambiguously in a three-dimensional context, which leads to this illusion. The Ponzo illusion perfectly demonstrates how spatial perception can affect our perception of size.