What Can Illusions Tell Us About How Consciousness Works?

There is no strict answer to the question, “What are illusions?” There are many types of illusions, and their origins can be explained from the perspectives of different scientific disciplines.

For example, there are visual illusions whose causes lie in the laws of physics: mirages, the halo effect, and so on. These types of illusions can be captured by a camera.

There are also illusions related to the structure of our visual organs. The blind spot is an area in the eye where there are no retinal receptors, and where nerve fibers and blood vessels are located. The presence of a blind spot allows us to experience the illusion of an object disappearing when we look at an image of two figures while covering one eye with a hand. At a certain distance from the image, one of the figures will fall into the blind spot, and we will stop seeing it until we change our position. Try looking at the right cross with your left eye while covering your right eye. Move closer and farther away until the image disappears.

The most interesting illusions are those that cannot be explained by the body’s physiology or by physical phenomena. These are connected to the workings of the mind and cognition. Debates about their origins are still ongoing. Such phenomena cannot be recorded by devices like cameras, nor can they be fully explained by the functioning of physiological organs and systems. These are called cognitive illusions.

Cognitive illusions often include size illusions: the famous Müller-Lyer, Ponzo, Ebbinghaus, and Oppel-Kundt illusions, among others. In all these illusions, objects of the same size appear different to us. Even knowing that two lines or two shapes are equal in size does not protect us from being visually deceived.

The Müller-Lyer Illusion

Scientists have long tried to unravel the mystery of perceptual illusions. For centuries, hypotheses have been proposed about the causes of the Moon illusion. It is well known that the Moon appears much larger near the horizon than at the zenith, but a camera does not capture this difference. This is not surprising: wherever the Moon is in the sky, its size remains the same, and the distance to Earth does not change enough to turn a small disk in the sky into a giant bright ball on the horizon.

For a camera, the Moon is always the same size, regardless of its position in the sky.

There are many attempts to explain this, involving the direction of gaze, the work of eye muscles when looking up or straight ahead, the role of objects in the line of sight when looking at the horizon (but not upward), the position of the body relative to the object’s line, and so on. Unfortunately, none of these explanations are sufficient so far. Scientists would also like to find a unified explanation for illusions, to identify at least some general patterns and mechanisms for their occurrence, and to build a single classification. However, this has not yet been achieved—there are more questions than answers.

Undoubtedly, searching for the causes of illusions and the factors that increase or decrease them will help us better understand how our perception works, how we comprehend the visible world, what mistakes we make in judging size, in visual search, in detecting and recognizing objects, and why we make these mistakes. It could also help us learn how to help people avoid such errors, reduce the strength of illusions, or even increase them, for example, for camouflage purposes.

The Ponzo Illusion and Its Variations

It is very important to understand that visual illusions are just one example of how our consciousness can alter the world around us. This fascinating phenomenon reminds us that we do not always see what we think we see. We are surrounded by an incredible number of objects, located at different distances and angles, with shapes that can be very complex and multidimensional. At every moment, a person must choose what will be consciously perceived and how that part of reality will be interpreted. What will the perceived, not the real, shape or size be, and what must happen for what was previously unconscious to become conscious?

The Delboeuf Illusion

Perceptual illusions can serve as unique material for studying the patterns of consciousness. After all, an illusion can be stronger or weaker. Sometimes this depends on unexpected things, like the units of measurement—you can estimate the difference in centimeters, grams, or even in “parrots,” as in the famous cartoon. In certain states, people stop experiencing illusions; the assessment of objects causing illusions can differ depending on whether they are evaluated verbally (with words) or motorically (by grasping the object or drawing a line of the corresponding length). Sometimes, how we perceive the size of an object matters more than its actual size: there are experiments where people were more accurate at hitting a target simply because it appeared larger. By knowing about such illusions, studying them, and trying to understand their mechanisms, we can not only advance the study of our consciousness but also find interesting ways to use perceptual illusions to improve our effectiveness.