Usually the eyes, or more generally the visual apparatus, are used to perceive the reality that surrounds us as it is. However, it may happen that some characteristics of the images we observe are such as to deceive our senses and the brain, giving us the impression of a distorted reality or one that does not exist at all. This is the case of optical and cognitive illusions, which most often rely on the assumptions that the human perceptive system continuously makes according to the principle of economy at the level of brain effort.
We have collected below 20 of the many existing optical illusions, selecting the most curious ones and also telling them through a scientific explanation of why our brain is deceived by them. Some are images that have made history and have rightfully entered cognitive psychology manuals, while others are illustrations that have gone viral on social media in recent years, sparking funny jokes.
What is an optical illusion
There are many types of optical illusions, and a clear categorization is difficult to make, especially since sometimes the nature of the illusion is not fully understood. An important point of reference may be the categorization of Richard Gregory, a British psychologist and neuroscientist. According to Gregory, there are three main classes of optical illusions: physical, physiological, and cognitive.
Physical
optical illusionsdepend on physical phenomena in the world. For example, in conditions of low humidity a mountain may appear much closer to us than it actually is. Since haze is a cue that our brain uses to calculate depth perception, we can be “fooled” when there is particularly little depth perception. Another classic example of physical optical illusion is the one classically described by Ptolemy: a stick half immersed in water appears bent.
Physiological
optical illusionsthey are the result of the effects of excessive or unusual stimulation on the brain (or eyes). It is believed that each feature of a stimulus follows its own neural pathway in the early stages of visual processing. Particularly intense or repetitive activity regarding some characteristics, or interaction with adjacent pathways, can cause an imbalance that alters perception. The Hermann Grid Illusion , which you will see below, is one example: receptive fields for different colors of the grid compete and try to inhibit each other, resulting in the illusion of non-existent gray dots.
Cognitive
optical illusionsFinally, they are caused by unconscious inferences of various kinds that the brain makes about the world. An example is the famous rabbit-duck illusion : the ambiguous interpretation of the image has the ability to change continuously thanks to our knowledge of the world.
In addition to these three classes, and within each of them, there are four types of illusions: ambiguity, distortion, paradox, and fiction.
Ambiguity
is that optical illusion that can elicit a change, a “switch” between one interpretation and another (for example, precisely, the rabbit-duck illusion or the Necker cube). The distortion illusion it concerns, precisely, a perceptive distortion as regards the size, length, position or curvature of an object or a stimulus.
The paradox is that optical illusion given by the vision of an impossible object, which cannot exist in the real world. Sometimes it can be brought about by cognitive disagreements. An example is the Penrose triangle.
Fiction
This is the illusion that makes one perceive a figure even if it is not present in the stimulus. Finally, as regards the interpretation of the stimuli of our world, it is good to keep in mind that there are two types of processing by the brain. Bottom-up processing.
Optical Illusions
The Impossible Trident (or Impossible Fork) by DH Schuster
It appears to have three spikes but the top has two. A shape that violates the rules of Euclidean geometry and that can almost become frustrating to observe because, like all impossible objects, it disregards the expectations of our mind.
It is an illusion of a paradoxical type and of a cognitive class.
Dots with illusory movement
The picture is static, but the dots seem to fluctuate. The cause, according to the Barrow Neurological Institute, are the micro-movements of the pupil: combined with the particular conformation of the image, they create the illusory movement you see. It is therefore probably a distortion at a physiological level, being due to a characteristic of our visual system.
The center circle on the left looks bigger than the center circle on the right, and you’ll be surprised if we tell you they’re exactly the same size. The reason they look different to us is that the circle on the left is surrounded by other smaller circles, while the other one is surrounded by larger circles.
This is a cognitive class bias, because it is due to an inference of the visual system about the size of the circles, based on their context.
Cafe wall illusion
It is so called because this optical illusion was first observed on the outside wall of a café. It was Richard Gregory himself who discovered it in the 1970s. The gray lines look slanted, but they’re not! The effect is due to the black and white blocks, which are slightly offset from the one in the bottom row. The blocks cause the retina to focus on different parts of the gray lines, due to the strong contrast between them (as in the case of white color with black color).
Also in this case we have a distortion, but of a physiological class.
Ebbinghaus illusion
The center circle on the left looks bigger than the center circle on the right, and you’ll be surprised if we tell you they’re exactly the same size. The reason they look different to us is that the circle on the left is surrounded by other smaller circles, while the other one is surrounded by larger circles. This is a cognitive class bias, because it is due to an inference of the visual system about the size of the circles, based on their context.
Hermann grid
We named it by introducing the physiological illusions. The Hermann grid dates back to 1870 and generates an effect due to a neural process called lateral inhibition. It concerns the ability of an excited neuron to reduce the activity of adjacent ones. The white bands produce a lot of light which strikes the neurons of the retina. But the intensity perceived in a point of the visual field is not the result of the elaboration of a single receptor, but the result of a group of receptors. The receptor in the center, for that particular point, excites the corresponding neuron while those around it inhibit it. Thus the dots at the intersections appear darker, due to increased inhibition.
Neon color diffusion illusion
As you can see, the color of the blue colored lines seems to radiate in a circle, “illuminating” also the portion not occupied by the lines, which however is actually of the same white present throughout the rest of the image. The illusion was developed by the psychologist Dario Varin.
There are still not enough theories to explain the phenomenon.
Kanisza Triangle
Kanisza’s triangle is more of a non-triangle. It is the brain that fills in the spaces in the black lines and circles, creating the illusion of a triangle (a phenomenon also known as a “ghost edge”).
According to Gregory, this optical illusion is of cognitive class and of fictitious type. It is our brain that creates the triangle based on an assumption it has about what the world should be like.
Muller-Lyer illusion
The lines inside the arrows are all of equal length, although it doesn’t appear that way. What deceives is the orientation of the arrows. The line with arrows pointing “out” appears longer than the line with arrows pointing in the middle.
According to some theories, the effect is due to our visual system, which modifies the perceived shape of the corners of the arrows, so as to move the vertex within the corner itself.
This is a cognitive class bias.
Rotating snakes
If you dwell on this image you will see it move. But it’s not a gif: This illusion, invented in 2003 by Akiyoshi Kitaoka, is caused by a type of illusory motion called peripheral drift. If we focus on a part of the image, however, we will not see that same part move, but rather the parts in the peripheral vision area.
The class of this optical illusion is physiological.
Ambiguous duck-rabbit figure
The original drawing of this figure was annotated by Joseph Jastrow in 1900. The rabbit-duck hybrid is one of the most classic ambiguous images. Thanks to top-down processing we can impose two alternative interpretations: one is that of a rabbit (with its nose to the right), the other is that of a duck (with its beak where otherwise we would have seen the rabbit’s ears).
This illusion is a cognitive ambiguity, as Gregory notes.
Broken Inaglory
In the figure you see a variant of Rubin’s vase, the famous illusion invented in 1915 by the Danish Edgar Rubino. Focusing on the black part, we see two faces staring at each other. Conversely, the white part shows a vase. As in the case of the duck-rabbit figure, we can impose different interpretations top-down. It is also a cognitive ambiguity.
Afterimage illusion
If you stare at this image for a while (at least 20 seconds), and then move your gaze to an empty wall, you may see the image of Jesus imprinted on your retina. This happens because in some cases, especially after strong stimulations, the activation of vision persists for a certain time. An everyday example of this phenomenon is a flash that sometimes stays in the eye for a few seconds.
It is a physiological illusion of the fiction type.
Boring figure
Edwin Boring drew this famous double portrait in 1930. It depicts both his wife and mother-in-law, depending on how you interpret the image. But we can only see one at a time. To guide your vision, consider that the wife’s neckline is the mother-in-law’s chin, while the wife’s ear is the mother-in-law’s eye.
This too is a cognitive ambiguous interpretation.
Troxler fade
If you stare at the dot in the center for 20 seconds, the circle will disappear. This effect was discovered in 1804 by Troxler, and is due to the tendency of the eye to give priority to certain portions of the visual field.
Ames’ room
Ames’ room is a distortedly shaped room that creates an optical illusion that radically alters perspective. Seen from the front, this chamber appears to have the normal shape of a parallelepiped, but in reality it has the shape of a trapezoid, and the floor and ceiling are inclined, and the walls diverge. A person standing in one corner of the room looks gigantic when viewed from the right angle, while a person in the opposite corner looks tiny.
