- Optical illusions have been amusing and frustrating people for decades.
- Though often seen as a light source of fun, there’s a lot that optical illusions can tell us about our vision and neural systems.
- But which are the best examples?
Below are some of the most puzzling optical illusions of all time and the explanations behind how they trick your mind.
1. The Ebbinghaus Illusion: it is all about context
Also known as the Titchener Circles, this illusion tests your perception of size. Though discovered by and named after Edward B. Titchener created, Hermann Ebbinghaus, the most common depiction of this illusion.
Titchener’s depiction features two circles that are equal in size. One circle is surrounded by a ring of larger circles, while the other circle is surrounded by a ring of smaller circles. Though both central circles are the same size, one appears smaller than the other with the addition of additional circles.
It is believed that the cause of this discrepancy lies in how we perceive size. Research has suggested that our perception of size is dependent on context. By changing the context in which both circles are shown, our perception of their relative sizes changes.
2. The Lilac Chaser: afterimage and Troxler’s Fading
Popularly called the Pac-Man illusion, this image loop has had people scratching their heads since 2005. Created by Jeremy Hinton, the illusion features twelve dots, typically lilac or magenta. The dots are placed against a gray background, with a black cross in the center. One dot disappears for a fraction of a second before reappearing, an action that repeats itself in a clockwise motion throughout all the dots.
Viewers are instructed to stare at the cross in the middle of the ring; at this point, they observe two things happening: firstly, a green dot appears in the absence of the disappeared dot. Secondly, the green dot gradually wipes away the remaining lilac dots until all that remains is a green dot moving in a circuit.
The explanation behind this illusion is truly fascinating. The green dot appears thanks to an effect known as an afterimage. The rods and cones in our eyes adjust to the disappearance of the lilac dots after a few seconds and become tired. In the absence of the lilac dot, our eyes engage cones that process colors at the opposite end of the spectrum: green.
The ultimate disappearance of all the lilac dots is thanks to a phenomenon known as Troxler’s Fading. Because the lilac dots only appear in our peripheral vision, their movements are not significant enough to engage new neurons of our visual system and thus appear to fade away entirely.
3. The Motion Illusion: seeing movement where there is none
Optical illusions that create the illusion of movement are among the most common and popular. You might be surprised at the simplicity of the explanation behind these illusions.
Motion illusions typically present a pattern of high-contrast colors or tones. These contrasting aspects simultaneously trigger different neural signals, resulting in a motion-detecting effect when no motion is present.
4. The Spinning Dancer: control the dancer with your mind
One illusion that repeatedly goes viral online is the Spinning Dancer. The Spinning Dancer depicts the silhouette of a dancer spinning in one place. However, whether the dancer moves clockwise or counter-clockwise can be changed by the viewer at will.
This illusion was created in 2003 by Nobuyuki Kayahara and had been puzzling people online ever since. The key to the illusion is its lack of visual cues concerning depth and the ambiguity of the dancer’s anatomy. This visual ambiguity is known as multistable perception. The dancer’s body and environment are too ambiguous for our visual systems to perceive, so we perceive the image in alternating, conflicting states.
5. The Vertical-Horizontal Illusion: a surprisingly simple puzzle
The Vertical-Horizontal illusion is one of the most visually simple optical illusions you’ll likely encounter. It features a horizontal line bisected in the middle by a vertical line. Though most viewers perceive the vertical line as longer, they’re the same length.
Though the exact cause of this phenomenon is unknown, it has been suggested that the positioning of the vertical line triggers our depth perception – causing us to perceive the vertical line as being further away from us than the horizontal line and thus longer.
6. Kanizsa’s Triangle: illusory contours and depth perception
Kanizsa’s Triangle is a famous example of illusory contours. Illusory contours refer to the perceived presence of an edge or outline when there is none. This perception is created by the presence of separate shapes and edges arranged in a manner that suggests the presence of a defined contour.
In the case of Kanizsa’s Triangle, three so-called “Pac-Man configurations” and three open angles create the illusion of a white triangle. The illusory shape typically appears brighter and closer to the viewer in these illusions.
This is because the Pac-Man configurations trigger our depth perception, leading our visual systems to perceive the configurations as further away and, thus, darker than the triangle. These perceived depth cues establish the illusion of the triangle being close and bright.
7. Rubin’s Vase: figure-ground organization and edge-assignment
Developed by Edgar Rubin in 1915, Rubin’s Vase illusion is one of the most famous optical illusions in the world. The image depicts what can be perceived as either an ornamental vase or two faces in side profiles facing one another.
The key to this illusion is explained through the principle of figure-ground organization. This is what allows us to perceive objects as both figures and backgrounds. However, Rubin’s Vase illusion alters the figure with a perspective shift.
The vase becomes the figure when viewing the black area as the background. When viewing the white area as the background, the faces become the figure.
8. The Duck and Rabbit Illusion: ambiguous images and mid-level vision
This image first appeared in 1892 and has been puzzling and amusing people ever since. The duck-or-rabbit conundrum operates on the principles of how we perceive ambiguous images and the processes of mid-level vision.
Mid-level vision is the point at which our brains group visual information based on their ability to find edges in the image. However, in the case of ambiguous images, these edges are unclear, and we become capable of perceiving two contrary depictions within the same image. For this reason, we can see a rabbit or a duck, depending on which depiction we focus on.
9. The Müller-Lyer Illusion: which line is longer?
You’ve almost certainly come across this optical illusion in the past. Devised by Franz Carl Müller-Lyer in 1889, this series of lines is one of the most recognizable illusions ever developed.
Typically, the viewer is presented with three horizontal lines, each featuring a different configuration of arrowheads. Though the lines appear to be different lengths, they are all equal.
Studies have shown that this illusion is most effective among Western populations accustomed to “carpentered” surroundings, where straight lines and right angles are common. This could explain how the illusion works.
Someone familiar with intersecting lines in their daily lives would automatically register the arrowheads that angle inwards as representing a closer object. In contrast, the arrowheads that point outwards would suggest the object is further away. With this in mind, the object appearing further away would also appear longer.
10. The Checker Shadow Illusion: your eyes deceive you
Developed in 1995 by Edward H. Adelson, the Checker Shadow Illusion will have you doubting your vision. The image shows a checkerboard and an object resting upon it, casting a shadow. Two squares on the board are titled A and B. Though A and B appear to be different tones, they are identical.
The illusion operates by taking advantage of our perceptions of light constancy. Light constancy allows us to differentiate between dark objects in bright lighting and light objects in low lighting. In the Checker Shadow Illusion, we recognize that A is a dark object, and B is a light object in shadow. However, because we are differentiating the squares through our perceptions of light constancy, we cannot perceive them as they are – as being the same color.
11. The Impossible Trident: drawings that trick your brain
The Impossible Trident dates back to 1964 and is one of the most famous examples of an impossible object illusion. The image depicts what initially appears to be a three-pronged object whose prongs originate from only two sources.
This puzzling image tricks your brain into perceiving a two-dimensional drawing as a three-dimensional object. The impossible nature of the object only becomes clear after examining the image, as the brain attempts to conceive of the representation as a 3-D object that can exist in the natural world.
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