Eye Tricks: Why Our Eyes See Optical Illusions

Optical illusions reveal a great deal about the brain. Learn about some of the most famous eye tricks and illusions, and the science behind why they occur.

We’re often told that seeing is believing, but when it comes to vision, our eyes can deceive us. For centuries, people have been fascinated with optical illusions and eye tricks, and with good reason. While illusions are fun to look at — often challenging us to see things in new ways — they also reveal how the brain interprets what we see and shapes our perception of reality. Let’s explore the science behind the visuals that play tricks on our mind.

What Are Optical Illusions?

Research on optical illusions highlights one major point: Our visual system is too limited to accurately process all of the information that our eyes take in, so it works out the most likely interpretation. On rare occasions, the brain interprets incorrectly, causing us to perceive something inaccurately — an optical illusion. On other occasions, different people can perceive things in different ways. This is the basis of the the 2015 Internet phenomenon of “the dress.” A picture of a dress sparked widespread controversy when users could not agree on its color, splitting into factions of #whiteandgold or #blackandblue. The difference of opinion has to do with the difference in how people perceive light. It’s a reminder that not everyone experiences visual illusions in the same way.

Types of Optical Illusions

While illusions are fun for us to argue about or challenge each other with, they are actually a powerful tool to understand how our bodies and brains work. Psychologists, cognitive scientists, and neuroscientists have created and/or used optical illusions to figure out what exactly a trick of the eye means. In their studies, scientists have divided illusions into three categories based on how humans process them.

Physical Illusions

Sometimes the source of the illusion is a natural occurance. Image by Kuki Ladron de Guevara.

Physical illusions are created by stimuli in our environment. In other words, the source of the visual trick is external, in the physical world, rather than internal, created in our mind. The most famous example is a stick submerged in water appearing bent. That illusion is based on the refraction of light on the water.

Physiological Illusions

Physiological means having to do with the function of the body, so this type of illusion is about how the body perceives a stimulus. These illusions are often about overstimulation, using bright colors, contrast, and competing elements to overload our eyes or brain and trick us. Above you’ll see some famous physiological illusions, like the Hermann grid or the Zöllner illusions (learn more about them below). On top of fascinating us for centuries, these illusions have helped scientists understand how our eyes work, and how our eyes feed information to our brains.

Cognitive Illusions

The Rubin vase illusion. Image by Martin Janecek.

This type of illusion deals with how the brain perceives a stimulus. Unlike physiological illusions, which are largely based on the makeup of our eyes, cognitive illusions are based on the makeup of our brains and neural pathways. You’ve probably seen the image of the vase that is also an image of two faces. This famous trick shows how our brains switch between interpretations when we’re faced with an ambiguous visual.

From straight lines appearing crooked to still images moving on the screen, here are some of the most well-known optical illusions, along with scientific explanations of why they occur.

The Hermann Grid Illusion

eye tricks herman gridA classic Herman Grid illusion. Image by dipego.

The Herman Grid illusion was first reported by German physiologist Ludimar Hermann in 1870. As you look at the image, you’ll notice that dark dots quickly appear and disappear at the intersections between the grids. However, when you look directly at an intersection, the dots all but vanish.

The explanation behind this illusion has been the subject of debate for years, with many claiming it’s the result of “lateral inhibition” — the complex way the cells at the back of the retina respond to black and white. Though you can recreate grid illusions with color, nothing is as striking as a black and white design.

The Impossible Penrose Triangle

The Impossible Penrose TriangleImpossible shapes in vibrant colors. Image by Keep Calm and Vector.

The idea of the impossible triangle is based on a drawing originally created by physicist Roger Penrose in 1954. As we view the image, we flip back and forth between the equally possible perspectives of the object. The triangle starts here, or here, or here. When considered together, these multiple perspectives simply can’t exist. Try to trace a line from the outside to the inside corners of the triangle and you would have to trace it around three times before coming back to where you started.

This illusion plays on the eye’s interpretation of two-dimensional pictures as three-dimensional objects. Our eyes and brain are fooled because they assume that all the corners of the triangle are at the same distance from us. This leads us to perceive an impossible three-dimensional object.

Our brains want all of the corners to be the same distance from us. Image by elfinadesign.

The neat thing about the Penrose triangle is that you can draw it in any visual style, as long as it has two edges or contours on each side. You can also apply the Penrose illusion to other polygons, like squares, pentagons, or hexagons. While the other illusions on this list might require a very particular layout to work, you have much more freedom to try this illusion in any design you’re working on.

The Zöllner Illusion

The Zöllner IllusionParallel lines made from black and white pillows. Image by hunthomas.

The Zöllner IllusionOptical illusion with parallel lines. Image by diskoVisnja.

The Zöllner illusion, discovered by German astrophysicist Johann Karl Friedrich Zöllner in 1860, contains a series of horizontal lines crossed with either overlapping short lines or black and white cubes. As you look at the image, the horizontal lines appear to slope, as if they will eventually collide. In reality, the lines are perfectly parallel to each other.

There are a number of possible explanations for this illusion. One is that the angles between the short and long lines create an impression of depth, with one line appearing closer and the other further away. Another explanation is that the brain tries to increase the angle between the short and long lines, which makes it appear as if the lines are bending toward and away from each other. However you see it, it’s a testament to minimalist design — that simple lines can produce such a striking and mind-bending visual.

Motion Illusions

Motion dot IllusionsThe optical illusion of movement in the form of fluctuating pink and lilac polygons. Image bu Skripnichenko Tatiana.

Motion illusions rely on the rapid movements called “saccades” that our eyes make as we view something. To experience them, gently place your finger on your eyelid and look around the room. You should feel your eye making dart-like movements, as if its taking a series of snapshots.

Motion IllusionsMotion illusions take advantage of the natural movements our eyes make as they absorb information. Image by Mark Grenier.

Although our brain processes these movements as a smooth succession, sometimes they can fool us into thinking that still objects are moving. The rotating snakes illusion above does just that. The image is full of contrasting shapes and colors, composed in such a way that it switches on our motion sensors and tricks our mind.

These movement illusion graphics are intrinsically linked to psychedelic design from the ’60s and ’70s. The poster designs, album covers, tapestries, and leaflets from this era were a mesmerizing mix of colors, shapes, and psychedelic fonts. The art was meant to be experiential, reflecting the drug experimentation and counter-culture adventures pushed by hippies and free-love practitioners. With their kaleidoscopic colors and perceived flowing movement, motion illusions will always fit nicely into this design aesthetic.

Shadow Illusions

Shadow IllusionsOptical illusion with color. Image by diskoVisnja.

This impressive illusion was created by Edward H. Adelson from the Massachusetts Institute of Technology. In it, the two squares marked with dots appear to be two different shades of gray. In reality, they are actually identical in color.

Here, the lower square is cast in the shadow of the red cylinder, while the higher square is outside of the shadow. Although our eye can see two identical shades of gray, our brain reasons that if the square in the shadow reflects the same amount of light as the square outside of the shadow, then it must be a much lighter shade of gray. In effect, our brain alters our perception of the image so that we see the colors as they would likely appear in the real world. Again, it’s an illusion created by the differences in how we see light. This one is especially powerful because even when you squint — our only defense against illusions — our brains still can’t reason with what we’re seeing.

Cover image by Mykola Mazuryk.

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