10 Award-Winning Optical Illusions and Brain Puzzles

"The Spinning Disks Illusion"
"The Spinning Disks Illusion"
Used by permission of Johannes Zanker

When the new book Champions of Illusion: The Science Behind Mind-Boggling Images and Mystifying Brain Puzzles arrived at the Mental Floss offices, we couldn't flip through it—and flip our brains out—fast enough.

Created by Susana Martinez-Conde and Stephen Macknik, professors of ophthalmology, neurology, physiology, and pharmacology at SUNY Downstate Medical Center in Brooklyn, New York, the book is a fascinating compilation of award-winning images from the Best Illusion of the Year contest, which Martinez-Conde and Macknik first created for a neuroscience conference in 2005. Since then, the contest has produced some truly mind-bending mind tricks that challenge our sense of perception of the world around us. As the authors write:

Your brain creates a simulation of the world that may or may not match the real thing. The "reality" you experience is the result of your exclusive interaction with that simulation. We de­fine "illusions" as the phenomena in which your perception differs from physical reality in a way that is readily evident. You may see something that is not there, or fail to see something that is there, or see something in a way that does not reflect its physical properties.

Just as a painter creates the illusion of depth on a flat canvas, our brain creates the illusion of depth based on information arriving from our essentially two-dimensional retinas. Illusions show us that depth, color, brightness, and shape are not absolute terms but are subjective, relative experiences created actively by our brain's circuits. This is true not only of visual experiences but of any and all sensory perceptions, and even of how we ponder our emotions, thoughts, and memories. Whether we are experiencing the feeling of "redness," the appearance of "square­ness," or emotions such as love and hate, these are the result of the activity of neurons in our brain.

Yes, there is a real world out there, and you perceive events that occur around you, however incorrectly or incompletely. But you have never actually lived in the real world, in the sense that your experience never matches physical reality perfectly. Your brain instead gathers pieces of data from your sensory systems—some of which are quite imprecise or, frankly, wrong.

It's never been so fun to be wrong. Here are 10 of our favorite images from Champions of Illusion, accompanied by explanations from the book of how and why they work.


coffer illusion by Anthony Norcia, Stanford University
"Coffer Illusion"
Used by permission of Anthony Norcia, Stanford University

Information transmitted from the retina to the brain is constrained by physical limitations, such as the number of nerve fibers in the optic nerve (about a million wires). If each of these fibers was responsible for producing a pixel (a single point in a digital image), you should have lower resolution in your everyday vision than in the images from your iPhone camera, but of course this is not what we perceive.

One way our visual system overcomes these limitations—to present us with the perception of a fully realized world, despite the fundamental truth that our retinas are low-resolution imaging devices—is by disregarding redundant features in objects and scenes. Our brains preferentially extract, emphasize, and process those unique components that are critical to identifying an object. Sharp discontinuities in the contours of an object, such as corners, are less redundant—and therefore more critical to vision—because they contain more information than straight edges or soft curves. The perceptual result is that corners are more sa­lient than non-corners.

The Coffer Illusion contains sixteen circles that are invisible at first sight, obscured by the rectilinear shapes in the pattern. The illusion may be due, at least in part, to our brain's preoccupation with corners and angles.


"The Rotating Snakes Illusion"
Used by permission of Akiyoshi Kitaoka

This illusion is a magnificent example of how we perceive illusory motion from a stationary image. The "snakes" in the pattern appear to rotate as you move your eyes around the figure. In reality, nothing is moving other than your eyes!

If you hold your gaze steadily on one of the "snake" centers, the motion will slow down or even stop. Our research, conducted in collaboration with Jorge Otero-Millan, revealed that the jerky eye motions—such as microsaccades, larger saccades, and even blinks—that people make when looking at an image are among the key elements that produce illusions such as Kitaoka's Rotating Snakes.

Alex Fraser and Kimerly J. Wilcox discovered this type of illusory motion effect in 1979, when they developed an image showing repetitive spiral arrangements of luminance gradients that appeared to move. Fraser and Wilcox's illusion was not nearly as effective as Kitaoka's il­lusion, but it did spawn a number of related effects that eventually led to the Rotating Snakes. This family of perceptual phenomena is characterized by the periodic placement of colored or grayscale patches of particular brightnesses.

In 2005, Bevil Conway and his colleagues showed that Kitaoka's illusory layout drives the responses of motion-sensitive neurons in the visual cortex, providing a neural basis for why most people (but not all) perceive motion in the image: We see the snakes rotate because our visual neurons respond as if the snakes were actually in motion.

Why doesn't this illusion work for everyone? In a 2009 study, Jutta Billino, Kai Ham­burger, and Karl Gegenfurtner, of the Justus Liebig University in Giessen, Germany, tested 139 subjects—old and young—with a battery of illusions involving motion, including the Rotating Snakes pattern. They found that older people perceived less illusory rotation than younger subjects.


healing grid illusion by Ryota Kanai
"The Healing Grid"
Used by permission of Ryota Kanai

Let your eyes explore this image freely and you will see a regular pattern of intersecting horizontal and vertical lines in the center, flanked by an irregular grid of misaligned crosses to the left and right. Choose one of the intersections in the center of the image and stare at it for 30 seconds or so. You will see that the grid "heals" itself, becoming perfectly regular all the way through.

The illusion derives, in part, from "perceptual fading," the phenomenon in which an unchanging visual image fades from view. When you stare at the center of the pattern, the grid's outer parts fade more than its center due to the comparatively lower resolution of your peripheral vision. The ensuing neural guesstimates that your brain imposes to "reconstruct" the faded outer flanks are based on the available information from the center, as well as your nervous system's intrinsic tendency to seek structure and order, even when the sensory in­put is fundamentally disorganized.

Because chaos is inherently unordered and unpredictable, the brain must use a lot of energy and resources to process truly chaotic information (like white noise on your TV screen). By simplifying and imposing order on images like this one, the brain can reduce the amount of information it must process. For example, because the brain can store the image as a rectilinear framework of white rows and columns against a black background—rather than keeping track of every single cross's position—it saves energy and mental storage space. It also simplifies your interpretation of the meaning of such an object.


mask of love by Gianni Sarcone, Courtney Smith, and Marie-Jo Waeber
"Mask of Love"
Courtesy of Gianni Sarcone, Courtney Smith, and Marie-Jo Waeber. Copyright © Gianni A. Sarcone, giannisarcone.com. All rights reserved.

This illusion was discovered in an old photograph of two lovers sent to Archimedes' Laboratory, a consulting group in Italy that specializes in perceptual puzzles. Gianni Sarcone, the leader of the group, saw the image pinned to the wall and, being nearsighted, thought it was a single face. After putting on his eyeglasses, he realized what he was looking at. The team then superimposed the beautiful Venetian mask over the photograph to create the final effect.

This type of illusion is called "bistable" because, as in the classic Face/Vase illusion, you may see either a single face or a couple, but not both at once. Our visual system tends to see what it expects, and because only one mask is present, we assume at first glance that it surrounds a single face.


age is all in your head illusion by Victoria Skye
"Age Is All in Your Head"
Used by permission of Victoria Skye

The magician, photographer, and illusion creator Victoria Skye was having a hard time taking a picture of a photo portrait of her father as a teen. The strong overhead lighting was ruining the shot, so she tilted the camera to avoid the glare, first one way and then the other. As she moved her camera back and forth, she saw her father morph from teen to boy and then to adult.

Skye's illusion is an example of anamorphic perspective. By tilting her camera, she created two opposite vanishing points, producing the illusion of age progression and regression. In the case of age progression, the top of the head narrows and the bottom half of the face expands, creating a stronger chin and a more mature look. In the case of age regression, the opposite happens: the forehead expands and the chin narrows, producing a childlike appearance.

Skye thinks that her illusion may explain why, when we look at ourselves in the mirror, we sometimes see our parents, but not always. "I wonder if that is what happens to me when I look in the mirror and see my mom. Do I see her because I tilt my head and age myself just as I did with the camera and my dad?" she asked.


rotating tilted lines illusion by Simone Gori and Kai Hamburger
"The Rotating-Tilted-Lines Illusion"
Used by permission of Simone Gori and Kai Hamburger

To experience the illusion, move your head forward and backward as you fixate in the central area (or, alternatively, hold your head still and move the page). As you approach the image, notice that the radial lines appear to rotate counterclockwise. As you move away from the image, the lines appear to rotate clockwise. Vision scientists have shown that illusory motion activates brain areas that are also activated by real motion. This could help explain why our perception of illusory motion is qualitatively similar to our perception of real motion.


Pulsating Heart illusion by Gianni Sarcone, Courtney Smith, and Marie-Jo Waeber
"Pulsating Heart"
Courtesy of Gianni Sarcone, Courtney Smith, and Marie-Jo Waeber. Copyright © Gianni A. Sarcone, giannisarcone.com. All rights reserved.

This Op Art–inspired illusion produces the sensation of expanding motion from a completely stationary image. Static repetitive patterns with just the right mix of contrasts trick our visual system's motion-sensitive neurons into signaling movement. Here the parallel arrangement of opposing needle-shaped red and white lines makes us perceive an ever-expanding heart. Any other outline delimited in a similar fashion would also appear to pulsate and swell.


ghostly gaze illusion by Rob Jenkins
"Ghostly Gaze"
Used by permission of Rob Jenkins

Not knowing where a person is looking makes us uneasy. That's why speaking with somebody who is wearing dark sunglasses can be awkward. And it is why someone might wear dark sunglasses to look "mysterious." The Ghostly Gaze Illusion, created by Rob Jenkins, takes advantage of this unsettling effect. In this illusion, twin sisters appear to look at each other when seen from afar. But as you approach them, you realize that the sisters are looking directly at you!

The illusion is a hybrid image that combines two pictures of the same woman. The overlapping photos differ in two important ways: their spatial detail (fine or coarse) and the direction of their gaze (sideways or straight ahead). The images that look toward each other contain only coarse features, whereas the ones that look straight ahead are made up of sharp details. When you approach the pictures, you are able to see all the fine detail, and so the sisters seem to look straight ahead. But when you move away, the gross detail dominates, and the sisters appear to look into each other's eyes.


Elusive Arch illusion by Dejan Todorovic
"Elusive Arch"
Used by permission of Dejan Todorovic

Is this an image of three shiny oval tubes? Or is it three pairs of alternating ridges and grooves?

The left side of the figure appears to be three tubes, but the right side looks like a corrugated surface. This illusion occurs because our brain interprets the bright streaks on the figure's surface as either highlights at the peaks and troughs of the tubes or as inflections between the grooves. Determining the direction of the illumination is difficult: it depends on whether we consider the light as falling on a receding or an expanding surface.

Trying to determine where the image switches from tubes to grooves is maddening. In fact, there is no transition region: the whole image is both "tubes" and "grooves," but our brain can only settle on one or the other interpretation at a time. This seemingly simple task short-circuits our neural mechanisms for determining an object's shape.


floating star illusion by Joseph Hautman, aka Kaia Nao
"Floating Star"
Used by permission of Joseph Hautman, aka Kaia Nao. Copyright © Kaia Nao

This five-pointed star is static, but many observers experience the powerful illusion that it is rotating clockwise. Created by the artist Joseph Hautman, who moonlights as a graphic designer under the pseudonym "Kaia Nao," it is a variation on Kitaoka's Rotating Snakes Illusion. Hautman determined that an irregular pattern, unlike the geometric one Kitaoka used, was particularly effective for achieving illusory motion.

Here the dark blue jigsaw pieces have white and black borders against a lightly colored background. As you look around the image, your eye movements stimulate motion-sensitive neurons. These neurons signal motion by virtue of the shifting lightness and darkness boundaries that indicate an object's contour as it moves through space. Carefully arranged transitions between white, light-colored, black, and dark-colored regions fool the neurons into responding as if they were seeing continual motion in the same direction, rather than stationary edges.

15 Positively Reinforcing Facts About B.F. Skinner

Silly rabbit via Wikimedia Commons // CC BY 3.0
Silly rabbit via Wikimedia Commons // CC BY 3.0

Burrhus Frederic Skinner was one of the preeminent American psychologists of the 20th century. B.F. Skinner founded “radical behaviorism”—a twist on traditional behaviorism, a field of psychology that focused exclusively on observable human behavior. Thoughts, feelings, and perceptions were cast aside as unobservable.

B.F. Skinner dubbed his own method of observing behavior “operant conditioning,” which posited that behavior is determined solely by its consequences—either reinforcements or punishments. He also coined the term "positive reinforcement." 

To Skinner’s critics, the idea that these “principles of reinforcement,” as he called them, lead to easy “behavior modification” suggested that we do not have free will and are little more than automatons acting in response to stimuli. But his fans considered him visionary. Controversial to the end, B.F. Skinner was well known for his unconventional methods, unusual inventions, and utopian—some say dystopian—ideas about human society.

1. B.F. Skinner invented the "operant conditioning" or "Skinner" box.

Skinner believed that the best way to understand behavior is to look at the causes of an action and its consequences. He called this approach “operant conditioning.” Skinner began by studying rats interacting with an environment inside a box, where they were rewarded with a pellet of food for responding to a stimulus like light or sound with desired behavior. This simple experiment design would over the years take on dark metaphorical meaning: Any environment that had mechanisms in place to manipulate or control behavior could be called a "Skinner box." Recently, some have argued that social media is a sort of digital Skinner box: Likes, clicks, and shares are the pellet-like rewards we get for responding to our environment with certain behavior. Yes, we are the rats.

2. B.F. Skinner believed that all behavior was affected by one of three "operants."

Skinner proposed there were only three “operants” that had affected human behavior. Neutral operants were responses from the environment that had a benign effect on a behavior. Reinforcers were responses that increased the likelihood of a behavior’s repetition. And punishers decreased the likelihood of a behavior’s repetition. While he was correct that behavior can be modified via this system, it’s only one of many methods for doing so, and it failed to take into account how emotions, thoughts, and—as we learned eventually—the brain itself account for changes in behavior.

3. He's responsible for the term "positive reinforcement."

B.F. Skinner eventually moved on to studying pigeons in his Skinner box. The pigeons would peck at a disc to gain access to food at various intervals, and for completing certain tasks. From this Skinner concluded that some form of reinforcement was crucial in learning new behaviors. To his mind, positive reinforcement strengthens a behavior by providing a consequence an individual finds rewarding. He concluded that reinforced behavior tends to be repeated and strengthened.

4. Some critics felt "positive reinforcement" amounted to bribery.

Critics were dubious that Skinner's focus on behavior modification through positive reinforcement of desired behavior could actually change behavior for the long term, and that it was little more than temporary reward, like bribery, for a short-term behavioral change.

5. B.F. Skinner's idea of "negative reinforcement" isn't what you think.

Skinner believed negative reinforcement also helped to strengthen behavior; this doesn't mean exposing an animal or person to a negative stimulus, but rather removing an “unpleasant reinforcer.” The idea was that removing the negative stimulus would feel like a “reward” to the animal or person.

6. B.F. Skinner taught pigeons to play ping-pong.

As part of his research into positive reinforcement, he taught pigeons to play ping-pong as a first step in seeing how trainable they were. He ultimately wanted to teach them to guide bombs and missiles and even convinced the military to fund his research to that effect. He liked working with pigeons because they responded well to reinforcements and punishments, thus validating his theories. We know now that pigeons can be trained in a whole host of tasks, including distinguishing written words from nonsense and spotting cancer.

7. B.F. Skinner's first book, The Behavior of Organisms, broke new ground.

Published in 1938, Skinner’s debut book made the case that simple observation of cause and effect, reward and punishment, were as significant to understanding behavior as other “conceptual or neural processes.”

Skinner believed behavior was everything. Thoughts and feelings were just unreliable byproducts of behaviors, he argued—and therefore dismissed them. Many of his fellow psychologists disagreed. Regardless, Skinner’s theories contributed to a greater understanding of the relationship between stimuli and resulting behavior and may have even laid the groundwork for understanding the brain’s reward circuitry, which centers around the amygdala.

8. B.F. Skinner created the "baby tender."

Skinner was fond of inventions, and having children gave him a new outlet for his tendencies. He designed a special crib for his infant daughter called “the baby tender.” The clear box, with air holes, was heated so that the baby didn't need blankets. Unlike typical cribs, there were no slats in the sides, which he said prevented possible injury. Unsurprisingly, it did not catch on with the public.

9. B.F. Skinner also developed his own "teaching machine."

Silly rabbit via Wikimedia Commons // CC BY 3.0

You may have Skinner to thank for modern school workbooks and test-taking procedures. In 1954 Skinner visited his daughter’s classroom and found himself frustrated with the “inefficiencies” of the teaching procedures. His first "teaching machine"—a very basic program to improve teaching methods for spelling, math, and other school subjects—was little more than a fill-in-the-blank method on workbook or computer. It’s now considered a precursor to computer-assisted learning programs.

10. Skinner imaged an ideal society based on his theories of human behavior.

Skinner admired Henry David Thoreau’s famous book Walden, in which Thoreau writes about his retreat to the woods to get in greater contact with his inner nature. Skinner's "Ten Commandments" for a utopian world include: “(1) No way of life is inevitable. Examine your own closely. (2) If you do not like it, change it. (3) But do not try to change it through political action. Even if you succeed in gaining power, you will not likely be able to use it any more wisely than your predecessors. (4) Ask only to be left alone to solve your problems in your own way. (5) Simplify your needs. Learn how to be happy with fewer possessions.”

11. B.F. Skinner wrote a utopian novel, Walden Two.

Though inspired by Walden, Skinner also felt the book was too self-indulgent, so he wrote his own fictional follow-up with the 1948 novel Walden Two. The book proposed a type of utopian—some say dystopian—society that employed a system of behavior modification based on operant conditioning. This system of rewards and punishments would, Skinner proposed, make people into good citizens:

“We can achieve a sort of control under which the controlled, though they are following a code much more scrupulously than was ever the case under the old system, nevertheless feel free. They are doing what they want to do, not what they are forced to do. That's the source of the tremendous power of positive reinforcement—there's no restraint and no revolt. By careful cultural design, we control not the final behavior, but the inclination to behave—the motives, desires, the wishes.”

12. Some felt Skinner's ideas were reductionist ...

Critics, of which there were many, felt he reduced human behavior to a series of actions and reactions: that an individual human “mind” only existed in a social context, and that humans could be easily manipulated by external cues. He did not put much store in his critics. Even at age 83, just three years before he died, he told Daniel Goleman in a 1987 New York Times article, “I think cognitive psychology is a great hoax and a fraud, and that goes for brain science, too. They are nowhere near answering the important questions about behavior.”

13. ... and others were horrified by Walden Two.

Astronomer and colleague JK Jessup wrote, “Skinner's utopian vision could change the nature of Western civilization more disastrously than the nuclear physicists and biochemists combined.”

14. B.F. Skinner implied that humans had no free will or individual consciousness.

In the late 1960s and early '70s, Skinner wrote several works applying his behavioral theories to society, including Beyond Freedom and Dignity (1971). He drew fire for implying that humans had no free will or individual consciousness but could simply be controlled by reward and punishment. His critics shouldn't have been surprised: this was the very essence of his behaviorism. He, however, was unconcerned with criticism. His daughter Julie S. Vargas has written that “Skinner felt that by answering critics (a) you showed that their criticism affected you; and (b) you gave them attention, thus raising their reputation. So he left replies to others.”

15. He died convinced that the fate of humanity lay in applying his methods of behavioral science to society.

In 1990, he died of leukemia at age 86 after receiving a Lifetime Achievement Award from the American Psychological Association. Proud of his work, he was nonetheless concerned about the fate of humanity and worried “about daily life in Western culture, international conflict and peace, and why people were not acting to save the world.”

A New DNA Test Will Break Down Your Cat's Breed


Modern DNA testing kits can reveal a lot of information about you just by sending your spit off to a lab for analysis. As a result, it's easier than ever to learn about your personal ancestry and health risks. And now, the same goes for your cat, too.

Basepaws is now offering what it calls the "world's first DNA test for cats," which can tell you which breeds your beloved fur baby likely descended from, in addition to other information about their characteristics. The CatKit will reveal whether your little Simba is more similar to an American Shorthair, Abyssinian, or one of the other 30 breeds on record, as well as determining which of the "big cats" (think lions) your kitty has the most in common with.

Here's how it works: After receiving your kit in the mail, you will be asked to collect a DNA sample from your feline friend. The current kit includes adhesives for collecting cat hair, but Basepaws will soon roll out new kits that call for saliva samples instead. (This will provide a more consistent DNA sample, while also allowing staff to process more samples at once, according to a company spokesperson. It also will make it easier to collect samples from hairless cats like Sphinxes.)

A cat DNA test result

Once you collect the sample, just mail it in and wait eight to 12 weeks for your report. Basepaws uses sequencing machines to "read" your kitty's genetic code, comparing it to the sequences of other cats in its network. "More than 99 percent of your cat's genetic sequence will be similar to every other cat; it's the small differences that make your cat unique," Basepaws writes on its website.

In the future, Basepaws will also be able to determine your cat's predisposition for certain diseases, as well as their personality and physical traits. The company holds on to your cat's genetic data, allowing it to provide updates about your cat as the Basepaws database continues to grow.

Order a kit on the Basepaws website for $95. Enter the code "MEOWRCH-I5W3RH" at the checkout for a 10 percent discount.

And don't feel left out if you're a dog lover rather than a cat person—Wisdom Panel offers a similar service for canine companions. Its kit is available for $73 on Amazon.