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.

1. "THE COFFER ILLUSION," ANTHONY NORCIA, SMITH-KETTLEWELL EYE RESEARCH INSTITUTE, U.S.A., 2007 FINALIST

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.

2. "THE ROTATING SNAKES ILLUSION," AKIYOSHI KITAOKA, RITSUMEIKAN UNIVERSITY, JAPAN, 2005 FINALIST

"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.

3. "THE HEALING GRID," RYOTA KANAI, UTRECHT UNIVERSITY, THE NETHERLANDS, 2005 FINALIST

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.

4. "MASK OF LOVE," GIANNI SARCONE, COURTNEY SMITH, AND MARIE-JO WAEBER, ARCHIMEDES LABORATORY PROJECT, ITALY, 2011 FINALIST

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.

5. "AGE IS ALL IN YOUR HEAD," VICTORIA SKYE, U.S.A., 2014 FINALIST

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.

6. "THE ROTATING-TILTED-LINES ILLUSION," SIMONE GORI AND KAI HAMBURGER

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.

7. "PULSATING HEART," GIANNI SARCONE, COURTNEY SMITH, AND MARIE-JO WAEBER, ARCHIMEDES LABORATORY PROJECT, ITALY, 2014 FINALIST

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.

8. "GHOSTLY GAZE," ROB JENKINS, UNIVERSITY OF GLASGOW, UK, 2008 SECOND PRIZE

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.

9. "ELUSIVE ARCH," DEJAN TODOROVIC, UNIVERSITY OF BELGRADE, SERBIA, 2005 FINALIST

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.

10. "FLOATING STAR," JOSEPH HAUTMAN / KAIA NAO, 2012 FINALIST

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.

The Weird, Disturbing World of Snail Sex

iStock
iStock

Romance is rare in the animal kingdom. Instead of wooing their partners before copulating, male ducks force themselves onto females, depositing genetic material with spiky, corkscrew penises. Then, there's tardigrade sex, which is less violent but not exactly heartwarming. Females lay eggs into a husk of dead skin. The male then ejaculates onto the eggs while stroking the female, and the whole process can take up to an hour.

But you can't talk about disturbing mating rituals in nature without mentioning snails. If you're unfamiliar with snail sexuality, you may assume that snail sex falls on the vanilla side: The mollusks, after all, are famous for being slow-moving and they don't even have limbs. But if you have the patience to watch a pair of snails going at it, you'll notice that things get interesting.

The first factor that complicates snail sex is their genitalia. Snails are hermaphrodites, meaning individuals have both a male set and female set of parts, and any two snails can reproduce with each other regardless of sex. But in order for a couple of snails to make little snail babies, one of them needs to take on the role of the female. That's where the love dart comes in.

The love dart, technically called a gypsobelum, isn't exactly the Cupid's arrow the name suggests. It's a nail-clipping-sized spike that snails jab into their partners about 30 minutes before the actual sex act takes place. The sliver is packed with hormones that prepare the receiving snail's body for sperm. Depending on the species, only one snail might release the dart, or they both might in an attempt to avoid becoming the female of the pair. You can watch the action in the video below.

For sex to be successful, both snails must insert their penises into the other's vaginal tracts at the same time. Both snails deposit sperm, and the strength of the love dart ultimately determines whether or not that sperm fertilizes their partner's eggs.

That's assuming the snail survives the little love-stab. In human proportions, the love dart is the equivalent of a 15-inch knife. Fortunately, snails are resilient creatures, and gastropod researcher Joris Koene tells KQED he's only ever seen one snail die from the transfer.

Snails also have a way of making it up to their partners after skewering them with a hormone stick. Their sperm deposit contains a dose of fortifying nutrients, something scientists refer to as a nuptial gift. It may not equal the energy expended during sex, but its enough to give them a small post-coital boost.

10 Facts About Rosacea

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iStock

Rosacea, a skin condition characterized by redness and swelling, is incredibly common: A recent study found that an estimated 300 million people worldwide suffer from it. Here’s what you need to know about the condition.

1. IT HAS A LONG HISTORY.

According to the National Rosacea Society (NRS), rosacea was first described in the 14th century by a French surgeon named Dr. Guy de Chauliac; he called it goutterose (“pink drop” in French) or couperose and noted that it was characterized by “red lesions in the face, particularly on the nose and cheeks.”

2. SCIENTISTS AREN’T SURE WHAT CAUSES IT ...

But they have some theories. According to the NRS, “most experts believe it is a vascular disorder that seems to be related to flushing.” Scientists also think that because rosacea seems to run in families, it might be genetic. Other things—like mites that live on the skin, an intestinal bug called H pylori (common in those who have rosacea), and a reaction to a bacterium called bacillus oleronius—could also play a role in causing the condition. One 2015 study suggested an increased risk among smokers.

3. … BUT SOME PEOPLE ARE MORE LIKELY TO HAVE IT THAN OTHERS.

Though people of all ages and skin tones can get rosacea, fair skinned people between the ages of 30 and 50 with Celtic and Scandinavian ancestry and a family history of rosacea are more likely to develop the condition. Women are more likely to have rosacea than men, though their symptoms tend to be less severe than men’s. But men are more likely to suffer from a rare rosacea side effect known as rhinophyma, which causes the skin of the nose to thicken and become bulbous. It’s commonly—and mistakenly—associated with heavy drinking, but what exactly causes rhinophyma is unclear. According to the NRS, “The swelling that often follows a flushing reaction may, over time, lead to the growth of excess tissue (fibroplasia) around the nose as plasma proteins accumulate when the damaged lymphatic system fails to clear them. Leakage of a substance called blood coagulation factor XIII is also believed to be a potential cause of excess tissue.” Thankfully, those who have rhinophyma have options available for treatment, including surgery and laser therapy.

4. THERE ARE FOUR SUBTYPES.

According to the American Academy of Dermatology (AAD), rosacea “often begins with a tendency to blush or flush more easily than other people.” All rosacea involves redness of some kind (typically on the nose, cheeks, chin, and forehead), but other symptoms allow the condition to be divided into four subtypes: Erythematotelangiectatic rosacea is characterized by persistent redness and sometimes visible blood vessels; Papulopustular rosacea involves swelling and “acne-like breakouts”; Phymatous rosacea is characterized by thick and bumpy skin; and Ocular rosacea involves red eyes (that sometimes burn and itch, or feel like they have sand in them [PDF]), swollen eyelids, and stye-like growths.

5. IT’S NOT THE SAME AS ACNE.

Though rosacea was once considered a form of acne—"acne rosacea" first appeared in medical literature in 1814—today doctors know it’s a different condition altogether. Though there are similarities (like acne, some forms of rosacea are characterized by small, pus-filled bumps) there are key differences: Acne involves blackheads, typically occurs in the teen years, and can appear all over the body; rosacea is a chronic condition that occurs mainly on the face and the chest and typically shows up later in life.

6. YOU CAN FIND IT IN CLASSIC ART AND LITERATURE.

Both Chaucer and Shakespeare likely made references to rosacea. Domenico Ghirlandaio’s 1490 painting An Old Man and His Grandson seems to depict rhinophyma, and some believe that Rembrandt’s 1659 self-portrait shows that the artist had rosacea and rhinophyma.

7. IT MAY BE TRIGGERED BY CERTAIN FOODS AND ACTIVITIES.

According to the National Institutes of Health (NIH) [PDF], people report that everything from the weather to what you eat can cause rosacea to flare up: Heat, cold, sunlight, and wind, strenuous exercise, spicy food, alcohol consumption, menopause, stress, and use of steroids on the skin are all triggers.

8. THERE ARE A NUMBER OF MYTHS ABOUT ROSACEA.

No, it’s not caused by caffeine and coffee (flare ups, if they occur, are due to the heat of your coffee) or by heavy drinking (though alcohol does exacerbate the condition). Rosacea isn’t caused by poor hygiene, and it’s not contagious.

9. THERE ARE SOME PRETTY FAMOUS PEOPLE WITH ROSACEA.

Sophia Bush, Cynthia Nixon, Kristin Chenoweth, Bill Clinton, and Sam Smith all have rosacea. Diana, Princess of Wales had it, too. W.C. Fields had rosacea and rhinophyma, and Andy Warhol may also have suffered from those conditions.

10. IT CAN’T BE CURED—BUT IT CAN BE TREATED.

The NRS reports that “nearly 90 percent of rosacea patients [surveyed by NRS] said this condition had lowered their self-confidence and self-esteem, and 41 percent reported it had caused them to avoid public contact or cancel social engagements.” Dr. Uwe Gieler, a professor of dermatology at the Justus-Liebig-University in Giessen, Germany, and one of the authors of the report Rosacea: Beyond the Visible, said in a press release that "People with rosacea are often judged on their appearance, which impacts them greatly in daily life. If their rosacea is severe, the symptoms are likely to be more significant also, from itching and burning to a permanently red central facial area. However, even people with less severe rosacea report a significant impact on quality of life."

Which makes it all the more unfortunate that there’s not a cure for the condition. Thankfully, though, there are treatments available.

There are no tests that will diagnose rosacea; that’s up to your doctor, who will examine your medical history and go over your symptoms. Doctors advise that those with rosacea pay attention to what triggers flare-ups, which will help them figure out how to treat the condition. Antibiotics might be prescribed; laser therapy might be used. Anyone with rosacea should always wear sunscreen [PDF] and treat their skin very, very gently—don't scrub or exfoliate it. The AAD recommends moisturizing daily and avoiding products that contain things like urea, alcohol, and glycolic and lactic acids.

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