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.

Sorry, Plant Parents: Study Shows Houseplants Don’t Improve Air Quality

sagarmanis/iStock via Getty Images
sagarmanis/iStock via Getty Images

Sometimes accepted wisdom needs a more thorough vetting process. Case in point: If you’ve ever heard that owning plants can improve indoor air quality in your home or office and act as a kind of organic air purifier or cleaner, you may be disappointed to learn that there’s not a whole lot of science to back that theory up. In fact, plants will do virtually nothing for you in that respect.

This botanic bummer comes from Drexel University researchers, who just published a study in the Journal of Exposure Science and Environmental Epidemiology. Examining 30 years of previous findings, Michael Waring, an associate professor of architectural and environmental engineering, found only scant evidence that plants do anything to filter contaminants from indoor air.

Many of these studies were limited, the study says, by unrealistic conditions. Plants would often be placed in a sealed chamber, with a single volatile organic compound (VOC) introduced to contaminate the air inside. While the VOCs decreased over a period of hours or days, Waring found that the studies placed little emphasis on measuring the clean air delivery rate (CADR), or how effectively an air purifier can “clean” the space. When Waring converted the studies' results to CADR, the plants's ability to filter contaminants was much weaker than simply introducing fresh air to disperse VOCs. (Additionally, no one is likely to live in a sealed chamber.)

The notion of plants as natural air filters likely stemmed from a NASA experiment in 1989 which argued that plants could remove certain compounds from the air. As with the other studies, it took place in a sealed environment, which made the results difficult to translate to a real-world environment.

Plants can clean air, but their efficiency is so minimal that Waring believes it would take between 10 and 1000 of them per square meter of floor space to have the same effect as simply opening a window or turning on the HVAC system to create an air exchange. Enjoy all the plants you like for their beauty, but it’s probably unrealistic to expect them to help you breathe any easier.

10 Radiant Facts About Marie Curie

Photo Illustration by Mental Floss. Curie: Hulton Archive, Getty Images. Background: iStock
Photo Illustration by Mental Floss. Curie: Hulton Archive, Getty Images. Background: iStock

Born Maria Salomea Skłodowska in Poland in 1867, Marie Curie grew up to become one of the most noteworthy scientists of all time. Her long list of accolades is proof of her far-reaching influence, but not every stride she made in the fields of chemistry, physics, and medicine was recognized with an award. Here are some facts you might not know about the iconic researcher.

1. Marie Curie's parents were teachers.

Maria Skłodowska was the fifth and youngest child of two Polish educators. Her parents placed a high value on learning and insisted that all their children—including their daughters—receive a quality education at home and at school. Maria received extra science training from her father, and when she graduated from high school at age 15, she was first in her class.

2. Marie Curie had to seek out alternative education for women.

After collecting her high school diploma, Maria had hoped to study at the University of Warsaw with her sister, Bronia. Because the school didn't accept women, the siblings instead enrolled at the Flying University, a Polish college that welcomed female students. It was still illegal for women to receive higher education at the time so the institution was constantly changing locations to avoid detection from authorities. In 1891 Maria moved to Paris to live with her sister, where she enrolled at the Sorbonne to continue her education.

3. Marie Curie is the only person to win Nobel Prizes in two separate sciences.

Marie Curie and her husband, Pierre Curie, in 1902.
Marie Curie and her husband, Pierre Curie, in 1902.
Agence France Presse, Getty Images

In 1903, Marie Curie made history when she won the Nobel Prize in physics with her husband, Pierre, and with physicist Henri Becquerel for their work on radioactivity, making her the first woman to receive the honor. The second Nobel Prize she took home in 1911 was even more historic: With that win in the chemistry category, she became the first person to win the award twice. And she remains the only person to ever receive Nobel Prizes for two different sciences.

4. Marie Curie added two elements to the Periodic Table.

The second Nobel Prize Marie Curie received recognized her discovery and research of two elements: radium and polonium. The former element was named for the Latin word for ray and the latter was a nod to her home country, Poland.

5. Nobel Prize-winning ran in Marie Curie's family.

Marie Curie's daughter Irène Joliot-Curie, and her husband, Frédéric Joliot-Curie, circa 1940.
Marie Curie's daughter Irène Joliot-Curie, and her husband, Frédéric Joliot-Curie, circa 1940.
Central Press, Hulton Archive // Getty Images

When Marie Curie and her husband, Pierre, won their Nobel Prize in 1903, their daughter Irène was only 6 years old. She would grow up to follow in her parents' footsteps by jointly winning the Nobel Prize for chemistry with her husband, Frédéric Joliot-Curie, in 1935. They were recognized for their discovery of "artificial" radioactivity, a breakthrough made possible by Irène's parents years earlier. Marie and Pierre's other son-in-law, Henry Labouisse, who married their younger daughter, Ève Curie, accepted a Nobel Prize for Peace on behalf of UNICEF, of which he was the executive director, in 1965. This brought the family's total up to five.

6. Marie Curie did her most important work in a shed.

The research that won Marie Curie her first Nobel Prize required hours of physical labor. In order to prove they had discovered new elements, she and her husband had to produce numerous examples of them by breaking down ore into its chemical components. Their regular labs weren't big enough to accommodate the process, so they moved their work into an old shed behind the school where Pierre worked. According to Curie, the space was a hothouse in the summer and drafty in the winter, with a glass roof that didn't fully protect them from the rain. After the famed German chemist Wilhelm Ostwald visited the Curies' shed to see the place where radium was discovered, he described it as being "a cross between a stable and a potato shed, and if I had not seen the worktable and items of chemical apparatus, I would have thought that I was been played a practical joke."

7. Marie Curie's notebooks are still radioactive.

Marie Curie's journals
Hulton Archive, Getty Images

When Marie Curie was performing her most important research on radiation in the early 20th century, she had no idea of the effects it would have on her health. It wasn't unusual for her to walk around her lab with bottles of polonium and radium in her pockets. She even described storing the radioactive material out in the open in her autobiography. "One of our joys was to go into our workroom at night; we then perceived on all sides the feebly luminous silhouettes of the bottles of capsules containing our products […] The glowing tubes looked like faint, fairy lights."

It's no surprise then that Marie Curie died of aplastic anemia, likely caused by prolonged exposure to radiation, in 1934. Even her notebooks are still radioactive a century later. Today they're stored in lead-lined boxes, and will likely remain radioactive for another 1500 years.

8. Marie Curie offered to donate her medals to the war effort.

Marie Curie had only been a double-Nobel Laureate for a few years when she considered parting ways with her medals. At the start of World War I, France put out a call for gold to fund the war effort, so Curie offered to have her two medals melted down. When bank officials refused to accept them, she settled for donating her prize money to purchase war bonds.

9. Marie Curie developed a portable X-ray to treat soldiers.

Marie Curie circa 1930
Marie Curie, circa 1930.
Keystone, Getty Images

Marie's desire to help her adopted country fight the new war didn't end there. After making the donation, she developed an interest in x-rays—not a far jump from her previous work with radium—and it didn't take her long to realize that the emerging technology could be used to aid soldiers on the battlefield. Curie convinced the French government to name her Director of the Red Cross Radiology Service and persuaded her wealthy friends to fund her idea for a mobile x-ray machine. She learned to drive and operate the vehicle herself and treated wounded soldiers at the Battle of the Marne, ignoring protests from skeptical military doctors. Her invention was proven effective at saving lives, and ultimately 20 "petite Curies," as the x-ray machines were called, were built for the war.

10. Marie Curie founded centers for medical research.

Following World War I, Marie Curie embarked on a different fundraising mission, this time with the goal of supporting her research centers in Paris and Warsaw. Curie's radium institutes were the site of important work, like the discovery of a new element, francium, by Marguerite Perey, and the development of artificial radioactivity by Irène and Frederic Joliot-Curie. The centers, now known as Institut Curie, are still used as spaces for vital cancer treatment research today.

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