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BRIGHTNESS AND CONTRAST ILLUSIONS
Nothing is more fundamental to our vision than how we see the brightness of an object. But even so, our visual system plays fast and loose with reality and serves up—for your viewing pleasure—monstrously bizarre and perplexingly inaccurate interpretations of the physical world. And this raises the question that constantly cycles through the brains of vision scientists: Why doesn’t human vision faithfully represent the world we see? The answer, as any student of Darwin should agree, is that illusions must help us survive (or at the very least not hinder our survival). If illusions were harmful, it is likely that they would have been weeded out of the gene pool by now. Mutations that work against survival—and reproductive success—are self-limiting.
But how can a visual illusion be useful? To illustrate, we’ll do an experiment. Go to a dark room in your domicile with a cell phone and a book (an actual book, made of paper). Then dimly illuminate the pages of your book, using your phone, just enough to see the letters. White pages, black text—looks like a book, right? After you have completed this part of the experiment, head outside on a sunny day with the same book. Under direct sunlight, look at the same page; it looks identical, right? If you think it through, that’s impossible, because the physical reality under the two lighting conditions is very different! When you read black text on a page lit by a dim cell phone, the amount of light reflected by the white paper is around 100,000 times lower than the amount of light reflected by the black letters in direct sunlight. So why don’t the black letters seem super-white (100,000 times brighter than white) outside? The reason is that your brain doesn’t care about light levels; it cares about the contrast between the lightness of objects. It interprets the letters as black because they are darker than the rest of the page, no matter the lighting conditions.
The illusion that allows us to identify an object as being the same under different lighting conditions is a very useful one. It helps us survive. For one thing, it might have allowed our ancestors to recognize their children inside and outside the cave … and therefore not eat them!
THE WORLD’S LARGEST BRIGHTNESS ILLUSION
BY BARTON ANDERSON AND JONATHAN WINAWER
UNIVERSITY OF SOUTH WALES, AUSTRALIA, AND MASSACHUSETTS INSTITUTE OF TECHNOLOGY, U.S.A.
2005 FINALIST
Our brain does not perceive the true brightness of an object in the world (for instance, measured with a photometer), but instead compares it with that of other nearby objects. For instance, the same gray square will look lighter when surrounded by black than when it is surrounded by white. This illusion by Anderson and Winawer extends this concept dramatically. In the images, the four sets of chess pieces are identical. The backgrounds are the only things that change: the images on the two bottom rows show the same chess pieces as the images on the top two rows, only with the backgrounds removed. We perceive the first-row pieces as white and the second-row pieces as black because of the variations in the clouds engulfing them. Checkmate!
HERE COMES THE SUN
BY ALAN STUBBS
UNIVERSITY OF MAINE, U.S.A.
2006 FINALIST
Hold this book at a comfortable distance from your eyes while looking at the picture. Then bring the book gradually closer. As the image approaches, you should notice that its brightness seems to increase. Move the book back and forth to make the brightness increase and decrease repeatedly. The neural bases of this effect are not yet understood, but the explanation may reside in how our visual system reacts to expanding versus contracting objects as a function of their distance from the observer. Some motion-sensitive neurons of the visual pathway become selectively activated when visual objects either loom (expand) or recede (contract). It could be that the ghostly, transparent white cloud radiating from the center of the image appears less salient to those neurons than the highly visible red-blue background. If so, when the cloud and the background expand and contract together, your neurons may signal a difference in the relative amounts of expansion and contraction—so that one element appears to loom or recede more than the other, even though no difference actually exists.
THE PRIMAL FLASHLIGHT
BY LOTHAR SPILLMANN, JOE HARDY, PETER DELAHUNT, BAINGIO PINNA, AND JOHN WERNER
UC DAVIS MEDICAL CENTER, U.S.A.; UNIVERSITY OF FREIBURG, GERMANY; POSIT SCIENCE, U.S.A.; UNIVERSITY OF SASSARI, ITALY
2009 FINALIST
All you will need to experience this illusion is a cardboard tube, such as a paper-towel roll or a poster tube. Look through the tube with one eye, and keep the other eye open. Point the tube at a bright wall. After just a few seconds, the circle you see through the tube will look much brighter than the rest of the wall! If you look at a textured surface, the illusion will enhance not only the brightness and color but also the details in the pattern. Vision scientists do not fully understand this phenomenon, but it could be that the dark inner cardboard walls enhance the brightness of the scene trapped inside the tube, compared with the rest of the visual field. Another nonexclusive possibility is that looking through the tube helps focus your attention in one eye more than in the other.
WEAVES AND THE HERMANN GRID
BY KAI HAMBURGER AND ARTHUR G. SHAPIRO
UNIVERSITY OF GIESSEN, GERMANY, AND BUCKNELL UNIVERSITY, U.S.A.
2007 FINALIST
The Hermann Grid Illusion, reported by the German physiologist Ludimar Hermann in 1870, consists of a white grid on a black background or a black grid on a white background. Faint, ghostly spots appear at the grid’s crossings, even though there is nothing there. The classical explanation for this illusion is based on experiments carried out by the neurophysiologist Günter Baumgartner in 1960. Baumgartner proposed that neighboring neurons of the early visual system—the first areas of the brain to respond to visual information—enhance the perceptual contrast at the grid’s crossings by suppressing each other’s activity. This process—known as lateral inhibition—was first described in neurons in the horseshoe crab’s eye, a discovery that led to Keffer Hartline’s 1967 Nobel Prize. Hamburger and Shapiro’s novel variant of the Hermann Grid Illusion interlaces light gray vertical columns with dark gray horizontal rows (or the other way around, however you prefer to think about it). As the lightness of the background varies from black to white, the apparent brightness of the illusory spots at the crossings reverses.
BRIGHT PATCHES
BY ROB VAN LIER AND MARK VERGEER
RADBOUD UNIVERSITY, NIJMEGEN, THE NETHERLANDS
2006 FINALIST
This perceptual effect is another novel variation of the classic Hermann Grid Illusion. The “patches” at the intersections of the grid above look brighter than the rest of the grid. And yet the entire grid is in a single tone of gray.
THE ILLUSION OF SEX
BY RICHARD RUSSELL
HARVARD UNIVERSITY, U.S.A.
2009 THIRD PRIZE
You may perceive these two side-by-side faces as female (left) and male (right). But both are versions of the same androgynous face. The two images are identical, except that the contrast between the eyes and mouth and the rest of the face is higher for the one on the left than for the one on the right. This illusion shows that contrast is an important cue for determining gender: low-contrast faces appear male, and high-contrast faces appear female. It may also help explain why females in many cultures darken their eyes and mouths with cosmetics: a made-up face looks more feminine than a face without makeup.
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COLOR ILLUSIONS
Take a look at the red chips on the top two Rubik’s cubes on the opposite page. They are actually orange on the left and purple on the right, if you look at them in isolation. They only appear more or less equally red across the images because your brain is interpreting them as red chips lit by either yellow or blue light. This kind of misperception is an example of “color constancy,” the mechanism that allows you to recognize an object as being the same in different environments, and under very div
erse lighting conditions.
Color constancy is a remarkable skill, because an object’s surface can be colored only by virtue of the photons it reflects from a light source. That is, every object is tinted by both the color of its surface (defined as its ability to reflect light of a particular wavelength distribution) and the color of the light source (which limits which wavelengths are actually present to be reflected). A tricky prospect for the brain.
The two cubes also demonstrate another perceptual principle. See the blue chips on the top of the left cube, and the yellow chips on the top of the right cube? They are identical, and appear as plain gray when the surrounding colors are removed. This phenomenon, called “color contrast,” causes red apples to appear redder against a background of green leaves. More generally, it makes equal colors look different because of context.
Color contrast and color constancy show that our perception of color is not all that is seems. This chapter presents several of the color illusions that competed in the contest, as well as their neural bases.
BIRDS IN A CAGE
When you stare at a color image, its afterimage takes on a shade of its own. Afterimages are the consequence of a neural process called adaptation, by which neurons decrease their responses to unchanging sensory inputs. Once neurons have adapted, it takes a while for them to reset to their previous, responsive state. It is during this period that illusory afterimages appear. We see such images every day when we experience a temporary dark spot in our field of vision after briefly looking at the sun or at a bright lightbulb, or after being momentarily blinded by a camera flash. Gazing at any colored surface can also induce a vivid afterimage of the complementary color—that is, red versus green, or blue versus yellow. Imagine staring at a red surface. The cells in your retina that respond to red light will reduce their activity to save energy and to prepare themselves for detecting any future changes in redness. So, when you look away to a white background, your retina remains adapted to the red environment for a few seconds. With the red “subtracted” from the white, you will see red’s opposite: green.
To try it out, stare at the red parrot for thirty seconds, then immediately look at the center of the empty birdcage. You should see a ghostly greenish parrot inside. Try the same with the green cardinal, and you should see a pink bird. A similar illusion is part of an exhibit at the Exploratorium museum in San Francisco.
COLOR DOVE ILLUSION
BY YUVAL BARKAN AND HEDVA SPITZER
TEL AVIV UNIVERSITY, ISRAEL
2009 SECOND PRIZE
Positive afterimages—which have the same color as the inducing image, rather than its opposite—can be captured from a surrounding color. Stare for about thirty seconds at the “target” on the bird in the left panel. While you keep your eyes on the dot, you may notice that the red background causes the white bird to fill in with a complementary blue-green color. Then look immediately at the same spot on the bird in the right panel. Removing the red background gives rise to a surprisingly strong and long-lasting red afterimage of the bird. To experience an even more striking version of this illusion with a “flying” bird, visit the Best Illusion of the Year Contest website.
SHAPE-SPECIFIC AFTERIMAGES
BY ROB VAN LIER AND MARK VERGEER
RADBOUD UNIVERSITY, NIJMEGEN, THE NETHERLANDS
2008 FIRST PRIZE
In this illusion, a single multicolored image produces two afterimages of different colors, with each afterimage confined to a different shape. Fix your gaze on the black dot between the colored stars in the middle panel and stare at it for a full minute without moving your eyes. Then look at the empty outlines in the top panel. The left one fills in with a ghostly blue-green, and the right one looks reddish. When you look at the bottom panel, the colors are reversed. How does one image produce two afterimages of different colors? And how does the shape of the outline determine the filled-in color? This illusion demonstrates that patches of an afterimage can spread and merge to fill the contours of an outlined shape. The shape on the upper right takes on a reddish hue because it has the same outline as the complementary blue-green patches in the original color image. Likewise, the blue-green-tinged shape on the upper left matches the red patches in the original color image.
FLEXIBLE COLORS
BY MARK VERGEER, STUART ANSTIS, AND ROB VAN LIER
UNIVERSITY OF LEUVEN, BELGIUM; UNIVERSITY OF CALIFORNIA, SAN DIEGO, U.S.A.; RADBOUD UNIVERSITY, NIJMEGEN, THE NETHERLANDS
2014 SECOND PRIZE
Six years after they bagged the 2008 first prize for their work with afterimages, Vergeer and his colleagues extended the focus of their research to study how the contours of an image influence the way we see its colors. Paintings by Pablo Picasso and other artists suggest that coloring within the lines is not a strict requirement for our ability to assign color to shape. Vergeer and his colleagues set out to explore this in detail, and proved that our brain has an extraordinary ability to ascribe colors to relevant shapes in an image. Their illusion shows that a single image can lead to diametrically different color impressions. The left and right colored images in the middle row are identical, constructed by combining the color profiles of a picture of a forest and a picture of the Manhattan skyline. When semitransparent grayscale images (images in monochromous gray shades ranging from black to white—as seen, for instance, in black-and-white movies) of the forest or the skyline are overlaid on the color images (top row), our visual neurons seamlessly match the appropriate colors to the relevant outlines, ignoring the dissonant colors, and we clearly see one or the other—a suitably colored forest or a skyline—even though the color profiles of both scenes are still present in each image.
WHITE’S EFFECT AND THE ILLUSION OF THE YEAR LOGO
In 1979, the vision scientist Michael White described an illusion that changed visual science. In the top image, the gray bars on the left look darker than the gray bars on the right. In fact, all the gray bars are identical. Before White discovered this effect, brightness illusions were thought to result from opponent processes—that is, a gray object should look dark when surrounded by light, and light when surrounded by dark. But in this illusion, the darker-looking gray bars are surrounded by black, and the lighter-looking gray bars are surrounded by white. Although the brain mechanisms underlying White’s Effect remain unknown, the illusion has bolstered new research avenues in visual perception—pioneered by the neuroscientist Dale Purves—based on the idea that we see things according to our brains’ expectations.
Purves’s idea is that the visual system has evolved to interpret the world according to empirical probability. In other words, our perception is consistent with the way things are most likely to be. Our experience of illusions such as White’s Effect, it follows, depends on the context in which we view them, and the probability of all the possible interactions that our ancestors had with similar scenarios.
White’s Effect also changes the appearance of colors. The logo for the Best Illusion of the Year Contest is an homage to both White’s Effect (the trophy appears to be different colors behind the two curtains, though it’s actually the same color throughout) and the famous Face/Vase Illusion, but with a trophy in place of the vase.
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SIZE ILLUSIONS
As both a tiny person in the country of Brobdingnag and a giant on the island of Lilliput, Lemuel Gulliver—the protagonist of Jonathan Swift’s Gulliver’s Travels—experiences firsthand that size is relative. When we cast a neuroscientific light on this classic book, it is clear to us that Swift, a satirist, essayist, and poet, knew a few things about the mind, too. Absolute size is meaningless to our brain: we gauge size by context. The same medium-sized circle will appear smaller when surrounded by large circles and bigger when surrounded by tiny ones, a phenomenon discovered by the German psychologist Hermann Ebbinghaus. Social and psychological contexts also cause us to misperceive size. Research shows that spiders appear larger to people who suffer from arachnophobia than to those who are unafra
id of bugs, and that men holding weapons seem taller and stronger than men who are holding tools. Here we present a collection of illusions that will expand your horizons and shrink your confidence in what is real. Try these out for size!
DYNAMIC SIZE CONTRAST ILLUSION
BY GIDEON CAPLOVITZ AND RYAN MRUCZEK
UNIVERSITY OF NEVADA, RENO, AND SWARTHMORE COLLEGE, U.S.A.
2013 FINALIST
This illusion shows that a viewer’s perception of an object’s size radically varies as a function of background motion. When the background is growing, the object appears to shrink. Conversely, when the background is shrinking, the object appears to grow. Caplovitz and Mruczek’s discovery suggests that our visual system integrates multiple sources of sensory information to determine the perceived size of an object. For example, the brain must consider the projected size of the object’s image on the retina as well as its size relative to other objects in the scene. Extracting the information from the visual image is particularly challenging when the object is in motion, and doubly so when the observer’s eyes pursue the moving object. One reason that might help explain why the Dynamic Size Contrast Illusion is so powerful is that items that move draw our attention more forcefully than stationary ones. Our own research showed that during magic performances, spectators tend to focus their attention on fluidly moving elements, such as the magician’s hand, instead of the parts of the stage that remain still. In the Dynamic Size Contrast Illusion, our attention may focus preferentially on the moving background, maximizing our perception of the differences between the background and the object in the foreground. See a dynamic demonstration of this illusion at the Best Illusion of the Year Contest website.
Champions of Illusion Page 2
