THE DYNAMIC EBBINGHAUS ILLUSION
BY CHRISTOPHER BLAIR, GIDEON CAPLOVITZ, AND RYAN MRUCZEK
UNIVERSITY OF NEVADA, RENO, U.S.A.
2014 FIRST PRIZE
Earlier we mentioned the classic Ebbinghaus Illusion, in which a circle surrounded by smaller circles looks bigger than the same circle surrounded by larger circles. This perceptual phenomenon proves that our experience of an object’s size is relative to its visual context. Blair, Caplovitz, and Mruczek created an Ebbinghaus Illusion on steroids by combining it with the principles behind the Dynamic Size Contrast Illusion. The new and improved Ebbinghaus effect consists of a dynamic display with surrounding circles that expand and shrink while the central circle’s size stays constant. The authors explained that the Dynamic Ebbinghaus Illusion is at least twice as strong as the original, static Ebbinghaus Illusion.
The illusion is even more powerful when you look at it from the corner of your eye, rather than directly. The reason could be that your peripheral vision is more sensitive to motion than your central—also called foveal—vision.
Your retina has a high-resolution central area called the fovea. The fovea sees an extremely small part of our visual field—just 0.1 percent, or the size of your thumbnail at arm’s length—but its function is critical for everyday life, and particularly so for investigating fine details and small objects. Without a fovea, you wouldn’t be able to read the newspaper, watch TV, drive a car, or interpret facial expressions.
The main reason you make eye movements is to target your fovea to the right place at the right time, a necessary condition to see in detail. Many of the neural calculations that control and direct your gaze rely on information from the non-foveal 99.9 percent of your visual field: the retinal periphery.
The periphery of your retina—though it produces lower-resolution vision than the fovea—has certain special abilities that help you to decide where to look next. For example, the periphery detects motion and gross details better than the fovea, and is more sensitive to flickering light and changes in brightness and contrast. This is the reason why fluorescent lighting sometimes flickers when you see it out of the corner of your eye, but not if you look at the light fixtures directly. So it should come as no surprise that the brain circuits processing peripheral visual information are susceptible to special kinds of illusions, especially those that involve motion and other dynamic changes. See this illusion in action at the Best Illusion of the Year Contest website.
FAT FACE THIN ILLUSION
BY PETER THOMPSON
UNIVERSITY OF YORK, U.K.
2010 FINALIST
The Fat Face Thin Illusion shows two photographs that are identical, although the upside-down face appears strikingly slimmer than the right-side-up version. One possible explanation is that it is easier for the brain to recognize distinctive facial features, such as chubby cheeks, when they are viewed in the normal upright position. Research has shown that face-selective neurons of the human brain respond best to upright faces—perhaps because there has been no evolutionary pressure to recognize faces upside-down. These same neurons may encode various facial properties—like chubbiness—and be less capable of doing so accurately when faces are upside-down. If so, all upside-down faces could end up looking more similar to one another than if they were upright.
HEAD SIZE ILLUSION
BY KAZUNORI MORIKAWA AND ERI ISHII
OSAKA UNIVERSITY, JAPAN
2012 FINALIST
The two Barack Obama portraits above are identical except that the one on the left has a wider jaw and fuller face than in reality. The top of the head appears fatter, too, but it is not. The Head Size Illusion demonstrates that the brain does not determine the size of visual stimuli in isolation from one another; it considers objects and features in relation to those nearby in the scene. The illusion occurs in everyday life, Morikawa said, and offers an opportunity for those who wish to alter their appearance. “If one part of your face or body appears wider or thinner than average, other parts appear wider or thinner, too,” he explained. “You can take advantage of such illusions to make yourself look better, using effective makeup and clothing.”
4
SHAPE ILLUSIONS
Visual perception begins when our retinas locate the edges of objects in the world. “Downstream” neural mechanisms, which are activated after visual signals leave the retina, analyze the information at the edges of objects to fill in their insides. That’s how we perceive surfaces. But what happens when these edges—the fundamental building blocks of our visual reality—are tweaked? Our brain’s ability to represent reality accurately no longer functions, and the surfaces of objects appear to warp. Seemingly small mistakes in edge alignment lead to the distorted perception of an illusory world. The illusions in this chapter reveal how these mental building blocks work together to assemble an object. Surprising inaccuracies in some of these mechanisms result in weird and wonderful misinterpretations of the shapes we see in our lives.
IT’S A CIRCLE, HONEST!
BY DAVID WHITAKER
UNIVERSITY OF BRADFORD, U.K.
2007 FINALIST
Subtle local effects can have major global consequences on how we perceive a shape, even one as simple as a circle. The circle at the top appears round only if you look directly at it. If you view it through your peripheral vision, it has corners! When you use the center of your vision—where your visual neurons have small, high-resolution windows on the world that scientists call “receptive fields”—you can see the curves that form the circle, and also the checkerboard pattern on its surface. In the periphery of your vision, however, visual neurons see the world through larger, low-resolution receptive fields that poorly appreciate the circle’s subtle curves while favoring its high-contrast large checks. And because the checks form diagonal lines when blurred, you see a diamond shape instead of a circle out of the corner of your eye. In contrast, when you view the lower circle through the center of your vision, you perceive it as roughly circular with a checkerboard surface. But when you view it peripherally, it looks much more rounded. That’s because the smaller elements that form the circle smear out to gray in the larger peripheral receptive fields, and so the circular interpretation of the ring dominates your perception.
COFFER ILLUSION
BY ANTHONY NORCIA
SMITH-KETTLEWELL EYE RESEARCH INSTITUTE, U.S.A.
2007 FINALIST
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 were 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 HEALING GRID
BY RYOTA KANAI
UTRECHT UNIVERSITY, THE NETHERLANDS
2005 FINALIST
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 imag
e and stare at it for thirty 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.
SHEPARD’S SARCOPHAGI
The shape on the upper right appears longer and narrower than the one on the upper left. Or is it? Photocopy this page, and then grab a pair of scissors and cut around the trapezoid shapes containing each sarcophagus. Now line them up and superimpose them. Both shapes are exactly the same size. The effect is a version of the classic Shepard Tabletop Illusion, also featured here, named after its creator, the cognitive scientist Roger Newland Shepard. The illusion consists of two polygons with very different appearances (one looks much longer and thinner than the other), even though the two shapes are identical. The confusion results from our perceptual inferences about actual objects. If the two polygons corresponded to real tabletops, they would have to be physically different. The Shepard Tabletop Illusion is, in a sense, an illusion of perspective. The inferences we make about the shapes of the polygons also apply to any images within them. This is why the sarcophagi inside the trapezoids appear distorted too.
ANOTHER TURN
BY LYDIA MANIATIS
AMERICAN UNIVERSITY, U.S.A.
2009 FINALIST
The vision researcher Lydia Maniatis presented a novel variant of the Shepard Tabletop Illusion, in which not just two but three identical polygons look diametrically different from each other. The three red-and-blue-colored box tops are identical: all the blue lines are the same length, and so are all the red lines. If you don’t believe it, measure the lines, or cut and rotate the boxes, to convince yourself!
STRETCHING OUT IN THE TUB
BY LYDIA MANIATIS
AMERICAN UNIVERSITY, U.S.A.
2010 FINALIST
Lydia Maniatis discovered this illusion serendipitously. Walking down the street one day, she noticed an odd effect as she passed a bathtub company’s billboard. As she proceeded from one end of the huge image to the other, the bathtub seemed, impossibly, to stretch; when approached from the other direction, it appeared to shrink. Intrigued, she walked past the ad again and again. From one end, the foreshortened tub looked like a large sink. But as Maniatis approached the other end, the “sink” slowly stretched back into a tub. Her visual sys-tem made assumptions about the identity of the object from each angle, giving rise to different 3-D perceptions at each location along the billboard.
THE MORE-OR-LESS MORPHING FACE ILLUSION
BY ROB VAN LIER AND ARNO KONING
DONDERS INSTITUTE, THE NETHERLANDS
2011 FINALIST
Rob van Lier and Arno Koning of the Donders Institute for Brain, Cognition and Behaviour, a research center in The Netherlands, showed the audience the image of a face. They superimposed a circling red dot over the face and asked the audience to keep their eyes on the dot. After a short while, the dot disappeared and the picture appeared to morph from a male face to a female face and back to male. Then the red dot reappeared, and the face stopped morphing. In reality, the face had been morphing continuously the entire time. It only appeared unchanging when the viewers focused on the moving red dot. Van Lier demonstrated that this illusion also works for a face with shifting emotions and ages, and even when he used a famous face, such as Barack Obama’s, morphing between happy and sad expressions. The misperception could be due to how various visual features engage our attention. Our own research has shown that when we focus on a moving object, attention mechanisms in our visual cortex work to suppress everything else. For this illusion, closely watching the moving dot may suppress neural activity in the fusiform gyrus, the part of the brain that processes faces, causing us to overlook the changing characteristics.
YANG’S IRIS ILLUSION
BY JISIEN YANG AND ADRIAN SCHWANINGER
UNIVERSITY OF ZURICH, SWITZERLAND; NATIONAL CHUNG-CHENG UNIVERSITY, TAIWAN; MAX PLANCK INSTITUTE, GERMANY
2008 FINALIST
This illusion shows that overall context affects how we perceive specific facial details. The shape of the eyelids and face in these two photos seems to affect the distance between the irises. Consider the pair of Asian faces: the distance between the left eye of the right face and the right eye of the left face seems short. For the Caucasian faces, the separation seems wider. If you compare the photos with the diagrams of the eyes and irises with all facial context removed, it is clear that the irises are equally spaced.
ILLUSORY PYRAMID
BY PIETRO GUARDINI AND LUCIANO GAMBERINI
UNIVERSITY OF PADUA, ITALY
2007 SECOND PRIZE
The Illusory Pyramid is a novel variant of the classic Kanizsa Triangle, in which a phantom triangle arises from the placement of three Pac-Man shapes at its imaginary corners. Guardini and Gamberini’s illusion adds a background formed by three patches of different shades of gray. One variation of the three gray segments produces the illusory triangle; another variation produces an illusory pyramid. We perceive the 3-D shape of the pyramid only because our brains impose this imaginary shape on a very sparse field of 2-D data. This illusion shows that much of our perception is an elaborate construct, based on scarce physical foundations.
SKYSCRAPERS AND CLOUDS
BY SANDRO BETTELLA, CLARA CASCO, AND SERGIO RONCATO
UNIVERSITY OF PADUA, ITALY
2008 FINALIST
The vertical sides of the skyscrapers seem to bulge out and contract relative to the clouds in the background, but the width of each building is actually constant. The neural bases of this illusion are unknown, but the visual neuroscientist Jens Kremkow, working with Jose Manuel Alonso and Qasim Zaidi at the State University of New York, discovered a similar phenomenon that goes back to the Italian astronomer Galileo Galilei. It turns out that our eyes see lightness and darkness in a surprising way.
Galileo was puzzled by the fact that planets appeared different depending on whether he looked at them with the naked eye or through a telescope. He wrote that the planets seemed “expanded” and had “a radiant crown” when viewed directly, making Venus look eight to ten times larger than Jupiter, even though Jupiter is the larger planet. Though Galileo realized that his eyes, rather than the planets themselves, were responsible for the change, he did not understand why or how. He mused over whether the effect occurred “either because [the planets’] light is refracted in the moisture that covers the pupil, or because it is reflected from the edges of the eyelids and these reflected rays are diffused over the pupil, or for some other reason.” The Renaissance artist and engineer Leonardo da Vinci had noticed a similar effect. While looking at a canvas, he observed that dark objects on a light background seemed more defined than light objects on a dark background. And more than three centuries later, Hermann von Helmholtz—the venerable nineteenth-century German physician-physicist—also described the phenomenon in his Treatise on Physiological Optics as the “irradiation effect.”
It took modern neurophysiologi
cal research to understand the neural mechanisms that affect the perceived size of black and white objects. Kremkow and his colleagues examined the responses of neurons in the visual system of the brain—to both light stimuli and dark stimuli—and discovered that whereas dark stimuli generate faithful neural responses that accurately represent their size, light stimuli produce exaggerated neural responses that make the objects appear larger. So white spots on a black background look bigger than black spots of the same size on a white background, and Galileo’s glowing planets are not really as big as they might appear to the unaided eye.
The irradiation effect influences our perception of all sorts of objects and textures, and explains why it is easier to read this very page with black-on-white lettering, rather than white-on-black (a well-known and previously unexplained fact). It could even illuminate (pun intended) why skyscrapers seem to expand and shrink depending on their background. See a dynamic version of this illusion at the Best Illusion of the Year Contest website.
Champions of Illusion Page 3
