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!
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.
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.
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.
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.
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.
