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105 Mind-Bending Illusions p. 8 p. 70 p. 66 p. 82 p. 22 p. 74 REPORTSspecial edition on perception www.SciAm.com Mind- 105 Bending Illusions What They Reveal about Your Brain See impossible objects Feel a phantom hand Watch as a gorilla disappears © 2008 SCIENTIFIC AMERICAN, INC. Page Intentionally Blank SCIENTIFIC AMERICAN Digital (from the editor) REPORTS Scientific American Reports is published by the staff of Scientific American, with project management by: EDITOR IN CHIEF: John Rennie EXECUTIVE EDITOR: Mariette DiChristina ISSUE EDITOR: Dawn Stover ART DIRECTOR: Patricia Nemoto ISSUE PHOTOGRAPHY EDITOR: Bridget Gerety Small PRODUCTION EDITOR: Richard Hunt COPY DIRECTOR: Maria-Christina Keller COPY CHIEF: Daniel C. Schlenoff ASSISTANT COPY CHIEF: John Matson COPY AND RESEARCH: Rachel Dvoskin, The Aaron Fagan, Aaron Shattuck, Kenneth Silber, Kevin Singer, Michelle Wright EDITORIAL ADMINISTRATOR: Avonelle Wing “Real” SENIOR SECRETARY: Maya Harty ASSOCIATE PUBLISHER, PRODUCTION: William Sherman World MANUFACTURING MANAGER: Janet Cermak ADVERTISING PRODUCTION MANAGER: “The camera does not lie,” the saying goes. And we tend to think of our eyes and Carl Cherebin our other sensory organs as video equipment, faithfully recording all the details PREPRESS AND QUALITY MANAGER: Silvia De Santis of our busy lives. As you will learn from the articles on illusions collected in this PRODUCTION MANAGER: Christina Hippeli special issue, however, we see with our brains, not with our eyes. And our brains CUSTOM PUBLISHING MANAGER: make instant value judgments about the jumble of incoming sensory informa- Madelyn Keyes-Milch tion, depending on what is important at that moment to us, to create a sensible ASSOCIATE PUBLISHER, CIRCULATION: Simon Aronin narrative of the world around us. CIRCULATION DIRECTOR: Christian Dorbandt Rather than pondering every bit of light that enters our orbs, the brain quick- RENEWALS MANAGER: Karen Singer FULFILLMENT AND DISTRIBUTION MANAGER: ly jumps to conclusions, based on millions of years of evolution. Humans are Rosa Davis intensely visual creatures, and we have developed an incredible apparatus for VICE PRESIDENT AND PUBLISHER: detecting things that are critical to our survival, such as predators, prey and Bruce Brandfon mates. For instance, we can instantly mentally assemble several tiny patches of SALES DEVELOPMENT MANAGER: David Tirpack SALES REPRESENTATIVES: Jeffrey Crennan, orange with stripes peeking through dense foliage: “tiger!” As we glance around Stephen Dudley, Stan Schmidt a room, the image bounces on the retina (the light-receiving tissue at the back of ASSOCIATE PUBLISHER, STRATEGIC PLANNING: the eye) as various areas of the scene excite different groups of cells. Yet the world Laura Salant appears stable to us, the view a smooth pan across our surroundings. The brain PROMOTION MANAGER: Diane Schube RESEARCH MANAGER: Aida Dadurian even fills in missing bits of picture in the eye’s blind spot, where the optic nerve PROMOTION DESIGN MANAGER: Nancy Mongelli pierces the retina. VICE PRESIDENT, FINANCE, AND On the other hand, we do not see everything. Something that is irrelevant to GENERAL MANAGER: Michael Florek a particular task will not make it to our conscious awareness. In one telling ex- BUSINESS MANAGER: Marie Maher MANAGER, ADVERTISING ACCOUNTING AND periment, volunteers had to count how many times a basketball got passed be- COORDINATION: Constance Holmes tween players. A person in a gorilla suit then strutted across the room. Concen- DIRECTOR, SPECIAL PROJECTS: trating on those ball passes, about half the volunteers did not see the gorilla. Barth David Schwartz Of course, the brain cannot actually tell us about what it is thinking as it DIRECTOR, WEB TECHNOLOGIES, ONLINE: processes sensory inputs, focusing on certain items and ignoring others. But our Vincent Ma SALES REPRESENTATIVES, ONLINE: responses to illusions can be just as revealing. Scientists have long used these Gary Bronson, Thomas Nolan disarmingly simple—and fun—sensory tricks to probe the mind’s inner work- DIRECTOR, ANCILLARY PRODUCTS: ings. This special edition offers an amazing collection of such illusions and the Diane McGarvey lessons that they teach us about the brain. We can promise you one thing: you PERMISSIONS MANAGER: Linda Hertz won’t believe your eyes. CHAIRMAN: Brian Napack PRESIDENT: Steven Yee VICE PRESIDENT AND MANAGING DIRECTOR, Mariette DiChristina INTERNATIONAL: Dean Sanderson VICE PRESIDENT: Frances Newburg Executive Editor AKIYOSHI KITAOKA 2003 CHAIRMAN EMERITUS: John J. Hanley [email protected] www.SciAmMind.com SCIENTIFIC AMERICAN REPORTS 1 © 2008 SCIENTIFIC AMERICAN, INC. (contents) Volume 18, Number 2, 2008 REPORTS 00 SHADES OF TRUTH 22 >> Right Side Up Studies of perception show the importance of being upright. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN 26 >> Seeing Is Believing 2-D or not 2-D, that is the question: test yourself to learn what shapes formed by shading reveal about the brain. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN 30 >> Seeing in Black & White Why it’s not so cut-and-dried. BY ALAN GILCHRIST 38 >> Transparently Obvious How the brain sees through the perceptual hurdles of tinted glass, shadows and all things transparent. 4 BY VILAYANUR S. RAMACHANDRAN AND 4 DIANE ROGERS-RAMACHANDRAN >> From the Editor 1 MORE THAN MEETS THE EYE >> 4 Mind the Gap The brain, like nature, abhors a vacuum. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN >> 8 Stability of the Visual World When your eyes scan a room, why doesn’t the world appear to bounce like the real image on your retina? BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN >> 12 When the Two Eyes Clash A tale of binocular rivalry. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN >> 16 How Blind Are We? We have eyes, yet we do not see. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN >> 18 Hidden in Plain Sight Camouflage in fish and other animals provides insights into visual perception. BY VILAYANUR S. RAMACHANDRAN AND 38 DIANE ROGERS-RAMACHANDRAN 2 SCIENTIFIC AMERICAN REPORTS ILLUSIONS © 2008 SCIENTIFIC AMERICAN, INC. >> 48 64 The Phantom Hand The feeling of being touched on a fake hand illuminates how the brain makes assumptions about the world. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN >> 66 It’s All Done with Mirrors Reflections on the familiar and yet deeply enigmatic nature of the looking glass. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN 70 >> Paradoxical Perceptions How does the brain sort out contradictory images? MORE REAL THAN REALITY BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN 42 >> The Reality of Illusory Contours How can an imaginary square look more real THE EYE OF THE BEHOLDER than a box with actual lines? BY VILAYANUR S. RAMACHANDRAN AND >> DIANE ROGERS-RAMACHANDRAN 74 The Neurology of Aesthetics How visual-processing systems shape our feelings about what we see. >> 44 The Quirks of Constancy BY VILAYANUR S. RAMACHANDRAN AND Even when we consciously know two lines are DIANE ROGERS-RAMACHANDRAN the same length, why can’t we help seeing them as different? >> BY VILAYANUR S. RAMACHANDRAN AND 78 Cracking the da Vinci Code DIANE ROGERS-RAMACHANDRAN What do the Mona Lisa and Abraham Lincoln have in common? >> BY VILAYANUR S. RAMACHANDRAN AND 48 Sizing Things Up DIANE ROGERS-RAMACHANDRAN When you hoist two items of equal weight, your brain may be doing some heavy lifting. >> BY VILAYANUR S. RAMACHANDRAN AND 82 Illusory Color & the Brain DIANE ROGERS-RAMACHANDRAN Novel illusions suggest that the brain does not separate perception of color from perception of form and depth. >> 52 A Moving Experience BY JOHN S. WERNER, BAINGIO PINNA AND How the eyes can see movement where it LOTHAR SPILLMANN does not exist. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN MIXED MESSAGES >> 56 Ambiguities & Perception What uncertainty tells us about the brain. BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN 60 >> Touching Illusions 74 Startling deceptions demonstrate how tactile information is processed in the brain. BY VILAYANUR S. RAMACHANDRAN AND Articles in this special edition are updated from previous issues DIANE ROGERS-RAMACHANDRAN of Scientific American Mind and Scientific American. Scientific American Special (ISSN 1936-1513), Volume 18, Number 2, 2008, published by Scientific American, Inc., 415 Madison Avenue, New York, NY 10017-1111. Copyright © 2008 by Scientific American, Inc. All rights reserved. No part of this issue may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy- ing and recording for public or private use, or by any information storage or retrieval system, without the prior written permission of the publisher. Canadian BN No. 127387652RT; QST No. Q1015332537. To purchase additional quantities: U.S., $10.95 each; elsewhere, $13.95 each. Send payment to Scientific American, Dept. IL2008, 415 Madison Avenue, New York, NY 10017-1111. Inquiries: fax 212-355-0408 or telephone 212-451-8442. Printed in U.S.A. www.SciAmMind.com SCIENTIFIC AMERICAN REPORTS 3 © 2008 SCIENTIFIC AMERICAN, INC. © 2008 SCIENTIFIC AMERICAN, INC. Mind the Gap The brain, like nature, abhors a vacuum BY VILAYANUR S. RAMACHANDRAN AND DIANE ROGERS-RAMACHANDRAN OUR PERCEPTION of the world depends, to a surprising degree, on intelligent guesswork by the brain. An oval- shaped white image exciting your retina could be produced by an egg, or a perfectly circular, flat tilted disk, or an infi- nite number of intermediate shapes, each angled to the right degree. Yet your brain “homes in” instantly on the correct in- terpretation of the image. It does this by using certain uncon- scious assumptions about the statistics of the natural world—sup- Opositions that can be revealed by visual illusions. Your retina has The manner in which the brain deals with page about a foot away from your face and slowly a blind spot where inexplicable gaps in the retinal image—a process move it toward you and away from you. At a cer- the optic nerve called filling in—provides a striking example of tain distance the disk on the left vanishes.
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