I imagine most readers have seen Edward Adelson’s checkershadow illusion, because it’s done the rounds of a few blogs. If you haven’t seen it it’s worth looking at, because it’s really quite remarkable.
I always find these things fascinating because of what they tell us about the boundary between perception and inference. So I was interested to see how many similar illusions he has posted, many of them as part of the very good HTML version of Lightness Perception and Lightness Illusions.
I had always thought that most of the illumination illusions were all trading in some way or other on either simultaneous contrast or perceived three-dimensionality. But it turns out that this can’t be the whole story, at least if simultaneous contrast is construed fairly narrowly.
In the snake illusion two of the squares that appear to be different shades are (a) the same shade and (b) surrounded by areas that are also the same shade. If there’s a contrast effect, it’s from areas that surround the areas that surround the squares. Either there are many more contrast effects than seemed a priori plausible (not that contrast effects were given much weight back when philosophers tried to do psychology a priori) or something other than contrast effects are active here. (The best Edelman can do with it is to suggest that straight lines are better than curved lines at marking off contrast areas, which seems to describe the phenomenon more than explain it.)
I’m no expert in this area, so I don’t know someone has explained what’s going on with the snake in the 7 or so years since it appeared, but it seems very mysterious to me.
I put up a card with holes over all but the A and B parts of the checkshaddow illusion image, and the squares continue to look different in brightness to me. Shouldn’t the masking of the rest of the image abolish the illusion?
That’s surprising. Yes, that should eliminate the illusion.
It could be because you’re at some level remembering what is under the card. Also there’s going to be a mild contrast effect between the squares and the letters inside them, but it’s hard to imagine that could explain everything. (In other words I don’t know how to explain your result and I’m guessing a bit.)
I wasn’t convinced the squares were the same shade until I threw the picture into Photoshop and deleted everything but the squares, which I guess is something like covering the board apart from the squares. They looked the same shade then. (This does seem to undercut what I said last paragraph about the letters causing a contrast effect, doesn’t it?)
Try covering up the letters A and B as well—the fact that one is black and the other white is an additional contrast cue. I found that once I did that they did indeed appear to be the same shade of gray (which is what my little web-design color selecter tool insisted despite the evidence of my lying eyes).
Save the image and load it into Photoshop or Paint or the GIMP. Use the eyedropper to sample the colour from one, and paint it over the other.
Try using the paintbrush to slowly expand the squares, and see how much you need to do to get them to appear the same.
It took me a while, but the REALLY ARE IDENTICAL.
Covering the intervening space with my finger - making both squares contrast to the same object, in other words - also convinced me they were the same color.
But I now have to suppose that didn’t prove anything, did it? Hmmm.
I believe in this instance, the illusion may be suffering from the limitations of the medium involved. The illusion is certainly for real; I’ve seen it many times before (it’s understandably a favorite in perceptual psychology). But the image at this link does not reproduce the illusion accurately; the two squares, on my lcd screen, are demonstrably not the same.
cerebrocrat,
I agreed that there was no way the squares were the same colour but copying and pasting into paint and then erasing everything but the two squares I am convinced.
Which all just goes to show how great the illusion is.
Edwin Land? Retinex theory? I thought Oliver Sacks gave it a thought. The eye integrates shading across boundaries, or something.
This is the second time I’ve seen this illusion. The first time I envisioned using a hole punch to make two carefully spaced apertures to test it. Both times I wondered why I trusted my monitor to give a calibrated result, and didn’t bother.
Ok, I cut uniform blocks of the A and B squares into a blank canvas, and yes they are the same, wow!
I also did the test with small blocks containing the letters A and B, and there was a little difference between the colors, but nothing as large as the actual shadow illusion.
Amazing.
It made a great demo in class the other day. “They are the same colour! Who believes me?” No one, it would seem. Using GIMP’s pencil tool, I fill in the area between the squares with the same colour. 300 students go “Oooh”.
Computers can be fooled too, if not so subtly. Some of the people who insist that the squares in the checkershadow illusion are provably not the same color just might be viewing them on computers that actually are giving them different colors, because they’re using some sort of error-diffusion dither that can be led astray by local contrast effects.
They’re the same color on my screen, though.
À Gauche
Jeremy Alder
Amaravati
Anggarrgoon
Audhumlan Conspiracy
H.E. Baber
Philip Blosser
Paul Broderick
Matt Brown
Diana Buccafurni
Brandon Butler
Keith Burgess-Jackson
Certain Doubts
David Chalmers
Noam Chomsky
The Conservative Philosopher
Desert Landscapes
Denis Dutton
David Efird
Karl Elliott
David Estlund
Experimental Philosophy
Fake Barn County
Kai von Fintel
Russell Arben Fox
Garden of Forking Paths
Roger Gathman
Michael Green
Scott Hagaman
Helen Habermann
David Hildebrand
John Holbo
Christopher Grau
Jonathan Ichikawa
Tom Irish
Michelle Jenkins
Adam Kotsko
Barry Lam
Language Hat
Language Log
Christian Lee
Brian Leiter
Stephen Lenhart
Clayton Littlejohn
Roderick T. Long
Joshua Macy
Mad Grad
Jonathan Martin
Matthew McGrattan
Marc Moffett
Geoffrey Nunberg
Orange Philosophy
Philosophy Carnival
Philosophy, et cetera
Philosophy of Art
Douglas Portmore
Philosophy from the 617 (moribund)
Jeremy Pierce
Punishment Theory
Geoff Pynn
Timothy Quigley (moribund?)
Conor Roddy
Sappho's Breathing
Anders Schoubye
Wolfgang Schwartz
Scribo
Michael Sevel
Tom Stoneham (moribund)
Adam Swenson
Peter Suber
Eddie Thomas
Joe Ulatowski
Bruce Umbaugh
What is the name ...
Matt Weiner
Will Wilkinson
Jessica Wilson
Young Hegelian
Richard Zach
Psychology
Donyell Coleman
Deborah Frisch
Milt Rosenberg
Tom Stafford
Law
Ann Althouse
Stephen Bainbridge
Jack Balkin
Douglass A. Berman
Francesca Bignami
BlunkettWatch
Jack Bogdanski
Paul L. Caron
Conglomerate
Jeff Cooper
Disability Law
Displacement of Concepts
Wayne Eastman
Eric Fink
Victor Fleischer (on hiatus)
Peter Friedman
Michael Froomkin
Bernard Hibbitts
Walter Hutchens
InstaPundit
Andis Kaulins
Lawmeme
Edward Lee
Karl-Friedrich Lenz
Larry Lessig
Mirror of Justice
Eric Muller
Nathan Oman
Opinio Juris
John Palfrey
Ken Parish
Punishment Theory
Larry Ribstein
The Right Coast
D. Gordon Smith
Lawrence Solum
Peter Tillers
Transatlantic Assembly
Lawrence Velvel
David Wagner
Kim Weatherall
Yale Constitution Society
Tun Yin
History
Blogenspiel
Timothy Burke
Rebunk
Naomi Chana
Chapati Mystery
Cliopatria
Juan Cole
Cranky Professor
Greg Daly
James Davila
Sherman Dorn
Michael Drout
Frog in a Well
Frogs and Ravens
Early Modern Notes
Evan Garcia
George Mason History bloggers
Ghost in the Machine
Rebecca Goetz
Invisible Adjunct (inactive)
Jason Kuznicki
Konrad Mitchell Lawson
Danny Loss
Liberty and Power
Danny Loss
Ether MacAllum Stewart
Pam Mack
Heather Mathews
James Meadway
Medieval Studies
H.D. Miller
Caleb McDaniel
Marc Mulholland
Received Ideas
Renaissance Weblog
Nathaniel Robinson
Jacob Remes (moribund?)
Christopher Sheil
Red Ted
Time Travelling Is Easy
Brian Ulrich
Shana Worthen
Computers/media/communication
Lauren Andreacchi (moribund)
Eric Behrens
Joseph Bosco
Danah Boyd
David Brake
Collin Brooke
Maximilian Dornseif (moribund)
Jeff Erickson
Ed Felten
Lance Fortnow
Louise Ferguson
Anne Galloway
Jason Gallo
Josh Greenberg
Alex Halavais
Sariel Har-Peled
Tracy Kennedy
Tim Lambert
Liz Lawley
Michael O'Foghlu
Jose Luis Orihuela (moribund)
Alex Pang
Sebastian Paquet
Fernando Pereira
Pink Bunny of Battle
Ranting Professors
Jay Rosen
Ken Rufo
Douglas Rushkoff
Vika Safrin
Rob Schaap (Blogorrhoea)
Frank Schaap
Robert A. Stewart
Suresh Venkatasubramanian
Ray Trygstad
Jill Walker
Phil Windley
Siva Vaidahyanathan
Anthropology
Kerim Friedman
Alex Golub
Martijn de Koning
Nicholas Packwood
Geography
Stentor Danielson
Benjamin Heumann
Scott Whitlock
Education
Edward Bilodeau
Jenny D.
Richard Kahn
Progressive Teachers
Kelvin Thompson (defunct?)
Mark Byron
Business administration
Michael Watkins (moribund)
Literature, language, culture
Mike Arnzen
Brandon Barr
Michael Berube
The Blogora
Colin Brayton
John Bruce
Miriam Burstein
Chris Cagle
Jean Chu
Hans Coppens
Tyler Curtain
Cultural Revolution
Terry Dean
Joseph Duemer
Flaschenpost
Kathleen Fitzpatrick
Jonathan Goodwin
Rachael Groner
Alison Hale
Household Opera
Dennis Jerz
Jason Jones
Miriam Jones
Matthew Kirschenbaum
Steven Krause
Lilliputian Lilith
Catherine Liu
John Lovas
Gerald Lucas
Making Contact
Barry Mauer
Erin O'Connor
Print Culture
Clancy Ratcliff
Matthias Rip
A.G. Rud
Amardeep Singh
Steve Shaviro
Thanks ... Zombie
Vera Tobin
Chuck Tryon
University Diaries
Classics
Michael Hendry
David Meadows
Religion
AKM Adam
Ryan Overbey
Telford Work (moribund)
Library Science
Norma Bruce
Music
Kyle Gann
ionarts
Tim Rutherford-Johnson
Greg Sandow
Scott Spiegelberg
Biology/Medicine
Pradeep Atluri
Bloviator
Anthony Cox
Susan Ferrari (moribund)
Amy Greenwood
La Di Da
John M. Lynch
Charles Murtaugh (moribund)
Paul Z. Myers
Respectful of Otters
Josh Rosenau
Universal Acid
Amity Wilczek (moribund)
Theodore Wong (moribund)
Physics/Applied Physics
Trish Amuntrud
Sean Carroll
Jacques Distler
Stephen Hsu
Irascible Professor
Andrew Jaffe
Michael Nielsen
Chad Orzel
String Coffee Table
Math/Statistics
Dead Parrots
Andrew Gelman
Christopher Genovese
Moment, Linger on
Jason Rosenhouse
Vlorbik
Peter Woit
Complex Systems
Petter Holme
Luis Rocha
Cosma Shalizi
Bill Tozier
Chemistry
"Keneth Miles"
Engineering
Zack Amjal
Chris Hall
University Administration
Frank Admissions (moribund?)
Architecture/Urban development
City Comforts (urban planning)
Unfolio
Panchromatica
Earth Sciences
Our Take
Who Knows?
Bitch Ph.D.
Just Tenured
Playing School
Professor Goose
This Academic Life
Other sources of information
Arts and Letters Daily
Boston Review
Imprints
Political Theory Daily Review
Science and Technology Daily Review