Visual Illusions in Art and Science

The fol­low­ing is an excerpt from “Sleights of Minds: What the Neu­ro­science of Mag­ic Reveals about Our Every­day Decep­tions.” by Stephen L. Mack­nik and Susana Mar­tinez-Conde with San­dra Blakeslee, pub­lished by Hen­ry Holt and Co., LLC © 2010 by  Stephen L. Mack­nik and Susana Martinez-Conde.

Visu­al illu­sions are fas­ci­nat­ing. They have both been used by artists and stud­ied by sci­en­tists. Read on to learn how art can help sci­ence under­stand the secrets of how we per­ceive the world around use.

Chap­ter 3. The Broth­er who Faked a Dome
Visu­al Illu­sions in Art and Science

Vision sci­en­tists like us seek to under­stand how we see, from both a psy­cho­log­i­cal and a bio­log­i­cal per­spec­tive, and our dis­ci­pline has a long tra­di­tion of study­ing visu­al artists such as painters and sculp­tors. Sci­en­tists did not invent the vast major­i­ty of visu­al illu­sions— painters did. The visu­al arts often pre­ced­ed the visu­al sci­ences in the dis­cov­ery of fun­da­men­tal vision prin­ci­ples, through the appli­ca­tion of method­i­cal— although per­haps more intu­itive— research techniques.
Like­wise, magi­cians— as the world’s pre­mier artists of atten­tion and aware­ness— have made their own dis­cov­er­ies. This is what drew us to their foot­lights, card tables, and street per­for­mances. We want magi­cians to help us under­stand cog­ni­tive illu­sions in the same way that artists have revealed insights about visu­al illu­sions. And in fact visu­al illu­sions are a bit like mag­ic tricks on the page. In this chap­ter we’ll take a brief tour of some of our favorites.

In the ear­ly decades of the sev­en­teenth cen­tu­ry Dutch painters devel­oped still-life easel paint­ings with trompe l’oeil real­ism. (The Attrib­ut­es of the Painter by Cor­nelius N. Gys­brechts. Réu­nion des Musées Nationaux / Art Resource, N.Y.)

Artists have been uti­liz­ing visu­al illu­sions since the ?fteenth cen­tu­ry, when Renais­sance painters invent­ed tech­niques to trick your brain into think­ing that a ?at can­vas is three-dimen­sion­al or that a series of brush­strokes in a still life is a bowl of lus­cious fruit. They ?gured out lin­ear per­spec­tive— the notion that par­al­lel lines can be rep­re­sent­ed as con­verg­ing so as to give the illu­sion of depth and dis­tance. (Again, think of train tracks head­ing toward the hori­zon.) They real­ized they could manip­u­late atmos­pher­ic effects by mak­ing tones weak­en and col­ors pale as they recede from view. They used the hori­zon or eye lev­el as a ref­er­ence point to judge the size and dis­tance of objects in rela­tion to the view­er. They used shad­ing, occlu­sion, and van­ish­ing points to make their paint­ings hyperrealistic.
Trompe l’oeil is a French term that means “trick the eye.” It ?our­ished in the sev­en­teenth cen­tu­ry in the Nether­lands. The life­like pic­tures appeared to jump from the frame.

The “dome” of Saint Ignatius church looks like a real dome from this van­tage point. (

Trompe l’oeil is some­times used on a large scale to sug­gest entire parts of build­ings that do not actu­al­ly exist. The archi­tect of the Saint Ignatius church in Rome, Horace Gras­si, had planned to build a cupo­la but died before ?nish­ing the church, and the mon­ey for the cupo­la was used for some­thing  else. Thir­ty years lat­er, in 1685, the Jesuit artist Andrea Poz­zo was asked to paint a fake dome on the ceil­ing over the altar. Poz­zo was already con­sid­ered a mas­ter in the art of per­spec­tive, and yet what he accom­plished could hard­ly be believed. Even today, many vis­i­tors to Saint Ignatius’s are amazed to ?nd out that the spec­tac­u­lar cupo­la is not real but an illusion.

Archi­tects soon real­ized that they, too, could manip­u­late real­i­ty by warp­ing per­spec­tive and depth cues to cre­ate illu­so­ry struc­tures that de?ed per­cep­tion. Need a big room in one- fourth the space? No prob­lem. Francesco Bor­ro­mi­ni accom­plished just that at the Palaz­zo Spa­da, a palace in Rome that we vis­it­ed a few years ago. Bor­ro­mi­ni cre­at­ed the illu­sion of a court­yard gallery 121 feet long in a 26- foot space. There’s even a life- size sculp­ture at the end of the arch­way. Not real­ly. The sculp­ture looks life-size but is actu­al­ly just two feet tall.

This hall­way is much short­er and the sculp­ture is much small­er than they appear. (

Clos­er to home and to mag­ic is the Grand Canal con­course at the Venet­ian Hotel and Casi­no in Las Vegas. The ?rst time you step onto the con­course, you feel a sud­den onset of twi­light. That’s exact­ly what Susana’s moth­er, Lau­ra, expe­ri­enced when we ?rst took her to Las Vegas while plan­ning our con­fer­ence. We descend­ed from our suite after a room ser­vice lunch. Step­ping out of the ele­va­tors and onto the con­course, she said, “Oh, it’s got­ten so dark out­side.” Susana asked her what she meant. “The sky,” Lau­ra said. “It’s got­ten dark so ear­ly.” “But, Mamá,” Susana explained, “we’re still inside. You see the black spots in the sky? They are sprin­kler heads.” Mouth agape, Lau­ra exam­ined the incred­i­ble illu­so­ry sky, with its ?ve shades of roco­co blue— pea­cock, azure, cerulean, turquoise, and aqua­ma­rine— and wisps of mare’s tails, stra­tus, and cir­rocu­mu­lus clouds. Lau­ra con­sid­ered it for a minute before turn­ing to Susana and say­ing, “Well, why did you tell me so soon? I would have liked to enjoy it a lit­tle longer.”

Anoth­er great illu­sion­ist is the Dutch lith­o­g­ra­ph­er and wood­cut artist Mau­rits Cor­nelis (bet­ter known as M. C.) Esch­er. Ear­ly in his career, Esch­er carved real­is­tic scenes based on his obser­va­tions and trav­els. Lat­er, he turned to his imag­i­na­tion, ren­der­ing some of the most bril­liant visu­al illu­sions in the his­to­ry of art. When he was in high school, one of Steve’s favorite posters was an Esch­er print of the nev­er- end­ing stair­case (Ascend­ing and Descend­ing, 1960), in which a group of robed monks per­pet­u­al­ly climb or descend an impos­si­ble stair­case sit­u­at­ed at the top of a tem­ple. It was impos­si­ble because it cir­cled around on itself and nev­er end­ed. So how could it be drawn if it was phys­i­cal­ly impos­si­ble? Esch­er must have cheat­ed some­where in the print and failed to depict the prop­er struc­ture of a real stair­case. But Steve  couldn’t ?nd it, no mat­ter how close­ly he looked. He real­ized he should exam­ine the struc­ture as a  whole to see if there was a small sys­tem­at­ic warp along the entire struc­ture that allowed for the illusion.
And that’s when Steve found that he  couldn’t look at the struc­ture glob­al­ly. He could only real­ly see one area of the stair­case at a time. His vision could process the details of the stair­case when he cen­tered his gaze on a speci?c part. But when he did that, every oth­er area of the stair­case, in his visu­al periph­ery, was left in a blur. And he real­ized that that was how Esch­er must have done it: since you can see only one local area at any giv­en time, small, grad­ual errors along the entire struc­ture could not be seen with the naked eye.

This effect chal­lenges our hard-earned per­cep­tion that the world around us fol­lows cer­tain invi­o­lable rules. It also reveals that our brains con­struct the feel­ing of a glob­al per­cept by sewing togeth­er mul­ti­ple local per­cepts. As long as the local rela­tion between sur­faces and objects fol­lows the rules of nature, our brains don’t seem to mind that the glob­al per­cept is impossible.
Susana’s for­mal intro­duc­tion to visu­al illu­sions came in 1997 when she arrived at Har­vard Uni­ver­si­ty to study under David Hubel and Mar­garet Liv­ing­stone. At the time, Har­vard was the mec­ca for the study of illu­sions, and in fact this is where she met Steve. Not only were Liv­ing­stone and Hubel lead­ing the ?eld in the study of illu­sions in the brain, but a num­ber of Har­vard psy­chol­o­gists were dis­cov­er­ing an array of com­plete­ly new phenomena.

As part of her post­doc­tor­al train­ing, Susana decid­ed to choose a visu­al illu­sion and inves­ti­gate its effects. Lea?ng through an art book, she found the per­fect play­ground for her curios­i­ty: op art, a ?eld that explores many aspects of visu­al per­cep­tion, such as the rela­tions between geo­met­ri­cal shapes, vari­a­tions on “impos­si­ble” ?gures that can­not occur in real­i­ty, and illu­sions involv­ing bright­ness, col­or, and shape perception.

Susana set­tled on op artist Vic­tor Vasare­ly, whose Nest­ed Squares series exhib­it­ed an odd illu­sion: the cor­ners of the squares looked brighter than their straight-edged sides. But the effect wasn’t just about the light­ness of the cor­ners, because if Vasare­ly reversed the order of the nest­ed squares from white- to- black (cen­ter to exte­ri­or) to black- to- white, now the cor­ners  were dark­er than the sides. So it seemed to be an illu­sion con­cern­ing con­trast, and not light­ness per se.

Susana searched the vision research lit­er­a­ture and found that only a cou­ple of peo­ple had dis­cussed this effect pre­vi­ous­ly and nobody had inves­ti­gat­ed its neur­al bases. And no one had looked at shapes oth­er than squares. Squares are a spe­cial type of shape in which all of the cor­ners are con­vex (all point away from the cen­ter of the square). But nobody had exam­ined the effect for non­square shapes with con­cave cor­ners or for shapes with cor­ner angles oth­er than 90 degrees. Susana real­ized there  were many aspects of this illu­sion that she could study per­cep­tu­al­ly, fol­lowed by phys­i­o­log­i­cal research in the brain.

Vasarely’s Utem (1981). Nest­ed squares of increas­ing or decreas­ing lumi­nance pro­duce illu­so­ry diag­o­nals that look brighter or dark­er than the rest of the squares. (Cour­tesy of Michèle Vasarely)

After sev­er­al years, ?rst as a trainee at Har­vard and lat­er as the direc­tor of her own research team, Susana learned one of the most fun­da­men­tal secrets of the visu­al sys­tem. The pre­vi­ous dog­ma in the ?eld had been that neu­rons in the ?rst few stages of the visu­al sys­tem  were most sen­si­tive to the edges of object sur­faces. Susana’s results showed instead that neu­rons of the visu­al sys­tem are more sen­si­tive to the cor­ners, curves, and dis­con­ti­nu­ities in the edges of sur­faces, as opposed to the straight edges that had pre­vi­ous­ly been thought to reign.

Op artists  were also inter­est­ed in kinet­ic or motion illu­sions. In these eye tricks, sta­tion­ary pat­terns give rise to the pow­er­ful but sub­jec­tive per­cep­tion of illu­so­ry motion. An exam­ple is Enig­ma by Isia Leviant.

Rein­ter­pre­ta­tion of Enig­ma (Cre­at­ed by and cour­tesy of Jorge Otero- Mil­lan, Mar­tinez- Conde Lab­o­ra­to­ry, Bar­row Neu­ro­log­i­cal Institute)

This sta­t­ic image of reg­u­lar pat­terns elic­its pow­er­ful illu­so­ry motion in most of us and has gen­er­at­ed an enor­mous amount of inter­est in the visu­al sci­ences since it was cre­at­ed in 1981. How­ev­er, the ori­gin of the illu­sion— is it the brain, the eye, or a com­bi­na­tion of both?— remains, appro­pri­ate­ly, an enigma.

—> Inter­est­ed in read­ing more? Please check out the new book “Sleights of Minds: What the Neu­ro­science of Mag­ic Reveals about Our Every­day Decep­tions.” by Stephen L. Mack­nik and Susana Mar­tinez-Conde with San­dra Blakeslee, pub­lished by Hen­ry Holt and Co., LLC © 2010 by  Stephen L. Mack­nik and Susana Martinez-Conde.

About SharpBrains

SHARPBRAINS is an independent think-tank and consulting firm providing services at the frontier of applied neuroscience, health, leadership and innovation.
SHARPBRAINS es un think-tank y consultoría independiente proporcionando servicios para la neurociencia aplicada, salud, liderazgo e innovación.

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