"Long before it's in the papers"
May 18, 2015


Scientists weigh in on optical illusion that went viral

May 18, 2015
Courtesy of the University of Bradford
and World Science staff

Last Feb­ru­ary, a snap­shot on the In­ter­net of a lace-decorated dress went vi­ral as it sparked a de­bate. Some peo­ple said they saw a blue and black dress; oth­ers, white and gold.

Now, per­cep­tu­al psy­chol­o­gists claim to have an ex­plana­t­ion from the wildly di­verg­ing per­cep­tions. The sci­en­tists in­ves­t­i­gated the is­sue by pre­sent­ing the pho­to, along with oth­ers, to a sam­ple of view­ers.

This photo of a dress, sold by the retailer Roman Originals, became an Internet sensation in late February as viewers wildly disagreed about what its colors were. The photo was said to have originated with a Scottish couple who planned to use the dress at a wedding. (Courtesy of Wikipedia).

The re­search­ers found that all par­ti­ci­pants bas­ic­ally per­ceived si­m­i­lar col­or shades, only var­y­ing in light­ness. For one col­or, the per­cep­tion ranged from a very light blue—al­most white—to bright mid blue. For the oth­er, it ranged from yel­low and gold to an in­tense and dark brown, al­most black.

All these per­ceived col­ors have some­thing in com­mon: they’re all part of the “day­light lo­cus”—col­ors of day­light, in their var­i­ous shades. De­pend­ing on the po­si­tion of the sun, day­light tends to be rath­er blu­ish, at noon, or rath­er yel­lowish in the morn­ing and eve­ning.

Usu­al­ly, peo­ple un­con­sciously fil­ter that ef­fect so that ever­yone per­ceives the same col­ors. But to do so, we re­quire col­ors out­side the “day­light lo­cus” as ref­er­ence points. Yet no such col­ors are per­ceived in the pho­to, so it lacks rel­e­vant in­forma­t­ion on the scene’s illu­mina­t­ion, the re­search­ers said.

Some view­ers thus per­ceived the dress as “a white dress... ex­posed to cool blu­ish light,” said Gegen­furt­ner, and oth­ers as “a blue dress which was overex­posed by warm light.”

The fact that it is in­deed a blue/black dress is only a sec­ond­ary mat­ter, ac­cord­ing to the re­search­ers.

But “this would not have hap­pened with a red dress,” since red and green are out­side the day­light lo­cus, said Karl Gegen­furt­ner of Jus­tus Lie­big Uni­vers­ity Gies­sen in Ger­ma­ny, one of the re­search­ers.

The find­ings are pub­lished in the jour­nal Cur­rent Bi­ol­o­gy.

Pre­vi­ous stud­ies, the sci­en­tists added, have shown that peo­ple have trou­ble per­ceiv­ing col­ors along the day­light lo­cus cor­rect­ly. Test sub­jects are for in­stance rarely able to set a com­pletely neu­tral grey on the screen with­out tend­ing to a slightly blu­ish or yel­lowish tinge. De­via­t­ions con­cern­ing red or green how­ev­er are rare, they added, ex­plain­ing why when they pre­sented view­ers with a ver­sion of the pho­to us­ing a red and green dress, there was no dis­a­gree­ment.

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Last February, a snapshot on the Internet of a lace-decorated dress went viral as it sparked a debate. Some people said they saw a blue and black dress; others, white and gold. Now, perceptual psychologists claim to have an explanation from the wildly diverging perceptions. The scientists investigated the issue by presenting the photo, along with others, to a sample of viewers. The researchers found that all participants basically perceived similar color shades, only varying in lightness. For one color, the perception ranged from a very light blue—almost white—to bright mid blue. For the other, it ranged from yellow and gold to an intense and dark brown, almost black. All these perceived colors have something in common: they’re all part of the “daylight locus”—colors of daylight, in their various shades. Depending on the position of the sun, daylight tends to be rather bluish, at noon, or rather yellowish in the morning and evening. Usually, people unconsciously filter that effect so that everyone perceives the same colors. But to do so, we require colors outside the “daylight locus” as reference points. Yet no such colors are perceived in the photo, so it lacks relevant information on the scene’s illumination, the researchers said. Some viewers thus perceived the dress as “a white dress was exposed to cool bluish light,” said Gegenfurtner, and others as “a blue dress which was overexposed by warm light.” The fact that it is indeed a blue/black dress is only a secondary matter, according to the researchers. But “this would not have happened with a red dress,” since red and green are outside the daylight locus, said Karl Gegenfurtner of Justus Liebig University Giessen in Germany, one of the researchers. The findings are published in the journal Current Biology. Previous studies, the scientists added, have shown that people have trouble perceiving colors along the daylight locus correctly. Test subjects are for instance rarely able to set a completely neutral grey on the screen without tending to a slightly bluish or yellowish tinge. Deviations concerning red or green however are rare, they added, explaining why when they presented viewers with a version of the photo using a red and green dress, there was no disagreement.