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Yes, we have no blue bananas

Oct. 15, 2006
Courtesy Nature Publishing Group
and World Science staff

A ba­nan­a in black and white (a­bove) and with a slight blue tinge (be­low.)

If we see a pure­ly black-and-white pic­ture of a ba­nan­a, it will look just a tad yel­low to us. So finds a new study in the No­vem­ber is­sue of the re­search jour­nal Na­ture Neu­ro­science.

This shows that col­or per­cep­tion de­pends not on­ly on an ob­ject’s true hue, but al­so on our know­l­edge of what it’s sup­posed to look like, the re­search­ers said.

Karl Ge­gen­furt­ner of Jus­tus-Lie­big Uni­ver­sity in Gies­sen, Ger­ma­ny, and col­leagues showed peo­ple im­ages of com­mon fruits and ve­g­e­ta­bles. 

The view­ers were al­lowed to ma­ni­p­u­late the co­l­ors in each im­age, and asked to al­ter them so as to make them ap­pear to lack any col­or. 

Pre­con­cep­tions about a fruit’s nat­u­ral col­or in­f­lu­enced ob­ser­v­ers’ per­cep­tions of its ac­tu­al co­l­or, the re­search­ers found. For ex­am­ple, to make a ba­na­n­a look black and white, par­ti­ci­pants ad­just­ed it to have a slight tinge of blue. 

This sug­gests the view­ers saw per­fect­ly black-and-white ba­na­nas as slight­ly yel­low, and had to com­pen­sate by tun­ing the im­age to­ward the “op­posite” col­or, the re­search­ers said. For let­tuce to ap­pear black and white, it had to be slight­ly red. 

“These re­sults,” the team wrote, “show that co­l­or sen­sa­tions are not de­ter­mined by the in­com­ing sen­so­ry da­ta alone, but are sig­nif­i­cantly mo­d­u­lat­ed by high-lev­el vi­s­u­al me­m­o­ry,” that is, ex­pec­ta­tions.

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If we see a purely black-and-white picture of a banana, it will look just a tad yellow to us. So concludes a new study in the November issue of the research journal Nature Neuroscience. Color perception depends not only on the pigmentation of an object, but also on our knowledge of what the object is supposed to look like, the researchers say. Karl Gegenfurtner of Justus-Liebig-University in Giessen, Germany, and colleagues showed people images of common fruits and vegetables. The participants were allowed to manipulate the color content in each image, and asked to alter the hues so as to make the fruits appear to lack color. The results show that the observers’ preconceptions about the fruit’s natural color influenced their perception of its actual color, the researchers said. For example, to make a banana appear black and white, subjects adjusted it to be slightly blue. That suggests that the viewers saw a perfectly black-and-white picture as slightly yellow, and they had to compensate for that by tuning the image toward the “opposite” color, the researchers said. For a head of lettuce to appear black and white, it had to be slightly red. “These results,” the team wrote, “show that color sensations are not determined by the incoming sensory data alone, but are significantly modulated by high-level visual memory,” or the brain’s expectations.