Brain teasers

Con­grats! You and two friends won a free va­ca­tion to a re­sort is­land. But while wan­der­ing the trop­i­cal isle, you get su­per lost. The path back is full of ob­sta­cles; can you make it through?

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IF A SIGN TELLS YOU TO FOL­LOW THE pur­ple skull to your des­ti­na­tion, the an­swer seems sim­ple: Veer left. But iso­late the stripes that make up the skulls, and you’ll find nei­ther skull has pur­ple bones; in fact, all the bones are the same color. Slot them back into the banded set­ting, and they shift to pur­ple and or­ange.

The pig­ments morph be­cause of the Munker-White il­lu­sion, which shifts the per­cep­tion of two iden­ti­cal color tones when they’re placed against dif­fer­ent sur­round­ing hues. No one knows for sure, but the il­lu­sion prob­a­bly results from what David Novick, a com­puter sci­en­tist at the Uni­ver­sity of Texas at El Paso, calls the color-com­ple­tion ef­fect. The phe­nom­e­non causes an im­age to skew to­ward the color of the ob­jects that sur­round it. In a black-and-white im­age, a gray el­e­ment would ap­pear lighter when it’s striped with white, and darker when banded with black.

Many neu­ro­sci­en­tists think that neu­ral sig­nals in charge of re­lay­ing in­for­ma­tion about the pig­ments in our visual field get av­er­aged— cre­at­ing a color some­where in the mid­dle. Here, one skull is cov­ered by blue stripes in the fore­ground and the other with yel­low ones. When the orig­i­nal skulls take on the char­ac­ter­is­tics of the sep­a­rate sur­round­ings, they look like dif­fer­ent col­ors en­tirely. Don’t be fooled: Fol­low both skulls by go­ing straight.

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