er scientists had a true understanding of how the
newfound illusions might come about.
Pascal Wallisch, a neuroscientist at New York
University, believes that the key to the puzzle is
observers’ previous knowledge of lighting sourc-
es and materials such as fabrics—what psycholo-
gy researchers call “priors.” To prove it, Wallisch
and his New York University collaborator Michael
Karlovich devised a method of creating color illu-
sions that are just as confounding as those previ-
ously found by chance. The Crocs and socks pho-
tograph at the beginning of this article is one ex-
ample. To create the image, Wallisch and
Karlovich started with an object that looks pink
under white light (a pair of “Ballerina Pink Classic
Crocs”) and instead illuminated it with green light,
equalizing its appearance to gray. Then, they
made the background pitch-black, removing any
contextual color cues that the visual system
might utilize. As a result, the Crocs might be any
color or at least any of the 28 different hues that
you might find at your favorite Crocs retailer.
Depending on your past familiarity with white
tube socks (your prior), your visual system may cor-
rectly conclude that the socks are truly white but
illuminated by green lighting. If so, you may be able
to retrieve the Crocs’ original pink color in your per-
ception. Observers who lack the white sock prior
may instead perceive the Crocs as grayish.
People believe that they see things “how they
really are,” Wallisch says. “But does this mean the
colors of the pixels in isolation or of the whole
shoe in context? Those two [interpretations] can
be different for different people.”
ILLUSIONS