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See Inside June/July 2006

Seeing in Black and White

Why it's not so cut-and-dried

How many times have you heard people say that something is "black and white," meaning it is simple or crystal-clear? And because black and white are so obviously distinct, it would be only natural for us to assume that understanding how we see them must be equally straightforward.

We would be wrong. The seeming ease of perceiving the two color extremes hides a formidable challenge confronting the brain every time we look at a surface. For instance, under the same illumination, white reflects much more light to the eye than black does. But a white surface in shadow often reflects less light to the eye than a black surface in sun. Nevertheless, somehow we can accurately discern which is which. How? Clearly, the brain uses the surrounding context to make such judgments. The specific program used to interpret that context is fraught with mystery for neuroscientists like me.

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