Earth's creatures have eyes so they can see the light—at least that's been the thinking for several millennia. But new research suggests that our eyes are actually geared to pick out darkness. Animal eyes have two ways of processing light-dark contrasts. "On" cells pick out bright spots on dark backgrounds (left image), and "off" cells pick out dark spots on bright backgrounds (right image). Our eyes have almost twice as many "off" cells as "on" cells. University of Pennsylvania researchers wanted to find out if our environment could explain the discrepancy. After analyzing 50 photos ranging from downtown Philadelphia to the plains of Botswana, they discovered that in natural and human-made settings the "off" contrasts (dark on light) outnumber the "on" contrasts (light on dark) by 10% to 20%. A bit of mathematics showed that the ideal ratio of "off" to "on" cells to take in these contrasts is roughly what's seen in our eyes, the researchers report online  today in Proceedings of the National Academy of Sciences.
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