Help 閱讀識字 English Reading Made Easy

SIGHT

The human eye is an exceptional tool for gathering light; it can distinguish among 500 shades of gray and spot the light of a candle 14 miles away. But for all the eye’s extraordinary ability, seeing is a function of the brain—humans’ visual cortex is more developed than that of any other mammal. Vision demands that the brain differentiate foreground from background, and edges from lines—skills that even the world’s most powerful computers have been unable to match. Your brain makes sense of shapes and symbols by putting them together like a jigsaw puzzle, organizing fragments into a coherent whole. (Movies work because the brain stitches a ­series of frames into an unbroken stream.) We develop this ability as infants—which explains why patients who grew up blind but have had their sight restored often struggle to understand what they are seeing: Their brains can’t initially distinguish an object from its background, or accurately separate two over­lapping objects. (A chair and desk would be perceived as a single flat image, like an abstract painting made of rectangles.) Optical illusions work by exploiting the mind’s tendency to try to find order in patterns—a face in the pockmarks on a cement wall—or make sense of an impossible image, like the endless climbing staircase in an Escher drawing.