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12/6/11

The Puzzle of Face Space: Your Eyes May See When Your Brain Does Not

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In Oliver Sacks' classic The Man Who Mistook His Wife For A Hat the title is comic and almost absurd but his book provides fascinating explanation of a neurological condition in which a man does mistake his wife for a hat without realizing it. Sacks introduces the lay reader to another phenomenon--how a stroke leaves a person unable to see things on one side of the visual field and how the stroke victim also has no inkling that anything is wrong or missing with his perception. In fact, in both cases, nothing is wrong with the eyes, only with the way the brain processes visual information.

There is a name for this condition, prosopagnosia, and it describes a small fraction of people who cannot recognize faces--even the faces of loved ones  The condition puzzles researchers but they are beginning to understand how facial recognition works, which may help those with prosopagnosia.  With her colleagues, Marlene Behrmann at Carnegie Mellon University has gathered some pieces of the puzzle by comparing the brains of individuals who are face-blind to those who are face-sighted. Their results hint at how we recognize faces not in a flash of insight, not as a kind of photographic image, but by building up recognition on a neurological assembly line. Recognition is encoded in the brain as face space.
~"Behrmann has been testing a model for face recognition that was first proposed 25 years ago by Tim Valentine and Vicki Bruce, two psychologists at the University of Nottingham in England. Valentine and Bruce argued that our brains do not store a photographic image of every face we see. Instead, they carry out a mathematical transformation of each face, encoding it as a point in a multidimensional 'face space'.

~On a map of face space, you might imagine the north-south axis being replaced with a small-mouth-to-wide-mouth axis. But instead of three different dimensions, like the space we’re familiar with, face space may have many dimensions, each representing some important feature of the human face. Just as the ancient cosmos was centered on Earth, Valentine and Bruce argued that the facial universe is centered on the perfectly average face. The farther a face is from this average center, the more extreme it becomes.

~By reducing a face to a point—creating a compact code for representing an infinite number of faces—our brains need to store only the distance and direction of that point from the center of face space.

~Face space also explains why the favorite trick of editorial cartoonists works so well. By exaggerating features on a politician’s face—Bush’s eyebrows, Obama’s ears .

~Imagine that an eccentric psychologist accosts you. In his hand is a piece of paper with 20 pictures of roses. One of the pictures shows a rose in the flower bed you just passed, he says, and he asks you to pick its picture out from his lineup. The challenge would seem absurd—but if you were to change the roses to faces, nearly everyone could meet it."  MoreBookmark and Share
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