30 August 2010 by David Robson
Can you tell a snake from a pretzel? Some can't – and their experiences are revealing how the brain builds up a coherent picture of the world
AFTER her minor stroke, BP started to feel as if her eyes were playing tricks on her. TV shows became confusing: in one film, she was surprised to see a character reel as if punched by an invisible man. Sometimes BP would miss seeing things that were right before her eyes, causing her to bump into furniture or people.
BP's stroke had damaged a key part of her visual system, giving rise to a rare disorder called simultanagnosia. This meant that she often saw just one object at a time. When looking at her place setting on the dinner table, for example, BP might see just a spoon, with everything else a blur (Brain, vol 114, p 1523).
BP's problems are just one example of a group of disorders known collectively as visual agnosias, usually caused by some kind of brain damage. Another form results in people having trouble recognising and naming objects, as experienced by the agnosic immortalised in the title of Oliver Sacks's 1985 best-seller The Man Who Mistook His Wife for a Hat.
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http://www.newscientist.com/article/mg20727751.200-the-minds-eye-how-the-brain-sorts-out-what-you-see.html