for a 
Localized damage to the brain can selectively affect particular brain modules, brain circuits that specialize in performing particular, circumscribed functions. Such damage creates deficits in particular sorts of information processing --color, motion, etc, in the absence of a problem with vision in general. Excellent examples of such cases are given in Oliver Sacks' book, The Man Who Mistook his Wife for a Hat (1985, Summit Books).
Oliver Sacks is a neurologist who, as his writing shows, has a wonderful capacity for putting himself in the patient's shoes. Many of Sacks' essays deal with odd, unexpected "fault lines" created by the brain's modularity of function. For example in the book's title essay, Sacks describes the strange case of Dr. P.:
"Dr. P. was a musician of distinction, well-known... as a teacher. Sometimes a student would present himself, and Dr. P. would not recognise him; or, specifically, would not recognise his face. The moment the student spoke, he would be recognised by his voice. Such incidents multiplied, causing embarrassment, perplexity, fear --and, sometimes, comedy. For not only did Dr. P. increasingly fail to see faces, but he saw faces where there were no faces to see: genially, Magoo-like, when in the street, he might pat the heads of water-hydrants and parking-meters, taking there to be the heads of children; he would amiably address carved knobs on the furniture, and be astounded when they did not reply..."
Later, when the examination had ended, Dr. P.
"started to look round for his hat. He reached out his hand and took hold of his wife's head, tried to lift it off, and to put it on. He had apparently mistaken his wife for a hat! His wife looked as if she was used to such things."
Sacks tested Dr. P patient with three dimensional simple geometrical objects --cubes, dodecahedrons, etc. After inspecting them, Dr. P. identified each one correctly. But he had problems with complex, real objects. For example, Sacks handed him a red rose. Dr. P. examined it as though it were a specimen of some strange, unknown object. Dr. P. described this ordinary red rose
"as six inches in length; a convoluted red form with a linear green attachment,"but could not say what it was. Once he had smelled it, he recognized it immediately as a rose. Dr. P was equally baffled by a glove, describing it as "A continuous surface, infolded on itself. It appears to have five outpouchings. A container of some sort. It could be a change purse for coins of five sizes...."