There are different parts of the brain specialised for different functions such as discriminating sounds, decoding letters, producing speech, reading fluency,  passage comprehension, integrating visual and audio...and they all have to work together. (Prof S Dehaene)

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Problems can occur in any area, as well as in the connections between them.

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 As well, while speech a "natural" for humans, reading is not, so we have to use parts of the brain designed for other things, to help us read.