Walsh & Hoyt: Central Dyslexias

Further types of more subtle acquired dyslexic deficits have been described. Many of these occur in association with other aphasic features and so might be classified as aphasic alexias; however, they occasionally occur as isolated dyslexias also. These reading defects are sometimes labeled central dyslexias, as they reflect dysfunction of central reading processes rather than ""peripheral"" attentional or visual processes. Central dyslexias are formulated in terms of reading models derived from cognitive neuropsychology. One of the main concepts involved is that of parallel information processing in reading. The perception of visual features and the abstraction of letter identity are followed by at least two distinct modes of processing. One route is a ""direct"" phonologic process, in which the units of a letter string are converted into units of sound (graphemephoneme correspondence), using the generic pronunciation rules of a given language. These components are then assembled into the pronounced whole word, found in an internal dictionary of word sounds (phonologic lexicon) and linked to information about word meaning in a semantic lexicon. Another route is an ""indirect"" lexical one, in which the whole word is perceived and identified in an internal dictionary of written words (orthographic lexicon): when the correct word form is activated, a corresponding entry in the semantic lexicon is also activated, leading in turn to access to the phonologic lexicon and the pronunciation of the word. A possible third ""rule-based"" phonologic route for pronunciation of non-words and unfamiliar words is postulated but controversial. The anatomic correlates of these different lexical and phonologic reading routes are still uncertain.