Reading: dissociation of the lexical and phonologic mechanisms

Three distinct reading mechanisms have been proposed: the phonologic conversion system in which letters are converted to their sound equivalents (phonemes) before meaning is established, and two lexical or whole-word mechanisms in which a word (or root morpheme) is identified as a single unit rather than by phonologic conversion. The two lexical mechanisms differ in that one is inextricably linked to semantics but the other is not. The phonologic conversion system may depend more on the perisylvian phonologic system than do the lexical mechanisms, which may be mediated by left parietal or occipital areas or by the right hemisphere. Described here is a patient who had an infarction that partially isolated the perisylvian speech areas and disconnected the partially preserved lexical systems from the phonologic mechanism. He read visually presented orthographically irregular words but could not pronounce orally spelled irregular words. Although he could not read nonwords, he could pronounce orally spelled nonwords and orthographically regular words. These observations suggest that the lexical and phonologic conversion systems are functionally and anatomically distinct.