The contemporary study of acquired dyslexias has largely focused on impairments in the ability to read single words aloud. One model of the mechanisms involved in reading a word aloud claims that there are three separate and partially independent routines in the brain for converting a written word into its spoken form: The first pathway (the semantic route) involves recognizing a word visually, gaining access to its meaning, and then activating the sound of the word from its meaning. The second pathway (the whole word route) translates the orthography of the entire word directly into a pronunciation, without first contacting the word's meaning. Finally, a third pathway (the sublexical route) decomposes the word into orthographic segments (graphemes and other spelling units) and derives a pronunciation by assigning each of them a spoken (phonemic) value. Other models of the reading-aloud process combine the second and third processes into one highly interactive system.
Brain damage can selectively affect a particular routine without impairing the function of the remaining branches of the system. Certain patients (phonological alexics) lose the operation of the sublexical route, which acts on subword units to assemble a response, leaving the semantic and whole word routes available. A patient with damage to the sublexical route is impaired in the reading aloud of nonwords, whereas legitimate words are mostly read correctly. Other reading disorders appear to be the outcome of severe damage to both the whole word route and the sublexical route. Patients with this pattern or impairment, termed deep dyslexics, are unable to pronounce written nonwords (indicating severe impairment to the sublexical route) but also make semantic paralexias (e.g., they read "chair" as "table") and are poor at reading abstract words aloud relative to concrete words. The failure to read nonwords combined with the presence of semantic errors and the influence of a semantic variable on the ability to read a written word aloud is consistent with the interpretation that the patient has lost the use of both the routines from whole words and subword units to sound and is forced to use a defective routine from the visual description of the word through meaning to pronunciation. Yet other dyslexic readers (surface dyslexics) have lost the ability to use both the semantically mediated reading route and the whole-word route but retain the translation of subword units into sound (the sublexical route). These patients can still read words aloud that obey regular correspondences between spelling and sound (e.g., "hint," "mint," and "stint"), but not those that do not (e.g., "pint"). Finally, some patients with dementia show the ability to use the whole-word reading route without the benefit of semantics. These patients can read irregularly spelled words correctly that they do not understand.
The fact that a patient has difficulties in reading words and/or nonwords aloud does not necessarily imply that he or she does not recognize or understand a printed word. Some patients have a severe disturbance of reading aloud known as "letter-by-letter reading" because they name each letter in a word before attempting (often with incomplete success) to pronounce the word. These patients take longer to read longer words, and it can take them many seconds to pronounce a printed word. Several of these patients have been tested for their abilities to recognize and comprehend words that are presented for short periods of time—far less than the time needed for them to read the words aloud. One such patient could recognize words and familiar letter strings as visual patterns, and other letter-by-letter readers can extract at least some semantic information from briefly presented words. Most of these patients denied that they were able to recognize or understand these briefly presented words. These performances suggest that some reading problems arise after words have been recognized, and also that some alexic subjects, such as the patients with disturbances affecting auditory-oral processing described previously, may retain abilities to recognize and understand words without being aware that they do so.
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