First described in 1979, phonologic dyslexia is perhaps the "purest" of the central dyslexias in that, at least by some accounts, the syndrome is attributable to a selective deficit in the procedure mediating the translation from print to sound. Thus, although in many respects less arresting than deep dyslexia, phonological dyslexia is of considerable theoretical interest.
Phonologic dyslexia is a disorder in which reading of real words may be nearly intact or only mildly impaired. Patients with this disorder, for example, correctly read 85-95% of real words. Some patients read all different types of words with equal facility, whereas other patients are relatively impaired in the reading of functors (or "little words"). Unlike patients with surface dyslexia described later, the regularity of print-to-sound correspondences is not relevant to their performance; thus, phonologic dyslexics are as likely to correctly pronounce orthographically irregular words such as "colonel" as words with standard print-to-sound correspondences such as "administer." Most errors in response to real words bear a visual similarity to the target word (e.g., "topple" read as ''table'').
The striking and theoretically relevant aspect of the performance of phonologic dyslexics is a substantial impairment in the oral reading of nonword letter strings. We have examined patients with this disorder, for example, who read > 90% of real words of all types but correctly pronounce only approximately 10% of nonwords. Most errors to nonwords involve the substitution of a visually similar real word (e.g., ''phope'' read as ''phone'') or the incorrect application of print-to-sound correspondences [e.g., ''stime'' read as ''stim'' (to rhyme with ''him'')].
Within the context of the reading model depicted in Fig. 1, the account for this disorder is relatively straightforward. Good performance with real words suggests that the processes involved in normal lexical reading (i.e., visual analysis, the visual word form system, semantics, and the phonological output lexicon) are at least relatively preserved. The impairment in nonword reading suggests that the print-to-sound translation procedure is disrupted.
Recent explorations of the processes involved in nonword reading have identified many distinct procedures involved in this task. If these distinct procedures may be selectively impaired by brain injury, one might expect to observe different subtypes of phonologic dyslexia. Although the details are beyond the scope of this article, there is evidence suggesting that different subtypes of phonologic dyslexia may be observed.
Lastly, it should be noted that several investigators have suggested that phonologic dyslexia is not attributable to a disruption of a reading-specific component of the cognitive architecture but, rather, to a more general phonologic deficit. Support for this assertion comes from the observation that the vast majority of phonologic dyslexics are impaired on a wide variety of nonreading tasks assessing phonology.
In certain respects, phonologic dyslexia is similar to deep dyslexia, the critical difference being that semantic errors are not observed in phonologic dyslexia. Citing the similarity of reading performance and the fact that deep dyslexics may evolve into phonologic dyslexics as they improve, it has been argued that deep and phonologic dyslexia are on a continuum of severity.
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