Deep dyslexia, initially described by Marshall and Newcombe in 1973, is the most extensively investigated of the central dyslexias and, in many respects, the most compelling. Interest in deep dyslexia is due in large part to the intrinsically interesting hallmark of the syndrome—the production of semantic errors. Shown the word ''castle,'' a deep dyslexic may respond ''knight''; shown the word ''bird,'' the patient may respond ''canary.'' At least for some deep dyslexics, it is clear that these errors are not circumlocutions. Semantic errors may represent the most frequent error type in some deep dyslexics, whereas in other patients they comprise a small proportion of reading errors. Deep dyslexics also typically produce frequent ''visual'' errors (e.g., ''skate'' read as ''scale'') and morphological errors in which a prefix or suffix is added, deleted, or substituted (e.g., ''scolded'' read as ''scolds'' and ''governor'' read as ''government'').
Additional features of the syndrome include a greater success in reading words of high compared to low imageability. Thus, words such as ''table,'' ''chair,'' ''ceiling,'' and ''buttercup,'' the referent of which is concrete or imageable, are read more successfully than words such as ''fate,'' ''destiny,'' ''wish,'' and ''universal,'' which denote abstract concepts.
Another characteristic feature of deep dyslexia is a part of speech effect such that nouns are typically read more reliably than modifiers (adjectives and adverbs), which are in turn read more accurately than verbs. Deep dyslexics manifest particular difficulty in the reading of functors (a class of words that includes pronouns, prepositions, conjunctions, and interrogatives including "that," "which," "they," "because," and "under"). The striking nature of the part of speech effect may be illustrated by the patient who correctly read the word "chrysanthemum" but was unable to read the word "the." Most errors to functors involve the substitution of a different functor ("that" read as "which") rather than the production of words of a different class, such as nouns or verbs. Because functors are in general less imageable than nouns, some investigators have claimed that the apparent effect of part of speech is in reality a manifestation of the pervasive imageability effect. There is no consensus on this point because other investigators have suggested that the part of speech effect is observed even if stimuli are matched for imageability.
All deep dyslexics exhibit a substantial impairment in the reading of nonwords. When confronted with letter strings such as "flig" or "churt," deep dyslexics are typically unable to employ print-to-sound correspondences to derive phonology; nonwords frequently elicit "lexicalization" errors (e.g., "flig" read as "flag"), perhaps reflecting a reliance on lexical reading in the absence of access to reliable print-to-sound correspondences.
Finally, it should be noted that the accuracy of oral reading may be determined by context. This is illustrated by the fact that a patient was able to read aloud the word "car" when it was a noun but not when the same letter string was a conjunction. Thus, when presented the sentence ''Le car ralentit car le moteur chauffe'' ("The car slowed down because the motor overheated''), the patient correctly pronounced only the first instance of "car." Recently, three deep dyslexics were demonstrated to read function and content words better in a sentence context than when presented alone.
How can deep dyslexia be accommodated by the information processing model of reading illustrated in Fig. 1? Several alternative explanations have been proposed. Some investigators have argued that the reading of deep dyslexics is mediated by a damaged form of the left hemisphere-based system employed in normal reading. In such a hypothesis, multiple processing deficits must be hypothesized to accommodate the full range of symptoms characteristic of deep dyslexia.
First, the strikingly impaired performance in reading nonwords and other tasks assessing phonologic function suggests that the print-to-sound conversion procedure is disrupted. Second, the presence of semantic errors and the effects of imageability (a variable thought to influence processing at the level of semantics) suggest that these patients also suffer from a semantic impairment. Lastly, the production of visual errors suggests that these patients suffer from impairment in the visual word form system or in the processes mediating access to the visual word form system.
Other investigators have argued that deep dyslexics' reading is mediated by a system not normally used in reading (i.e., the right hemisphere). Finally, citing evidence from functional imaging studies demonstrating that deep dyslexic subjects exhibit increased activation in both the right hemisphere and non-perisylvian areas of the left hemisphere, other investigators have suggested that deep dyslexia reflects the recruitment of both right and left hemisphere processes.
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