Reading

Reading impairment in AD can be better understood in relation to cognitive neuropsychological models of reading processes. A major feature of these models is the identification of three main routes for reading. First, there is a sublexical route that connects the visual analysis of the word via grapheme to phoneme conversion and is used in learning to read. Second, a lexical route involves accessing the word lexicon, but with direct access to the language output system, bypassing analysis of the semantic system. This allows for the identification of words and pronunciation, but without accessing meaning. Finally, there is input via lexical analysis to the semantic system and then language output.

In AD, the impairment of semantic processing contributes heavily to problems with reading and there is evidence that semantic memory deteriorates before lexical access. For example, in AD, there is more impairment in defining words than in reading comprehension, followed by oral reading of single words. Regular words can be read successfully, but problems occur when attempting to match words to pictures. In AD, there is preservation of reading irregular words, and this is exemplified in the National Adult Reading Test (NART), which is used to estimate the premorbid intelligence of a person with AD. Here, the irregular spellings mean that the grapheme-to-phoneme route cannot be used successfully, with reliance on either the semantic or the lexical route. NART performance is much less affected in AD than performance on other tests that would normally measure intellectual function; thus, it can be used as a premorbid predictor. Nevertheless, with moderate dementia, NART performance does decline and the errors made tend to reflect resorting to using the grapheme-to-phoneme conversion rules to arrive at an incorrect pronunciation. There is also evidence that the ability of AD patients to read NART words can be enhanced by placing them in the context of sentences, as in the Cambridge Contextual Reading Test. This is interpreted as the sentence context either facilitating the reader to identify the word as familiar, which then improves access to the lexical entry, or facilitating the use ofa semantic route, resulting in a greater likelihood of correct reading. The reading of nonwords is impaired in AD, unless reading is done in circumstances in which it is possible to draw on orthographi-cally similar words to construct pronunciation. A decline in nonword reading also follows the severity of dementia, indicating that the sublexical route is compromised in line with the dementing process.

Taken together, the pattern of reading ability in AD indicates that the lexical system shows less marked deterioration, with more reliance on the lexical rather than semantic or sublexical route.

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