Oral reading in dementia.

Reading has been thought to consist of three main processing components: the orthographic, phonological, and semantic lexicons. In traditional psycholinguistic models, these components have been treated independently such that the selective dysfunction of one does not necessarily imply the breakdown of another. Recently, it has been proposed that a word's semantic representation is essential to oral reading such that a disturbance within the semantic lexicon will disrupt processing within the orthographic and/or phonological lexicons. From this view, semantic deterioration should lead to fragmentation of the other systems contributing to reading, resulting in a specific pattern of errors during oral reading. This would include (1) a larger than normal advantage for reading words with regular spelling-to-sound correspondence over words with exception spelling, as well as the production of "regularization errors" when reading exception words; and (2) a smaller than normal difference between reading real words and pronounceable nonwords, or pseudowords (PW's). We found that patients with Semantic Dementia generally conformed to these hypothesized patterns of reading difficulty. Despite the presence of a semantic impairment, however, patients with Alzheimer's Disease, Frontotemporal Dementia, and Progressive Non-Fluent Aphasia did not demonstrate these patterns of reading difficulty. Our findings suggest that not all semantic impairments invariably lead to the disruption of the orthographic and phonological lexicons.