Semantic impairment with and without surface dyslexia: Implications for models of reading

Abstract

The two best-developed computational models of reading aloud, the DRC model of Coltheart and colleagues and the connectionist attractor model of Plaut and colleagues, offer very different views about the degree to which semantic knowledge is involved in lexical processing, and hence make differing predictions about how semantic impairment (as seen, for example, in semantic dementia) will impact on lexical processing in clinical cases. Two cases meeting the criteria for semantic dementia, PC and EM, were given a battery of tests comprising comprehension tasks, a reading task, and a visual word recognition (lexical decision) task. All tasks used the same target words allowing cross-test and cross-patient comparisons. Both cases showed significant impairment of semantic memory, and word comprehension was found to be related to the word frequency of the target words. PC demonstrated poor reading of irregular words, with a surface dyslexic pattern of reading aloud, and he performed poorly on the visual lexical decision task. His ability to read irregular words was related to their frequency and to his ability to comprehend them. In contrast, his visual lexical decision performance was not reliably influenced by his comprehension of the same words or by their frequency. EM demonstrated essentially perfect reading aloud of irregular words and essentially perfect visual lexical decision, despite her severe semantic impairment. The pattern of performance shown by EM is consistent with the DRC model of reading, but inconsistent with the connectionist attractor model and with the view, associated with that model, that orthographic and phonological processes cannot remain intact when semantic representations are degraded.

Acknowledgments

The first author would like to thank PC and EM for their patience and stamina, and participation in ongoing research. Thanks also goes to Dr Alan Taylor for statistical advice, and Dr Matthew Dobson for helpful comments regarding this research and data management support. Finally, the first author wishes thank Dr David Basic for his referral of EM, and the Aged Care Unit, Liverpool Hospital, Sydney, Australia for their support of this research.

Notes

Our emphasis.

That is not to say that in normal circumstances the semantic system cannot influence lexical decision according to the DRC model: On the contrary, feedback from the semantic system to the phonological or orthographic lexicon is viewed as responsible for semantic priming of auditory or visual lexical decision, and for the explanation of imageability effects upon lexical decision performance.

Since the completion of our study, an additional study of visual lexical decision in semantic dementia has been reported: Rogers, Lambon Ralph, Hodges, and Patterson (2004). As discussed by Coltheart (2004), two of the 22 patients of this study, MG and LS, performed within the normal range on visual lexical decision.

This was in case any incorrect translation from print to speech interfered with the visual lexical decision task, and also to give the patients the best chance to avoid false positives to the pseudohomophones.

One might have expected PC to seek to perform this task by translating orthography to phonology on those trials where neither item in a pair succeeded in activating an entry in the orthographic lexicon. This should produce better lexical decision performance with regular than with irregular words, and indeed there was a clear but not significant trend in this direction: 81% correct vs. 69.8% correct.

We are assuming here, in common with virtually all theorists interested in modelling the object and word comprehension system, that there is a single systems of semantic representations used for understanding printed words, spoken words, seen objects, and pictures.