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Abstract: Within the connectionist triangle model of reading aloud, interaction between semantic and phonological representations occurs for all words but is particularly important for correct pronunciation of lower-frequency exception words. This framework therefore predicts that (a) semantic dementia, which compromises semantic knowledge, should be accompanied by surface dyslexia, a frequency-modulated deficit in exception word reading; and (b) there should be a significant relationship between the severity of semantic degradation and of surface dyslexia. We evaluated these claims with reference to 100 observations of reading data from 51 cases of semantic dementia. Surface dyslexia was rampant, and a simple composite semantic measure accounted for half of the variance in low-frequency exception word reading. Although in three cases initial testing revealed a moderate semantic impairment but normal exception word reading, all of these became surface dyslexic as their semantic knowledge deteriorated further. The connectionist account attributes such cases to pre-morbid individual variation in semantic reliance for accurate exception word reading. These results provide a striking demonstration of the association between semantic dementia (SD) and surface dyslexia (SD), a phenomenon that we have dubbed SD-squared.

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