How damaged brains repeat words: a computational approach.

Two routes have been proposed for auditory repetition: a lexical route which activates a lexical item and retrieves its phonology, and a nonlexical route which maps input phonology directly onto output phonology. But when is the nonlexical route recruited? In a sample of 103 aphasic patients, we use computational models to select patients who do and do not recruit the nonlexical route, and compare them in light of three hypotheses: 1 - Lexical-phonological hypothesis: when the lexical route is weak, the nonlexical route is recruited. 2 - Nonlexical hypothesis: when the nonlexical route is weak, it is abandoned. 3 - Semantic-access hypothesis: when access to meaning fails, the nonlexical route is recruited. In neurocognitive terms, hypotheses 1 and 2 identify different aspects of the intactness of the dorsal stream, while the third hypothesis focuses on the ventral stream. Our findings (and a subsequent meta-analysis of four studies) support hypotheses 2 and 3. Ultimately, we claim that the choice about whether to recruit the nonlexical route is guided, not by assessment of production abilities that support repetition, but instead by relying on accessible cues, namely whether the speaker understands the word, or can remember its sequence of phonemes.