Repeating without semantics: surface dysphasia?
We describe our investigations of MNA, who had a progressive, severe and global loss of semantic knowledge (semantic dementia). Her verbal vocabulary was restricted to a few common words and she was also unable to recognize common objects from sight. By contrast, she had a well-preserved digit span (7-8 digits). In this series of experiments, we focused on her ability to repeat lists and sentences in which familiarity, meaningfulness, morphology and syntactic structure were manipulated. In list repetition tasks, we found that MNA showed a reliable effect of phonological similarity, word frequency and stimulus lexicality, but was unaffected by linguistic complexity, word length, semantic coherence or the status of individual stimuli as "known" or "unknown". In sentence repetition, her performance was not influenced by any semantic variables. However, there was a substantial effect of the frequency of the constituent vocabulary, even for words outside the range of her retained vocabulary. The influence of syntax was restricted to minor effects of morphology. The phonemes of syllables and the syllables of words are bound by their co-occurrence rather than their meaning. We conclude that the phonological representation of words is functionally independent of the semantic system.