Measuring the lexical semantics of picture description in aphasia

Gordon, J. K. (2008). Aphasiology, 22:7, 839-852.

Individuals with non-fluent aphasia have difficulty producing syntactically laden words, such as function words, whereas individuals with fluent aphasia often have difficulty producing semantically specific words. It is hypothesised that such dissociations arise, at least in part, from a trade-off between syntactic and semantic sources of input to lexical retrieval. The aims of this study were (a) to identify quantitative measures of the semantic content of narrative for people with aphasia that are reliable indicators of semantic competence, independent of overall aphasia severity; (b) to determine whether these measures distinguish between fluent and non-fluent aphasia; and (c) to assess whether individuals with fluent and non-fluent aphasia show a trade-off between measures of syntactic and semantic production. The study gathered connected speech samples from 16 participants with aphasia, 8 fluent and 8 non-fluent, and analysed the samples’ semantic sufficiency. The results show some evidence for a trade-off between syntactic and semantic inputs to word retrieval, at least among non-fluent participants. The heavy-light verb ratio provides information about semantic specificity, beyond what is provided by the CIU (correct information unit) or TTR (type-token ratio) measures.