Measuring the lexical semantics of picture description in aphasia
AbstractBackground : 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. Aims : 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. Methods & Procedures : Connected speech samples were elicited from 16 participants with aphasia, 8 fluent and 8 non-fluent. The semantic sufficiency of the samples was analysed by measuring the proportion of correct information units (CIUs), the type-token ratios (TTRs) of content words, and the proportion of semantically specific (“heavy”) to semantically general (“light”) verbs produced. These measures were then correlated with syntactic measures from the QPA (Berndt, Wayland, Rochon, Saffran, & Schwartz, 2000) across and within participant groups. Outcomes & Results : CIUs were found to reflect primarily aphasia severity, and not to differentiate between fluent and non-fluent groups. TTRs were also strongly influenced by severity among fluent, but not non-fluent, participants. The ratio of heavy to light verbs reliably distinguished the groups, and showed different patterns of correlation with the syntactic measures. Conclusions: 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 or TTR measures.