RESUMO
Background: Contemporary models of aphasia predominantly attribute lexical retrieval deficits to impaired access and/or maintenance of semantic, lexical, and phonological representations of words. A central hypothesis of language-emergent models of verbal short-term memory (STM) is that temporary storage and maintenance of verbal information arises from activation of linguistic representations in long-term memory. This close relationship between short-term retention and linguistic representations has prompted accounts of aphasia that include impairments to both these components. Aims: We investigated associations between measures of input semantic and phonological verbal STM and corresponding output processing measures. We hypothesised that both input and output functions of verbal STM rely on a common substrate (i.e., temporary activation and maintenance of long-term linguistic representations). Methods & Procedure: Twenty adults with aphasia completed a series of semantic and phonological probe spans. Results were compared with naming performance in immediate and delayed conditions. The data were analysed using correlations, principal components analysis and linear regressions. Results & Discussion: Input semantic and phonological verbal STM abilities were predictive of naming accuracy. Greater input semantic and phonological STM spans were associated with fewer semantic and phonological naming errors. Latent factors identified by principal components analysis of probe span data were consistent with a two-step interactive model of word retrieval. Probe spans measuring access to semantic and initial consonant-vowel representations aligned with lexical-semantic abilities (lexical-semantic factor). Probe spans assessing access to the rhyme component of a word measured lexical-phonological abilities (lexical-phonological factor). The principal components analysis indicated that stronger lexical-semantic abilities were associated with fewer semantic and nonword errors, and stronger lexical phonological abilities were associated with fewer formal and unrelated errors. In addition, our results were consistent with models that postulate serial access to phonology, proceeding from initial to final phonemes. The span measuring access to initial consonant-vowel was associated with lexical selection, while the span measuring access to rhyme information was associated with phonological selection. Conclusion: Performance on input semantic and phonological tasks predicts accuracy of picture naming performance and types of errors made by people with aphasia. These results indicate overlap in input and output semantic and phonological processing, which must be accounted for in models of lexical processing. These findings also have implications for approaches to diagnosis and treatments for lexical comprehension and production that capitalise on the overlap of input and output processing.
RESUMO
Aphasia has had a profound influence on our understanding of how language is instantiated within the human brain. Historically, aphasia has yielded an in vivo model for elucidating the effects of impaired lexical-semantic access on language comprehension and production. Aphasiology has focused intensively on single word dissociations. Yet, less is known about the integrity of combinatorial semantic processes required to construct well-formed narratives. Here we addressed the question of how controlled lexical-semantic retrieval deficits (a hallmark of aphasia) might compound over the course of longer narratives. We specifically examined word-by-word flow of taxonomic vs. thematic semantic distance in the storytelling narratives of individuals with chronic post-stroke aphasia (n = 259) relative to age-matched controls (n = 203). We first parsed raw transcribed narratives into content words and computed inter-word semantic distances for every running pair of words in each narrative (N = 232,490 word transitions). The narratives of people with aphasia showed significant reductions in taxonomic and thematic semantic distance relative to controls. Both distance metrics were strongly predictive of offline measures of semantic impairment and aphasia severity. Since individuals with aphasia often exhibit perseverative language output (i.e., repetitions), we performed additional analyses with repetitions excluded. When repetitions were excluded, group differences in semantic distances persisted and thematic distance was still predictive of semantic impairment, although some findings changed. These results demonstrate the cumulative impact of deficits in controlled word retrieval over the course of narrative production. We discuss the nature of semantic flow between words as a novel metric of characterizing discourse and elucidating the nature of lexical-semantic access impairment in aphasia at multiword levels.