The effect of lexical content on sentence production in nonfluent aphasia

Individuals with nonfluent aphasia are able to produce many words in isolation, but have great difficulty producing sentences. Most research to date has compared accuracy across different types of sentence structures, focussing on grammatical aspects that may be compromised in nonfluent aphasia. However, based on the premise that lexical elements activate their associated grammatical frames as well as vice versa, lexical content may also be of vital importance. For example, rapid access to lexical elements – particularly ones appearing early in the sentence - may be crucial, especially if the sentence plan is weakly activated or rapidly decaying. The current study investigated the effect of different aspects of lexical content on nonfluent aphasic sentence production. Five participants with nonfluent aphasia, four participants with fluent aphasia and eight controls completed two picture description tasks eliciting subject-verb-object sentences (e.g., the dog is chasing the fox). Based on existing evidence suggesting that common words are accessed more rapidly than rarer ones, Experiment 1 manipulated the frequency of sentence nouns, thereby varying their speed of lexical retrieval by varying the frequency of sentence nouns. Nonfluent participants' accuracy was consistently higher for sentences commencing with a high frequency subject noun, even when errors on those nouns were themselves excluded. This was not the case for the fluent participants. Experiment 2 manipulated the semantic relationship between subject and object nouns. Previous research suggests that phrases containing related words may be challenging for individuals with nonfluent aphasia, possibly because lexical representations are inadequately tied to appropriate structural representations. The nonfluent participants produced sentences less accurately when they contained related lexical items, even when those items were in different noun phrases. The fluent participants exhibited the opposite trend. Finally, the relationship between the patterns observed in Experiment 1 and 2 and lesion location in the aphasic participants was explored by analysing magnetic resonance scans. We discuss the implications of our findings for theoretical accounts of sentence production more generally, and of nonfluent aphasia in particular. More precisely, we propose that individuals with nonfluent aphasia are disproportionately reliant on activated lexical representations to drive the sentence generation process, an idea we call the Content Drives Structure (COST) hypothesis.