Why Can't Individuals with Nonfluent Aphasia Produce Sentences? Exploring the Role of Lexical Availability

Nonfluent aphasia is a language disorder characterised by sparse, fragmented speech. Individuals with this disorder often produce single words accurately (for example, they can name pictured objects), but have great difficulty producing sentences. An important research goal is to understand why sentences are so difficult for these individuals. To produce a sentence, a speaker must not only retrieve its lexical elements, but also integrate them into a grammatically well-formed sentence. Indeed, most research to date has focused on this grammatical integration process. However, recent studies suggest that the noun and/or verb content of the sentence can also be an important determinant of success (e.g., Raymer & Kohen, 2006; Speer & Wilshire, 2014). In this thesis, I explore the role of noun availability on sentence production accuracy using an identity priming paradigm. Participants are asked to describe a pictured event using a single sentence (e.g., “The fish is kissing the turtle”). In the critical condition, an auditory prime word is presented just prior to the picture, which is identical to one of the nouns in the target sentence (e.g., fish). The rationale is that the prime will enhance the availability of its counterpart when the person comes to produce the target sentence. Participants were four individuals with mild nonfluent aphasia, two individuals with fluent aphasia, and six older, healthy controls. Consistent with our hypotheses, the nonfluent participants as a group were more accurate at producing sentences when one of its nouns – either the subject or object - was primed in this way. Importantly, in the primed subject noun condition, these results held even when accuracy on the primed element itself was excluded, suggesting it had a broad effect on sentence production accuracy. The primed nouns had no effect on sentence production accuracy for the fluent individuals or the controls. We interpret these findings within models of sentence production that allow for considerable interplay between the processes of lexical content retrieval and sentence structure generation.