Linguistic mechanisms of coherence in aphasic and non-aphasic discourse

Background: Coherence is the quality that distinguishes discourse from a random collection of sentences. People with aphasia have been reported to produce less-coherent discourse than non-language-impaired speakers. It is largely unclear how coherence is established in natural language and what leads to its impairment in aphasia. Aims: This paper presents a cross-methodological investigation on coherence in the discourse of Russian native speakers with and without aphasia. The purpose of this study was to examine the connection between language impairments in aphasia and different aspects of discourse coherence in order to determine the linguistic mechanisms that could be involved in establishing and maintaining it. Methods & Procedures: Coherence was operationalised as a combination of four aspects: informativeness, clarity, connectedness, and understandability. Twenty participants were asked to retell the content of a short movie. The retellings were annotated using Rhetorical Structure Theory (RST), a formalistic framework for discourse-structure analysis. Next, they were evaluated for coherence on a four-point scale by trained raters. The ratings were compared between groups. A classification analysis was performed to determine whether the ratings could be predicted based on the macrolinguistic variables collected from the RST annotations and several microlinguistic variables previously linked to coherence. Results: Retellings produced by speakers with aphasia received lower ratings than those of control participants on all aspects of coherence. The results indicate that different combinations of microlinguistic and discourse-structure variables play a role in establishing each of the coherence aspects. Conclusions: Our results provided supporting evidence on coherence impairment in aphasia. Perception of a discourse as more or less coherent was associated with both microlinguistic and macrolinguistic variables, with different combinations of variables relevant for each of the aspects. Furthermore, we found that discourse structure plays an important role, especially for understandability. We speculate that pragmatic knowledge shared by interlocutors may boost the coherence of aphasic discourse.