Evidence for an integrated bilingual language system from discourse tasks in aphasia.

Background: The extent to which bilingual individuals represent and process their two languages within a shared or partially distinct neural architecture remains a topic of ongoing debate. While both parallel and divergent patterns of impairment have been reported in bilingual aphasia, such findings likely reflect a spectrum of representational overlap influenced by dominance, proficiency, and task demands. Critically, few studies have examined how breakdown manifests across multiple levels of linguistic structure using ecologically valid, discourse-based tasks. Aims: This study investigates whether Spanish-English bilinguals with aphasia exhibit parallel or dissociable patterns of impairment across their two languages, focusing on naturalistic narrative production and fine-grained analysis of speech error types and code-switching. Methods & Procedures: Thirteen bilingual individuals with aphasia following acquired brain injury produced story retellings in both languages. Speech samples were transcribed and coded for phonological, morphological, syntactic, and semantic errors, and for the word type at which they occurred. Code-switches were also identified and categorized along the same dimensions. Analyses included generalized linear modeling, unsupervised clustering, and supervised machine learning methods. Outcomes & Results: While participants made more errors in their non-dominant than in their dominant language, the structure and distribution of errors were highly similar across languages. Clustering algorithms and supervised classification analyses revealed that impairments were parallel across dominant and non-dominant languages. Code-switching occurred more frequently from the non-dominant to the dominant language, consistent with activation-based lexical selection. Conclusions: Findings support an integrated bilingual language system that spans multiple levels of linguistic representation, modulated by language dominance. Naturalistic discourse tasks allow for richer characterization of bilingual language breakdown and may better inform both theoretical models and clinical management of bilingual aphasia.