Distinct patterns of syntactic errors doubly dissociate in chronic post-stroke aphasia

Background: The lesion correlates of syntactic deficits in aphasia remain poorly understood. Previous studies have suggested that distinct error types in expressive syntax, such as paragrammatic and agrammatic speech, may be associated with damage to different brain regions, but the specific lesion correlates of these errors have not been fully delineated.Objective: To identify the lesion correlates associated with distinct expressive syntactic error types in individuals with chronic post-stroke aphasia, using a novel utterance-level analysis of spontaneous speech errors.Methods: We analyzed spontaneous speech samples from individuals with chronic aphasia, categorizing errors into hierarchical (paragrammatic-like) and linearization (agrammatic-like) errors. Lesion-symptom mapping was conducted to identify brain regions associated with these error types. The analysis was based on a two-stage model of sentence production, with hierarchical processing and linearization analyzed as distinct stages.Results: Lesion clusters in the medial superior temporal sulcus and inferior parietal lobe were associated with hierarchical errors, while large frontal lesions, including those in the inferior and middle frontal lobe, were associated with linearization errors. These results provide support for a two-stage model of syntactic encoding, with distinct neural correlates for hierarchical and linearization processes. Conclusion: Our findings suggest a dichotomy of the lesion basis for syntactic deficits in aphasia, suggesting that distinct brain regions contribute to different stages of syntactic encoding. Hierarchical processing seems to be supported by posterior temporal-parietal regions, while linearization seems to be supported by frontal regions. These results suggest that the diagnosis and treatment of aphasia should take into account the existence of distinct syntactic production deficits resulting from different patterns of brain damage.