Distinct Patterns of Syntactic Errors Doubly Dissociate in Chronic Poststroke Aphasia

The lesion correlates of syntactic deficits in aphasia remain poorly understood. Previous studies have suggested that distinct error types in expressive syntax may be associated with damage to different brain regions, but the specific lesion correlates of these errors have not been fully delineated. To address this gap, 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. Lesion clusters in the medial superior temporal sulcus and inferior parietal lobe were associated with hierarchical errors, whereas 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. Our findings suggest that distinct brain regions contribute to different stages of syntactic encoding. Hierarchical processing seems to be supported by posterior temporal-parietal regions, whereas linearization seems to be supported by frontal regions. These results suggest that the diagnosis and treatment of aphasia could take into account the existence of distinct syntactic production deficits resulting from different patterns of brain damage.