Left inferior frontal cortex and syntax: function, structure and behaviour in patients with left hemisphere damage

TLDR

Neurobiological models of language have long debated the role of key brain regions, particularly the left inferior frontal gyrus, in syntactic processing. This study examined whether the left inferior frontal gyrus is essential for syntactic processing by combining functional activity, grey matter integrity, and behavioural performance data from patients with left hemisphere damage and healthy controls. Participants underwent functional neuroimaging while listening to spoken sentences that were either syntactically ambiguous or unambiguous, and behavioural performance on three syntactic tests was subsequently collected. Controls showed co‑activation of left Brodmann areas 45/47 and posterior middle temporal gyrus during syntactic processing, with left parietal activity reflecting working‑memory demands; voxel‑based correlational analyses revealed that tissue integrity and neural activity in left Brodmann area 45 and posterior middle temporal gyrus correlated with preserved syntactic performance, whereas patients were insensitive to syntactic preferences, indicating a syntactic deficit and underscoring the essential role of the left inferior frontal gyrus within a network that includes left Brodmann area 45 and posterior middle temporal gyrus.

Abstract

For the past 150 years, neurobiological models of language have debated the role of key brain regions in language function. One consistently debated set of issues concern the role of the left inferior frontal gyrus in syntactic processing. Here we combine measures of functional activity, grey matter integrity and performance in patients with left hemisphere damage and healthy participants to ask whether the left inferior frontal gyrus is essential for syntactic processing. In a functional neuroimaging study, participants listened to spoken sentences that either contained a syntactically ambiguous or matched unambiguous phrase. Behavioural data on three tests of syntactic processing were subsequently collected. In controls, syntactic processing co-activated left hemisphere Brodmann areas 45/47 and posterior middle temporal gyrus. Activity in a left parietal cluster was sensitive to working memory demands in both patients and controls. Exploiting the variability in lesion location and performance in the patients, voxel-based correlational analyses showed that tissue integrity and neural activity—primarily in left Brodmann area 45 and posterior middle temporal gyrus—were correlated with preserved syntactic performance, but unlike the controls, patients were insensitive to syntactic preferences, reflecting their syntactic deficit. These results argue for the essential contribution of the left inferior frontal gyrus in syntactic analysis and highlight the functional relationship between left Brodmann area 45 and the left posterior middle temporal gyrus, suggesting that when this relationship breaks down, through damage to either region or to the connections between them, syntactic processing is impaired. On this view, the left inferior frontal gyrus may not itself be specialized for syntactic processing, but plays an essential role in the neural network that carries out syntactic computations.