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

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.

Two principles of organization in the prefrontal cortex are cognitive hierarchy and degree of automaticity

“…A large body of studies of L1 using English as a test language found enhanced activity in Broca's area (that is, left BA 44 and BA 45 extending to BA 47), suggesting that this area is activated by syntactic manipulations and task demands 33 . However, it has been also postulated that BA 47, usually combined with BA 45, supports the retrieval of semantic information and the processing of semantic relationships between words in sentences 34,35 .…”

“…It has been known that BA 47/45 is connected to the temporal lobe via the ECFS, as a ventral pathway, supporting the processing of semantic information 37 and syntactically simple sentences 33 . As mentioned above, the task of BRAN in L2 may be related to processing semantic information, as well as building up a sentence structure.…”

Nodes and networks in the neural architecture for language: Broca's region and beyond

“…A third position attributes both core language functions to part of Broca's region (though not necessarily specifically to syntax), as well a multiple demand system characteristics to other parts of Broca's region [42]. In the MUC framework [5 ] Broca's region is not language-specific, but realizes its language-relevant unification function in connection to language-relevant areas in temporal and inferior parietal cortex that support the storage of different types of linguistic knowledge [43 ]. Recently, a similar network view has been advocated by .…”

Brain and Language Specialty: Insights from Aphasiology and Neuroimaging

“…Recently, the key role of the left inferior frontal gyrus (Broca's area) in syntactic processing has been questioned on the grounds that studies typically involve task and stimulus demands may activate brain regions that overlap with those involved in linguistic computations, making it difficult to differentiate between linguistic and non-linguistic processes (Tyler et al 2011). As reviewed by Stowe, Haverkort & Zwarts (2005), recent neuroimaging evidence show that the left inferior frontal gyrus plays some role in comprehending sentences when processing is more difficult, but combined with the simple sentence results, they suggest that the left inferior frontal gyrus is not exclusively dedicated for syntactic processing.…”

“…As reviewed by Stowe, Haverkort & Zwarts (2005), recent neuroimaging evidence show that the left inferior frontal gyrus plays some role in comprehending sentences when processing is more difficult, but combined with the simple sentence results, they suggest that the left inferior frontal gyrus is not exclusively dedicated for syntactic processing. More recently, in a functional neuroimaging study of the linguistic performance in patients with left hemisphere damage and healthy participants, Tyler et al (2011) have found that 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. This net involves a variety of brain areas co-activated with the left inferior frontal gyrus, including the right inferior frontal gyrus, bilateral superior temporal gyrus, left middle temporal gyrus and a more posterior temporoparietal cluster including left inferior parietal lobule, left angular gyrus and left supramarginal gyrus.…”