Anterior and Posterior Left Inferior Frontal Gyrus Contribute to the Implementation of Grammatical Determiners During Language Production

Byurakn Ishkhanyan,Jesper Mogensen,Gesa Hartwigsen,Violaine Michel Lange,Kasper Boye,Hartwig Roman Siebner,Anke Karabanov

doi:10.3389/fpsyg.2020.00685

Abstract

The left inferior frontal gyrus (IFG) is a key region for language comprehension and production. Previous studies point to a preferential involvement of left anterior IFG (aIFG) in lexical and semantic processes, while the posterior IFG (pIFG) has been implicated in supporting syntactic and phonological processes. Here we used focal neuronavigated transcranial magnetic stimulation (TMS) to probe the functional involvement of left IFG in lexical and grammatical processing at the sentence level. We applied 10 Hz TMS effective or sham bursts to left aIFG and pIFG, while healthy volunteers performed an adjective-noun production task contrasting grammatical and lexical determiners. For each trial, we measured the time from the stimulus onset to the moment of articulation (response time) and the time from articulation onset to the end of articulation (duration). Focal TMS of IFG generally delayed response times. The TMS-induced delay in response times was relatively stronger for the grammatical condition compared to the lexical condition, when TMS targeted aIFG. Articulation of the determiner was generally shorter in trials presenting grammatical determiners relative to lexical determiners. The shorter articulation time for grammar determiners was facilitated by effective TMS to pIFG. Together, the effects of TMS on task performance provide novel evidence for a joint involvement of anterior and posterior parts of left IFG in implementing grammatical determiners during language production, suggesting an involvement of aIFG in the initiation and pIFG in the production of grammatically appropriate verbal responses at the sentence level.

Highlights

The left inferior frontal gyrus (IFG) is a key node of the neural networks engaged in language processing in the human brain and has been subdivided in several areas crucial for different aspects of language processing
Inspection of the group data showed that the interaction was caused by a relative delay in mean logRT during effective transcranial magnetic stimulation (TMS) trials relative to sham TMS trials [main effect of TMS condition: [b = 0.06 ± 0.01, t(4629) = -5.6, p < 0.001], with the delays being stronger when anterior IFG (aIFG) was stimulated, compared to posterior IFG (pIFG) (TMS condition by TMS site interaction: [b = -0.08 ± 0.02, t(4629) = 5.6, p < 0.001] (Figure 3A)
There was no evidence for logRTs being differentially affected by effective TMS over pIFG in the grammatical task relative to the lexical task [ logRT = -0.03 ± 0.01, t(4630) = -2.4, p = 0.15]

Summary

Introduction

The left inferior frontal gyrus (IFG) is a key node of the neural networks engaged in language processing in the human brain and has been subdivided in several areas crucial for different aspects of language processing. Neuroimaging studies have shown that the anterior part of the left IFG [aIFG, pars orbitalis, Brodmann’s area (BA) 47, and pars triangularis, BA 45] is associated with semantic processing both during language comprehension and production while the posterior part (pIFG; pars opercularis BA 44) is engaged in syntactic aspects of language comprehension and production (Caplan et al, 1998; Dapretto and Bookheimer, 1999; Crosson et al, 2001; Gaillard et al, 2003; Hagoort et al, 2004; Haller et al, 2005; Friederici, 2009; Heim et al, 2009a). Damage to the left IFG can cause severe language deficits and is often associated with Broca’s aphasia (e.g., Druks, 2016), Broca’s aphasia does not necessarily include a lesion of the IFG (Dronkers et al, 2004; Fridriksson et al, 2018) and damage to Broca’s area alone is not sufficient to produce Broca’s aphasia (Dronkers, 2000; Mohr, 1976)

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Conclusion