Language Tasks and the Network Control Role of the Left Inferior Frontal Gyrus.

Recent work has combined cognitive neuroscience and control theory to make predictions about cognitive control functions. Here, we test a link between whole-brain theories of semantics and the role of the left inferior frontal gyrus (LIFG) in controlled language performance using network control theory, a branch of systems engineering. Specifically, we examined whether two properties of node controllability - boundary and modal controllability - were linked to semantic selection and retrieval on sentence completion and verb generation tasks. We tested whether the controllability of the left IFG moderated language selection and retrieval costs and the effects of continuous theta burst stimulation (cTBS), an inhibitory form of transcranial magnetic stimulation (TMS) on behavior in 41 human subjects (25 active, 16 sham). We predicted that boundary controllability - a measure of the theoretical ability of a node to integrate and segregate brain networks - would be linked to word selection in the contextually-rich sentence completion task. In contrast, we expected that modal controllability - a measure of the theoretical ability of a node to drive the brain into specifically hard-to-reach states - would be linked to retrieval on the low-context verb generation task. Boundary controllability was linked to selection and to the ability of TMS to reduce response latencies on the sentence completion task. In contrast, modal controllability was not linked to performance on the tasks or TMS effects. Overall, our results suggest a link between the network integrating role of the LIFG and selection and the overall semantic demands of sentence completion.Significance StatementOur understanding of language systems and responses to neural stimulation is incomplete. Here, we demonstrate that the effects of neuromodulation (transcranial magnetic stimulation, TMS) on verbal language production are linked to the role of the left inferior frontal gyrus in mediating communication across white matter anatomical networks. We replicate prior findings in weighted anatomical networks, and further identify a link between the role of the LIFG in word selection demands. These findings provide a critical basis to reconcile local and whole brain models of language in the brain.