Transcranial Ultrasound Stimulation Unravels the Causal Role of the Left Inferior Frontal Gyrus in Syntactic Processing

Hierarchical syntactic processing is a fundamental ability for human language processing. Previous studies dispute the left inferior frontal gyrus (LIFG) and the left posterior temporal lobe (LpTL as its possible neural underpinnings, however, direct comparisons of the role of these regions during sentence comprehension are rare. We are among the first to use transcranial ultrasound stimulation (TUS), a non-invasive brain stimulation technique with high precision, to test the causal roles of the different brain regions in processing complex sentences (with embedded relative clauses) and simpler coordinated sentences. Participants completed three lab visits (7 days apart), with LIFG, LpTL, or vertex (control) stimulated each time. Our TUS experiment provides consistent evidence for a causal role of the LIFG in syntactic processing: LIFG TUS significantly decreased d-prime (d’) sensitivity, prolonged reaction times (RTs), and impaired evidence accumulation during decision-making revealed by the hierarchical drift-diffusion modeling. Our findings besides demonstrating the TUS efficacy in sentence-level syntactic processing strengthen the crucial role of LIFG (not LpTL) for hierarchical syntactic processing, thereby deepen the understanding of this core language faculty’s causal neural mechanisms.