Causal Contributions and Interhemispheric Interactions of the Left and Right Supramarginal Gyri in Vocal Feedback Control: Insights From Dual-Site Transcranial Magnetic Stimulation.

The bilateral supramarginal gyri (SMGs) have been implicated in sensorimotor control of speech production, yet their precise roles and interhemispheric interactions are poorly understood. This event-related potential study employed dual-site continuous theta burst stimulation (c-TBS) over the bilateral SMGs simultaneously to investigate their functional dynamics in vocal motor control. Following unilateral and bilateral c-TBS over the SMG as well as sham stimulation, participants vocalized the vowel sounds while exposed to unexpected pitch perturbations in auditory feedback. Unilateral real c-TBS paired with contralateral sham stimulation led to reduced vocal compensation magnitudes and latencies and decreased P2 responses compared to bilateral sham stimulation, with no differences between left and right SMG stimulation. Source localization revealed that decreased P2 responses following left SMG stimulation localized to left-lateralized dorsolateral prefrontal cortex, supplementary motor area, SMG, middle temporal gyrus, and temporo-parietal junction, whereas such decreases following right SMG stimulation involved left-lateralized primary motor cortex, premotor cortex, and middle temporal gyrus. These findings suggest that both SMGs are causally involved in vocal feedback control through distinct but interconnected networks. Surprisingly, dual-site c-TBS over the bilateral SMG did not alter vocal compensation or cortical activity, suggesting an interhemispheric balancing mechanism for fine-tuning vocal production. Our results offer novel insights into the bihemispheric coordination of auditory-vocal integration, highlighting potential treatment for speech disorders by modulating interhemispheric interactions.