Supplementary motor area: A promising neurostimulation target to improve speech production

The supplementary motor area (SMA) works in concert with several cortical and subcortical regions to produce speech in a timely, accurate, and fluent manner. Despite the SMA's critical role in speech motor control, it has been inadequately addressed in neurostimulation studies, and its exact mechanism remains unexplored. We investigated the SMA's function through the lenses of various theoretical models, as well as lesion and neuroimaging studies. This review synthesizes the current evidence on the potential of SMA-targeted non-invasive neurostimulation techniques, including repetitive transcranial magnetic stimulation (rTMS), continuous theta burst stimulation (cTBS), and high-definition transcranial direct current stimulation (HD-tDCS). Current evidence supports the integrative role of the SMA in coordinating planning-related and sensory-guided aspects of speech motor control and demonstrates that both excitatory and inhibitory stimulation of the SMA modulate speech-related processes, including motor planning, temporal coordination, auditory-vocal integration, and vocal compensation, in healthy individuals and clinical populations, particularly those with Parkinson's disease and stuttering. Collectively, the SMA holds promise for improving speech motor function and motivates future studies using diverse stimulation techniques and speech production tasks to inform therapeutic applications.