Personalized beta band HD-tACS over the left SMA improves speech and limb movement by modulating prefrontal delta oscillations in neurotypical young adults

<i>Objective</i>. The supplementary motor area (SMA) demonstrates abnormal beta activity (13-30 Hz) during speech and limb movement tasks in neurological conditions such as Parkinson's disease (PD). Transcranial alternating current stimulation (tACS) has demonstrated promising improvement in motor and non-motor functions by entraining endogenous neural oscillations. We conducted an exploratory study on the modulatory effects of personalized beta high-definition (HD)-tACS over the left SMA on speech production and limb movement.<i>Approach.</i>In a repeated-measures experiment, twenty-two neurotypical young adults were recruited to participate in four stimulation conditions: sham, HD transcranial random noise stimulation (HD-tRNS), and HD-tACS tuned to each individual's frequency of maximal SMA beta activity (identified using source-localized EEG) during speech (tuned-to-speech, TtS) and limb movement (tuned-to-limb, TtL). All participants completed a 25 min sham/active stimulation over the left SMA, followed by an interleaved speech production and limb movement task.<i>Main results</i>. Behavioral results showed that active stimulation resulted in more pronounced improvements in reaction times compared to the sham condition, regardless of the active stimulation type. The neural correlates of this aftereffect were indicated by a prominent modulation in delta power in prefrontal and frontocentral electrodes during speech and limb movement tasks following personalized beta TtS and TtL HD-tACS, relative to sham and tRNS.<i>Significance</i>. Personalized beta HD-tACS modulated delta oscillations, rather than beta rhythms, in a task-specific manner, highlighting the brain's adaptive response. These findings have implications for neurological conditions such as PD, which are characterized by deficits in speech production and limb motor coordination.