Personalized Beta HD-tACS Over the Left SMA Surpasses HD-tDCS in Improving Speech and Limb Reaction Times

Purpose: Beta activity (13-30 Hz) in the supplementary motor area (SMA) plays a crucial role in the planning and execution of speech and limb movements. Abnormal beta oscillations within SMA have been associated with impaired speech and limb movement in neurological conditions wherein speech and limb movements are impaired. Normalizing aberrant neural activity through non-invasive technique such as transcranial direct/alternating current stimulation over SMA demonstrated promising effects in ameliorating motor and non-motor functions. However, these improvements are somewhat limited due to significant between-subject variability, highlighting the need for personalized brain stimulation protocols. This study examined the modulatory effects of personalized beta (13-30 Hz) high-definition transcranial alternating current stimulation (HD-tACS) vs. High-definition transcranial direct current stimulation (HD-tDCS) targeting the left SMA on speech and limb reaction times.Method: In a sham-controlled dual-experiment design, two groups of neurologically intact adult participants underwent multiple stimulation sessions: Experiment 1) sham HD-tACS, HD-tACS tuned to each individual’s frequency of maximal SMA beta activity during speech (tuned-to-speech) or limb (tuned-to-limb), or high-definition transcranial random noise stimulation (HD-tRNS); Experiment 2) sham, anodal or cathodal HD-tDCS. For the experiment 1 source-localized EEG was employed to identify each individual’s peak beta activity within the left SMA.Results: Findings revealed that personalized beta HD-tACS – but not HD-tDCS – over left SMA significantly improved reaction times for both speech and limb movement compared to sham. Moreover, reaction times significantly improved for HD-tACS vs. anodal and cathodal HD-tDCS. However, there was no difference in reaction times between sham HD-tACS and HD-tDCS. Conclusion: Overall, these findings demonstrated superior neuromodulatory effects of personalized beta HD-tACS over HD-tDCS in improving speech and limb reaction times which may have clinical implications for protocols aimed to alleviate neurological conditions.Keywords: Personalized HD-tACS, HD-tDCS, SMA, Speech Production, Limb Movement