Effects of personalized vs. non-personalized neurostimulation protocols in improving speech and limb reaction times.

Abnormal activity within supplementary motor area (SMA) has been associated with impaired speech and limb movement in neurological conditions. Normalizing aberrant neural activity through non-invasive neuromodulation techniques over SMA has demonstrated promising effects in ameliorating motor and non-motor functions. However, there is limited research on the application of transcranial electrical stimulation (tES) over the left SMA as a potential non-invasive protocol to improve speech production. In this study, we examined the effects of several tES protocols, including high-definition transcranial alternating current stimulation (HD-tACS), transcranial random noise stimulation (tRNS), and direct current stimulation (HD-tDCS), targeting the left SMA in neurotypical adults, on speech and limb reaction times. 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 (15-30 Hz) during speech (tuned-to-speech) or limb (tuned-to-limb), or HD-tRNS; Experiment 2) sham, anodal or cathodal HD-tDCS. Following the stimulation, the participants were instructed to perform a speech-limb interleaved task. Personalized beta HD-tACS and HD-tRNS - but not HD-tDCS - over the left SMA significantly improved reaction times for both speech and limb movement compared to sham. There was no difference in reaction times between HD-tACS and HD-tRNS for either speech or limb movement. These findings demonstrate comparable neuromodulatory effects of HD-tACS and HD-tRNS in improving speech and limb reaction times in younger adults. This study is exploratory and warrants replication with a larger sample within a single-group design.