Personalized Beta Band HD-tACS Improves Speech Reaction Times by Modulating Low-Frequency Prefrontal Oscillations

Abstract

<p>Abnormal beta activity (15-30 Hz) within the supplementary motor area (SMA) has been associated with speech impairments in neurological conditions such as Parkinson’s disease (PD). Therefore, stimulation of the SMA targeted toward normalizing beta band oscillations may improve speech production in neurological disorders. Transcranial Alternating Current Stimulation (tACS) demonstrated promising improvement in motor and non-motor functions by entraining endogenous neural oscillations associated with a specific task. However, these improvements are somewhat limited due to significant variability between subjects, highlighting the importance of personalized protocols. Furthermore, in general the effects of personalized protocols on speech remain understudied. We conducted an exploratory study on the modulatory effects of personalized beta high-definition transcranial alternating current stimulation (HD-tACS) over the left SMA on speech production. In a sham-controlled dual-experiment design, 22 neurotypical young adults (mean age = 19.875 ±1.541 years; 5 males) were recruited to participate in an experiment comprised of 3 stimulation sessions: sham HD-tACS, high-definition transcranial random noise stimulation (HD-tRNS; untuned), and HD-tACS tuned to each individual’s frequency of maximal SMA beta activity during speech (tuned-to-speech) production. Using a repeated-measures approach, all participants completed a 25-minute sham/active stimulation session over left SMA, followed by a speech production in which participants were instructed to vocalize vowels in response to visual cues on the screen. Behavioral results showed that active stimulation resulted in more pronounced improvements in speech reaction times than sham condition, regardless of the active stimulation type; however, no significant differences were found between “tuned-to-speech” and “untuned” stimulation. EEG data revealed that beta HD-tACS tuned-to-speech over the left SMA induced significant delta (1-3 Hz) desynchronization over the prefrontal electrodes during speech production, compared to other stimulation conditions. We speculate that personalized beta HD-tACS of left SMA accelerates reaction times by modulating slow prefrontal oscillations associated with cognitive control during speech production.</p>