HD-tACS over the Left Frontal Aslant Tract Entrains Theta Activity Associated with Speech Motor Control

Objective: Transient disruption or permanent damage to the left Frontal Aslant Tract (FAT) is associated with deficits in speech production. The present study examined the application of theta (4 Hz) high-definition transcranial alternating current stimulation (HD-tACS) over the left SMA and IFG – as a part of FAT- as a potential multisite protocol to modulate neural and behavioral correlates of speech motor control.Methods: Twenty-one young adults participated in three counterbalanced sessions in which they received in-phase, anti-phase and sham theta HD-tACS. In each session, 4 Hz stimulation was applied over the left IFG and SMA, and subsequently EEG data was recorded while participants performed a speech Go/No-Go task.Results: Relative to sham and anti-phase, in-phase HD-tACS significantly improved speech reaction time. Neural data showed an increase in the power of frontal theta activity prior to speech initiation for the in-phase condition compared to sham. Moreover, in-phase stimulation increased the phase synchrony of theta activity between the left central and frontal electrodes. For speech inhibition, the power of theta activity increased following in-phase condition over frontocentral electrodes. Furthermore, the in-phase condition enhanced the connectivity between the left central and frontal electrodes.Conclusion: Overall findings suggest that in-phase 4Hz stimulation of left SMA and IFG entrained the power of theta activity and enhanced the connectivity associated with speech motor control.Significance: Multi-site theta HD-tACS hold a promise to improve speech motor control deficits in neurogenic communication disorders such as apraxia of speech and stuttering where the integrity of FAT is compromised.