HD-tACS over the left frontal aslant tract entrains theta activity associated with speech motor control

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. 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. 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 the in-phase condition over frontocentral electrodes. Furthermore, the in-phase condition enhanced the connectivity between the left central and frontal electrodes. Overall findings suggest that in-phase theta HD-tACS of FAT enhanced the neural markers of cognitive control required for motor preparation and inhibition during a speech task and have translational implications.