Phase and Task Dependent Effects of Theta HD-tACS Over the Frontal Aslant Tract on Speech Production

Clinical and experimental evidence supports the role of Frontal Aslant tract (FAT) in fluent speech. However, there is limited research on developing non-invasive brain stimulation protocols that could improve speech motor control deficits caused by disruption in the FAT. In a recent study (Johari & Tabari, 2025), we showed that stimulation of FAT improves the preparatory and inhibitory mechanisms of simple vocalization task by modulating prefrontal theta activity in neurologically intact adults. In the present study, we aimed to examine whether 4 Hz HD-tACS over the left FAT would differentially affect neural and behavioral correlates of a more complex speech task (i.e., syllable repetition varying in length). Behavioral results did not show an effect of stimulation, perhaps due to limited room for improvement in young neurotypical adults. However, neural results showed an increase in the power of theta activity following anti-phase stimulation relative to sham and in-phase conditions. The findings were in contrast with our recent study indicating the effects of theta HD-tACS over left FAT on speech motor control is phase- and task- dependent. These results highlight the sensitivity of speech motor networks to task demand and the phase of stimulation