HD-tACS Over Left Frontal Aslant Tract Enhances Prefrontal Theta Activity During Action Verbal Fluency in Parkinson’s Disease

While invasive brain stimulation of subcortical structures improves motor symptoms in Parkinson’s disease (PD), its effects on speech and language impairments in PD remain inconsistent. Accumulating evidence supports the application of transcranial alternating current stimulation (tACS) to enhance both motor and non-motor functions in the disorder. We conducted a sham-controlled, multisite 4 Hz HD-tACS over the left frontal aslant tract (FAT) in 17 non-demented patients. Following stimulation, EEG data were recorded while participants performed action fluency (AF), phonemic fluency (PF), and semantic fluency (SF) tasks. Patients generated significantly fewer items in AF compared to PF and SF, regardless of stimulation, suggesting a relative decline in action word retrieval in PD. Relative to sham, the power of prefrontal theta activity significantly increased following actual stimulation for AF; this effect was not observed for PF and SF. Moreover, AF elicited a significant increase in prefrontal theta activity relative to PF and SF following actual stimulation but not sham stimulation. Based on these findings, we propose that stimulation of the left FAT in PD modulates oscillatory markers of word retrieval for motion-related words (i.e., action fluency). This selective modulation of prefrontal theta activity does not indicate a general improvement in cognitive control but rather suggests that stimulation specifically enhances the executive control required for action word retrieval in PD. These results suggest that 4 Hz stimulation of the FAT may serve as a non-invasive protocol to improve action semantics, which is typically compromised in PD.