Left Frontal HD-tACS 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 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 central and frontal regions in 17 non-demented PD patients. Following stimulation, EEG data were recorded while participants performed action fluency (AF), phonemic fluency (PF), and semantic fluency (SF) tasks. Patients did not show improvement in the number of generated words following stimulation. 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. These findings suggest concurrent stimulation of left central and frontal regions selectively modulates oscillatory markers of word retrieval for motion-related words in PD (i.e., action fluency). This modulation does not indicate a general improvement in cognitive control but may suggest that stimulation specifically enhances the neural mechanisms associated with action word retrieval in PD. These results provide novel insights into the application of a non-invasive brain stimulation protocol to enhance neural activity associated with action verb retrieval, which is typically compromised in PD.