Non-Invasive Brain Stimulation Improves Word Retrieval Deficits in Parkinson’s Disease: A Meta Analysis of Clinical Trials

Background: Verbal fluency (VF) requires both linguistic processing and executive control to facilitate word retrieval. VF is an integral component of neuropsychological assessment in clinical settings to diagnose neurological conditions. Non-invasive brain stimulation (NiBS) techniques have shown potential modulatory effects on neural substrates of VF, though findings have been mixed. Objectives: This meta-analysis aimed to consolidate the results of randomized controlled trials (RCTs) that investigated the effects of various NiBS on VF in individuals with Parkinson’s disease (PD), focusing on the target, modality, and fluency types (i.e., semantic or phonemic VF). Method: A total of 1126 studies were retrieved from academic databases comprising PubMed, MEDLINE, EMBASE, SCOPUS, and Cochrane Library. After comprehensive screening of 626 studies involving PD participants who underwent one form of NiBS, including repetitive transcranial magnetic stimulation (rTMS), transcranial direct stimulation (tDCS), and random noise stimulation (tRNS), 7 RCTs with 105 PD patients (mean age: 66.40 ± 8.03) met inclusion criteria and were included in our final analysis. Results: tDCS applied over the left dorsolateral prefrontal cortex (DLPFC) yielded the most consistent improvements in both semantic and phonemic VF. rTMS and tRNS showed minimal and null effects. Phonemic VF showed immediate but short-lived enhancement, while semantic VF showed a gradual but sustained improvement. Conclusion: This meta-analysis bolsters the application of tDCS, particularly over the DLPFC, as a non-invasive protocol to improve word retrieval deficits in PD.