A meta-analysis of NIBS studies on verbal fluency

Non-invasive brain stimulation (NIBS) has emerged as a promising tool to modulate neural circuits involved in language production, yet the magnitude and variability of its effects on VF remain to be systematically evaluated. This meta-analysis aimed to quantify the effects of various NIBS techniques on VF performance and to examine the moderating roles of stimulation parameters, target regions, task domains, population characteristics, and language.

From 1,667 records, 38 studies comprising 91 experimental comparisons met inclusion criteria. Data were extracted on stimulation modality, target site, intensity, duration, task type, and participant demographics. Hedges' g was used to compute effect sizes. Moderator analyses included both meta-regression (for age, gender, stimulation intensity, duration, sessions) and subgroup comparisons (for modality, region, diagnosis, fluency domain, and language).

NIBS had a small-to-moderate overall effect on VF (g = 0.18), with significant heterogeneity. Greater improvements were associated with stimulation over the left M1, left DLPFC, and right IFG. iTBS and tACS showed the strongest effects. Semantic VF showed greater responsiveness than phonemic VF. Clinical groups showed greater improvement than healthy individuals. Significant moderators included stimulation intensity, duration, number of sessions, and participant age.

NIBS can enhance verbal fluency, especially when applied with attention to neural target, modality, dose, and clinical profile. These findings support the design of targeted NIBS protocols for cognitive rehabilitation and highlight key variables to optimize therapeutic efficacy. Future studies with longitudinal follow-up designs are needed to clarify the durability and functional relevance of these effects.