Facilitatory Effects of Leftward Prism Adaptation on Verbal Fluency in Japanese Speakers: A Randomized Controlled Trial

Background Prism adaptation (PA) is a classical paradigm known to induce sensorimotor plasticity, and accumulating evidence suggests that it may also influence language networks. In particular, leftward prism adaptation (L-PA) has been proposed to modulate language-related functions through alterations in motor cortical excitability and interhemispheric inhibition. However, its effects on native Japanese speakers remain unclear. Objective This study aimed to investigate the effects of L-PA on performance in phonemic fluency tasks (PFT) and category fluency tasks (CFT) in healthy adults whose native language is Japanese. Methods A randomized controlled trial was conducted on 57 right-handed healthy adults, who were undergraduate or graduate students at Kumamoto Health Science University (Kumamoto, Japan) and volunteered without financial compensation. Participants were assigned to one of three groups using a virtual reality-based prism adaptation system (VRPA): the L-PA group, in which visual space was shifted leftward; the R-PA group, in which visual space was shifted rightward; or the control group, with no visual displacement. Both PFT and CFT were administered before and after the intervention. The dependent variable was the number of correct words generated within one minute. The primary analysis tested the interaction between group (L-PA/R-PA/control) and time (pre-/post-intervention) using split-plot ANOVA. Results In total, nine participants who failed to exhibit an aftereffect were excluded, leaving 48 for analysis. No significant differences were observed among groups at baseline. In the L-PA group, performance significantly improved after the intervention in both PFT (p = 0.0065) and CFT (p = 0.0404). No significant changes were found in the R-PA or control group. Conclusion These findings suggest that L-PA may transiently enhance both phonemic and semantic verbal fluency in Japanese speakers. This study provides preliminary evidence that L-PA can modulate language functions through plasticity of language networks. Future research should address the underlying neural mechanisms, the durability of the effects, and validation in larger clinical trials.