1 Department of Medicine, Universidad Autónoma de Barcelona (UAB), Barcelona, Spain.
2 Movement Disorders Unit, Neurology Department, Santa Creu i Sant Pau Hospital, Barcelona, Spain.
3 Sant Pau Biomedical Research Institute (IIB Sant Pau), Barcelona, Spain.
4 Neuropsychology Unit, Neurology Department, Santa Creu i Sant Pau Hospital, Barcelona, Spain.
5 Center for Networked Biomedical Research in Neurodegenerative Diseases (CIBERNED), Instituto de Salud Carlos III, Madrid, Spain.
6 Faculty of Health Sciences, Universitat Oberta de Catalunya (UOC), Barcelona, Spain.
7 Department of Psychology at Concordia University and Centre for Research on Brain, Language & Music in Montreal, Montreal, Canada.

Objective: The effects of stimulation frequency on verbal fluency (VF) following subthalamic nucleus deep brain stimulation (STN-DBS) in Parkinson's disease (PD) are not well understood. The present study examines the impact stimulation frequency has on VF following bilateral STN-DBS in PD.

Methods: Prospective study of 38 consecutive patients with PD with low frequency STN-DBS (LFS) (n = 10) and high frequency STN-DBS (HFS) (n = 14), and a non-operated PD control group consisting of patients with fluctuating response to dopaminergic medication (n = 14) homogeneous in age, education, disease duration, and global cognitive function. Patients were evaluated on VF tasks (letter, semantic, action verbs, alternating). A one-way analysis of variance (ANOVA) was conducted to assess distinctions between groups. Pre- and post-surgical comparisons of fluencies were performed for operated groups. A mixed ANOVA was applied to the data to evaluate the interaction between treatment (HFS vs. LFS) and time (pre- vs. post-surgery). Strategy use (clustering and switching) was evaluated.

Results: Semantic and letter fluency performance revealed significant differences between HFS and LFS groups. Pre- and post-surgical comparisons revealed HFS negatively affected letter, semantic, and action fluencies, but LFS had no effect on VF. No interaction effect or main effect of treatment was found. Main effect of time was significant for semantic and action fluencies indicating a decrease in postoperative fluency performance. Patients with LFS produced larger average cluster sizes than patients with HFS.

Conclusion: LFS may be less detrimental to VF, but these findings suggest that VF decline following STN-DBS is not caused by stimulation frequency alone.