Differential effects of deep brain stimulation on verbal fluency

• Verbal fluency is reduced by DBS in the vicinity of the thalamic ventral intermediate nucleus.
• DBS impacts predominantly on left-hemispheric phonemic word processing.
• This is presumably due to current spread to anteromedial thalamic nuclei.
• DBS in the vicinity of the subthalamic nucleus unfolds subtle, opposite effects on phonemic verbal fluency.
• Relations to electrode localization are of conceptual and therapeutic interest.

We aimed at gaining insights into principles of subcortical lexical processing. Therefore, effects of deep brain stimulation (DBS) in different target structures on verbal fluency (VF) were tested.VF was assessed with active vs. inactivated DBS in 13 and 14 patients with DBS in the vicinity of the thalamic ventral intermediate nucleus (VIM) and, respectively, of the subthalamic nucleus (STN). Results were correlated to electrode localizations in postoperative MRI, and compared to those of 12 age-matched healthy controls.Patients’ VF performance was generally below normal. However, while activation of DBS in the vicinity of VIM provoked marked VF decline, it induced subtle phonemic VF enhancement in the vicinity of STN. The effects correlated with electrode localizations in left hemispheric stimulation sites.The results show distinct dependencies of VF on DBS in the vicinity of VIM vs. STN. Particular risks for deterioration occur in patients with relatively ventromedial thalamic electrodes.