Speech motor program maintenance, but not switching, is enhanced by left-hemispheric deep brain stimulation in Parkinson's disease.

Speech reaction time (SRT) was measured in a response priming protocol in 12 participants with Parkinson's disease (PD) and hypokinetic dysarthria "on" and "off" left-hemispheric deep brain stimulation (DBS). Speech preparation was measured during speech motor programming in two randomly ordered speech conditions: speech maintenance and switching. Double blind testing was completed in participants with DBS of globus pallidus pars interna (GPi) (n = 5) or subthalamic nucleus (STN) (n = 7). SRT was significantly faster in the maintenance vs switch task, regardless of DBS state. SRT was faster in the speech maintenance task "on" stimulation, while there was no difference in speech switching "on" and "off" DBS. These data suggest that left-hemispheric DBS may have differential effects on aspects of speech preparation in PD. It is hypothesized that speech maintenance improvements may result from DBS-induced cortical enhancements, while the lack of difference in switching may be related to inhibition deficits mediated by the right-hemisphere. Alternatively, DBS may have little influence on the higher level motor processes (i.e., motor planning) which it is believed the switch task engaged to a greater extent than the maintenance task.