To move or not to move: Subthalamic deep brain stimulation effects on implicit motor simulation

We explored implicit motor simulation processes in Parkinson's Disease (PD) patients with ON-OFF subthalamic deep brain stimulation (DBS) of the sub-thalamic nucleus (STN). Participants made lexical decisions about hand action-related verbs, abstract verbs, and pseudowords presented either within a positive (e.g., "Do.") or a negative (e.g., "Don't.") sentence context. Healthy controls showed significantly slower responses for hand-action verbs (vs. abstract verbs) in the negative (vs. positive) context, which suggests that negative contexts may suppress motor simulation or preparation processes. The STN-DBS improves cortical motor functions, thus patients are expected to perform at the same level as unimpaired subjects in the ON condition. By contrast, the 50% reduced DBS is expected to result in a reduced activation for motor information, which in turn might cause a reduced, if not absent, context modulation. PD patients exhibited the same pattern as controls when their DBS was at 100% ON; however, reducing the DBS to 50% had a deleterious outcome on the positive faster than negative context effect, suggesting that the altered inhibition mechanism in PD could be responsible for the missed effect. In addition, our results confirm the view that implicit motor simulation mechanisms behind action-related verb processing are flexible and context-dependent.