Excessive synchronization of basal ganglia neuronal activity at ~20 Hz is characteristic of patients with untreated Parkinson's disease (PD). Correlative evidence suggests that this activity may contribute to bradykinesia. Attempts to demonstrate causality through stimulation imposed synchronization at 20 Hz in the region of the subthalamic nucleus (STN) have had limited success. Finger-tapping is slowed by about 8% and only in those PD patients that have a relatively normal baseline performance in this task. Here we investigate whether greater performance decrements might be seen in a reaction time grip task. We studied 32 sides in 16 patients with PD after overnight withdrawal of medication. Patients were asked to grip as hard and as fast as possible without STN stimulation and during bilateral stimulation at 5 Hz, 10 Hz, 20 Hz, 50 Hz and 130 Hz. Stimulation at 20 Hz slowed the development of force by 14.7±8.3% (P=0.044) across all patients. Slowing increased by 22±7% (P=0.005) in those patients with the best performance in the task without stimulation. The effect was frequency specific. These data provide direct interventional evidence of a mechanistic link between excessive neuronal synchronization in the beta range and motor impairment in PD.
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