Eusebio et al, 2009:  Dopamine damps oscillations in basal-ganglia cortical loops in patients with Parkinson’s Disease. Patients were stimulated through their deep brain electrodes at different frequencies and the evoked cortical response recorded. When patients were off the dopamine prodrug levodopa, the cortical response was large, particularly during stimulation at 20 Hz. This resonance phenomenon was attenuated when the same experiment was repeated with patients on levodopa.