STN, subthalamic nucleus; PD, Parkinson's disease; FOG, freezing of gait; EEG, electroencephalography; DBS, deep brain stimulation; LFP, local field potential; UPDRS, unified Parkinson's disease rating scale; CT, computerized tomography; MR, magnetic resonance; SMA, supplementary motor area; PPN, pedunculopontine nucleus; SPECT, photon emission tomography; HM-PAO, technetium-99m-hexamethyl-propyleneamine oxime