Coherence analysis of local field potentials in the subthalamic nucleus: differences in parkinsonian rest and postural tremor

Implantation of electrodes in the subthalamic nucleus (STN) for deep brain stimulation is a well-established method to ameliorate motor symptoms in patients suffering from Parkinson's disease (PD). This study investigated the pathophysiology of rest and postural tremor in PD. In 14 patients with PD, we recorded intraoperatively local field potentials (LFPs) in the STN (at different recording depths) and electromyographic signals (EMGs) of the contralateral forearm. Using coherence analysis we analysed tremor epochs both at rest and hold conditions in patients of the akinetic-rigid or of the tremor-dominant PD subtype. Data analysis revealed significant LFP-EMG coherence during periods of rest and postural tremor. However, strong differences between both tremor types were observed: local maxima (cluster) of rest and postural tremor did not match. Additionally, during rest tremor coherence occurred significantly more frequently at single tremor frequency than at double tremor frequency in tremor-dominant as well as in akinetic-rigid patients. In contrast, during postural tremor in patients with akinetic-rigid PD coherence was predominantly at double tremor frequency. The data suggest a specific topography of 'tremor clusters' for rest and postural tremor. Furthermore, we presume that the same tremor mechanisms exist in patients with tremor-dominant and akinetic-rigid PD, but to different degrees.