We simultaneously recorded local field potentials (LFPs) in the subthalamic nucleus (STN) and surface electromyographic signals (EMGs) from the extensor and flexor muscles of the contralateral forearm in eight patients with idiopathic tremor-dominant Parkinson's disease (resting tremor) during the bilateral implantation of deep brain stimulation electrodes. Recordings were made at different heights (in 0.5- to 2.0-mm steps beginning outside the STN) using up to five concentrically configured macroelectrodes (2 mm apart). The patients were instructed to relax their contralateral forearm (rest condition). We analysed the coherence between tremor EMGs and STN LFPs, which showed significant tremor-associated coupling at single tremor and double tremor frequencies. Moreover, the EMG-LFP coherences were characterised by differences between antagonistic muscles (flexor, extensor) and by the spatial distribution of LFPs within the STN. Coherence at single and double tremor frequencies occurred significantly more frequently within STN than above STN (in the zona incerta). In this study, we were able to show that, within STN, tremor-associated LFP activity varied with spatial distribution and with the contralateral antagonistic forearm muscles. These findings suggest the existence of distribution- and muscle-specific tremor-associated LFP activity at different tremor frequencies and an organisation of tremor-related subloops within the STN.

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Thalamomuscular coherence in essential tremor (ET) has consistently been detected in numerous neurophysiological studies. Thereby, spatial properties of coherence indicate a differentiated, somatotopic organization; so far, however, little attention has been paid to temporal aspects of this interdependency. Further insight into the relationship between tremor onset and the onset of coherence could pave the way to more efficient deep brain stimulation (DBS) algorithms for tremor. We studied 10 severely affected ET patients (six females, four males) during surgery for DBS-electrode implantation and simultaneously recorded local field potentials (LFPs) and surface electromyographic signals (EMGs) from the extensor and flexor muscles of the contralateral forearm during its elevation. The temporal relationship between the onset of significant wavelet cross spectrum (WCS) and tremor onset was determined. Moreover, we examined the influence of electrode location within one recording depth on this latency and the coincidence of coherence and tremor for depths with strong overall coherence ("tremor clusters") and those without. Data analysis revealed tremor onset occurring 220 Ϯ 460 ms before the start of significant LFP-EMG coherence. Furthermore, we could detect an anterolateral gradient of WCS onset within one recording depth. Finally, the coincidence of tremor and coherence was significantly higher in tremor clusters. We conclude that tremor onset precedes the beginning of coherence. Besides, within one recording depth there is a spread of the tremor signal. This reflects the importance of somatosensory feedback for ET and questions the suitability of thalamomuscular coherence as a biomarker for "closedloop" DBS systems to prevent tremor emergence.

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