Task-related high-frequency synchrony in the subthalamic nucleus (STN) during precision grip

The STN is a pivotal element in the neuronal network exhibiting abnormal firing patterns in Parkinson's disease (PD). The STN has become the most effective target of therapeutic deep brain stimulation (DBS) in PD. In the present study, neuronal synchrony was studied by EEG coherence analysis in 6 PD patients during a precision grip task. The goal of the experiments was to assess if there are any task-related modulations of neuronal synchrony inside the STN and/or between STN, motor cortex and muscle. EEG was recorded from 12 scalp electrodes (FC1, C1, CP1, FC3, C3, CP3, FC2, C2, CP2, FC4, C4, CP4), and 4 subcortical channels. EMG was recorded from small hand muscles (first digital interosseus [FDI] and abductor pollicis brevis [APB] muscle). EEG coherence was computed between 2 bipolar channels inside the STN, and between STN and scalp electrodes [cortex], and between cortex and muscle [corticomuscular coherence]. The recording sessions took place preoperatively, 1 and 6–8 days after the operation. Data were acquired during rest and while performing a precision grip task in which a constant force of 2 N had to be maintained (pinch grip). Visual feedback was only provided between single recording trials, but not while EEG and EMG data were registered. Task-related coherence [TRCoh] was computed by subtracting coherence during rest from coherence during the motor task. The main result was a 47% increase of TRCoh between bipolar channels inside the STN. This task-related synchrony occurred specifically in a high frequency range from 60–100Hz. The results in lower frequency bands showed high inter-subject variability. The STN high-frequency synchrony at 60–100Hz was not present in the STN-cortex or cortex-muscle coherence data. We conclude that high-frequency synchrony occurs as a task-related neuronal firing pattern during voluntary motor behavior in the STN. Since this 60–100Hz synchrony was restricted to the local signals (ie, local field potentials) in the STN, this finding would be consistent with local motor signal processing in the STN. These data support the concept that synchrony of neuronal assemblies is implemented at high frequencies in local or regional circuits while long-range synchrony occurs at lower frequencies (e.g., beta- or alpha band, cf. Hummel & Gerloff, Cereb Cortex. 2005 May;15(5):670–8).