Coherence and imaginary part of coherency identifies cortico-muscular and cortico-thalamic coupling

The understanding of the interaction between muscle control and cortical areas and between subcortical and cortical areas is important for the effective treatment of patients with movement disorders. The combination of coherence (COH) and the imaginary part of coherency (iCOH) is applied here to electrophysiological data from patients with a movement disorder and to data from healthy subjects performing finger movements. The COH and iCOH between magnetoencephalographic (MEG) and electromyographic (EMG) signals of the healthy subjects yields the expected result for cortico-muscular coupling. Based on this the COH and iCOH between sub-thalamic nucleus local field potentials (STN-LFP) and MEG signals are assessed for deep brain stimulation patients with externalized LFP electrodes. The results suggest interactions in the 10 to 20 Hz range. Artificially mimicking volume conduction by re-referencing the STN electrodes to a surface EEG electrode leads to large changes in the COH and iCOH. This suggests that volume conduction is not important for the analysis of interactions between MEG and bipolar STN electrodes.