Spatial variability in cortex – muscle coherence investigated with magnetoencephalography 1 and high - density surface electromyography 2

19 Cortex–muscle coherence (CMC) reflects coupling between magnetoencephalography (MEG) 20 and surface electromyography (sEMG), being strongest during isometric contraction, but absent, 21 for unknown reasons, in some individuals. 22 We used a novel non-magnetic high-density sEMG (HD-sEMG) electrode grid (36 mm x 12 23 mm; 60 electrodes separated by 3 mm) to study effects of sEMG recording site, electrode 24 derivation and rectification on the strength of CMC. Monopolar sEMG from right thenar and 25 306-channel whole-scalp MEG were recorded from 14 subjects during 4-min isometric thumb 26 abduction. CMC was computed for 60 monopolar, 55 bipolar, and 32 Laplacian HD-sEMG 27 derivations, and two derivations were computed to mimic 'macroscopic' monopolar and bipolar 28 sEMG (electrode diameter 9 mm; inter-electrode distance 21 mm). 29 With unrectified sEMG, 12 subjects showed statistically significant CMC in 91–95% of the 30 HD-sEMG channels, with maximum coherence at ~25 Hz. CMC was about a fifth stronger for 31 monopolar than bipolar and Laplacian derivations. Monopolar derivations resulted in most 32 uniform CMC distributions across the thenar and in most tight cortical source clusters in the left 33 rolandic hand area. CMC was 19–27% stronger for HD-sEMG than for ‘macroscopic’ 34 monopolar or bipolar derivations. EMG rectification reduced the CMC peak by a quarter, 35 resulted in a more uniformly distributed CMC across the thenar, and provided more tightly 36 clustered cortical sources than unrectifed sEMGs. Moreover, it revealed CMC at ~12 Hz. 37 We conclude that HD-sEMG, especially with monopolar derivation, can facilitate detection of 38 CMC and that the individual muscle anatomy cannot explain the high inter-individual CMC 39 variability. 40