Electroencephalographic analysis of cortico-muscular coherence: reference effect, volume conduction and generator mechanism

Objective: To measure the synchrony between cortical and muscle oscillatory activities, the coherence estimate between EEG and EMG was computed. Methods: The multichannel electroencephalogram (EEG) and electromyogram (EMG) of the right abductor pollicis brevis muscle were recorded in 5 normal volunteers. Various types of EEG derivation methods were systematically compared to establish a standard method to study cortico-muscular coupling. Results: The use of a reference-free EEG derivation (current source density) greatly improved cortico-muscular coherence. In all subjects, EEGs over the left sensorimotor cortex were coherent with EMG (mean peak frequency: 18.7 Hz, mean highest coherence: 0.124). The time lag from cortex to muscle in 14–50 Hz was 14.3 ms. EEG source derivation revealed that both radial and tangential generators in the precentral cortex might contribute to this phenomenon. In the EEG signals using common average reference, an artifactual coherence peak over the medial frontal area was observed, which might largely be explained by volume conduction from the primary sensorimotor cortex. Conclusions: We conclude that the current source density or its approximation is preferable to estimate the cortico-muscular coherence and that the interpretation of such coherence using referenced EEGs should be taken with care.