Contributions of descending and ascending pathways to corticomuscular coherence in humans

Corticomuscular coherence in the beta frequency band (15-30 Hz) has been demonstrated in both humans and monkeys, but its origin and functional role are still unclear. Phase-frequency plots produced by traditional coherence analysis are often complex. Some subjects show a clear linear phase-frequency relationship (indicative of a fixed delay) but give shorter delays than expected; others show a constant phase across frequencies. Recent evidence suggests that oscillations may be travelling around a peripheral sensorimotor loop. We recorded sensorimotor EEGs and EMGs from three intrinsic hand muscles in human subjects performing a precision grip task, and applied directed coherence (Granger causality) analysis to explore this system. Directed coherence was significant in both descending (EEG -> EMG) and ascending (EMG -> EEG) directions at beta frequencies. Average phase delays of 26.4 ms for the EEG -> EMG direction and 29.5 ms for the EMG -> EEG direction were closer to the expected conduction times for these pathways than the average delays estimated from coherence phase (7.9 ms). Subjects were sub-divided into different groups, based on the sign of the slope of the linear relation between corticomuscular coherence phase and frequency (positive, negative or zero). Analysis separated by these groups suggested that different relative magnitudes of EEG -> EMG and EMG -> EEG directed coherence might underlie the observed inter-individual differences in coherence phase. These results confirm the complex nature of corticomuscular coherence with contributions from both descending and ascending pathways.