It is not all about phase: amplitude dynamics in corticomuscular interactions

Corticomuscular interactions are studied mostly with EEG/EMG coherence, which, however, does not allow quantification of amplitude dynamics of sensorimotor oscillations. Here, we investigated the amplitude dynamics of sensorimotor EEG beta oscillations during an isometric task and their relation to corticomuscular coherence (CMC). We used amplitude envelopes of beta oscillations, derived from multichannel EEG and EMG recordings, as a measure of local cortical and spinal-cord synchronization. In general, we showed that the amplitude of cortical beta oscillations can influence CMC in two ways. First, we showed that the signal-to-noise ratio of pre-stimulus beta oscillations affects CMC. Second, we demonstrated that the attenuation of beta oscillations upon imperative stimulus correlated with the CMC strength. Attenuation of cortical beta oscillations was previously hypothesized to reflect increased motor cortex excitability. Consequently, this correlation might indicate that high cortical excitability, produced by imperative stimulus, facilitates the recruitment of neuronal networks responsible for establishing reliable corticospinal control manifested in larger CMC. Critically, we demonstrated that the amplitude envelopes of beta oscillations in EEG and EMG are positively correlated on time scales ranging from 50 to 1000 ms. Such correlations indicate that the amplitude of cortical beta oscillations might relate to the rhythmic spiking output of both corticospinal neurons and their spinal targets. Compared to CMC, however, amplitude-envelope correlations were detected in fewer cases, which might relate to a higher susceptibility of these correlations to signal-to-noise ratio. We conclude that EEG beta oscillations, originating from the sensorimotor cortex, can transmit not only their phase but also amplitude dynamics through the spinal motoneurons down to peripheral effectors.