Amplitude dynamics in corticospinal interactions

Corticomuscular interactions are investigated mostly with phase coherence of EEG/EMG which, however, is not informative about the local dynamics of interacting cortical and spinal neuronal populations. Here, we investigated the amplitude dynamics of sensorimotor EEG beta oscillations during an isometric task and their relation to corticomuscular coherence (CMC). The amplitude-envelopes of beta oscillations, obtained from multichannel EEG and EMG recordings, respectively, were used as a measure of local cortical and spinal cord synchronization. Upon an imperative stimulus, the amplitude of cortical beta oscillations showed an initial attenuation which correlated with the CMC strength. Our results also indicated that this correlation relates to the magnitude of pre-stimulus relative spectral power, which itself is correlated with both CMC and the attenuation of beta oscillations, rendering these two measures spuriously correlated. Therefore, a correlation of relative beta power with CMC provides a plausible explanation for the previously unaccounted variability of CMC strength across subjects. Critically, we demonstrated that the amplitude-envelopes of beta oscillations in EEG and EMG are correlated on time scales ranging from 50 to 1000 ms. Thus, the amplitude of ongoing cortical beta oscillations might directly contribute to the rhythmic spiking output of both corticospinal neurons and their motoneural spinal targets. 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.

Supported by the Center for Stroke Research Berlin and the Bernstein Center for Computational Neuroscience Berlin.