J. Gross et al., Cortico-muscular synchronization during isometric muscle contraction in humans as revealed by magnetoencephalography, J PHYSL LON, 527(3), 2000, pp. 623-631
1. Magnetoencephalographic (MEG) and electromyographic (EMG;) signals were
recorded from six subjects during isometric contraction of four different m
uscles.
2. Cortical sources were located from the MEG signal which was averaged tim
elocked to the onset of motor unit potentials. A spatial filtering algorith
m was used to estimate the source activity. Sources were found in the prima
ry motor cortex (M1) contralateral to the contracted muscle. Significant co
herence between rectified EMG and M1 activity was seen in the 20 Hz frequen
cy range in all subjects.
3. Interactions between the motor cortex and spinal motoneuron pool were in
vestigated by separately studying the non-stationary phase and amplitude dy
namics of M1 and EMG signals.
4. Delays: between M1 and EMG signals, computed from their phase difference
, were found to be in agreement with conduction times from the primary moto
r cortex to the respective muscle. The time-dependent cortico-muscular phas
e synchronization was found to be correlated with the time course of both M
1 and EMG signals.
5. The findings demonstrate that the coupling between the primary motor cor
tex and motoneuron pool is at least partly due to phase synchronization of
20 Hz oscillations which varies over time. Furthermore, the consistent phas
e lag between M1 and EMG signals, compatible with conduction time between M
1 and the respective muscle with the M1 activity preceding EMG activity, su
pports the conjecture that the motor cortex drives the motoneuron pool.