Introduction:
Voluntary motor control is subserved by a neural network involving the interaction between the two primary motor cortices (M1). It is well established that the pathway connecting the right and the left M1 has a very important role in movement execution allowing single muscle control. Specifically, the contralateral M1 exerts a powerful inhibitory influence over the ipsilateral M1 through transcallosal connections and it is functionally relevant and correlated with motor control. Moreover, the pathway can be examined in humans; by stimulating the contralateral M1 shortly (6 to 8 ms) before the ipsilateral M1, it is possible to influence how activity in the ipsilateral M1 evolves. When this is done repeatedly, the influence that contralateral M1 exerts over ipsilateral M1 is modulated. Such a procedure is often referred to as corticocortical paired associative stimulation (ccPAS).
Aim:
In this study we used ccPAS over the right and the left M1, to selectively manipulate synaptic plasticity in the pathway connecting such regions, and then measured changes in cortical excitability and oscillatory connectivity in the human brain of 37 healthy adults (aged 18-40 years). All participants had no personal or familial history of neurological or psychiatric disease, were right-handed and gave written informed consent (Department of Psychology Research Ethics Committee, – ref numb: ETH2324-0189)
Methods:
In two separate blocks, we stimulated over the left and the right M1 with either single pulse TMS, paired pulse TMS (12ms inter-pulse interval – IPI), or dual-site paired pulse TMS over both left and right M1 (8ms IPI) and measured the levels of motor-related M1 excitability, from the first dorsal interosseous muscle with electromyography. Moreover, we measured motor electrophysiological (EEG) responses at rest. Between the two blocks, we applied the ccPAS protocol by repeatedly stimulating over left and right M1 with the same 8ms IPI at 0.1Hz during a 15minture interval. This ccPAS protocol is proven to evoke synchronous pre- and post-synaptic activity and to strength interregional connectivity between the left and right primary motor cortex in a Hebbian-like manner.
Results:
The results of the study showed selective changes in the motor-evoked potentials (MEPs) and EEG oscillatory activity after applying ccPAS over the left and then the right M1 (8ms IPI); we observed an enhancement of local right M1 excitability (F1,36 = 4.12, p < 0.05). However, no changes in cortical inhibition were observed (p > 0.05). The changes in cortical excitability were observed in tandem with an increase of phase synchrony in beta (Monte Carlo p value < 0.01), while no phase changes were observed in the alpha or theta bands (p > 0.05). Conversely, administration of the control ccPAS protocol (1ms IPI) showed no alterations neither in EEG phase activity nor in M1 cortical activity.
Conclusions:
These findings demonstrate a clear link between physiology of the motor control network and the resonant frequencies mediating its interactions.