Abstract
Background: Neural oscillations in the primary motor cortex (M1) shape corticospinal excitability. Power and phase of ongoing mu (8–13 Hz) and beta (14–30 Hz) activity may mediate motor cortical excitability. However, the functional dynamics of both mu and beta phase and power relationships and their interaction, are yet to be determined.
Objective: We employ recently developed real-time targeting of the mu and beta rhythm, to apply phase-specific brain stimulation and probe motor corticospinal excitability non-invasively. We investigate the functional significance of phase and power in the mu and beta oscillations.
Methods: Twenty healthy adults took part in this double-blind randomized crossover study. We applied suprathreshold single-pulse transcranial magnetic stimulation (TMS) to M1 based on the instantaneous analysis of ongoing motor oscillations, targeting four different phases (0°, 90°, 180°, and 270°) of mu and beta rhythms. Resulting motor evoked potentials (MEPs) in the right first dorsal interossei muscle were recorded.
Results: Mixed model regression analyses showed significant phase-dependent modulation of corticospinal output in both mu and beta rhythms. These modulations exhibit a double dissociation. In the mu band, MEPs are larger in the trough and rising phases and smaller at the peak and falling phases. We found the opposite behavior for the beta rhythm. Also, mu power had a weak positive correlation with the corticospinal output. No correlation was found for the beta power. Power and phase effects did not interact for either rhythm, suggesting independence between these aspects of oscillations.
Conclusion: Our results provide insights into real-time motor cortical oscillation dynamics, which offers the opportunity to improve the stimulation effectiveness through personalized state-dependent TMS.