Abnormal mu rhythm state-related cortical and corticospinal responses in chronic stroke

Objective: The motor cortex's activity is state-dependent. Specifically, the sensorimotor mu rhythm phase relates to the variability of primary motor cortex (M1) excitability, previously demonstrated in young and healthy volunteers. It is unknown whether this observation is generalizable to individuals with stroke-related brain lesions. Methods: We investigated the phase relationship between mu oscillations and cortical excitability by combining real-time processing of electroencephalography (EEG) and transcranial magnetic stimulation (TMS) of M1. In N = 23 volunteers (chronic stroke survivors and healthy controls), we applied TMS to M1 at four phases of the mu oscillation. We investigated motor-evoked (MEP) and TMS-evoked potential (TEP) amplitudes. Results: MEP amplitude in stroke survivors and older volunteers showed a phase-dependency with increased MEPs at the trough and decreased MEPs at the peak of the mu rhythm. However, individuals with stronger stroke-related motor symptoms showed a decreased phase preference. Phase-dependency of TEPs was reduced in the stroke-affected hemisphere, compared to the non-affected hemisphere. In healthy volunteers, no hemispheric difference was found. Conclusion: Our preliminary results indicate that the strength of phase preference of TMS motor responses could indicate the severity of motor impairment. Significance: These results could enable the development of improved TMS paradigms for recovery of motor impairment after stroke.