Motor cortex excitability states in chronic stroke patients probed by EEG-TMS

• In healthy subjects, trough vs. no-trough phase of the µ-rhythm correspond to high- vs. low-excitability of the motor cortex (M1) • We tested this differentiation in the ipsilesional (iM1) and contralesional (cM1) M1 of chronic stroke patients. • The degree of excitability differentiation of iM1 correlated with impairment, suggesting its functional relevance for motor recovery. In healthy subjects, the trough vs. no-trough phases of the sensorimotor µ-rhythm correspond to high- vs. low-excitability states of the motor cortex (M1). We tested this excitability differentiation in the ipsilesional (iM1) and contralesional M1 (cM1) of chronic stroke patients. 19 chronic stroke patients received single-pulse transcranial magnetic stimulation (TMS), separately over the iM1 and cM1, during EEG recordings. High and low M1 excitability states were defined by binning a post-hoc estimate of the µ-phase at TMS delivery. TMS-evoked EEG potentials (TEPs) and time–frequency responses were characterized for excitability states and hemispheres. The motor function of the affected arm was tested by the Fugl-Meyer Assessment Upper Extremity (FMA-UE. In cM1, TMS at the high- vs. low-excitability state resulted in larger TEP amplitudes and increased post-pulse power in the beta band. In iM1, these modulations were not significant except for post-pulse beta power. This retained excitability differentiation significantly correlated with FMA-UE. The degree of excitability differentiation in iM1 depending on phase of the sensorimotor µ-rhythm correlates with individual affected upper extremity motor function. The degree of excitability differentiation in iM1 might serve as a new independent marker of motor recovery.