EEG oscillations and magnetically evoked motor potentials reflect motor system excitability in overlapping neuronal populations.

OBJECTIVE To understand the relationship between neuronal excitability reflected by transcranial magnetic stimulation (TMS) evoked motor potentials (MEPs) and spontaneous oscillation amplitude and phase. METHODS We combined spontaneous EEG measurement with motor cortex TMS and recorded MEP amplitudes from abductor digiti minimi (ADM). RESULTS Midrange-beta oscillations over the stimulated left motor cortex were, on average, weaker before large- than small-amplitude MEPs. The phase of occipital midrange-beta oscillations was related to the MEP amplitudes. CONCLUSIONS The present results support the view that MEP and Rolandic beta oscillation amplitudes are associated with motor cortical excitability. However, oscillations seen in EEG reflect the excitability of a large population of cortical neurons, and MEP amplitude is affected also by spinal excitability and action potential desynchronization. Thus, MEP and EEG oscillation amplitudes are not strongly correlated. In addition, even during rest, motor system excitability appears to be related to activity in occipital areas at frequency ranges associated with visuomotor processing. SIGNIFICANCE The ability of spontaneous oscillations and MEPs to inform us about cortical excitability is clarified. For example, it is suggested that oscillatory activity at non-motor sites might be related to motor system excitability at rest.