State-dependent brain stimulation: Power or phase?

Background: Intrinsic motor cortex activity modifies corticospinal excitability (CSE) in accordance with both oscillatory power fluctuations and phase-specific modulation along the oscillatory beta cycle, particularly in the 16-17 Hz frequency bin. Objective: To determine the magnitude of CSE and the relevance of stimulation timing for input gain mediated by either oscillatory power or phase. Methods: We applied single-pulse transcranial magnetic stimulation (TMS) over the primary motor cortex of healthy subjects at rest during electroencephalography recordings. The corticospinal gain modulation was indexed by the amplitude variability of the induced motor-evoked potentials recorded from the forearm muscle. Results: Low compared to high beta power led to a robust 40-70% CSE increase over a wide range of power values. By contrast, the phase modulation was critically dependent on the precise timing of the stimuli to the rising phase of the oscillatory beta cycle, but could then achieve CSE increases of 180%. Conclusion: These findings can influence closed-loop, state-dependent stimulation in the context of neurorehabilitation. (C) 2018 Elsevier Inc. All rights reserved.