Mu-Rhythm Phase Modulates Cortical Reactivity to Subthreshold TMS: A TMS-EEG Study

Area of Science:

• Neuroscience
• Neurophysiology
• Brain Stimulation

Background:

• Cortical reactivity is influenced by electroencephalogram (EEG) signal phase.
• Subthreshold transcranial magnetic stimulation (TMS) research is limited due to inability to induce motor evoked potentials (MEPs).
• Investigating phase-dependent responses under subthreshold stimulation is crucial for understanding brain excitability.

Purpose of the Study:

• To examine how ongoing EEG phase influences cortical responses to subthreshold TMS.
• To characterize these phase-dependent responses using temporal, spatial, and spectral features.
• To explore the potential of EEG phase for closed-loop neuromodulation.

Main Methods:

• Combined TMS and EEG in 34 healthy adults.
• Delivered subthreshold single-pulse TMS at the motor hotspot during 64-channel EEG recording.
• Estimated and binned mu-phase (0°, 90°, 180°, 270°) to assess cortical responses (ITPC, TEPs, GMFP, ERSP).

Main Results:

• Reliable distinction of four mu-phase bins.
• Significant phase dependence in subthreshold TMS-EEG responses, including early inter-trial phase coherence (ITPC) and TMS-evoked potentials (TEPs).
• Stronger responses observed at 180° and 270° phases; greatest TMS-induced mu-rhythms at 180°.

Conclusions:

• EEG phase significantly modulates cortical reactivity even at subthreshold stimulation levels.
• Mu-phase-based closed-loop TMS is a promising strategy for precise neuromodulation.
• Findings advance understanding of brain excitability and TMS protocols.