Immediate TMS-EEG responses reveal motor cortex excitability.

Area of Science:

• Neuroscience
• Neurophysiology
• Brain Stimulation

Background:

• Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) traditionally assesses network-level cortical excitability.
• Local excitability measures using TMS-EEG were previously limited.
• Recent findings suggest immediate TMS-evoked potentials (i-TEPs) following primary motor cortex (M1) stimulation, but their physiological basis is unclear.

Purpose of the Study:

• To replicate and validate the physiological origin of i-TEPs.
• To determine if i-TEPs are related to M1 excitability.
• To investigate the oscillatory components contributing to i-TEPs.

Main Methods:

• Stimulation of the primary motor cortex (M1) in 28 healthy participants using TMS with two current directions.
• Minimal preprocessing of TMS-EEG data, with sample division based on muscle artifacts (Muscle/NoMuscle groups).
• Source localization and computation of immediate TMS-related power (i-TRP) to analyze oscillatory components (100-200 Hz and 600-800 Hz).

Main Results:

• i-TEPs were successfully replicated for both stimulation directions.
• Source localization confirmed i-TEPs originate from the precentral gyrus.
• A significant positive association was found between i-TRP in the 600-800 Hz range and motor evoked potentials (MEPs) amplitude in the NoMuscle group.

Conclusions:

• Immediate TMS-EEG responses are physiological.
• The 600-800 Hz i-TRP component likely reflects M1 cortical excitability.
• i-TEPs offer a novel way to probe local cortical excitability.