Preconditioning Stimulus Intensity Alters Paired-Pulse TMS Evoked Potentials

Area of Science:

• Neuroscience
• Neurophysiology
• Brain Stimulation

Background:

• Motor cortex (M1) paired-pulse transcranial magnetic stimulation (ppTMS) using motor-evoked potentials (MEPs) assesses intracortical dynamics.
• MEPs confound cortical and spinal excitability, limiting precise cortical function assessment.
• Concurrent TMS-EEG offers a peripheral confound-free measure of cortical function via TMS-evoked potentials (TEPs).

Purpose of the Study:

• To investigate the effect of varying preconditioning stimulus intensities in ppTMS on TEPs.
• To compare the sensitivity of TEPs versus MEPs to intracortical inhibition across different stimulus intensities.

Main Methods:

• Single-pulse and ppTMS paradigms were applied to the left M1.
• Three ppTMS conditions used preconditioning stimulus intensities of 70%, 80%, and 90% of resting motor threshold.
• Motor-evoked potentials (MEPs) and TMS-evoked potentials (TEPs) were recorded concurrently.

Main Results:

• Corticospinal inhibition was observed across all ppTMS conditions.
• ppTMS reduced TEP peaks primarily at 70% intensity, with less reduction at 80% and no reduction at 90%.
• A significant negative correlation was found between MEPs and the N45 TEP peak, particularly at lower intensities.

Conclusions:

• Unlike MEPs, ppTMS TEPs do not show inhibition across all tested intensities.
• TEP responses to M1 ppTMS are intensity-dependent, with lower intensities yielding better discriminability.
• Minimizing preconditioning stimulus intensity is recommended for ppTMS-EEG studies probing intracortical inhibition.