Effects of pulse width, waveform and current direction in the cortex: A combined cTMS-EEG study

Area of Science:

• Neuroscience
• Biomedical Engineering
• Electrophysiology

Background:

• Transcranial Magnetic Stimulation (TMS) parameters like pulse width, waveform, and current direction are crucial for outcomes.
• Previous studies indirectly assessed these factors using motor-evoked potentials (MEP).
• Combining TMS with electroencephalography (EEG) allows direct examination of cortical responses.

Purpose of the Study:

• To systematically investigate the effects of varying TMS pulse width, waveform, and current direction on TMS-evoked EEG responses.
• To compare these effects on global and local cortical activity with MEP.

Main Methods:

• Utilized a controllable TMS (cTMS) device to manipulate pulse parameters.
• Measured resting motor threshold (RMT), global mean field power (GMFP), and local mean field power (LMFP) in 19 healthy volunteers.
• Analyzed TMS-evoked EEG responses to assess global and local cortical excitability.

Main Results:

• Lower RMT was observed with longer pulse width monophasic posterior-to-anterior (PA) pulses.
• Monophasic pulses resulted in higher GMFP, and longer pulse widths in higher LMFP, even after RMT normalization.
• Early TMS-evoked potential (TEP) polarity was dependent on current direction.

Conclusions:

• TMS-evoked EEG responses (both local and global) vary significantly with pulse parameters, independent of RMT.
• Adjusting TMS parameters based solely on MEP threshold may not guarantee consistent neuronal activation within a cortical area.
• This highlights the importance of optimizing TMS pulse parameters for targeted cortical stimulation.