Effects of electric field direction on TMS-based motor cortex mapping

Area of Science:

• Neuroscience
• Biophysics
• Computational modeling

Background:

• Transcranial magnetic stimulation (TMS) is used for brain mapping, but predicting cortical responses is challenging.
• Current TMS motor mapping often uses electric field (E-field) magnitude or its normal component, neglecting neuronal factors.

Purpose of the Study:

• To refine TMS motor mapping by developing a model incorporating both E-field magnitude and directional sensitivity.
• To compare the performance of different E-field models in predicting cortical excitability and motor-evoked potentials (MEPs).

Main Methods:

• Conducted regression-based TMS mapping in 14 participants targeting the first dorsal interosseous (FDI) muscle.
• Estimated neuronal firing thresholds and regressed MEPs against E-field magnitude, normal component (cosine), and an orientation-adjusted effective E-field (neuron model).
• Validated models using optimized coil placements in 10 additional participants.

Main Results:

• The E-field magnitude and neuron models showed similar, robust performance in predicting MEPs.
• The cosine model explained less variance, required more TMS pulses, and yielded weaker MEPs.
• E-field magnitude was identified as the dominant factor in motor cortex activation.

Conclusions:

• E-field magnitude is a primary driver of motor cortex activation during TMS.
• While neuronal orientation plays a role, it is secondary to E-field magnitude for accurate TMS motor mapping.
• The findings suggest improved models for TMS-based neurostimulation and mapping.