Differences in TMS-Induced Electric Fields Between Motor and Dorsolateral Prefrontal Cortices Revealed by...

Area of Science:

• Neuroscience
• Biomedical Engineering
• Computational Modeling

Background:

• Clinical transcranial magnetic stimulation (TMS) often scales parameters from motor threshold (MT) measured at the primary motor cortex (M1).
• The efficacy of this approach for complex brain regions like the dorsolateral prefrontal cortex (DLPFC) remains uncertain due to anatomical differences.

Purpose of the Study:

• To investigate if scaling TMS parameters from M1 MT provides comparable electric field (E-field) stimulation at the DLPFC.
• To evaluate the impact of anatomical variations on E-field distribution in different brain regions.

Main Methods:

• Constructed personalized head models from MRI scans of 20 healthy adults.
• Performed TMS E-field simulations using SimNIBS 4.1 for M1 and DLPFC targets.
• Compared E-field characteristics under fixed-output and MT-equivalent reference output frameworks.

Main Results:

• Fixed-output TMS showed significantly lower E-field magnitudes at the DLPFC compared to M1.
• While normal E-field components were similar, tangential components were weaker at the DLPFC.
• MT-equivalent scaling reduced magnitude differences but did not fully align E-field component balance between M1 and DLPFC.

Conclusions:

• Standard M1-referenced TMS parameter scaling does not ensure equivalent E-field stimulation at the DLPFC.
• Target-specific, E-field-informed TMS parameter selection is recommended.
• Future studies should validate modeled E-fields with experimental and clinical outcomes.