1000 Section - Differences in TMS-Induced Electric Fields between Motor and Dorsolateral Prefrontal Cortices Revealed by Personalized Modeling

Objectives : To investigate whether the common clinical practice of setting Transcranial Magnetic Stimulation (TMS) parameters by scaling from the motor threshold (MT) measured at the primary motor cortex (M1) can be expected to yield comparable stimulation at complex brain regions such as the dorsolateral prefrontal cortex (DLPFC).

Design: Personalized head models were constructed from T1- and T2-weighted MRI scans from 20 healthy older adults from a public Chinese dataset. SimNIBS 4.1 was used for tissue segmentation, mesh generation, and TMS electric-field (E-field) simulations. A MagVenture MC-B70 figure-of-eight coil was used to simulate stimulation over M1 and DLPFC (C3/F3, 10–10 EEG system). First, both targets were simulated at a fixed intensity of 60% of maximum stimulator output (MSO) to isolate anatomical contributions. Second, an MT-equivalent reference output framework was implemented by calibrating subject-specific output to an M1 reference E-field level of 60 mV/mm, using two calibration tracks (Mean-based and Max-based), and DLPFC was evaluated at 80–120% of the subject-specific reference output.

Results: Under fixed-output stimulation, the mean and robust maximum E-field magnitudes on the gray matter surface and within gray matter volume were significantly lower in the DLPFC ROI than in the M1 ROI (PFDR < 0.05). The normal (surface-perpendicular) component did not differ significantly between targets (PFDR > 0.05), whereas the tangential (surface-parallel) component was significantly weaker at DLPFC (PFDR < 0.05). Under MT-equivalent scaling, increasing intensity reduced the overall magnitude gap, with approximate parity around 110% of the reference output, but a component trade-off persisted: scaling that brought tangential fields closer to the M1 reference tended to increase normal fields beyond the M1 reference.

Conclusion:

Conclusion: Fixed-output simulations reveal target-wise differences in E-field magnitude and component balance between M1 and DLPFC. MT-equivalent, M1-referenced scaling reduces magnitude differences but does not necessarily align component balance, motivating target-specific, E-field–informed parameter selection and prospective validation.