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

Background: 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). Methods: Personalized head models were constructed from T1- and T2-weighted magnetic resonance imaging (MRI) scans from 20 healthy elderly 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 the M1 and DLPFC (C3/F3, 10–10 electroencephalography (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 the 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 region of interest (ROI) than in the M1 ROI (PFDR 0.05), whereas the tangential (surface-parallel) component was significantly weaker at the 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: Fixed-output simulations show target-wise differences in both E-field magnitude and component balance between the M1 and DLPFC. An M1-referenced, MT-equivalent scaling framework can reduce magnitude differences but does not necessarily align component balance at the DLPFC. These findings support target-specific, E-field–informed parameter selection and should be tested in prospective studies linking modeled E-fields to experimental readouts and clinical outcomes.