Abstract
The strength of a given transcranial magnetic stimulation (TMS) pulse decays rapidly with distance. Male and female bone structure reliably differs by the shape of the frontal bone, mandible, and inion. Given the morphology of these structures constitutes much of the scalp-to-cortex distance (STCD), we hypothesized that females have shorter STCDs and thereby receive stronger TMS electrical field strengths, relative to males. Head models (nā=ā411; 197 female, 214 male) were constructed from MRIs of healthy participants (ages 18ā90). STCD and peak electrical field strength were measured at 50 EEG 10ā20 sites (SimNIBSv3.2). Linear models (bootstrapped and Benajamini-Hochberg multiple comparison-corrected) evaluated the influence of sex on STCD and electrical field strength. Females had significantly shorter STCDs at 27/50 sites and stronger TMS electrical fields at 18/50. When normalized by data collected at the motor cortex, females had significantly shorter STCD at 40/49 sites and stronger TMS electrical fields at 29/49 sites. The largest effect size differences were detected at the frontal, temporal, and occipital poles, and the cerebellum. Interestingly, STCD at the motor cortex was not different between sexes, suggesting the motor cortex-based dosing strategies produce unequal electrical fields between sexes. These data provide a mathematically grounded explanation for sex-differences in clinical outcome and may be relevant to other modalities that depend on electromagnetic signals (e.g., EEG, MEG).
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Data availability
MR images from the ADNI and ICBM data sets are publicly available. MR data from the Stanford data set can be made available upon request. Processed STCD and electrical field strength data for all data sets are available upon request.
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Acknowledgements
Data used in preparation of this article were obtained from the Alzheimerās Disease Neuroimaging Initiative (ADNI) database (adni.loni.usc.edu). As such, the investigators within the ADNI contributed to the design and implementation of ADNI and/or provided data but did not participate in analysis or writing of this report. A complete listing of ADNI investigators can be found at: http://adni.loni.usc.edu/wp-content/uploads/how_to_apply/ADNI_Acknowledgement_List.pdf. Data were collected for this manuscript from the Alzheimerās Disease Neuroimaging Initiative (ADNI) (National Institutes of Health Grant U01 AG024904) and DOD ADNI (Department of Defense award number Q81XWH-12-2-0012). ADNI is funded by the National Institute on Aging, the National Institute of Biomedical Imaging and Bioengineering, and through generous contributions from the following: AbbVie, Alzheimerās Association; Alzheimerās Drug Discovery Foundation; Araclon Biotech; BioClinica, Inc.; Biogen; Bristol-Myers Squibb Company; CereSpir, Inc.; Cogstate; Eisai Inc.; Elan Pharmaceuticals, Inc.; Eli Lilly and Company; EuroImmun; F. Hoffmann-La Roche Ltd and its affiliated company Genentech, Inc.; Fujirebio; GE Heathcare;IXICO Ltd.; Jannsen Alzheimer Immunotherapy Research & Development, LLC.; Johnson & Johnson Pharmaceutical Research & Development LLC.; Lumosity; Lundbeck; Merck & Co, Inc.; Meso Scale Diagnostics, LLC.; NeuroRx Research; Neurotrack Technologies; Novartis Company; and Transition Therapeutics. The Canadian Institutes of Health Research is providing funds to support ADNI clinical sites in Canada. Private sector contributions are facilitated by the Foundation for the National Institutes of Health (www.fnih.org). The grantee organization is the Northern California Institute for Research and Education, and the study is coordinated by the Alzheimerās Therapeutic Research Institute at the University of Southern California. ADNI data are disseminated by the Laboratory for Neuro Imaging at the University of Southern California. Data were also collected from the International Consortium of Brain Mapping (ICBM). The ICBM project (Principal Investigator John Mazziotta, M.D., University of California, Los Angeles) is supported by the National Institute of Biomedical Imaging and BioEngineering. ICBM is the result of efforts of co-investigators from UCLA, Montreal Neurologic Institute, University of Texas at San Antonia, and the Institute of Medicine, Juelich/Heinrich Heine University, Germany. This work was supported by the Stanford Neurochoice initiative and the National Institute for Alcohol Abuse and Alcoholism (McCalley, K99AA031508).
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DMM and CBP were responsible for experimental design, data analysis, interpretation, manuscript writing, funding acquisition, and data collection. NJC, FW, and L-TT contributed meaningfully to data analysis and interpretation as well as data collection. BK contributed to data analysis, collection, and interpretation, as well as manuscript writing. All authors reviewed and edited the manuscript in full prior to submission for publication.
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McCalley, D.M., Cadicamo, N.J., Weijerman, F. et al. Females have shorter scalp-to-cortex distances and receive stronger TMS electrical fields: Implications for clinical treatment. Neuropsychopharmacol. (2025). https://doi.org/10.1038/s41386-025-02299-6
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DOI: https://doi.org/10.1038/s41386-025-02299-6

