Individual head models for estimating the TMS-induced electric field in rat brain

Lari M. Koponen*, Matti Stenroos, Jaakko O. Nieminen, Kimmo Jokivarsi, Olli Gröhn, Risto J. Ilmoniemi

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)


In transcranial magnetic stimulation (TMS), the initial cortical activation due to stimulation is determined by the state of the brain and the magnitude, waveform, and direction of the induced electric field (E-field) in the cortex. The E-field distribution depends on the conductivity geometry of the head. The effects of deviations from a spherically symmetric conductivity profile have been studied in detail in humans. In small mammals, such as rats, these effects are more pronounced due to their less spherical head, proportionally much thicker neck region, and overall much smaller size compared to the TMS coils. In this study, we describe a simple method for building individual realistically shaped head models for rats from high-resolution X-ray tomography images. We computed the TMS-induced E-field with the boundary element method and assessed the effect of head-model simplifications on the estimated E-field. The deviations from spherical symmetry have large, non-trivial effects on the E-field distribution: for some coil orientations, the strongest stimulation is in the brainstem even when the coil is over the motor cortex. With modelling prior to an experiment, such problematic coil orientations can be avoided for more accurate targeting.

Original languageEnglish
Article number17397
JournalScientific Reports
Issue number1
Publication statusPublished - 1 Dec 2020

Bibliographical note

Funding Information:
This research was supported by the Finnish Cultural Foundation, the Jane and Aatos Erkko Foundation, and the Academy of Finland (Decisions Nos. 255347, 265680, 283105, and 294625).

Publisher Copyright:
© 2020, The Author(s).

ASJC Scopus subject areas

  • General


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