Comparative modeling of transcranial magnetic and electric stimulation in mouse, monkey, and human

Abstract

Transcranial magnetic stimulation (TMS) and transcranial electric stimulation (TES) are increasingly popular methods to noninvasively affect brain activity. However, their mechanism of action and dose-response characteristics remain under active investigation. Translational studies in animals play a pivotal role in these efforts due to a larger neuroscientific toolset enabled by invasive recordings. In order to translate knowledge gained in animal studies to humans, it is crucial to generate comparable stimulation conditions with respect to the induced electric field in the brain. Here, we conduct a finite element method (FEM) modeling study of TMS and TES electric fields in a mouse, capuchin and macaque monkeys, and a human model. We systematically evaluate the induced electric fields and analyze their relationship to head and brain anatomy. We find that with increasing head size, TMS-induced electric field strength first increases and then decreases according to a two-term exponential function. TES-induced electric field strength strongly decreases from smaller to larger specimen with up to 100x fold differences across species. Our results can serve as a basis to compare and match stimulation parameters across studies in animals and humans.

Keywords: Animal model; Finite element modeling; Neuromodulation; Transcranial electric stimulation; Transcranial magnetic stimulation.

Copyright © 2019 Elsevier Inc. All rights reserved.

Figures

Figure B1.

One wire loop figure-8 coil in a spherical coordinate system.

Figure 1.

Anatomically accurate FEM models of mouse, monkey, and human. The following tissues are considered: skin and soft tissues, skull and bones, eyes, CSF, grey matter, and white matter. On the top row are the brain surfaces, middle row – skull surfaces, bottom row – horizontal cut of FEMs.

Figure 2.

Distribution of the electric field strength in the grey matter volume for a 70 mm (A) and 25 mm (B) figure-8 coil. Blue color corresponds to the caudal-medial or CM coil orientation, red color – medial or M, and yellow – caudal-lateral or CL coil orientation. The grey zone shows the range of the data on the x-axis. The same plots on the individual axes are shown in Figure S4. (C) Normalized electric fields on the brain surfaces for the CM orientation. See other montages in Figures S1, S2.

Figure 3.

Distribution of the tangential (A-C) and perpendicular (D-F) components of the electric field for the 70 mm figure-8 coil oriented caudal-medial for mouse (top row), monkey (middle row), and human (bottom row). For each figure in the panel, both the anatomical realistic surface (left) and the inflated surface (right) is shown.

Figure 4.

(A-C) TMS summary statistics across species. Color encodes the coil orientation: blue – caudal-medial, red – medial, and yellow – caudal-lateral. (D-F) TES summary statistics across species. Color encodes the electrode montage: blue – rostral-caudal, red – left-right, and yellow – dorsal-ventral. The shape of the data points indicates the species: round – mouse, square – monkey, triangle – human. The top row depicts the robust maximum of the electric field (Emax), the middle row – the affected brain volume (L½max), and the bottom row – the ratio of the medians of the tangential to perpendicular field components.

Figure 5.

(A) Distribution of the electric field magnitude in the grey matter volume during TES with the following electrode montages: blue color – rostral-caudal, red – left-right (L-R), and yellow – dorsal-ventral. Note that the x-axes are scaled to the model-specific maximum values. (B) Normalized electric fields on the brain surface for the L-R montage. See the results for the other montages in Figure S3.

Figure 6.

Distribution of the tangential (A-C) and perpendicular (D-F) components of the electric field on the brain surface due to TES with the left-right electrode montage. On the top row is mouse, middle row – monkey, and bottom row – human. For each figure in the panel, normal surface is on the left and its inflated version is on the right.

Figure 7.

TES electric fields in grey matter for different sizes of stimulation electrodes (d = 36 mm, 15 mm, and 3 mm). (A) Frontal view for the rostral-caudal electrode montage. (B) Lateral view for the left-right montage. (C) Transverse view for the dorsal-ventral montage.