Abstract
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.
Highlights
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
We studied all tangential coil orientations with a 10° step size, resulting in 630 unique coil placements
Compared to the reference model, the cortical E-field in the 2C model with spine had a relative error (RE) of 7.6 ± 1.6% over all coil positions and orientations and 4.7 ± 2.4% in the region of the strongest fields; omitting the spine, the error increased to 9.6 ± 2.4% (5.1 ± 2.2% in the region of the strongest fields)
Summary
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. Sometimes a simple VCM is entirely adequate: For example, for human TMS with the coil near the vertex, the location of the resulting E-field maximum can be computed quite accurately with a spherically symmetric VCM fitted to the local radius of c urvature[16]. Even for human TMS, such a spherical model can be inadequate when stimulating the frontal or occipital lobes: The E-field-magnitude prediction can be off by tens of percent—even when only considering the error relative to the E-field-magnitude prediction in the motor cortex, i.e., the prediction error relevant to a typical experiment where the desired stimulation intensity is proportional to individual motor threshold[16]. As more invasive recording techniques can be used in rodents than in humans, and as brain tissue is available for histological and molecular analysis, rodent TMS studies are valuable, e.g., in studying brain
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