To better understand the biophysics of TMS, computational models can be applied. Individual characteristics in brain anatomy, affecting TMS effects, can be better accounted for using modeling. Further, neuronal activation, and combination of temporal and spatial effects may be simulated for better understanding of neural plasticity and neuromodulation. To enable optimization of stimulation routines, computational modeling may be of help in neuromodulation therapy or for basic neuroscience.
A model geometry can be generated and TMS effects simulated using SimNIBS pipeline. Using experimental data from nTMS mapping, coil location, rotation and tilt can be implement for agreement between computational and experimental setups. Coil characteristics can be implemented based on the used coil geometry (from X-ray) and measured pulse characteristics. (Figure 1).
|Figure 1. Modeling of nTMS. A model of the head is constructed automatically from the anatomical MRI using SimNIBS pipeline. For TMS simulations, coil geometry and properties are implemented with help of an X-ray. Stimulus locations are taken from experiments to simulate TMS-induced electric field.|
Using simulations based on real head geometry, we can generate subject-specific models to estimate e.g., the extent of the activated cortical area, or the quality of presurgical mapping method. During presurgical mapping, it is essential to use appropriate intensity with nTMS, as the electric field induced in the cortex will spread far from targeted area if the intensity is too high (Figure 2).
|Figure 2. The spread of the TMS-induced electric field becomes larger when stimulus intensity is increased with the activating electric field exciting neurons further and further away from the stimulus focus.|