How anatomical details affect noninvasive brain stimulation in computational models

Abstract

Noninvasive brain stimulation (NIBS) is an exciting field of study that is becoming increasingly popular for its many therapeutic uses. Two of the most widely used types of NIBS are transcranial electric and magnetic stimulation (TES and TMS). NIBS takes advantage of the electrical properties of neurons by modifying neuronal behavior through externally applied electric fields. This is achieved by either passing a current through two or more electrodes (TES) or inducing electric fields via a time varying magnetic field (TMS). Today, the biggest problems facing the NIBS field are the variability of responses in experiments and clinical settings and translating findings from animal studies to humans. To work to address these problems, we employ the power of computational modeling, specifically finite element method (FEM) modeling. FEM modeling allows us to build head models and simulate TMS and TES induced electric fields. However, there are many factors that go into building accurate models and it is not always clear how important they are in estimating the NIBS induced electric fields. Therefore, in this dissertation I explain how we look at three different factors in building FEM models: inclusion of stroke lesions in pediatric models, changing head model size, and inclusion of muscle tissue. In this work we found that stroke lesions greatly influence variability of the TMS induced electric field, either increasing or decreasing the electric field strength depending on the TMS coil location. This indicates that individualized head models are key to planning future experiments because the complex morphology does not allow us to make a simple prediction about the electric field. Next, we found that head size plays a significant role in NIBS induced electric fields, both in spherical models and non-human primate (NHP) models. For TES the electric field strength exponentially decreases with increasing head size. But the TMS induced electric field strength first increases with head size and then decreases after a critical point based on the TMS coil size. Finally, we determined that muscle tissue is an important feature in NHP models for TES simulations and it increases the electric field strength, but the percent change can be influenced by anisotropic properties of the muscle. Overall, these results from modeling nonstandard cases suggest that individualized modeling with careful consideration of the model setup is vital to accurately predicting NIBS induced electric fields.