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Abstract

Introduction: Accurate modeling of the electric fields caused by transcranial direct current stimulation (tDCS) or transcranial magnetic stimulation (TMS) can help to better understand the spatial distribution of the stimulation effects in the brain (Opitz et al., accepted; Thielscher et al., 2011). SimNIBS (Simulation of Non-Invasive Brain Stimulation) is an open-source pipeline for simulating the electric fields both for TMS and tDCS based on the Finite-Element method (FEM) and individualized head models generated from MR images (Windhoff et al., 2013 ). Here, we present a strongly improved version of SimNIBS that has an easily accessible graphical user interface (GUI) for setting up the simulations, as well as a unique and highly flexible tool for modeling realistic tDCS electrodes. We also provide a range of TMS coil models. Methods: The automatic creation of the head models is based on T1- and T2-weighted structural MR images. Using FreeSurfer (Dale et al., 1999) and FSL (Jenkinson et al., 2012) tools, the images are segmented in 5 tissue types: white matter, gray matter, cerebrospinal fluid, skull and skin. A high-resolution tetrahedral mesh is than generated from these segmentations. SimNIBS is capable of integrating anisotropic conductivities in white matter, if DTI data is provided. The electrode definition for tDCS simulations is highly flexible, allowing modeling of multi-layered electrodes, user defined shapes as well as plugs and holes. The electrode orientations and positions are defined by the use by clicking on the head mesh. The simulations may have as many electrodes as the user specifies. The electrodes are then automatically added onto the skin surface. For the TMS simulations, the user can choose between a variety of coils. The user specifies the coil position and orientation via mouse click on the head model. The coil model is then automatically arranged perpendicular to the local curvature of the skin. Results: We used SimNIBS to exemplarily demonstrate the effect of electrode design on the electric field that is generated in cortical gray matter by tDCS. The example shows that, for the relatively large electrodes (7x5 cm²) modeled, proprieties such as the conductivity and the placement of the plugs may significantly alter the electric fields in gray matter. This points out to the benefit of more realistic electrode models and highlights the need for a detailed documentation of these parameters in practical tDCS studies. Conclusions: We provide a flexible, easy to use and open-source tool for non-invasive brain stimulation research.