Guidelines for precise and accurate computational models of tDCS.

Guidelines for precise and accurate computational models of tDCS To the Editor: During transcranial electrical stimulation, including trans-cranial direct current stimulation (tDCS), current is induced in the brain. Because different electrode montages result in distinct brain current flow, researchers and clinicians can adjust montage to target or avoid specific brain regions in an application specific manner. Though tDCS montage design often follow basic rules-of-thumb (e.g., increased/ decreased excitability ''under'' the anode/cathode), computational models of brain current flow during tDCS (also called ''forward'' models) provide more accurate insight into detailed current flow patterns, and in some cases challenge simplified electrode-placement assumptions. With the increased recognition of the value of computational forward models in informing tDCS montage design and interpretation of results, there have been recent advances in modeling tools and a proliferation of publications. 1-10 In considering new electrode montages, and especially in potentially vulnerable populations , 2,11-13 forward models are the main tool to relate the externally controllable dose parameters (electrode number, position, size, shape, current) with resulting brain current flow–the use and adequacy of these models is considered here. Computational models of tDCS range in complexity from concentric sphere models to high-resolution models based on individuals MRIs. The appropriate level of modeling detail depends on the clinical question being asked, as well as the available computational resources. Whereas simple geometries (e.g., spheres) may be solved analytically, 14 realistic geometries use numeric solvers, namely, finite element methods (FEM). Regardless of complexity, all forward models share the primary outcome of correctly predicting brain current flow during transcra-nial stimulation to guide clinical practice. Special effort has been recently directed toward increasing the precision of tDCS models, but complexity does not necessarily equate with accuracy or clinical value. To meaningfully guide clinical practice, attempts to enhance model precision must rationally balance detail (complexity) and accuracy. (1) Beginning with high-resolution (e.g., 1 mm) anatomic scans, the entire model work flow should preserve precision. Any human head model is limited by the precision and accuracy of tissue segmentation (''masks'') and of the assigned conductivity values. One hallmark of precision is that the cortical surface used in the final FEM solver should capture realistic sulci and gyri. In addition, with finer segmentation, reliable tissue conductivities at DC frequencies are needed. (2) Simultaneously, a priori knowledge of tissue anatomy and factors known to shape current flow should be applied to further refine segmentation. Particularly critical are discontinuities …