Use of electric field orientation as an index for estimating the contribution of model complexity in transcranial direct current stimulation forward head model development

The study evaluates the role of human skull composition and brain anisotropy in the context of transcranial direct current stimulation (tDCS) based predictive modeling. Four head models were developed and each proposed attribute (cancellous bone and brain anisotropy) was compared with the isotropic model. By employing a single high-definition montage, the efficacy of each attribute in shaping induced electric field was analyzed by its magnitude and orientation information. Relative error (RE) was used to estimate the variation in field magnitude. It was observed that for a given high-definition montage, the brain anisotropy contributed to 5% change (RE) in the strength of the gray matter (GM) electric field and 10% for the white matter (WM). Inclusion of diploe in the model resulted in 45% variation in the magnitude of the brain electric field. On average, brain anisotropy contributed to field deviations of up to 20 degrees in major WM fiber tracts. Skull heterogeneity caused field deviations of up to 35 degrees in diploe, 15 degrees in subcutaneous fat and marginal variations in brain regions. These simulation results demonstrated the importance of considering refinement in forward models of tDCS, especially; the role of diploe should be considered for more accurate field assessments.