Mohsen Mosayebi Samani - Department of Psychology and Neuroscience, Leibniz Research Center, Dortmund, Germany
Seyed Mohamad Firoozabadi - Department of Medical Physics, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran.
Hamed Ekhtiari - Neurocognitive Laboratory, Iranian National Center for Addiction Studies (INCAS), Tehran University of Medical Sciences, Tehran, Iran

Introduction: The response variability between subjects, which is one of the fundamental challenges facing transcranial direct current stimulation (tDCS), can be investigated by understanding how the current is distributed through the brain. This understanding can be obtained by means of computational methods utilizing finite element (FE) models. Materials and Methods: In this study, the effect of realistic geometry and white matter anisotropy on the head electrical current density intensity (CDI) distribution was measured using a magnetic resonance imaging (MRI)-derived FE model at the whole brain, below electrodes, and cellular levels. Results: The results revealed that on average, the real geometry changes the CDI in gray matter and the WM by 29% and 55%, respectively. In addition, WM anisotropy led to an 8% and 36% change of CDI across GM and WM, respectively. The results indicated that for this electrode configuration, the maximum CDI occurs not below the electrode, but somewhere between the electrodes, and its locus varies greatly between individuals.  In addition, by investigating the effect of current density components on cellular excitability, significant individual differences in the level of excitability were detected. Conclusion: Accordingly, consideration of the real geometry in computational modeling is vital. In addition, WM anisotropy does not significantly influence the CDI on the gray matter surface, however, it alters the CDI inside the brain; therefore, it can be taken into account, especially, when stimulation of brain’s internal regions is proposed. Finally, to predict the outcome result of tDCS, the examination of its effect at the cellular level is of great importance.

Brain, Finite Element, Individual Difference, tDCS