Appropriate tissue conductivities and their dependence on current and time in tDCS modeling

Determine which electrical conductivity values for each tissue should be used in finite element models of transcranial direct current stimulation and characterize how these tissue conductivities vary with stimulation current amplitude and over time during stimulation, given that most conductivity measurements are not performed with true direct-current test conditions and observed reductions in overall resistance suggest dynamic changes.

Background

The paper reviews the widespread use of the quasistatic approximation in neuromodulation modeling and emphasizes that most transcranial direct current stimulation models assign fixed, purely resistive conductivities to tissues. However, practical observations (e.g., changing impedance during stimulation) and the fact that many conductivity measurements are performed at non-dc frequencies raise concerns about the accuracy of these fixed choices.

In the discussion of tDCS modeling, the authors explicitly note unresolved issues in selecting appropriate conductivity values and understanding their potential changes with current and time, highlighting a need for better empirical grounding and modeling practices.