Impact of uncertain head tissue conductivity in the optimization of transcranial direct current stimulation for an auditory target

Abstract

Objective Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique to modify neural excitability. Using multi-array tDCS, we investigate the influence of inter-individually varying head tissue conductivity profiles on optimal electrode configurations for an auditory cortex stimulation. Approach In order to quantify the uncertainty of the optimal electrode configurations, multi-variate generalized Polynomial Chaos (gPC) expansions of the model solutions are used based on uncertain conductivity profiles of the compartments skin, skull, gray matter, and white matter. Stochastic measures, probability density functions, and sensitivity of the quantities of interest are investigated for each electrode and the current density at the target with the resulting stimulation protocols visualized on the head surface. Main results We demonstrate that the optimized stimulation protocols are only comprised of a few active electrodes, with tolerable deviations in the stimulation amplitude of the anode. However, large deviations in the order of the uncertainty in the conductivity profiles could be noted in the stimulation protocol of the compensating cathodes. Regarding these main stimulation electrodes, the stimulation protocol was most sensitive to uncertainty in skull conductivity. Finally, the probability that the current density amplitude in the auditory cortex target region is supra-threshold was below 50%. Significance The results suggest that an uncertain conductivity profile in computational models of tDCS can have a substantial influence on the prediction of optimal stimulation protocols for stimulation of the auditory cortex. The investigations carried out in this study present a possibility to predict the probability of providing a therapeutic effect with an optimized electrode system for future auditory clinical and experimental procedures of tDCS applications.

Modeling sequence and quasi-uniform assumption in computational neurostimulation

“…Here, the framework is well agreed-upon, if not the specific tissue parameters that should be used in any given case Opitz et al, 2011;Schmidt et al, 2015;Wagner et al, 2014). The tissue properties are generated in forward models by first dividing the anatomy into individual masks, such as gray and white matter.…”

Using reciprocity for relating the simulation of transcranial current stimulation to the EEG forward problem

“…which can be used to visualize the sensitivity of this electrode pair to sources in the brain in just a single image for S i and having Equation (13) in mind [31]. More importantly, for relating the AA to tCS simulation, the solution vector w i of the adjoint PDE (9) and (10) can additionally be used to simulate a current density vector field J : R 3 → R 3 for a current injected at a point electrode r i and removed at another point electrode r 0 as:…”

“…Furthermore, white matter anisotropy modeling seemed important only for deeper target regions. Even though the predicted current flow patterns often fit to experimental results [8] and even allow to calculate individually optimized stimulation protocols [4,13], the reliability of computer simulation studies needs to be validated by 30 its own. One important source of errors, which has not yet sufficiently been investigated in tCS modeling, are numerical errors that may strongly influence the simulation results.…”

“…In clinical practice, the exact knowledge of current density orientation and magnitude is very important. Minor changes in the cortical current density vector field might 505 even strongly influence the decision with respect to placement of the electrodes and/or strength of stimulation [4,13]. Therefore, using a geometry-adapted FE approach might substantially increase accuracy and reliability of tCS simulation results and help to find optimized stimulation protocols.…”

Scalp‐to‐cortex distance of left primary motor cortex and its computational head model: Implications for personalized neuromodulation

“…Noninvasive brain stimulation (NIBS), including transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), encompasses a broad array of treatments that target the disease‐specific regions or networks to achieve desired outcomes in neurorehabilitation . It should be noted that TMS and transcranial eletrical stimulation (tES) use coil or electrode placed on the scalp to deliver a magnetic or electrical current through the scalp to the cortex where the stimulation levels are supposed to be attenuated with the distance . As highlighted in the NIBS guidelines, scalp‐to‐cortex distance (SCD) has been reported to critically influence the focality and strength of electric field induced by NIBS …”