Anirban Dutta, Walter Paulus, and Michael A. Nitsche
transcranial DC stimulation, non-invasive brain stimulation, electric field, finite element analysis, activating function
Non-invasive brain stimulation (NIBS) such as transcranial direct current stimulation (tDCS) involves passing low currents through the brain and is a promising tool for inducing cortical excitability. However tDCS presents challenges in terms of optimization of the electrode placement and stimulation parameters especially in cases of heterogeneously damaged cortical structures. An 'objective method' is needed to determine the best tDCS protocol for cases such as stroke. In this study we investigated the effects of change in cathodal “return” electrode placement and anodal motor-cortex stimulating electrode size using an 'objective method' not explicitly based on the magnitude of current density or electric field but on an activating function which is the spatial change in the electric field along an axon. We assumed a simplified multi-shell finite element model of normative human head. We could conclude the following from our results: 1. An 'objective method' based on the electric field at a given target location is appropriate, 2. The direction of electric field is relevant especially when the target is the fiber tract, 3. The importance of the placement of the “return” electrode is more critical for smaller stimulating electrode. Moreover, optimizing tDCS with patient-specific 'relevant' but simple head-model may be necessary to make tDCS a viable option for stroke where heterogenously damaged cortical structures presents a challenge.
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