Abstract
A key challenge in multi-electrode transcranial electrical stimulation (TES) or transcranial direct current stimulation (tDCS) is to find a current injection pattern that delivers the necessary current density at a target and minimizes it in the rest of the head, which is mathematically modeled as an optimization problem. Such an optimization with the Least Squares (LS) or Linearly Constrained Minimum Variance (LCMV) algorithms is generally computationally expensive and requires multiple independent current sources. Based on the reciprocity principle in electroencephalography (EEG) and TES, it could be possible to find the optimal TES patterns quickly whenever the solution of the forward EEG problem is available for a brain region of interest. Here, we investigate the reciprocity principle as a guideline for finding optimal current injection patterns in TES that comply with safety constraints. We define four different trial cortical targets in a detailed seven-tissue finite element head model, and analyze the performance of the reciprocity family of TES methods in terms of electrode density, targeting error, focality, intensity, and directionality using the LS and LCMV solutions as the reference standards. It is found that the reciprocity algorithms show good performance comparable to the LCMV and LS solutions. Comparing the 128 and 256 electrode cases, we found that use of greater electrode density improves focality, directionality, and intensity parameters. The results show that reciprocity principle can be used to quickly determine optimal current injection patterns in TES and help to simplify TES protocols that are consistent with hardware and software availability and with safety constraints.
Highlights
Transcranial electrical stimulation (TES) is known as transcranial direct current stimulation and transcranial alternating current stimulation, depending on the nature of the applied current
Four trial cortical targets (Figure 5) have been selected to test the reciprocity-based targeting methods and compare them with the reference Least Squares (LS) and Linearly Constrained Minimum Variance (LCMV) methods: T1 is in the medial premotor cortex, T4 is in the premotor cortex as well, but more laterally, and T2 and T3 are in the primary motor cortex
We chose two targets to be on the crowns of gyri, where the cortex surface is almost parallel to the scalp, such that the targeting orientation is perpendicular to the scalp or radial (T1 and T3), and two targets on the walls of sulci, where the cortex is almost perpendicular to the scalp surface and the surface normal vectors are mostly tangential (T2 and T4)
Summary
Transcranial electrical stimulation (TES) is known as transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), depending on the nature of the applied current. Because the current levels are typically small (1–2 mA) and do not stimulate neuronal firing, the method is termed transcranial electrical neuromodulation (TEN). Even without stimulating neuronal firing, TES or TEN applications are capable to modify cortical excitability [1, 2] as well as brain rhythms and networks [3, 4]. TEN Optimization Based on Reciprocity magnetic stimulation (TMS), TES is a portable, cost-effective, and easy-to-use tool. It has been proposed to enhance cognitive skills such as memory or learning [11, 12]. This technique may become eventually an alternative for psychoactive drugs, as it ideally does not affect the entire brain indiscriminately, and it has minimal adverse side effects. The requirement for specific targeting of neural regions of interest (ROI) is to use a methodology minimizing, as much as possible, current applied to non-target areas
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