Abstract
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technology that modulates the excitability of the brain by delivering weak electric currents to the brain via scalp electrodes. Electrode configuration and injected current intensity are two important parameters in the tDCS design. This simulation study examined three commercially available electrode configurations, i.e. conventional low definition rectangular pad, high-definition Disc, and high-definition 4 x 1 with different electrode distances and different injected current intensity. Simulation results show that increasing the injected current intensity of HD-tDCS mainly increases the electrical field strength for all configurations. Both Disc and 4 x 1 high definition tDCS (HD-tDCS) have better focality than the conventional low-definition rectangular pad. Increasing the inter-electrode distance in HD-tDCS enlarges the electrical field strength and the depth of stimulation but reduces the focality. In motor rehabilitation, a trade-off needs to be made in the tDCS design to allow the electrical field reaching the white matter to facilitate the usage of the cortico-spinal tract without influencing other undesirable regions in the brain.
I. INTRODUCTION
Transcranial direct current stimulation (tDCS) is a neuro-modulatory technique that influences the excitability of the brain by delivering weak electric currents to the brain via scalp electrodes [2, 3]. Electrode configuration is a critical issue in the tDCS design. Conventional tDCS places large sponge electrodes, an anode and a cathode at a location on the scalp guided by the 10/20 EEG system. For motor rehabilitation, the anode is typically placed over the “target” area, such as primary motor area [4]. The cathode is typical placed at a distant location at the supraorbital fossa of contralateral hemisphere. In contrast to the conventional tDCS, a high-definition transcranial direct current stimulation (HD-tDCS) typically uses either specialized small electrodes in a 4 x 1 concentric configuration with one central electrode and four surrounding electrodes [5], or one central round electrode and one concentric large ring in a Disc configuration [6]. Previous modeling and simulation studies indicated that HD-tDCS improves spatial focality and may provide more targeted stimulation than the conventional tDCS [5]. However, no optimal stimulation paradigm regarding electrode configuration and amount of direct current applied is available for HD-tDCS. To address this problem, this study aims to examine the effects of electrode configuration (including electrode type and inter-electrode distance) and injected current intensity of tDCS on the spatial focality, strength of the electrical current field, as well as the depth of stimulation in the brain. The results from this simulation study may provide important reference for the practical use of HD-tDCS.
II. METHODS
This study uses computational models to examine the effects of different electrode localizations, distances and injected current intensities on current flow in the brain. Various software tools are available for these computations. We used the ROAST pipeline [1] in this study, because it is fully automated and allows for realistic modeling of the human head anatomy with relatively short run times. The ROAST pipeline is a realistic volumetric approach to simulate transcranial electric stimulation. It combines the segmentation algorithm of SPM12, a Matlab script for touch-up and automatic electrode placement, the finite element mesher iso2mesh and the solver getDP (see Fig. 1) [1].
Figure 1.
ROAST pipeline [1]
The different montages of tDCS examined in this study were simulated in ROAST. To run these simulations, a MNI152 head model was used. (Huang) The default electrical conductivities for each tissue were used: white matter (0.126 S/m); gray matter (0.276 S/m); CSF (1.65 S/m); bone (default 0.01 S/m); skin (0.465 S/m); air (2.5e-14 S/m); gel (0.3 S/m); electrode (5.9e7 S/m).
We simulated three different categories of electrode configurations for anodal tDCS over the left primary motor area (C3 in the standard10-20 EEG recording system):
1. Conventional low-definition (LD) rectangular pad: anodal sponge pad (50x70 mm) is placed over the target area (C3) with the long axis of anterior-posterior. Cathodal pad (50x70 mm) is on supraorbital fossa of contralateral hemisphere.
2. High definition Disc (HD Disc): one central round electrode (anodal, diameter 10 mm) is placed over the target area (C3), one concentric ring with opposite polarity (cathodal) is placed over the distant site. We simulated three different inner-outer diameters of the cathodal electrode: 30-35 (small), 35-40 (median), 40-45 (large) mm to examine the effect of inter-electrode distance on the electrical field of HD Disc.
3. High definition 4x1 (DH 4x1): one ring electrode (anodal, outer diameter 12mm, inner diameter 6 mm) is placed over the target area (C3), the other four are placed in a circle at equidistance with opposite polarity (cathodal). We simulated three different electrode distances, i.e., small, mediate, large, according to 10-5 (i.e., Cz, F3, T7, P3), 10-10 (i.e., C1, C5, FC3, CP3) and 10-20 (i.e., Cz, F3, T7, P3) EEG systems.
These montages were chosen in the simulation because they are already commercially available and relatively easy and safe to use [7]. All configurations were simulated for two different current intensities: 1 mA and 2 mA, since they are commonly used in experimental studies [8].
MRIcron was used to illustrate the electrical current in the different simulations. For visualization purposes, only the electric field in the grey and white matter are shown (the electric field in skin, skull, and CSF are set to zero). The peak values of the electric field in the grey and white matter were calculated using the max function in Matlab.
III. RESULTS AND DISCUSSION
Fig. 2 shows the simulation results for different configurations with different inter-electrode distances and injected current intensities. Table 1 shows the maximum electrical field strength in the grey matter for each setup. Table 2 shows the maximum electrical field strength in the white matter. Simulation results show that a larger injected current intensity yields larger electrical field strength with higher maximum electrical field strength in both grey matter ad white matter for all configurations. Increasing inter-electrode distance in HD-tDCS not only enhance the electrical field strength but also increase the depth of stimulation as shown in Fig. 2. However, a larger inter-electrode distance reduces the focality of the stimulation.
Figure 2.
Electrical field in the brain generated by different montages and different current intensity
TABLE I.
Maximum Electrical Field Strength (V/m) in Grey Matter
| 1 mA | 2 mA | |
|---|---|---|
| HD Disc small | 0.16 | 0.25 |
| HD Disc median | 0.17 | 0.34 |
| HD Disc large | 0.32 | 0.68 |
| HD 4x1 small | 0.08 | 0.17 |
| HD 4x1 median | 0.15 | 0.37 |
| HD 4x1 large | 0.33 | 0.49 |
| LD | 0.24 | 0.48 |
TABLE II.
Maximum Electrical Field Strength (V/m) in White Matter
| 1 mA | 2 mA | |
|---|---|---|
| HD Disc small | 0.12 | 0.24 |
| HD Disc median | 0.14 | 0.29 |
| HD Disc large | 0.16 | 0.33 |
| HD 4x1 small | 0.09 | 0.17 |
| HD 4x1 median | 0.12 | 0.25 |
| HD 4x1 large | 0.22 | 0.45 |
| LD | 0.25 | 0.49 |
In motor rehabilitation, a trade-off needs to be made in the tDCS design to allow the electrical field reaching the white matter to facilitate the usage of the cortico-spinal tract without influencing other undesirable regions in the brain. Fig. 3 shows the mean electrical field strength in the supplementary motor areas when the target area is the left primary motor cortex (the anodal electrode placed at C3). Both large distance HD 4 x1 and conventional low definition (LD) tDCS can result in large electrical field strength in the supplementary motor areas. In stroke rehabilitation, an anodal stimulation over the primary motor cortex in the lesioned hemisphere aims to increase the excitability of the primary motor cortex [9] and thereby improves the functional integrity of the damaged corticospinal tract. However, a spread electrical field over the secondary motor cortex, such as the supplementary motor cortex, may result in a side effect. This is because that anodal stimulation on the supplementary motor cortex may facilitate the usage of cortico-reticulospinal tract, which is known to be associated with post-stroke motor impairment in individuals suffering from moderate-to-severe stroke [10]. This simulation result may explain previous controversial finding in the use of conventional tDCS for stroke rehabilitation [11].
Figure 3.
Mean electrical field in supplementary motor area when the anodal electrode placed at C3
IV. CONCLUSION
This simulation study investigated the influence of electrode configuration and injected current intensity of tDCS on the electrical field strength in the brain. Our results show that increasing the injected current intensity of HD-tDCS mainly increases the electrical field strength for all configurations. Both Disc and 4 x 1 high-definition tDCS (HD-tDCS) have better focality than conventional low-definition rectangular pad. Increasing the inter-electrode distance in HD-tDCS enlarges the electrical field strength and the depth of stimulation but reduces the focality. In motor rehabilitation, a trade-off needs to be made in the tDCS design to allow an improved focality and sufficient stimulation depth.
Clinical Relevance—
Detailed simulation in this study provides important reference for the practice use of HD-tDCS in motor rehabilitation.
Acknowledgments
The research leading to these results has received funding support from the support of the Dixon Translational Research Grants Initiative at Northwestern Medicine and Northwestern University Clinical and Translational Sciences Institute (UL1TR001422) and NIH 1R21HD099710.
Contributor Information
Caroline Mackenbach, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL for her intern. She is currently with Delft University of Technology, Delft, The Netherlands.
Runfeng Tian, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Yuan Yang, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL.
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