Simulation Analyses of tDCS Montages for the Investigation of Dorsal and Ventral Pathways

Abstract

Modulating higher cognitive functions like reading with transcranial direct current stimulation (tDCS) can be challenging as reading involves regions in the dorsal and ventral cortical areas that lie in close proximity. If the two pathways are stimulated simultaneously, the function of dorsal pathway (predominantly used for graphophonological conversion) might interfere with the function of the ventral pathway (used for semantics), and vice-versa. To achieve functional specificity in tDCS for investigating the two pathways of reading, it is important to stimulate each pathway per session such that the spread of current across the cortical areas due to the two montages has minimal overlap. The present study intends to achieve this by introducing a systematic approach for tDCS analysis. We employed the COMETS2 software to simulate 10 montage configurations (5 for each pathway) for three electrode sizes: 5 × 5, 3 × 3, and 5 × 7 cm². This diversity in montage configuration is chosen since previous studies found the position and the size of anode and cathode to play an important role. The values of the magnitude of current density (MCD) obtained from the configuration were used to calculate: (i) average MCD in each cortical lobe, (ii) number of overlapping coordinates, and (iii) cortical areas with high MCD. The measures (i) and (iii) ascertained the current spread by each montage within a cortical lobe, and (ii) verified the overlap of the spread of current between a pair of montages. The analyses show that a montage using the electrode size of 5 × 5 cm² with the anode at CP5 and cathode at CZ, and another with anode at TP7 and cathode at nape of the neck are optimal choices for dorsal and ventral pathways, respectively. To verify, we cross-validated the results with ROAST. This systematic approach was helpful in reducing the ambiguity of montage selection prior to conducting a tDCS study.

Figure 1

(A) The location of the total number of nodes in Talairach space (grey color). The target region of interest for dorsal (supramarginal gyrus) and ventral pathway (middle/inferior temporal gyrus) are shown in yellow and cyan color dots, respectively. (B) The position of anode (orange square patches) at CP5 for dorsal pathway montages and at TP7 for ventral pathway montages for electrode for two electrode sizes 5 × 5 cm² (i) and 3 × 3 cm² in left lateral view of head model of COMETS (ii). Note in (i) that the two square patches of tDCS at anode positions CP5 and TP7 are overlapping with each other. The green dots over the head model are the 10–20 electroencephalography electrode positions. (C) The position of anode (red) and cathode (blue) on the head model for electrode size 5 × 5 cm² for dorsal (1) and ventral (2) pathways, respectively. (1) The position of anode is at CP5 and cathode is at (i) Cz, (ii) SO, (iii) contralateral maxilla, (iv) nape of the neck, and (v) contralateral homologous area CP4. (2) The anode (red) is placed at TP7 and the positions of the cathode (blue) were placed in the 5 locations as described in (1). (D) The COMETS output image showing the MCD distribution for a selected montage.

Figure 2

(A) The 5 dorsal pathway montages used in the study on anterior-posterior view of the head model of COMETS. The first column shows the position of anode (red) and cathode (blue) on the head model for electrode size 5 × 5 cm². The position of anode is at CP5 and cathode is at (i) Cz, (ii) SO, (iii) contralateral maxilla, (iv) nape of the neck, and (v) contralateral homologous area CP4. The second column shows the distribution of the MCD (magnitude of current density) across the cortex. The third column shows the distribution of the electric field potential. All the montages shows the ‘current source’ (areas of high potential marked in red) at the left hemisphere. The ‘current sink’ (areas of low potential marked in blue) were formed at (i) the vertex for cathode at CZ, (ii) anterior pole for cathode at SO, (iii) anterior-inferior cortex for cathode at maxilla, (iv) posterior-inferior cortex for cathode at nape of the neck, and (v) right hemisphere for cathode at contralateral homologous area. (B) Montages, MCD distributions, and electric field distributions for the 5 ventral pathway montages for electrode size 5 × 5 cm². The anode (red) is placed at TP7 and the positions of the cathode (blue) were placed in the 5 locations as described in (A).

Figure 3

(A,B) The lobe selectivity configuration analysis for dorsal (anode at CP5) and ventral (anode at TP7) route montages, respectively. The cathode was placed at (i) CZ, (ii) SO, (iii) Maxilla, (iv) Nape of Neck, (v) contralateral homologous areas. The montage CP5_CZ_5_5 and TP7_Neck_5_5 shows max_MCD (magnitude of current density) at left parietal lobe and temporal lobe (demarcated by *), respectively with least number of lobes crossing the avg_MCD line (−). (C,D) Shows the lobe selectivity configuration analysis for dorsal (CP5_CZ) and ventral (TP7_Neck) pathway montages for three electrode sizes (i) 5 × 5 cm², (ii) 3 × 3 cm², (iii) 5 × 7 cm². For CP5_CZ, there is significant difference in the max_MCD values seen at left parietal lobe (Red bar) with change in electrode size from (i) 5 × 5 cm², to (ii) 3 × 3 cm² (p < 0.01) (demarcated by *).

Figure 4

(A) The electric field intensity map of the simulated montages in the head model of ROAST for dorsal pathway with anode at CP5 and cathode at (i) CZ, (ii) SO, (iii) Maxilla, (iv) Nape of Neck, (v) Contralateral homologous areas. (B) The lobe selectivity configuration analysis with mean MEF (magnitude of electric field intensity) for dorsal pathway (anode at CP5). The cathode was placed at (i) CZ, (ii) SO, (iii) Maxilla, (iv) Nape of Neck, (v) contralateral homologous areas. The montage CP5_CZ_5_5 (i) shows max_MEF (magnitude of electric field) at left parietal lobe (demarcated by *) with least number of lobes crossing the avg_MEF line (−). (C,D) Same as (A) for ventral pathway montages. The montage with anode a TP7 and cathode at neck (iv) shows max_MEF at left temporal lobe (demarcated by *).

Figure 5

(A) The target regions of interest for dorsal and ventral pathways. (B) The highest number of overlapping coordinates is seen between CP5_CZ_5_5 and TP7_CZ_5_5. (C,D) Two poles of high MCD are formed for the ventral route montages TP7_SO_5_5 and TP7_TP8_5_5. (E) Moderate overlap is seen between CP5_CZ_5_5 and TP7_Maxilla_5_5. (F) Least overlap is seen between CP5_CZ_5_5 and TP7_Neck_5_5. The yellow and cyan color dots represent the coordinates that cross the 50% threshold limit for dorsal and ventral pathway montages. The red dots represent the overlap between these two montages.

Figure 6

(A,B) The number of overlapping coordinates for the montages CP5_CZ_5_5 and TP7_CZ_5_5 for two electrode sizes 3 × 3 cm² and 5 × 7 cm². (C,D) The number of overlapping coordinates for the montages CP5_CZ_5_5 and TP7_SO_5_5 for two electrode sizes 3 × 3 cm² and 5 × 7 cm². (E,F) The number of overlapping coordinates for the montages CP5_CZ_5_5 and TP7_Neck_5_5 for two electrode sizes 3 × 3 cm² and 5 × 7 cm². The yellow and cyan color dots represent the coordinates that cross the 50% threshold limit for dorsal and ventral pathway montages. The red dots represent the overlap between these two montages.

Figure 7

(A,B) Highest cluster magnitude of current density (CMCD) is seen at supramarginal and middle/inferior temporal gyrus for dorsal pathway montage (CP5_CZ_5_5) and ventral pathway montage (TP7_Neck_5_5), respectively. (C,D) Similar clusters are formed for two dorsal and ventral pathway montages (CP5_SO _5_5 and TP7_SO_5_5) where the cathode position is in the same SO position but the anode positions are different at CP5 and TP7.