Optimizing electrode placement for transcranial direct current stimulation in nonsuperficial cortical regions: a computational modeling study

Abstract

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique for modulating neuronal excitability by sending a weak current through electrodes attached to the scalp. For decades, the conventional tDCS electrode for stimulating the superficial cortex has been widely reported. However, the investigation of the optimal electrode to effectively stimulate the nonsuperficial cortex is still lacking. In the current study, the optimal tDCS electrode montage that can deliver the maximum electric field to nonsuperficial cortical regions is investigated. Two finite element head models were used for computational simulation to determine the optimal montage for four different nonsuperficial regions: the left foot motor cortex, the left dorsomedial prefrontal cortex (dmPFC), the left medial orbitofrontal cortex (mOFC), and the primary visual cortex (V1). Our findings showed a good consistency in the optimal montage between two models, which led to the anode and cathode being attached to C4-C3 for the foot motor, F4-F3 for the dmPFC, Fp2-F7 for the mOFC, and Oz-Cz for V1. Our suggested montages are expected to enhance the overall effectiveness of stimulation of nonsuperficial cortical areas.

Supplementary information: The online version contains supplementary material available at 10.1007/s13534-023-00335-2.

Keywords: Finite element method; Noninvasive brain stimulation; Nonsuperficial cortical region; Optimal electrode montage; Transcranial direct current stimulation.

Fig. 1

A Illustration of finite element head models of H_sub1 and H_sub2 with segmented five different tissues: Scalp (red), skull (green), CSF (blue), brain (yellow), and eyeball (purple). Illustration of regions of interest (ROIs): left foot motor cortex, left dorsomedial prefrontal cortex (dmPFC), left medial orbitofrontal cortex (mOFC), and primary visual cortex (V1). B Illustration of electrode candidates based on an international 10–20 EEG configuration. The red and the blue represent the candidate for the anode and cathode, respectively. These positions were manually selected with consideration of the location of each ROI. (Color figure online)

Fig. 2

The average electric field intensity of AE_absolute (left column) and AE_normal (right column) among various electrode montages for the stimulation of the left foot motor cortex in both head models. The different colors of bar graphs correspond to different positions of the anode depending on the position of the cathode. (Color figure online)

Fig. 3

A Distributions of electric fields with normal direction when applying the optimal and conventional electrode montage for stimulation of the left foot motor cortex in both head models. The placement of the anode (red) and cathode (blue) was shown beside the illustration of electric field distribution. B The streamlines of the electric field for the optimal electrode montage (left) and conventional electrode montage (right) for both head models. The location of the left foot motor cortex was shown with the black rectangle, and the color of the streamlines indicates the magnitude of electric fields. (Color figure online)

Fig. 4

The average electric field intensity of AE_absolute (left column) and AE_normal (right column) among various electrode montages for the stimulation of the left dmPFC in both head models. The different colors of bar graphs correspond to different positions of the anode depending on the position of the cathode

Fig. 5

A Distributions of electric fields with normal direction when applying the optimal and conventional electrode montage for stimulation of the left dmPFC in both head models. The placement of the anode (red) and cathode (blue) was shown beside the illustration of electric field distribution. B The streamlines of the electric field for the optimal electrode montage (left) and conventional electrode montage (right) for both head models. The location of the left dmPFC was shown with the black rectangle, and the color of the streamlines indicates the magnitude of electric fields. (Color figure online)

Fig. 6

The average electric field intensity of AE_absolute (left column) and AE_normal (right column) among various electrode montages for the stimulation of the left mOFC in both head models. The different colors of bar graphs correspond to different positions of the anode depending on the position of the cathode. (Color figure online)

Fig. 7

A Distributions of electric fields with normal direction when applying the optimal and conventional electrode montage for stimulation of the left mOFC in both head models. The placement of the anode (red) and cathode (blue) was shown beside the illustration of electric field distribution. B The streamlines of the electric field for the optimal electrode montage (left) and conventional electrode montage (right) for both head models. The location of the left mOFC was shown with the black rectangle, and the color of the streamlines indicates the magnitude of electric fields. (Color figure online)

Fig. 8

Distributions of electric fields in the normal direction for stimulating V1. The optimal montage (Oz–Cz), the montage determined by the highest AE_absolute (Oz–Fpz) and the electrode montage with placing cathode over extra-cephalic region (Oz-left cheek) were shown for both head models. The placement of the anode (red) and cathode (blue) were shown beside the illustrations of electric field distribution. Black squares in figures represent the location of the anode at Oz. (Color figure online)