Fig. 2

Integrated multi-channel transcranial direct current stimulation and functional near-infrared spectroscopy (fNIRS) system. (A) Participants wearing the integrated device. (B) Close-up view of an multi-channel tDCS-fNIRS module. The integrated module comprises a pair of fNIRS optodes (one source and one detector) and a tDCS electrode. The tDCS electrode, coupled with a saline-soaked, ring-shaped sponge, administers current to the scalp. A rubber guide encircles the sponge, ensuring its stable placement. (C) Layout of the multi-channel tDCS-fNIRS system on the head. The system configuration is composed of 32 integrated modules, each containing two anodes and five cathodes for multi-channel tDCS, along with 16 sources and 16 detectors forming 51 fNIRS measurement channels. (D) Simulation results demonstrate the distribution of the normal electric field component (V/m) during multi-channel tDCS application using a standard anatomical template. The simulation was performed using the Neurophet tES LAB software (Neurophet, Seoul, South Korea). Red circles represent anodes, and blue circles represent cathodes. The positive values represent the inward electric field relative to the cortical surface, while negative values represent the outward electric field. The electric field concentration is notably strong around the M1 and aIPS regions, aligning with the anode placements