Abstract

 

Prefrontal transcranial direct current stimulation (tDCS) is a main tDCS application that modulates cortical excitability in the prefrontal cortex to enhance cognitive functions. However, tDCS outcomes has been controversial due to a large interindividual variability. Studies indicated that the tDCS-induced spatial electric field (EF) distribution varies due to a subject-specific anatomical structure of the head/brain. And, the E-field strength (EFS), with its orientation, is regarded as a determinant for modulatory effect. Therefore, electrode positioning, based on EFS at the stimulation target and its vicinity, may help reduce the interindividual variability.

This in-silico study was aimed to determine the optimized position of anode to maximize EFS at the left dorsolateral prefrontal cortex (L-DLPFC) and analyze optimization-induced changes of EF in two regions, L-DLPFC and the left hemisphere (LH).

Structural MRI T1 images of 30 adults were retroactively collected. For simulation, a bipolar symmetric montage was applied by initially placing the anode and cathode over F3 and F4 of the 10-20 system. The current intensity, 1 mA, was injected through disc-shaped electrodes (r=2.26 cm). Then, the anode position was optimized by selecting the position that maximizes EFSL-DLPFC among several candidate positions around F3. The EF distributions at the ROIs were analyzed and compared between the initial and optimized montages using EF intensity (EFS) and focality. The statistical analysis was performed with Wilcoxon rank sum test (the significance level=1%).

The optimized anode positions were placed more laterally to the initial ones across all subjects. The optimization increased EFSL-DLPFC by 55.28% (P<0.0001) and EFLH by 45.22% (P<0.0001). In addition, the focality of L-DLPFC rose by 36.38% (P<0.0001). The group analysis in the ‘fsaverage’ space (of FreeSurfer) also showed the similar results. Further studies are planned to investigate if EFS-based anode position enhances clinical outcome by reducing the interindividual variability.