Background:
The dose of transcranial direct current Stimulation (tDCS) is defined by electrode montage and current, while the resulting brain current flow is more complex and varies across individuais. The left dorsolateral prefrontal cortex (1DLPFC) is a common target in neuropsychology and neuropsychiatry applications, with varied approaches used to position electrodes on subjects. TDCS is generally well tolerated; however, skin Sensation and burns caused by poor electrode assembly design have been reported.

Objective:
To predict brain current flow intensity and distribution using four conventional bicephalic frontal rectangular sponge-pad l x l electrode montages and novel HD-tDCS 3x1 and 4 xl disc electrode ring montages that nominally targeted the 1DLPFC. Furthermore, to investigate the impact of electrode assembly and skin micro-architecture on dermal current density (CD) clustering. Methods: Three MRI-scans were segmented in seven tissue compartments each in order to assign isotropic conductivities. Four electrode positioning methods for 5 x 5 cm2 rectangular sponge-pad electrodes over F₃-F₄were evaluated. Additionally, an EEG-tDCS cap layout containing HD electrodes was included to select electrode ring montages for Fß targeting. Also, a scalp model was derived to investigate dermal CD distribution. Results: The induced peak electric field in 1DLPFC decreases with an increase in overall head volume and by enlarging the electrode distance on the skin. Additionally, an electrode displacement towards thinner skull structures leads to the augmentation of cortically injected current. HD-tDCS provided enhanced spatial focality in combination with decreased Stimulation intensity. Electrode assembly designs with a housing that granted an extensive contact area and a large gel volume of a low conductivity resulted in decreased CD concentrations.

Conclusion:
Cortical electric field distribution fluctuations, for a given dose, are a function of inter-individual differences and pose the need for subject specific tDCS therapy. Novel, reproducible, ease-to-use electrode positioning methods were introduced to optimize 1DLPFC targeting. HD electrode ring montages for a novel closed loop EEG-tDCS device were found and electrode assembly design parameters were advanced.