tDCS Electrode Positioning
For most clinical applications, place the anode over the left dorsolateral prefrontal cortex (DLPFC) at position F3 using the 10-20 EEG system, with the cathode on the contralateral supraorbital region, using standard 5×7 cm electrodes at 2 mA for up to 30 minutes. 1, 2
Primary Target Selection
The DLPFC is the most frequently targeted region across clinical applications, with 77 out of 84 published studies selecting this target. 1 Specifically:
- Left DLPFC is the most common target, used in 19 tDCS studies 1
- Right DLPFC is the second most common, used in 17 tDCS studies 1
- Bilateral DLPFC stimulation is used in specialized protocols 1
Standard Electrode Configurations
For Left DLPFC Stimulation (Most Common)
For Right DLPFC Stimulation
- Anode placement: Over right DLPFC 1
- Cathode placement: Most commonly on left DLPFC (15 studies) or supraorbital region 1
Electrode Size and Current Parameters
Standard configuration uses 5×7 cm (35 cm²) electrodes at 2 mA current intensity for up to 30 minutes, which is considered safe based on behavioral and neuroimaging studies. 1, 2, 3
Alternative Optimized Configurations
For improved focality and specificity:
- Small electrode approach: 3.5×1 cm electrodes at 0.2 mA (maintaining same current density as standard) provides better specificity for targeted regions 4
- APPS-tDCS (Anterior Posterior Pad Surround): Using 1×1 cm electrodes positioned anterior and posterior to the target doubles on-target E-field magnitude (0.55 V/m vs 0.27 V/m with conventional montage) while reducing off-target stimulation 5
Critical Technical Considerations
Polarity Effects
- Anodal stimulation over the target enhances cortical excitability 1, 2, 3
- Cathodal stimulation over the target reduces cortical excitability 1, 2, 3
- These effects are mediated through long-term potentiation (LTP) and long-term depression (LTD) mechanisms 3
Localization Method Selection
The 10-20 EEG system (F3 for left DLPFC) is the standard clinical approach, though MRI-guided neuronavigation localizes the anode more latero-posteriorly, targeting the middle prefrontal gyrus rather than the intended DLPFC. 6 This discrepancy produces significantly different electric field distributions, though clinical significance remains under investigation. 6
Alternative Targets for Specific Applications
Beyond DLPFC, validated electrode positions include:
- Motor cortex (M1): Anode over C3/C4, cathode on contralateral supraorbital (2 studies) 1
- Inferior frontal gyrus: Anode over right IFG, cathode on supraorbital 1
- Temporoparietal junction: Cathodal stimulation with anode on occipital region (2 studies) 1
Motor Cortex Optimization
For leg motor area stimulation, a small anode (3.5×1 cm at 0.2 mA) with cathode at T7 provides superior specificity compared to conventional large-anode montages with contralateral supraorbital cathode. 4
For combined M1 and SMA stimulation, placing anodal and cathodal electrodes at FCz and POz respectively (A-P direction) enhances motor performance, while reversing polarity (P-A direction) modulates cortical excitability differently. 7
Common Pitfalls and Safety Considerations
Conventional anodal tDCS with large electrodes and contralateral supraorbital cathode often affects the contralateral hemisphere more strongly than intended, reducing specificity. 4
Anodal tDCS over motor cortex increases sympathetic nerve activity and shifts autonomic balance toward sympathetic predominance, warranting caution in patients with autonomic dysfunction. 8
The return electrode placement significantly impacts current distribution—supraorbital cathode placement is most common but may not optimize focality. 1
Reporting Requirements
Complete documentation should include electrode material, contact medium, size, geometry, placement relative to target, polarity, current density, and rationale for electrode location. 1 Modeling-based placement optimization should be described when utilized. 1