Electric field distribution in deep brain structures during transcranial direct current stimulation (tDCS) with extracephalic montages: a computational study

Background and aims: Insights from computational models have suggested that transcranial direct current stimulation (tDCS) with extracephalic montages and cervical transcutaneous spinal direct current stimulation (tsDCS) might induce noteworthy E amplitude in deep brain structures. Here, we compare the trend of electric field (E) distributions in 4 regions of interest (ROIs - grey matter, hippocampus, thalamus and mid-brain) during tDCS with extracephalic montages. Methods: Sponge electrodes (σ = 1.4 S/m, 25 cm2, 1 mm of thickness) were modelled with a conductor (σ = 5.9 × 107 S/m) over the surface and a uniform electrical potential (± 1 V). The following montages (according to 10–20 system) were tested: I) Anode over Cz, C3 and C4; cathode over right deltoid; II) Anode over Cz, C3 and C4; cathode over the tenth thoracic vertebra. As outcome variables, we considered: the “peak” (99th percentile), the median, the 25th and 75th percentile of the E amplitude distribution in the ROIs; the percentage of area of hippocampus, mid-brain and thalamus where E amplitude was greater than 25% (V25), 30% (V30), and 50% (V50) of the peak in the grey matter for the same simulation. All the values considered were normalized to the 99th percentile of E in the grey matter for each montage. Results: Both montage I) and II) revealed values in the deep structures comparable to grey matter, with normalized peak values around 70% of cortical ones. Also, percentual volumes were similar in both conditions for hippocampus (V25 = 100%; V30 ≅ 100%; V50 ≅ 35%), thalamus (V25 = 90%; V30 ≅ 80%; V50 ≅ 20%) and mid-brain (V25 = 85%; V30 ≅ 60%; V50 ≅ 20%). Conclusions: Our results suggest that tDCS with extracephalic montages might warrant further exploration in the context of non-invasive deep brain stimulation (NDBS).