Computational modeling of transcranial direct current stimulation (tDCS) in obesity: Impact of head fat and dose guidelines

Abstract

Recent studies show that acute neuromodulation of the prefrontal cortex with transcranial direct current stimulation (tDCS) can decrease food craving, attentional bias to food, and actual food intake. These data suggest potential clinical applications for tDCS in the field of obesity. However, optimal stimulation parameters in obese individuals are uncertain. One fundamental concern is whether a thick, low-conductivity layer of subcutaneous fat around the head can affect current density distribution and require dose adjustments during tDCS administration. The aim of this study was to investigate the role of head fat on the distribution of current during tDCS and evaluate whether dosing standards for tDCS developed for adult individuals in general are adequate for the obese population. We used MRI-derived high-resolution computational models that delineated fat layers in five human heads from subjects with body mass index (BMI) ranging from "normal-lean" to "super-obese" (20.9 to 53.5 kg/m(2)). Data derived from these simulations suggest that head fat influences tDCS current density across the brain, but its relative contribution is small when other components of head anatomy are added. Current density variability between subjects does not appear to have a direct and/or simple link to BMI. These results indicate that guidelines for the use of tDCS can be extrapolated to obese subjects without sacrificing efficacy and/or treatment safety; the recommended standard parameters can lead to the delivery of adequate current flow to induce neuromodulation of brain activity in the obese population.

Keywords: Body mass index; Finite-element modeling; Head fat; Obesity; tDCS.

Figures

Fig. 1

Segmentation of five subjects with varying BMI (S1, S2, S3, S4, S#), six tissue compartment models [skin, fat, skull, cerebral spinal fluid (CSF), gray matter and white matter]. High-resolution MRI scans were segmented using a combination of automated and manual techniques. Specific anatomical considerations, such as continuity of CSF, were verified or corrected. Images are shown on the same scale.

Fig. 2

Resulting peak electric field magnitude simulated in three montages (M1–SO, 4 × 1 HD-tDCS over M1, IFG-SO) across subjects. Variations in intensity occur across individuals, but these individual variations are consistent in ranking across montages (S3 

Fig. 3

Influence of fat thickness in isolation. Fat was dilated isometrically with 3 mm of skin cover; other tissues were unchanged. A moderate increase in the thickness of fat caused little change in peak electric field. There was a slight increase (5.7%) in S# and a slight decrease (8.9%) in S4. Increasing the thickness of fat beyond that physiologically observed led to noticeable decreases in intensity in both S# (15.6%) and S4 (25.7%). Varying the thickness of tissues surrounding the brain not only changes the overall conductance, but also changes the orientation of the electrodes with respect to the brain.

Fig. 4

Skin current density across subjects and montages. The largest change in current density magnitude was between the montages utilizing HD electrodes versus conventional pads; interindividual differences are relatively minor. Peak current density was calculated per mA of stimulation.