Why tDCS models cannot be trusted yet? — A simulation study

Transcranial direct current stimulation (tDCS) has gained increased interest over the past decades due to its affordability, ease of use and wide range of applications. However, its lack of consistency and reproducibility of published results is rising concerns.
A potential solution to improve the method is to tailor the stimulation for each subject based on individual measurements and models. Such model requires accurate information about the geometry of the tissues composing the head of the subjects, about their electric properties and about the electrode montage.
In the present simulation work, we evaluate the effect of an error on the placement of the anode and of the unknown physical properties of the tissues on the induced electric field for 6 experiments on 20 subjects.
In addition to confirming the concerning small tDCS effect size, we show that the uncertainty on the conductivity parameters prevents any other conclusion to be drawn from such models.