IS 1. The electric field in the brain during tDCS

Abstract

Transcranial brain stimulation involves the interaction between an applied electric field and cells in the brain. In transcranial direct current stimulation (tDCS), this field is created by passing a weak current between two or more electrodes, at least one of which is placed on the scalp. The spatial distribution of the electric field and its orientation relative to the brain cells are two important factors that influence the outcome of stimulation. To describe how the shape and the electrical properties of tissues affect the electric field distribution and to show to what extent this distribution can be altered by using multiple electrodes. We developed a realistic head model based on anatomical MR images to compute the electric field using the finite element method. The values for the conductivity of the tissues were obtained from the literature. The electrode montage typically used to stimulate M1 was chosen. The calculations show that the low conductivity of the skull and the high conductivity of the cerebrospinal fluid (CSF) have a strong impact on the electric field. As expected, the effect of the skull is to decrease the electric field intensity and to broaden its spatial variation in the brain. On the other hand, the presence of the CSF leads to very distinct distributions of the normal and tangential components of the electric field on the cortical surface. The normal component is strongest near the bottom of the sulci under the electrodes whereas the tangential component is strongest in the crowns of the gyri between electrodes. Whereas the high values of the normal component are confined to the bottom or walls of the sulci, the high values of the tangential component spread out over the crowns. One of the shortcomings of the traditional montages that employ two identical electrodes is that the electric field has practically the same magnitude (and opposite polarity) under the two electrodes. Using our model we show that one way to overcome this limitation efficiently is to use one “active” electrode and multiple return electrodes to reduce the magnitude of the electric field under the return electrodes relative to that under the “active” electrode. This work provides novel insights into the electric field distribution in the brain during tDCS. The most pressing issue for the future is the physiological validation of its predictions. Also, the accuracy of the calculations can be improved by incorporating the anisotropy of skull and white matter in the model. Likely future developments include the rapid generation of subject specific head models or the optimization of electric field delivery using multiple electrodes. This work was supported in part by the Foundation for Science and Technology (FCT), Portugal and by project HIVE. The project HIVE acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, FET-Open grant 222079.