The electric field in the cortex during transcranial current stimulation was calculated based on a realistic head model derived from structural MR images. The aim of this study was to investigate the effect of tissue heterogeneity and of the complex cortical geometry on the electric field distribution. To this end, the surfaces separating the different tissues were represented as accurately as possible, particularly the cortical surfaces. Our main finding was that the complex cortical geometry combined with the high conductivity of the CSF which covers the cortex and fills its sulci gives rise to a very distinctive electric field distribution in the cortex, with a strong normal component confined to the bottom of sulci under or near the electrodes and a weaker tangential component that covers large areas of the gyri that lie near each electrode in the direction of the other electrode. These general features are shaped by the details of the sulcal and gyral geometry under and between the electrodes. Smaller electrodes resulted in a significant improvement in the focality of the tangential component but not of the normal component, when focality is defined in terms of percentages of the maximum values in the cortex. Experimental validation of these predictions could provide a better understanding of the mechanisms underlying the acute effects of tCS.
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