Theoretical investigation of transcranial alternating current stimulation using laminar model

Transcranial alternating current stimulation (tACs) has been gaining an increased interest in the last few years due to its capacity to modulate non-invasively high-order cortical processes, such as decision-making, language and sensory perception. Nevertheless, the underlying mechanisms of activation of this brain stimulation technique are still poorly understood. Herein, we use a finite element modelling (FEM) technique to investigate the penetration and focality of tACs in comparison to a time invariant (DC) stimulation. We show that AC stimulations generate cerebral fields that are an order of magnitude larger in the radial direction, approximately 5 times larger in the tangential direction and more focused than DC stimulations. We argue that the basis for this effect is the reduced scalp's conductivity, which minimizes the surface shunting of the stimulating currents. The outcomes of this study may help tACs users to design better protocols and interpret experimental results.