Physics of effects of transcranial brain stimulation

The effects of transcranial stimulation are determined to a large extent by the spatial distribution and temporal variation of the electric field produced in the brain. In this chapter, we first describe some of the factors that affect the electric field in the brain generally, such as the effect of tissue heterogeneity and anisotropy, or focality. These are common to transcranial magnetic stimulation (TMS) and to several forms of transcranial current stimulation (tCS), such as transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial random noise stimulation (tRNS). Then the main features of the electric field in the human brain during TMS and tCS are presented. This knowledge is important to predict the location and extent of the stimulated region as well as the stimulation intensity in the brain.