Principles of transcranial direct current stimulation (tDCS): Introduction to the biophysics of tDCS

This chapter summarizes the current knowledge about the biophysics of transcranial direct current stimulation (tDCS). It begins by illustrating the basic physical principles by which weak electric currents applied transcranially induce an electric field inside the brain. This knowledge, mainly derived from computational models, is essential to estimating the intensity and distribution of the electric fields that neurons and non-neuronal elements experience during transcranial stimulation. This knowledge alone, however, is not sufficient to predict the effects of tDCS on brain function. The second part of the chapter provides a review of basic concepts, mainly derived from animal models, to explain how electrical stimulation may affect neurons. At the single neuron level, membrane polarization, firing rate and timing changes are described. The effects of weak electric currents on single neuron function are then extended to neuronal populations and modulation of synapses, both during and after the application of the stimulation. Finally, recent results describing how these changes may ultimately affect behavior are reviewed.