Physics and biophysics fundamentals of transcranial stimulation
Transcranial stimulation induces or modulates neural activity in the brain through basic physical and biophysical processes. Transcranial electrical stimulation and transcranial magnetic stimulation impose an exogenous electric field in the brain that is determined by the stimulation device and the geometric and electric parameters of the head. The imposed electric field drives an electric current through the brain tissue, which macroscopically behaves as a volume conductor. The electric field polarizes neuronal membranes as described by the cable equation, resulting in direct activation of individual neurons and neural networks or indirect modulation of intrinsic activity. Computational modeling can estimate the delivered electric field as well as the resultant responses of individual neurons. This dosimetric information can be used to optimize and individualize stimulation targeting. The field distributions of transcranial stimulation are well understood and characterized, whereas analysis and modeling of the neural responses require further investigation, especially at the network level.
