Calculating the activating function in the human brain by transcranial magnetic stimulation

It has been shown that the negative gradient of the component of the electric field, called the activating function, contributes to nerve excitation. In this paper, the induced electric field in a 3-D human head model by transcranial magnetic stimulation (TMS) has been calculated by employing the impedance method. The activating functions are then obtained by gradient operations to the three components of the induced electric field. The activating functions are scattered over the different brain tissues. Peaks of the activating functions are presented at the interfaces of the different tissues. The largest values of the activating functions are presented in the cerebral spinal fluid. (CSF). Results in this paper will be beneficial to the TMS applications in clinic as well as in the brain research.