Estimating the efficacy of transcranial magnetic stimulation in small animals

he efficacy of transcranial magnetic stimulation (TMS) is determined by the magnitude and direction of the induced electric field in the cortex. The electric field distribution is influenced by the conductivity structure, in particular, the size of the head and the shapes of conductivity boundaries. We show that neglecting the head size can result in overestimating the stimulus intensity by a factor of 5–8 in the case of the rat brain. In the current modelling literature, the TMS-induced electric field is estimated with detailed computational simulations; however, in many experimental studies, less attention is paid on modelling. We attempt to bridge this gap by suggesting the use of simple simulations, for example with the spherical head model, when studying bioelectromagnetic phenomena.