Studies of 3-dimensional voltage distributions induced in homogeneous media volume conductors by round and butterfly magnetic coils

Magnetic coils (MCs) for physiological stimulation of the human nervous system are discussed. Using a butterfly MC at threshold intensity over the motor cortex, focal movements can be elicited in contralateral digits. These observations can in part be explained by physical models of homogeneous volume conductors which allow measurement of induced voltage gradients in 3-D space. In an infinite-volume conductor, a round MC induces the greatest voltage gradient under the windings. The butterfly MC induces a sharp, central peak separated from smaller lateral peaks