Estimation of structures of neural fibers in the human brain by vectorial magnetic stimulation

A method of focal and vectorial magnetic stimulation has been developed. The basic idea is to concentrate induced eddy currents in a target by a pair of opposing pulsed magnetic fields which can be produced by a figure-of-eight coil. Since the concentrated eddy currents at the target beneath the intersection of the coil flow in the direction parallel to the tangent of the two circular coils, vectorial stimulation can be achieved. The neutral fibers can be excited easily when the fibers are stimulated by the eddy currents which flow parallel to the neural fibers. Based on this principle, functional maps of the human motor cortex related to the hand and foot areas have been obtained. It was observed that an optimal direction of stimulating currents for neural excitation exists in each functional area in the cortex. To explain these vectorial characteristics in magnetic brain stimulation, a model of neural excitation elicited by magnetic stimulation was developed. The model explains the observation that the direction of the induced current vectors reflects both the functional and anatomical organization of neural fibers in the brain