How TMS stimulates the cortex

Cortical stimulation vectors This diagram was prepared to illustrate how the phenomenon of directional specificity arises, in which a single coil at the vertex can stimulate muscles on either side of the body according to the direction of the current inside. The image shows how induced currents from a circular coil line up with the preferred stimulation directions of motor cortex, measured by Ueno (1996).

Presumed electric field in the cortex This image is a composite of electrical field measurements in a spherical model head (radius 7.5 cm) and a standard MRI slice. The red contour shows the field strength needed to depolarise neurons - the effective area of stimulation, which extends only as far as the outermost white matter. [Image provided by Dr Charles Epstein.]

Falloff of the induced electric field with distance The electric field induced by TMS coils of different sizes. [Image provided by Dr Charles Epstein.]

Induced current in an isotropic conductor The highly directional nature of the current induced by a double coil is clearly visible. This picture is adapted from a field diagram in Ueno (1996), and represents the field induced in an isotropic conductor by a double coil one unit above the plane. The field in real tissue would be broadly similar but different in many details.

Reference:

Ueno, S. Localized magnetic stimulation and nerve excitation models. In Nilsson J, Panizza M, Grandori F (eds). Advances in Magnetic Stimulation: Mathematical modeling and clinical applications. Advances in Occupational Medicine and Rehabilitation. Fondazione Salvatore Maugeri Edizioni, Pavia, Italy, Vol 2(2), May-August 1996.