TMS field model links physics and physiology

Stereotactic positioning devices allow to exactly navigate the position of a TMS coil with respect to the individual cortical architecture. However, the exact stimulation site and the size of the neuronal pool stimulated still remain unknown. We used a common spherical model to predict the cortical stimulation site in the motor system. In 4 subjects motor responses were registered in a small hand muscle. A stereotactic positioning system allowed to measure continually the position of the figure-of-eight coil with respect to the individual cortical anatomy visualized in an anatomical 3d MRI scan. The coil was oriented perpendicular to the central sulcus and a hot spot was determined. Then motor responses were measured at several stimulation sites from threshold levels up to maximal responses, increasing stimulus intensity in steps of 10 (input-output function). The stimulation sites were placed in a line perpendicular to the central sulcus. With maximal stimulation intensity motor responses were obtained about 5 cm apart from the hot spot. For each stimulation site a sigmoidal function was fitted to the input-output data and the stimulation intensity for half-maximal motor responses was calculated. The distribution of the electric field strength was calculated on the cortical surface for each site using the spherical model. Finally, the region on the cortical surface was calculated where all different stimulation sites produced the same electric field strength (variance < 7). In all subjects that region was found within the hand knob of the precentral gyrus. Using the field model a prediction of the effective field strength is possible