Dr.-Ing. Konstantin Weise | A novel approach to localize cortical TMS effects

Despite the widespread use of transcranial magnetic stimulation, the mechanism and precise cortical location underlying the physiological and behavioral stimulation effects are still only coarsely known. So far, mapping strategies that systematically measure TMS effects only determine the best-fitting coil position and localize the stimulated cortical site only approximately and indirectly. Focusing on the motor cortex, we present a new method to reliably determine the stimulated cortical site at the individual subject level. This approach combines measurements of the motor evoked potentials (MEP) at different coil positions and orientations with numerical modeling of induced electric fields within a formal statistical testing framework. We identify sharply bounded cortical areas around the gyral crown of the motor hand area as the origin of the MEPs and show that the tangential field component is most relevant for the observed effect. Importantly, unique results can be obtained with relatively few measurements at specific coil positions and orientations. We use extensive uncertainty and sensitivity analyses to verify the robustness of the method and optimize its efficiency. Our approach improves the localization of the cortex area stimulated by TMS. It is generic and can potentially be applied to other modalities such as language mapping.

Poster