How the brain tissue shapes the electric field induced by transcranial magnetic stimulation

► The gyrification strongly shapes the field induced by TMS in gray and white matter. ► Strong fields can occur rather deep in white matter (WM), but not gray matter (GM). ► Brain tissue anisotropy affects the electric field in WM, but only weakly in GM. ► Large down-projecting WM bundles systematically enhance the field. ► As a result, WM anisotropy can result in localized field increases of up to 40%.