The transcranial magnetic stimulation motor threshold depends on the distance from coil to underlying cortex: A replication in healthy adults comparing two methods of assessing the distance to cortex

Using transcranial magnetic stimulation (TMS), a hand-held electrified copp er coil against the scalp produces a powerful and rapidly oscillating magne tic field, which in turn induces electrical currents in the brain. The amou nt of electrical energy needed for TMS to induce motor movement (called the motor threshold [MT]), varies widely across individuals. The intensity of TMS is dosed relative to the MT. Kozel et al observed in a depressed cohort that MT increases as a function of distance from coil to cortex, This arti cle examines this relationship in a healthy cohort and compares the two met hods of assessing distance to cortex. Seventeen healthy adults had their TMS MT determined and marked with a fidu cial. Magnetic resonance images showed the fiducials marking motor cortex, allowing researchers to measure distance from scalp to motor and prefontal cortex using two methods: 1) measuring a line from scalp to the nearest cor tex and 2) sampling the distance from scalp to cortex of two 18-mm-square a reas. Confirming Kozel's previous finding, we observe that motor threshold increa ses as distance to motor cortex increased for both methods of measuring dis tance and that no significant correlation exists between MT and prefontal c ortex distance. Distance from TMS coil to motor cortex is an important determinant of MT in healthy and depressed adults. Distance to prefontal cortex is not correlat ed with MT, raising questions about the common practice of dosing prefontal stimulation using MT determined over motor cortex. Biol Psychiatry 2001;49 :454-459 (C) 2001 Society of Biological Psychiatry.