Localization of cortical TMS effects and accurate motor threshold determination

Abstract We describe a routine to precisely localize cortical muscle representations within the primary motor cortex with transcranial magnetic stimulation (TMS) based on the functional relation between induced electric fields at the cortical level and motor evoked potentials. Besides insights into structure-function relationships, this lays the foundation for TMS dosing metrics based on subject-specific cortical field thresholds. Motor evoked potentials (MEPs) for different coil positions/orientations are combined with electric field modelling. Compared to previous approaches this routine exploits the causal nature of neuronal activation functions to pinpoint MEPs to their cortical origin.After constructing an individual head model, MEPs are recorded, and the induced electric fields computed. Finally, the cortical muscle representations are determined by relating the induced cortical fields to MEP amplitudes. Subsequently, the coil position which optimally stimulates this cortical position can be identified.The protocol requires 2 hours of manual preparation and 10 hours for the automatized head model construction, one TMS session (2h), 12h of computational post-processing, and an optional second TMS session (0.5h). A basic level of computer science expertise and standard TMS neuronavigation equipment suffices.