Electric field spread contributes to stimulation intensity-dependent expansion of navigated transcranial magnetic stimulation motor map.

Objective: Navigated transcranial magnetic stimulation (nTMS) is used for preoperative motor mapping, yet mechanisms of stimulation-intensity (SI)-dependent map expansion remain unclear. We investigated whether the extent of the induced electric field (EF) contributes to map expansion, using anatomical cavities adjacent to motor cortex as a model. Methods: Two patients with postoperative and developmental cavities underwent nTMS mapping at 105% and 120% of resting motor threshold (rMT). Motor evoked potentials (MEPs) were recorded within cavities and intact cortex. Associations among distance from the cortical margin, rMT, and MEP amplitude were assessed. Map extension from reference points (RPs) was quantified toward cavities and intact tissue. In the postoperative case, finite-element modelling compared EF distributions in native and reconstructed anatomy. Results: MEPs were elicited within cortex-free cavities despite absent cortical tissue beneath stimulation sites. Increasing distance from the cortical margin was associated with higher rMT and lower MEP amplitude. Higher SI expanded motor maps toward cavities and intact tissue. At 120% rMT, map extent from RPs was greater toward intact tissue, although directional distances are not physiologically equivalent because the functional motor-map boundary over intact cortex is unknown. Finite-element modelling showed high concordance between native and reconstructed anatomies, with a mean absolute EF difference of 1.34 V/m across stimulation sites. Conclusions: Positive nTMS sites within cortex-free cavities demonstrate spatial dissociation between nominal stimulation sites and the neural elements generating MEPs. Findings support a contribution of EF spatial extent to SI-dependent map expansion. Significance: Increasing SI improves response detection but may reduce spatial specificity.