Spatial extent of cortical motor hotspot in navigated transcranial magnetic stimulation

Background: Motor mapping with navigated transcranial magnetic stimulation (nTMS) requires defining a “hotspot”, a stimulation site consistently producing the highest-amplitude motor-evoked potentials (MEPs). The exact location of the hotspot is difficult to determine, and the spatial extent of high-amplitude MEPs usually remains undefined due to MEP variability and the spread of the TMS-induced electric field (E-field). Therefore, here we aim to define the hotspot as a sub-region of a motor map. New method: We analyzed MEP amplitude distributions in motor mappings of 30 healthy subjects in two orthogonal directions on the motor cortex. Based on the widths of these distributions, the hotspot extent was estimated as an elliptic area. In addition, E-field distributions induced by motor map edge stimulations were simulated for ten subjects, and the E-field attenuation was analyzed to obtain another estimate for hotspot extent. Results: The median MEP-based hotspot area was 13 mm² (95% confidence interval (CI) = [10, 18] mm²). The mean E-field-based hotspot area was 26 mm² (95% CI = [13, 38] mm²). Comparison with existing methods: In contrast to the conventional hotspot, the new definition considers its spatial extent, indicating the most easily excited area where subsequent nTMS stimuli should be targeted for maximal response. The E-field-based hotspot provides an estimate for the extent of cortical structures where the E-field is close to its maximum. Conclusions: The nTMS hotspot should be considered as an area rather than a single qualitatively defined spot due to MEP variability and E-field spread.