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.

中文翻译：

---

在经颅经颅磁刺激中皮质运动热点的空间范围。

背景

带有经颅经颅磁刺激（nTMS）的运动图绘制需要定义“热点”，即始终产生最高振幅的运动诱发电位（MEP）的刺激部位。热点的确切位置很难确定，并且由于MEP的可变性和TMS感应电场（电场）的扩散，高振幅MEP的空间范围通常仍然不确定。因此，此处我们旨在将热点定义为汽车地图的子区域。

新方法

我们分析了运动皮层上两个正交方向上的30名健康受试者的运动映射中的MEP振幅分布。根据这些分布的宽度，将热点范围估计为椭圆区域。此外，针对十个对象模拟了由运动图边缘刺激引起的电场分布，并对电场衰减进行了分析以获得热点范围的另一个估计值。

结果

基于MEP的热点面积中位数为13 mm ²（95％置信区间（CI）= [10，18] mm ²）。基于电场的平均热点面积为26 mm ²（95％CI = [13，38] mm ²）。

与现有方法的比较

与常规热点相反，新定义考虑了其空间范围，指示了最容易激发的区域，应以随后的nTMS刺激为目标，以实现最大响应。基于电场的热点提供了对电场接近其最大值的皮质结构范围的估计。

结论

由于MEP的可变性和电场扩展，nTMS热点应被视为一个区域，而不是一个定性的斑点。