Is Navigated Transcranial Magnetic Stimulation Capable of Detecting Different Motor Cell Clusters Within the...

In 2021, Rossi et al. developed a cortical mapping algorithm that enabled a reliable mapping of an anatomical subdivision of the precentral gyrus. This approach allowed for the differentiation between an anterior subregion, characterized by lower excitability and slower motor responses, and a posterior subregion (named "M1 proper"), exhibiting higher excitability and faster motor responses. Based on these findings, we conducted a navigated transcranial magnetic stimulation (nTMS) study to investigate whether this anatomical subdivision can also be identified using a non-invasive mapping technique.

Fifteen healthy volunteers were examined. Using nTMS, the motor cortex at the hand knob area was subdivided into four distinct strips aligned parallel to the central sulcus: s, precentral non-proper, precentral proper, and postcentral. After determining the resting motor threshold (RMT), 10 stimulation points in each strip were stimulated with 150% RMT. Electromyographic (EMG) amplitudes and latencies were recorded and compared across the four stimulation strips to detect potential differences.

Although descriptively higher mean EMG amplitudes were observed in the precentral proper strip, generalized linear mixed-effects modeling revealed no statistically significant differences in EMG amplitude between the precentral proper and non-proper regions, nor across any of the four stimulation strips.

Using the applied protocol, a clear delineation between a functionally separated ventral and dorsal subdivision of the precentral gyrus could not be observed. However, it is noteworthy that the application of relatively high stimulation intensity may have masked subtle differences. Further examinations at the lower end of the stimulation spectrum (e.g., 90% or 100% RMT) or a specific paired pulse protocol may allow for a more precise differentiation between these two anatomical areas.