Background and Aims
Cortical mapping with transcranial magnetic stimulation (TMS) is a promising approach for non-invasive tracing of the changes in the motor cortex including post-stroke rehabilitation. Presently, however, only presurgical motor nTMS mapping has received a widespread use. There are several reasons for this situation. Firstly, apart from the parameters reflecting general corticospinal excitability, e.g., resting motor threshold (RMT), the reproducibility of other TMS metrics including most commonly reported cortical map’s area remains controversial. Secondly, despite the long history of TMS motor mapping there is still no general agreement considering the mapping protocol leading to the most reliable results. Thus, such questions as sufficient amount of the stimulation points and their repetitions are still under debate.
Here we studied a test-retest reliability of TMS cortical representations corresponding to multiple hand muscles. Besides standard parameters such as map`s area, hotspot and center of gravity (CoG) location we took into account an intricate muscle-specific complex profile of the cortical excitability potentially allowing a more refined estimation of the neuroplastic changes. Additionally, we were investigating the problem of the optimal mapping protocol based on the convergence of the TMS maps parameters including muscle-specific cortical excitability profile.
Method
The study included 18 young healthy right-handed male volunteers (19-33 y.o.). We used MRI-navigated TMS to stimulate left motor cortex in two mapping sessions separated by 5-10 days. Sessions consisted of 53-58 points on a grid covering left motor cortex stimulated in a pseudo-random order five times. Second-day session was an exact repetition of the first-day mapping session. Motor evoked potentials (MEP) were recorded using surface EMG from the tree hand muscles: abductor pollicis brevis (APB), abductor digiti minimi (ADM), extensor digitorum communis - (EDC). The analysis was performed using custom-made software TMSmap (http://tmsmap.ru/), which is illustrated in the video. We used intra-class correlation coefficient (ICC) to assess the reliability of map areas and volumes. For a quantitative comparison of the cortical excitability profiles of individual muscles, we utilized earth mover's distance metrics (EMD). 
Results
We found that RMT remained the same across two testing sessions in all but two subjects in whom it changed by only one percent. ICC for the same muscle representation could be considered as good for areas (0.73-0.85) and poor for volumes (0.45-0.49). An average shift for the hotspots was ~ 10 mm and for CoGs it was ~3 mm. When assessing individual excitability profiles, we found significantly smaller normalized EMD (higher reproducibility) for the same muscle representation across days than for the different muscles across days (Mann-Whitney criteria, P<0,0001). 
We also estimated a convergence of the TMS map's parameter depending on the stimulation points amount and showed that the sufficient amount of the stimulation points for stable size and topography parameters, including excitability profile of the motor map is roughly equal to 100 points. 
Conclusion
The obtained results provide evidence that not only general excitability and size characteristics (area) but also excitability profiles of the cortical muscle representation can be reliably traced with TMS motor mapping. This in turn indicates that the existence of complex cortical motor representations does not simply indicate stochastic fluctuations in the excitability during the TMS mapping procedure but rather demonstrates a possibility to probe with TMS cortical organization reflecting intricate descending projections relating to a specific muscle which may be useful for the longitudinal studies in stroke patients going through motor rehabilitation.

EXCITABILITY PROFILE OF THE MOTOR TMS MAP AS A NEW PARAMETER FOR DYNAMICAL INVESTIGATION OF THE HUMAN MOTOR CORTEX