Stability of cortical muscle representations: TMS motor mapping test-retest study
Cortical mapping with transcranial magnetic stimulation (TMS) is a promising approach for non-invasive investigation of the motor cortex in humans. However, apart from the parameters reflecting general corticospinal excitability, e.g., resting motor threshold (RMT) and mean amplitude of the motor evoked potentials, the reproducibility of other TMS motor maps metrics remains controversial or unknown. We studied a test-retest reliability of TMS cortical maps corresponding to multiple hand muscles. We took into account both standard size parameters such as map`s areas as well as novel parameters such as intricate muscle-specific excitability profiles. The study included 18 young healthy right-handed male volunteers. We used MRI-navigated TMS to stimulate left motor cortex in two mapping sessions separated by 5-10 days. For the mapping we used a grid of 53-58 points each being stimulated in a pseudo-random order five times. Second day TMS session was an exact repetition of the first day session. An analysis was performed using custom-made software TMSmap (http://tmsmap.ru/). We used intra-class correlation coefficient (ICC) to assess reliability of map areas, volumes and the extent of the different muscles overlap. For the quantative comparison of the cortical excitability profiles of individual muscles we utilized earth mover's distance metrics (EMD). We found that RMT remained the same across two testing sessions in all but two subjects in whom it changed by one percent. ICC for the same muscle representation could be considered as good (0.73-0.85) for areas, moderate for the extent of the different muscles maps overlap (0.7) and poor (0.45-0.49) for volumes. An average shift for hotspots was ~10 mm and for centers of gravity it was ~3 mm. When assessing individual excitability profiles, we found significantly smaller normalized EMD (higher reproducibility) for the same muscle representations across days than for the different muscle representations across days (P<0,0001). The obtained results provide evidence that not only general excitability but also other specific features including standard characteristics (areas, volume) and even excitability profiles of the cortical muscle representation can be reliably traced with TMS motor mapping. This in turn indicates that the existence of the complex TMS cortical representations doesn’t simply indicate stochastic fluctuations in the corticospinal excitability during TMS mapping procedure but rather demonstrates a possibility to probe with TMS cortical organization reflecting intricate descending projections relating to specific muscles.