Single-pulse transcranial magnetic stimulation (TMS) was delivered (Magstim 200 2 , Magstim Co.
Ltd) using a figure-of-eight shaped coil (
D70 2 Coil, Magstim Co. Ltd). Participants were seated and maintained their hand and forearm relaxed with the wrist pronated throughout the experiment.
With a swimming cap marked with a 1 × 1 cm stimulation grid and orientated to the vertex of the head, the coil was located over the left hemisphere at a 45-degree angle to the sagittal plane to induce current in a posterior-to-anterior direction (Schabrun et al., 2016) (
link). Using surface disposable silver/silver chloride adhesive recording electrodes (
Ambu Neuroline 720) bipolar mounted in parallel with the muscle fibre, MEPs were recorded over the right ECRB muscle. The reference electrode was located on the right olecranon. MEP signals were band-pass filtered at 5 Hz -1 kHz, sampled at 2 kHz, and digitized by a 16-bit data-acquisition card (National Instruments,
NI6122).
The optimal cortical site (hotspot) of the right ECRB muscle was determined as the coil position that evoked a maximal peak-to-peak MEP for a given stimulation intensity. At the beginning of each session on Day-0 and Day-5, two measures were collected at the hotspot: 1) Resting motor threshold (rMT), defined as the minimum stimulation intensity at which 5 out of 10 stimuli applied at the hotspot evoked a response with a peak-to-peak amplitude of a minimum 50 μV (Schabrun et al., 2016) (
link). 2) Based on the MEPs of 10 stimuli at 120% of rMT at the hotspot site, the peak-to-peak amplitudes were extracted and averaged for analysis (Schabrun et al., 2016) (
link).
Using a TMS intensity of 120% rMT, the motor cortical map was established based on MEPs evoked every 6 s with a total of 5 stimuli at each site on the stimulation grid (Schabrun et al., 2016; (
link)De Martino et al., 2018) (
link). All grid sites were pseudo randomly stimulated from the hotspot until no MEP was recorded (defined as <50 µV peak-to-peak amplitude) in all five stimuli at all border sites (Schabrun et al., 2016) (
link). The number of active map sites (map area) and map volume were calculated off-line. If the average peak-to-peak amplitude of the 5 MEPs evoked at that site was greater than 50 μV, the site was considered 'active' (Schabrun & Ridding, 2007) (
link). The averaged peak-to-peak MEP amplitudes at all active sites were summed to calculate the map volume. The center of gravity (CoG) was defined as the amplitude-weighted center of the map (Wassermann et al., 1992) and was calculated by ; where Vi represents mean MEP amplitude at each site with the coordinates Xi, Yi (Uy et al., 2002) . For each session, the average peak-to-peak MEP amplitude at all sites across subjects were linearly interpolated to generate the MEP maps used for illustration of group effects.
De Martino E., Seminowicz D.A., Schabrun S.M., Petrini L, & Graven-Nielsen T. (2019). High frequency repetitive transcranial magnetic stimulation to the left dorsolateral prefrontal cortex modulates sensorimotor cortex function in the transition to sustained muscle pain. NeuroImage, 186.
