Assessing Primary Motor Cortex Excitability and Excitability Modulation by Pairing Transcranial Magnetic Stimulation...

We describe a protocol to assess motor cortex excitability and its modulation, through paired use of electromyography (EMG) and repetitive transcranial magnetic stimulation (TMS) of the primary motor cortex (M1). EMG recordings use a custom-built system, and surface electrodes are placed over the right first dorsal interosseous (FDI) muscle, with a ground electrode on the left elbow. TMS is performed using a magnetic stimulator with a figure-eight coil, guided by a neuronavigation System. The motor hotspot for the FDI is identified by systematically applying single TMS pulses while adjusting coil placement, and is used as the target for all TMS procedures. The resting motor threshold (rMT) is defined as the minimum intensity required to elicit motor evoked potentials (MEPs) ≥ 50 µV in the EMG recording, in at least 5 out of 10 trials, while the active motor threshold (aMT) is the minimum intensity needed to evoke MEPs ≥ 200 µV in at least 5 out of 10 pulses while the participant maintains 10-20% of their maximal voluntary muscle activation. Baseline cortical excitability is defined as the mean MEP amplitude obtained from 40 single TMS pulses applied at 120% rMT, with pulses delivered at random intervals (6-10 s). Motor cortex excitability modulation is achieved using an intermittent theta-burst stimulation (iTBS) protocol (50 Hz pulse triplets, 80% aMT, 5 Hz for 2 s on/8 s off, repeated 20 times; 600 pulses total). Excitability modulation is assessed through repetition of MEP amplitude measurements at four post-stimulation time points: immediate (T0), 10 min (T10), 20 min (T20), and 30 min (T30). This protocol ensures precise and reproducible assessment of motor cortex excitability and captures the peak of the modulatory effects resulting from iTBS.