simHotpoint module · motor hotspot and motor threshold training
TMS · Motor hotspot · Step-by-step training
Choose the muscle to record from in a virtual subject and place the MC-B70 figure-of-eight coil with its holder; stimulate wherever and as strongly as you like to find that muscle's motor hotspot and determine its resting motor threshold (rMT). Each step builds on the one before.
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Pulse animation: the magnetic field was computed in these slices with SimNIBS's MC-B70 model; the time course is the real biphasic waveform of the MagPro X100 (peak current ≈ 3.6 kA at 80 A/µs). The ~0.3 ms pulse is slowed down to ~5 seconds. In reality the field builds up everywhere at once; the "spreading" you see is a given |B| value (the iso-lines) being pushed away from the coil as the current grows. The moving dots show the in-plane direction of the field lines and reverse when the current changes direction. The E-field is proportional not to |B| itself but to its rate of change (dI/dt): at peak current E is zero. The grey body is the coil and holder (the holder arm is schematic; the drawing is scaled to 80% for legibility, the field was computed with the full-size coil); the part lying over the head is drawn faded.
Model: the MEP is a sigmoid function of the effective stimulation under the coil, with trial-to-trial cortical excitability and amplitude variability added. Each muscle has its own somatotopic hotspot, threshold, Mmax and latency. The spatial fall-off (σ = 20 mm) is calibrated from a SimNIBS sensitivity analysis in the ernie head model (10 mm shift → ~13% drop at the target). Hotspots and thresholds change at random with every new subject. For teaching only.
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