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Longer cTMS pulse width switches 1 Hz inhibitory motor cortex rTMS aftereffects to excitation

      The motor evoked responses (MEP) to antero-posterior transcranial magnetic stimulation (TMS) have higher motor thresholds and longer latencies in comparison to PA current direction stimulation. Discrepancies extend to the plastic after effects of rTMS. These effects might be due to stimulation of different population of neurones or different segments of the neuron, because of their variable membrane properties. Here we investigate the effect of different pulse widths and phase directions of AP TMS on latencies, thresholds and plastic aftereffects. Using a controllable pulse stimulator TMS (cTMS) on 10 subjects we compared thresholds and MEP AP latencies among TMS pulses with different pulse durations (40 μs, 80 μs and 120 μs) using 0.2 M ratio pulses, and compared the two different pulse directions (0.2, 0.6 and 1.0 M ratios) with 80 μs wide pulses. We then applied 900 pulses of the selected pulse shapes at 1 Hz. As previously published, narrower pulse shapes had longer latencies and higher resting motor thresholds (RMT) while wider pulse shapes showed shorter latencies and lower RMT. Shorter pulse widths rTMS produced inhibition while the wider pulse shape exhibited excitatory after-effects. Unidirectional rTMS pulses produced inhibition, while bidirectional pulses showed inhibition during the stimulation but facilitatory after-effects. We conclude that the neuronal response to rTMS can be tuned by manipulating pulse width and directionality; we gather it might be attributed to the variable membrane properties of different neuronal segments. Pulse width plays a distinctive role already in measuring MT. Here we show that pulse width affects plasticity probably by targeting different segments of neurones.
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