P305 Effect of pulse width and phase on mep latency and its significance in inducing cortical plastic changes

Abstract

Introduction Repetitive transcranial magnetic stimulation (rTMS) has been found to be a promising non-invasive therapeutic tool for a variety of neuropsychiatric conditions. Variable pulse shapes can now be generated with a controllable pulse stimulator TMS (cTMS). We looked into two main characteristics of the pulse: the pulse width and the pulse direction. The pulse direction was expressed by the M ratio, which refers to the ratio between the amplitude of the main phase and the second phase i.e.: the lowest is monophasic while the highest is biphasic with two equal amplitude phases. Methods to investigate the effect of different pulse widths and phase directions in rTMS on plastic aftereffects, we compared MEP latencies among TMS pulses with different pulse durations (40 μs, 80 μs and 120 μs) using 0.2 M ratio pulses, and with different pulse directions (0.2, 0.6 and 1.0 M ratios) 80 μs wide pulses. We also examined effects of AP 900 pulses 1 Hz rTMS. Results During AP stimulation, results from 12 subjects showed that the narrower pulse shapes had longer onset and peak latencies while wider pulse shapes showed shorter latencies. rTMS results from 10 subjects showed that shorter pulse widths of 40 and 80 μs were inhibitory with the 40 μs rTMS having more consistent and persistent inhibitory effects. The widest pulse of 120 μs was excitatory in terms of the post stimulation MEP amplitude. The 0.2 M ratio rTMS produced inhibitory after-effects, while the 0.6 and the 1.0 ratio showed almost similar facilitatory effects. Conclusion The shorter pulse width stimuli caused preferentially inhibition as they stimulated a smaller number of neurons as a portion from the local circuit causing the stimulated neurons to desynchronize the firing of the local neuronal network. In comparison, the wider pulse stimuli likely stimulated a wider area setting the pace for the neuronal network firing rate. As for the phasicity, we believe that the inhibition brought by the monophasic 0.2 M ratio pulses could be accounted to repeated hyperpolarization of pyramidal cell, while the biphasic 0.6 and 1.0 M ratio pulses induce similar hyperpolarization followed by a steeper polarization thus producing facilitation. Figuring out the optimal pulse shape for either excitation or inhibition of the brain would help understand how its effect is actually propagating through local cortical networks and eventually being able to fine-tune its therapeutic efficacy.