Introduction:High frequency repetitive transcranial magnetic stimulation induces excitation when applied to the motor cortex, as measured by increased amplitudes of motor evoked potentials (MEPs) after the intervention. Different pulse widths induce different effects, likely by primarily stimulating distinct segments of neurons due to the different membrane properties of different neuronal segments.Here we focus on the aftereffects generated with high frequencies of controllable pulse TMS (cTMS).Objectives: To investigate the influence of different stimulation intensities, pulse widths and direction of high frequency rTMS on its excitatory plastic aftereffects as reflected by MEP amplitudes. Methods:Using a controllable pulse stimulator TMS (cTMS), we stimulated the hand motor cortex of 20 healthy subjects with 5 Hz rTMS with 1200 bidirectional pulses with the main component widths of 80 and 120 microseconds. 80% resting motor threshold (RMT) intensity was investigated for both directions anterior-posterior (AP) and vice versa (PA), and at 90% RMT in the AP direction for the second experiment.Results: 80% HF rTMS did not produce any significant excitation in both AP and PA directions. 90% RMT AP stimulation exhibited a significant excitation aftereffect with the 120 microseconds wide pulses, whereas the 80 microseconds failed to produce significant excitation. Conclusions:Higher intensity and longer HF pulse rTMS are more efficient in producing excitatory aftereffects, this may be due to the different membrane properties of the various neuronal segments such as dendrites whose activation plays a major role in long term potentiation.Significance: The findings here may contribute to better results in future clinical studies performed with longer cTMS pulses. Figure 1: Diagram of the experiment timeline for each session of the experiment. Methods: ParticipantsFor the first experiment, we recruited fourteen subjects, four males and ten females, mean age was 23.5 ± 2.6 SD years. For the second experiment, a different set of fifteen healthy volunteers was recruited, four males and eleven females, the mean age was 24.1 ± 3.1 SD years. All participants were right-handed and free from any neurological or psychiatric disorders, took no centrally acting medications, and had no contraindications to TMS. Because the power cap of the device and the standard coil for the wider pulse shapes prevented intensities above 50% of maximal stimulator output (MSO), a higher resting motor threshold, i.e. above 70% MSO for a Magstim 200 2 device, was an exclusion criterion.We obtained written informed consent from each subject before participation.The local ethics committee of the University Medical Center Göttingen approved the study protocol, which conformed to the Declaration of Helsinki. RecordingsMotor evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) muscle of the right hand with surface Ag-AgCl electrodes in a belly-tendon montage. The electromyography signals were amplified, band-pass filtered (2 Hz-2...
Introduction: High frequency repetitive transcranial magnetic stimulation applied to the motor cortex causes an increase in the amplitude of motor evoked potentials (MEPs) that persists after stimulation. Here, we focus on the aftereffects generated by high frequency controllable pulse TMS (cTMS) with different directions, intensities, and pulse durations.Objectives: To investigate the influence of pulse duration, direction, and amplitude in correlation to induced depolarization on the excitatory plastic aftereffects of 5 Hz repetitive transcranial magnetic stimulation (rTMS) using bidirectional cTMS pulses.Methods: We stimulated the hand motor cortex with 5 Hz rTMS applying 1,200 bidirectional pulses with the main component durations of 80, 100, and 120 μs using a controllable pulse stimulator TMS (cTMS). Fourteen healthy subjects were investigated in nine sessions with 80% resting motor threshold (RMT) for posterior-anterior (PA) and 80 and 90% RMT anterior-posterior (AP) induced current direction. We used a model approximating neuronal membranes as a linear first order low-pass filter to estimate the strength–duration time constant and to simulate the membrane polarization produced by each waveform.Results: PA and AP 5 Hz rTMS at 80% RMT produced no significant excitation. An exploratory analysis indicated that 90% RMT AP stimulation with 100 and 120 μs pulses but not 80 μs pulses led to significant excitation. We found a positive correlation between the plastic outcome of each session and the simulated peak neural membrane depolarization for time constants >100 μs. This correlation was strongest for neural elements that are depolarized by the main phase of the AP pulse, suggesting the effects were dependent on pulse direction.Conclusions: Among the tested conditions, only 5 Hz rTMS with higher intensity and wider pulses appeared to produce excitatory aftereffects. This correlated with the greater depolarization of neural elements with time constants slower than the directly activated neural elements responsible for producing the motor output (e.g., somatic or dendritic membrane).Significance: Higher intensities and wider pulses seem to be more efficient in inducing excitation. If confirmed, this observation could lead to better results in future clinical studies performed with wider pulses.
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