Goetz et al. (2016) used the above rTMS protocol to assess their cTMS device. The structure of the session involved an initial excitability probing train that lasted approximately 13 minutes. The train consisted of 80 pulses, comprising equal number of 4 different proving pulse conditions, to establish a baseline. The rTMS intervention was performed, utilising 1 of 4 pulse shapes in a 1 Hz, 1000 pulse train at 97.5% of the rMT. Finally, a corresponding probing train of 180 pulses was applied immediately after the rTMS intervention to quantify excitability changes.

The Elevate transcranial magnetic stimulation (TMS) devices offer the flexibility to modify rectangular pulse shapes, including monophasic or biphasic pulses, with adjustable pulse width. In one study, rTMS was applied at a frequency of 10Hz for 16 minutes, using four different pulse shapes targeted at the motor cortex (Goetz et al., 2016). The results indicated that while all monophasic pulse shapes had lasting inhibitory effects on motor evoked potential (MEP) amplitude, the biphasic pulse shape did not produce a similar effect. Among the monophasic pulses, the most effective in producing offline effects was the semi-monophasic pulse with a posterior-anterior (PA) current flow. These findings suggest that a biphasic pulse may not be optimal for inducing long-lasting changes in cortical excitability. 

Another experiment investigated the impact of varying pulse width, while keeping pulse frequency constant, on the offline effect of rTMS on cortical excitability (Halawa et al., 2019). In this study, a monophasic pulse with pulse widths of 40μs, 80μs, or 120μs was used, with 900 pulses delivered at a frequency of 1Hz. The results revealed that pulse width influenced both the direction and magnitude of the offline effect on MEP amplitude. A longer pulse width (120μs) led to an increase in MEP amplitude, while shorter pulse widths (40μs or 80μs) induced an inhibitory effect. Overall, these findings suggest that the presence of an inhibitory or excitatory effect depends on pulse width, while the magnitude of the overall effect is determined by pulse shape, with monophasic pulses inducing a current flow in the anterior-posterior (AP) direction appearing to be the optimal choice.

Furthermore, the effect of pulse shape and pulse width extends to the latency and amplitude of the motor threshold, indicating that different neural populations may be influenced depending on the magnetic pulse shape. Increasing the pulse width has been found to decrease MEP latency with AP currents (D’Ostilio et al., 2016; Halawa et al., 2019), while reducing the amplitude of the second phase of a biphasic pulse (making it more monophasic) reduces the motor threshold (Sommer et al., 2018). Similarly, decreasing the amplitude of the first phase of a biphasic pulse alters the MEP latency (Halawa et al., 2019; Sommer et al., 2018). These changes in MEP amplitude suggest that TMS pulse parameters can selectively target different populations of neurons, offering an opportunity to refine rTMS and single pulse protocols beyond the conventional biphasic sinusoidal waveform typically used in TMS research (Halawa et al., 2019; Sommer et al., 2018).

The effect of pulse shape and pulse width also affects the latency and amplitude of the motor threshold, suggesting that different neural populations might be affected depending on the shape of the magnetic pulse. Increasing the pulse width appears to reduce MEP latency with AP currents (D’Ostilio et al., 2016; Halawa et al., 2019) whereas reducing the amplitude of the second phase of the biphasic pulse (making it ‘more monophasic’) reduces the motor threshold (Sommer et al., 2018). Reducing the amplitude of the first phase of a biphasic pulse also appears to change the latency of the MEP (Halawa et al., 2019; Sommer et al., 2018). Such changes in MEP amplitude suggest that different populations of neurons could be selectively targeted by changing TMS pulse parameters (Halawa et al., 2019; Sommer et al., 2018), which can refine rTMS and single pulse protocols by going beyond the conventional biphasic sinusoidal waveform in TMS research.