Modulation-based TMS: plasticity effects of theta-burst stimulation
Wendt K., Sorkhabi MM., O'Shea J., Denison T.
Background: Theta-burst stimulation (TBS) is a widely-used plasticity induction protocol. Most TMS devices can only achieve TBS using biphasic pulses, due to the energy recovery required to sustain the repetition rate. However, monophasic pulses are hypothesised to more selectively recruit cortical neurons, which could increase the effectiveness of TBS. Pulse width modulation (PWM)-based devices recover more energy from each pulse, enabling monophasic TBS. Here, we use PWM-TMS and intermittent TBS to test the hypothesis that monophasic iTBS induces a stronger plasticity effect than biphasic iTBS. Methods: For this single-blinded within-subject study, monophasic and biphasic iTBS were applied to the primary motor cortex of 26 healthy right-handed volunteers (target N=30) using a custom-made PWM-based TMS device. Plasticity was quantified as % increase in the amplitude of motor evoked potentials (MEPs) post-TBS. Both protocols were applied at least one week apart in counter-balanced order. Two baseline blocks (30 pulses each) were applied at 120% of the resting motor threshold (RMT), followed by iTBS (600 pulses, mono-/biphasic) applied at 70% RMT. The predicted plasticity effect (increased MEPs) was assessed over a 1-hour follow-up period in 30-pulse blocks every 5-10 minutes. Results: In an interim analysis of n=26, rmANOVA revealed a significant effect of Time (F(8,200)=3.75; p<.01), reflecting the expected increase in MEP amplitudes after iTBS. There was a trend effect of Pulse Type (F(1,25)=3.67; p=.07), consistent with the hypothesis of a larger MEP increase after monophasic versus biphasic iTBS (mean difference: 7.2%). There was no interaction (Time*Pulse Type: F(8,200)=0.75; p=.65). Conclusions: Our study shows that using a PWM-based TMS device, monophasic and biphasic iTBS protocols can induce similar increases in plasticity. The interim analysis suggests a trend in the predicted direction of a larger plasticity induction effect with monophasic versus biphasic iTBS.