Quadripulse transcranial magnetic stimulation (QPS) is an innovative technique of repetitive transcranial magnetic stimulation (rTMS) that can modulate cortical excitability (Kaneko et al., 2020). It involves delivering four bursts of monophasic TMS from four independent magnetic stimulators, with each QPS burst followed by a five-second inter-burst interval (IBI). 

A thorough exploration of the inter-stimulus interval (ISI) within a QPS burst revealed that shorter ISIs led to increased motor evoked potential (MEP) amplitudes for a minimum of 75 minutes. Conversely, longer ISIs of 50ms in quadripulse stimulation suppressed MEPs for at least 75 minutes. The efficacy of QPS in modulating cortical excitability appears to be influenced by the inter-burst interval (IBI), with an optimal IBI of approximately 5 seconds (Hanajima et al., 2017). 

Compared to other non-invasive brain stimulation protocols, QPS exhibits reduced variability in its after-effects. Approximately 80% of younger participants demonstrate effects of QPS-50ms, with 69% displaying a measurable response through changes in MEPs (Hanajima et al., 2017; Nakamura et al., 2017). The reliability of QPS in modulating cortical excitability seems to stem from its utilisation of repetitive monophasic pulses instead of biphasic pulses. 

In a systematic comparison between monophasic and biphasic quadripulse QPS protocols with identical parameters, it was observed that the increases in motor MEP amplitude induced by QPS-5ms, and the decreases induced by QPS-50ms were greater when monophasic pulses were employed rather than biphasic pulses (Hamada et al., 2007). 

Monophasic pulses not only generate larger effects on the MEP but also exhibit sustained effects that persist beyond one hour. The utilisation of monophasic pulses in QPS may contribute to the reduced variability observed compared to other biphasic rTMS protocols. It also accounts for a higher number of responders following QPS application. 

Kimura et al., (2022) examined the effects of a newly developed QPS stimulator called DuoMAG MP-Quad and found that it was effective in inducing the desired after-effects. The study involved thirteen healthy participants who underwent two sessions of QPS using different parameters. The results showed significant modulation of motor evoked potentials (MEPs) after QPS, with QPS5 producing enlargement of MEPs and QPS50 resulting in reduction. The findings confirm the effectiveness of QPS in modulating brain activity and suggest that the newly developed stimulator is a promising tool for further research and treatment.