The influence of pulse shape and current direction of TMS on test–retest reliability, and variability of measurement outcomes

BACKGROUND: Pulse parameters of transcranial magnetic stimulation (TMS) critically affect the stimulation outcomes. There is, however, a lack of understanding on how these parameters influence test-retest reliability and variability of single-pulse TMS protocols. OBJECTIVES: This study aims to investigate the effects of four combinations of pulse shapes and current directions (TMS-waveform conditions) on outcome measurements, test-retest reliability, and variability. METHODS: Using robot-assisted neuronavigation, nineteen participants were stimulated with four TMS-waveform conditions in three repeated sessions within the same day. Sessions 1 and 2, and Sessions 1 and 3 were separated by 30 min, and approximately 7 h, respectively. The four TMS-waveform conditions were: biphasic and monophasic pulses delivered in either posterior-anterior (PA) or anterior-posterior (AP) current directions. TMS protocols investigated were resting/active motor thresholds, stimulus intensity for inducing 1 mV peak-to-peak motor-evoked potential (MEP) amplitude, corticospinal excitability measurements (MEP amplitudes and latencies) with three inter-stimulus intervals (ISIs) of 5, 10, and 15 s; input-output (I/O) curve, and cortical silent period. RESULTS: TMS-waveform influenced all spTMS protocol outcome measures except for the I/O. TMS pulses in the PA current direction generally induced less variable MEP amplitudes and latencies. Moderate to excellent test-retest reliability was also found for all protocols except the I/O, however TMS-waveform only influenced the reliability of the AMT and MEP latency protocols. Monophasic pulses in the PA direction were more reliable compared to pulses in AP for MEP latency, while biphasic pulses in the AP direction showed significantly lower reliability compared to other TMS-waveform conditions for the AMT. CONCLUSION: This systematic evaluation does shed more light on protocols and TMS pulse parameters that induce reliable and less variable measurements.