Objective: To characterize the behaviour of primary motor cortex and to determine appropriate measurement parameters for short-interval cortical inhibition (SICI) and intracortical facilitation (ICF) by paired-pulse transcranial magnetic stimulation (TMS) with the aid of MRI-based neuronavigation.
Methods: Paired-pulse TMS was targeted to the optimal cortical representation sites of the abductor pollicis brevis (APB) muscle in 48 healthy right-handed volunteers. Motor evoked potentials (MEPs) were recorded from the APB and abductor digiti minimi (ADM) muscles. The conditioning stimulus (CS) intensities were 80% and 90% and the test stimulus was 120% of the resting motor threshold (rMT). The interstimulus intervals (ISIs) were 3, 7, 13, 22 and 28 ms.
Results: Inhibition was observed at 3 ms with a CS of 80%. Facilitation emerged at ISIs of 7 and 13 ms with both CS intensities, more prominently with 90%. At ISI of 22 ms, facilitation was observed in ADM (p<0.01) but not in APB. No uniform amplitude change was observed at ISI of 28 ms. For both muscles, MEP latencies were shortened (p<0.01) at ISIs of 3 and 7 ms and prolonged (p<0.01) at 28 ms.
Conclusions: Inhibition is most prominent at ISI of 3 ms and CS of 80% of rMT, whereas CS of 90% of rMT and ISIs of 7 and 13 ms are preferable for facilitation. Latencies appear to be stable and independent indicators of both phenomena should be taken into account.
Significance: Both the latency and amplitude of MEPs are important parameters when paired-pulse paradigms are used in clinical studies.
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