Short- and intermediate-interval cortical inhibition and facilitation assessed by navigated transcranial magnetic...

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.

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.

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.

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.

Both the latency and amplitude of MEPs are important parameters when paired-pulse paradigms are used in clinical studies.