Long-interval facilitation and inhibition are differentially affected by conditioning stimulus intensity over different time courses

Intracortical facilitatory and inhibitory processes in the primary motor cortex (M1) play an important role in both the preparation and execution of motor tasks. Here we aimed to (1) confirm the existence of, and further characterise, intracortical facilitation at long conditioning-test stimulus intervals at subthreshold conditioning stimulus (CS) intensities and (2) identify the threshold for long-interval intracortical inhibition (LICI) at different inter-stimulus intervals (ISIs). To examine facilitation, stimulus-response curves at ISIs of 100 and 150 ms were obtained using a range of subthreshold CS intensities. LICI stimulus-response curves were also obtained using varying CS intensities at ISIs of 100 (LICI100) and 150 ms (LICI150). Facilitation of the conditioned MEP was observed at subthreshold CS intensities at an ISI of 100 ms. LICI100 was observed at a lower CS intensity than LICI150. First, we provide evidence of a long-interval facilitation and provide some evidence consistent with a cortical origin of this facilitation. Second, the lower threshold for evoking LICI100 than LICI150 suggests an intensity-duration effect whereby a more intense CS results in longer duration LICI. Investigation of the interaction between LICI and long-interval facilitation might help to elucidate the functional importance of these processes.