Effects of short latency afferent inhibition on short interval 1 intracortical inhibition

24 Peripheral nerve stimulation inhibits the motor cortex and the process has been termed 25 short latency afferent inhibition (SAI) at interstimulus intervals (ISI) of ~20 ms. The 26 objective of the present study is to test how SAI interacts with short interval intracortical 27 inhibition (SICI) under different stimulation conditions. We studied 20 healthy 28 volunteers. Surface electromyogram was recorded from the first dorsal interosseous 29 muscle. Using paired and triple pulse paradigms, how SAI interacts with SICI under 30 these different conditions was investigated. The effects of different conditioning stimulus 31 intensities (0.6 to 0.9 active motor threshold), SAI latencies (23 and 25ms) and ISI (2 and 32 3ms) for SICI were examined in rest and active conditions. SAI had inhibitory 33 interactions with SICI at different CS intensities for rest or active SICI, at SAI latencies 34 23 and 25ms. This interaction occurs at weak CS intensities for SICI when there was no 35 inhibition and SICI became facilitatory in the presence of SAI. This can be explained by 36 SICI inhibiting SAI and not by saturation of inhibition. The interaction between SAI and 37 SICI was greater for SICI at ISI of 3ms than 2ms, suggesting that different circuits may 38 be activated at these ISIs. We conclude that SAI and SICI have inhibitory interactions 39 which are influenced factors such as ISI and muscle activities, which should be 40 considered in design and interpretation of cortical interaction studies. 41 42