Short - and long - latency inter - hemispheric inhibitions are additive in the 1 human motor cortex 2

23 Transcranial magnetic stimulation (TMS) of the human primary motor cortex (M1) at 24 suprathreshold strength results in inhibition of M1 in the opposite hemisphere, a process termed 25 inter-hemispheric inhibition (IHI). Two phases of IHI, termed short-latency inter-hemispheric 26 inhibition (SIHI) and long-latency inter-hemispheric inhibition (LIHI), involving separate neural 27 circuits, have been identified. In this study we evaluated how these two inhibitory processes 28 interact with each other. We studied 10 right-handed healthy subjects. A test stimulus (TS) was 29 delivered to the left M1 and motor evoked potentials (MEPs) were recorded from the right first 30 dorsal interosseous muscle (FDI). Contralateral conditioning stimuli (CCS) were applied to the 31 right M1 either 10 ms or 50 ms prior to the TS, inducing SIHI and LIHI, respectively, in the left 32 M1. The effects of SIHI and LIHI alone, and SIHI and LIHI delivered together, were compared. 33 The TS was adjusted to produce 1 mV or 0.5 mV MEPs when applied alone or after a CCS 34 stimulus. SIHI and LIHI were found to be additive when delivered together, irrespective of the 35 strength of the TS. The interactions were neither affected by varying the strength of the 36 conditioning stimulus producing SIHI, nor by altering the current direction of the test stimulus. 37 Small or opposing interactions, however, may not have been detected. These results support 38 previous findings suggesting that SIHI and LIHI act through different neural circuits. Such 39 inhibitory processes may be used individually or additively during motor tasks, and should be 40 studied as separate processes in functional studies. 41