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. 2012 Dec 21;84(10):1161–1170. doi: 10.1136/jnnp-2012-304019

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(A) Short interval intracortical inhibition (SICI), defined as the stimulus intensity required to maintain a target motor evoked potential of 0.2 mV, as assessed by the threshold tracking transcranial magnetic stimulation technique. Intracortical inhibition is illustrated by an increase in the conditioned test stimulus intensity required to track the target response, while intracortical facilitation is indicated by a reduction in test stimulus intensity. In healthy controls, SICI develops between interstimulus intervals (ISI) of 1 and 7 ms, with two peaks evident at 1 and 3 ms as indicated by the arrows. Intracortical facilitation developed between ISIs of 10 and 30 ms. SICI is significantly reduced in both sporadic amyotrophic lateral sclerosis (SALS) and familial amyotrophic lateral sclerosis (FALS). (B) Averaged SICI, between ISI 1 and 7 ms, was reduced in two presymptomatic superoxide dismutase-1 (SOD-1) mutation carriers 6 months prior to the development of ALS. (C) Normalised SICI, expressed as a fraction of the SICI value measured at the first study, was reduced 8 months prior to development of ALS in a third presymptomatic SOD-1 mutation carrier.