Fig. 1.

(A) The tracking target of 0.2 mV ( ± 20%) lies in the steepest portion of the stimulus response curve (green color), such that a large variation in motor evoked potential (MEP) amplitude translates to a small variation in transcranial magnetic stimulation (TMS) intensity. (B) When the MEP amplitude is larger than the tracking target (a) the TMS intensity is reduced on subsequent stimulus, while when MEP is smaller (b) the TMS intensity is increased. (C) SICI is mediated by inhibitory GABAergic interneurons located within the primary motor cortex, mostly layers 2 and 3, projecting onto pyramidal cells. (D) Short interval intracortical inhibition (SICI) is reflected by conditioned-test stimulus intensity being greater than zero, while the converse is true for intracortical facilitation (ICF). In amyotrophic lateral sclerosis (ALS) there is a significant reduction in SICI and an increase in ICF, indicative of cortical hyperexcitability. (E) Treatment with the anti-glutamatergic agent riluzole partially normalizes SICI in ALS patients.