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Description: In a previous experiment, we used single-pulse TMS over M1 to examine corticospinal excitability (CSE) from an effector when it was task relevant (as a primary agonist) in a stop-signal task and when it was completely task-irrelevant. In 24 participants, CSE on successful stop trials was consitently suppressed relative to GO. However, compared to the task-irrelevant CSE suppression that did not defer across timepoints, greater CSE suppression was evident when the effector was task-relevant and this suppression increased from 150 to 250 ms. This finding is consistent with predictions from the pause-then-cancel model of action stopping in humans (Diesburg & Wessel, 2021). As CSE reflects the net excitatory and inhibitory influences to corticospinal neurons, it remains unclear to what extent the observed suppression arose from an increase in inhibitory neurons vs. a decrease in excitatory neurons. Short-interval intracortical inhibition (SICI) is a paired-pulse TMS technique that enables the indexing of GABA-A neural activity. Compared to Go trials, increased SICI has been found on Successful stop trials approximately 150 ms after signal onset and to similar degrees in a task-relevant effector and a highly proximal, but task-irrelevant effector (Coxon, Stinear, & Byblow, 2006, J Neurophysiol). Notably, this timepoint coincides with when global motor suppression (reduced CSE in distal, task-irrelevant effectors) is first evident, which is also the main signature of the proposed pause phase of action cancellation. Interestingly, the main signature of the cancel phase, the fronto-central P3 event-related potential has also been shown to be correlated with resting-state SICI (Hynd et al., 2021, J Neurophysiol). In the present experiment, we will examine SICI in an effector when it is task-relevant and -irrelevant to stopping. This study can thus inform whether GABA-A inhibitory neurons relate to the pause and cancel stages of action cancellation.