Comparing the speed of action initiation and action inhibition

Much work has examined the process of canceling or inhibiting an impending action. However, the converse process of initiating a movement has, comparatively, been much less studied. Action initiation and action inhibition are generally considered to be independent and qualitatively distinct processes, while the exact relationship between these processes remains unclear. One respect in which action initiation and action inhibition are often thought to differ is in their speed; action inhibition is typically considered to be faster than action initiation, which allows for impending actions to be inhibited before they are initiated. This apparent contrast is, however, largely observed in tasks in which there is much greater urgency to inhibit an action that there is to initiate an action. This asymmetry in the urgency between action initiation and action inhibition, as well as other asymmetries in how action initiation and inhibition are cued, make it impossible to compare their relative time courses. Here, we demonstrate that, when action initiation and action inhibition are measured under conditions that are matched as closely as possible, their speed is the same. In light of this, we suggest that action initiation and action inhibition may not necessarily be qualitatively distinct processes, but may instead reflect two opposing states of a single process supporting a decision about whether to act or not. This perspective carries significant implications for computational models and presumed neural mechanisms of action control.