Motor inhibition is a key control mechanism that allows humans to rapidly adapt their actions in response to environmental events. One of the hallmark signatures of rapidly exerted, reactive motor inhibition is the non-selective suppression of cortico-spinal excitability (CSE): unexpected sensory stimuli lead to a suppression of CSE across the entire motor system, even in muscles that are inactive. Theories suggest that this reflects a fast, automatic, and broad engagement of inhibitory control, which facilitates behavioral adaptations to unexpected changes in the sensory environment. However, it is an open question whether such non-selective CSE suppression is truly due to the unexpected nature of the sensory event, or whether it is sufficient for an event to be merely infrequent (but not unexpected). Here, we report data from two experiments in which human subjects experienced both unexpected and expected infrequent events during a two-alternative forced-choice reaction time task while CSE was measured from a task-unrelated muscle. We found that expected infrequent events can indeed produce non-selective CSE suppression—but only when they occur during movement initiation. In contrast, unexpected infrequent events produce non-selective CSE suppression relative to frequent, expected events even in the absence of movement initiation. Moreover, CSE suppression due to unexpected events occurs at shorter latencies compared to expected infrequent events. These findings demonstrate that unexpectedness and stimulus infrequency have qualitatively different suppressive effects on the motor system. They also have key implications for studies that seek to disentangle neural and psychological processes related to motor inhibition and stimulus detection.

运动抑制是一种关键的控制机制，它使人类能够根据环境事件快速调整自己的行为。快速施加的反应性运动抑制的标志之一是皮质脊髓兴奋性 (CSE) 的非选择性抑制：意外的感觉刺激导致整个运动系统的 CSE 抑制，即使在不活动的肌肉中也是如此。理论表明，这反映了抑制控制的快速、自动和广泛参与，这有助于对感官环境中意外变化的行为适应。然而，这种非选择性 CSE 抑制是否真的是由于感觉事件的意外性质，或者事件仅仅是不频繁（但并非意外）就足够了，这是一个悬而未决的问题。这里，我们报告了来自两个实验的数据，其中人类受试者在两个可选的强制选择反应时间任务中经历了意外和预期的偶发事件，而 CSE 是从与任务无关的肌肉测量的。我们发现预期的偶发事件确实可以产生非选择性 CSE 抑制——但仅当它们发生在运动开始时。相比之下，即使在没有运动启动的情况下，意外的偶发事件也会产生相对于频繁、预期事件的非选择性 CSE 抑制。此外，与预期的偶发事件相比，由于意外事件导致的 CSE 抑制发生在更短的延迟。这些发现表明，意外和刺激频率对运动系统有质的不同抑制作用。