Cancelling Movement or Pausing Involuntarily? Interpreting Behavioural Delays and Muscle Suppression in Inhibition Tasks

Abstract A prominent model of action stopping theorises that the reactive cancellation of movement is underpinned by two dissociable processes: a rapid, involuntary “pause” that transiently suppresses motor output, and a slower, voluntary cancellation of movement. As a fundamental component of motor control, the pause process has been posited to generalise broadly to infrequent and salient stimuli (irrespective of whether they bear an imperative to stop) and to be observable as suppression in electromyographical (EMG) recordings. Over two experiments (N = 24 in each), participants completed standard stop signal and flanker tasks, and novel task variants, where flanking arrows occurred infrequently, with or without a delay (relative to the central go stimulus), or coincident with a stop stimulus. Relative to a standard flanker task, presenting flankers infrequently specifically increased slowing in incongruent trials after at least three preceding trials without flanking stimuli, with no effect in congruent or neutral trials, indicating involuntary delays to infrequent events are sensitive to both stimulus features and the recency of perceptually similar events. Presenting flanker arrows of any congruence alongside a stop stimulus resulted in faster EMG suppression than stop stimuli alone, indicating that additional infrequent stimuli can facilitate inhibitory responses. However, presenting flanker stimuli at an equivalent frequency and delay outside of a stopping task did not reliably elicit EMG suppression or behavioural delays. The current findings challenge current models by demonstrating that physiological signatures previously interpreted as manifestations of a generalisable pause process are instead context specific, requiring the anticipation of action cancellation.