Dissociating Attentional Capture from Action Cancellation in the Stop Signal Task

Inhibiting ongoing responses when environmental demands change is a critical component of human motor control. Experimentally, the stop signal task (SST) represents the gold standard response inhibition paradigm. However, an emerging body of evidence suggests that the SST conflates two dissociable sources of inhibition, namely an involuntarily pause associated with attentional capture and the (subsequent) voluntary cancellation of action. The extent to which these processes also occur in other response tasks is unknown. 24 younger (20-35 years) and 23 older (60-85 years) adults completed a series of tasks involving rapid unimanual or bimanual responses to a visual stimulus. A subset of trials required cancellation of one component of an initial bimanual response (i.e., selective stop task; stop left response, continue with right response) or enacting an additional response (e.g., press left button as well as right button). Critically, both tasks involved some infrequent stimuli which bore no behavioural imperative (i.e., they had to be ignored). EMG recordings of voluntary responses during the stopping tasks revealed bimanual covert responses (i.e., muscle activation which was suppressed before a button press ensued), consistent with a pause process, following both stop and ignore stimuli, before the required response was subsequently enacted. Critically, we also observed the behavioural consequences of a similar involuntary pause in trials where action cancellation was not part of the response set (i.e., when the additional stimulus required additional action or ignoring, but not inhibition). The findings shed new light on the mechanisms of inhibition and their generalisability to other task contexts.