Has stopping stopped making sense? Disentangling the processes elicited by complex action cancellation paradigms

Action cancellation represents a fundamental aspect of human motor control. Nonetheless, the behavioural paradigms used to study it are methodologically diverse and capture an incompletely understood combination of processes. Advancing theoretical accounts of action cancellation have motivated increasingly complicated experimental designs, leading some authors to draw comparisons between "simple" and "complex" stopping tasks. The latter describes a mix of paradigms, which variably require increased working memory demands, stimulus discrimination, distractor interference, or more complex stimulus-response mappings. This narrative review provides an accessible explanation of commonly implemented complex stopping tasks, selected based on their relevance to current theoretical debates. Throughout, we describe the challenges associated with interpreting performance in these tasks and highlight future directions for research. Notably, much of the past work using these tasks has applied traditional methods of indexing the speed of stopping, which were developed specifically for simple stopping tasks, and do not generalise well beyond this context. Recent research using cognitive modelling and physiological approaches has failed to replicate earlier findings, raising fundamental questions regarding how action cancellation should be indexed. Furthermore, physiological observations in complex stopping tasks appear to be incompatible with current accounts of the neurological mechanisms that underpin action cancellation. We propose potential ways forward, highlighting how the current lack of well-controlled comparisons between task types means the literature risks extensive task-specific findings that fail to generalise between laboratories, and-critically-beyond the laboratory.