Abstract
Preparatory neural signals that precede movement onset are well established. However, whether they mark a commitment to act, align with conscious intention, or are subject to voluntary control remains under debate. We addressed these questions with a closed-loop experiment: a strategic timing game where a virtual opponent monitored intracortical sensorimotor activity and preempted participants' actions when their intention to move was detected. In three people with tetraplegia and preserved wrist extension, movement onset was reliably predicted from motor cortex 250-400 ms before action. Although participants attempted to act unpredictably, they were preempted on 94% of trials -and movement output generally followed despite the opponent's interruption. Participants reported intending to act on 88% of preemptions. The same decoder accurately predicted participants' actions in a non-competitive task, without retraining. These findings identify a late, task-independent motor-cortical state marking a voluntary commitment to act, which participants report as an intention when interrupted.