The transition from spontaneous to preparatory movement in infancy

Intention lies at the core of human behavior, allowing flexible and goal-directed interaction with the environment. However, how it emerges in infancy remains poorly understood. Here we show that intentional action can emerge before overt reaching, revealing that 3-month-old infants display pupil dilation preceding self-initiated actions during interaction with a suspended mobile toy. This phasic pupil dilation first appeared after movement onset but gradually shifted to precede it through environmental interaction, reflecting the emergence of action preparation and prediction of action outcomes. This shift also implies that infants are capable of anticipating the timing of their forthcoming movements. Furthermore, the antecedent pupil dilation occurred only with efficient movements characterised by reduced muscle co-contractions. These findings suggest that infants learn to predict outcomes of their own actions and initiate particular movements at self-determined timings, marking the emergence of intentional action.