Abstract
The distinction between acting jointly and acting side-by-side permeates our daily lives and is crucial for understanding the evolution and development of human sociality. While acting in parallel involves agents pursuing individual goals, acting jointly requires them to share a collective goal. We used dual EEG to investigate neural dynamics underlying these action types. We recorded event-related potentials (ERPs) from 20 dyads while they had to transport an object in a video game, either jointly or in parallel. Conditions were matched for task execution complexity, confirmed by equal success rates. Results revealed a distinctive pattern swap in ERPs during action preparation. Early preparation showed significantly higher amplitude during joint versus parallel action. This pattern reversed in late preparation, with significantly reduced ERP amplitude in joint compared to parallel action. Notably, decreased late ERPs correlated with higher RT variability in partners but not participants' own RT variability. This dynamic swap suggests different cognitive processes operate at distinct stages of action preparation. Sharing a collective goal may impose cognitive costs (reflected in higher early ERPs), but this is offset by facilitated late action preparation (as shown by reduced late ERPs), likely due to the enhanced predictability of partners' actions.