How Acting Jointly Differs from Acting Side-by-Side: A Dual EEG Study
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. Here, we used a dual electroencephalography (EEG) approach to explore the neural dynamics underlying joint and parallel action preparation. 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, or individually. Both conditions were carefully matched for coordination demands and performance complexity, as confirmed by equal success rates. Our results revealed a distinctive pattern swap in ERPs during action preparation. In the early preparation phase, ERPs showed significantly higher amplitude during joint action than parallel action. This pattern reversed in the late preparation phase, with significantly reduced ERP amplitude in the joint compared to parallel action. Notably, the decrease in late ERPs correlated with higher reaction time (RT) variability in partners but not with participants own RT variability. The dynamic swap in neural activity suggests that different cognitive processes operate at distinct stages of action preparation. While initially sharing a collective goal may impose cognitive costs (reflected in higher early ERPs), this is offset by facilitated late action preparation, likely due to enhanced predictability of partners actions. Significance StatementOur study reveals distinct neural signatures differentiating joint from side-by-side actions. Through dual EEG recordings of twenty dyads performing complexity-matched tasks, we identified a distinctive "swap" in event-related potentials during action preparation. Joint actions initially showed higher early-phase amplitudes but significantly reduced late-phase amplitudes compared to parallel actions. Notably, this late-phase reduction correlated specifically with the variability of partners behavior. This suggests that sharing collective goals initially requires cognitive resources but facilitates action preparation through enhanced predictability of partners. These findings provide a neural framework for understanding the distinction between acting jointly and acting in parallel --a distinction that pervades our daily experiences and is crucial for understanding the development and evolution of human sociality.