Co-representation of others' task constraints in joint action

Previous research has demonstrated that humans tend to represent each other's tasks even if no interpersonal coordination is required. The present study asked whether coactors in a joint action rely on task co-representation to achieve temporal coordination even if this implies increased movement effort for an unconstrained actor. Pairs of participants performed reaching movements back and forth between two targets, with the aim of synchronizing their landing times. One of the participants needed to move over an obstacle while the other had no obstacle. The results of four experiments showed that the participant without obstacle moved as if an obstacle was obstructing her way. Further amplifying the demands on interpersonal coordination led to a significant increase of this effect, indicating that unconstrained actors represented their coactor's task constraint and adjusted their own actions accordingly, particularly under high coordination demands. The findings also showed that unconstrained actors represented the object property constraining their coactor's movement rather than parameters of this movement. We conclude that joint action partners rely on task co-representation to achieve temporal coordination in a task with asymmetric task constraints.