Joint action partners modulate the first step of an action sequence to communicate a distal goal

When two co-actors perform a joint action, they often communicatively modulate their instrumental actions so as to facilitate each other's predictions of their immediate, proximal goals. Here, we ask whether co-actors would also engage in such "sensorimotor communication" for distal goals, specifically those that result from a two-step action sequence. To address this question, we asked pairs of participants to work together to deliver an animated box to one of two delivery locations displayed on a computer screen. This was done in two sequential steps such that one participant ("Sender") performed the first step and the other ("Receiver") performed the second step. Crucially, only the Sender was informed about the correct delivery location. Therefore, we expected Senders to modulate their movement to inform Receivers, who could observe them moving, about the correct location. Experiment 1 confirmed that Senders indeed modulated the timing of their movements to disambiguate between delivery locations. The stronger these modulations, the better they were understood by Receivers, thereby increasing coordination success. In Experiment 2, we explored how pairs agree on communicative mappings, and how such mappings develop as a result of alternating the roles of Sender and Receiver. Experiment 2 showed that communicative attempts that occur early on in the interaction are key to coordination success. Together, the present findings show that joint action partners produce and perceive temporal modulations of the first part of an action sequence, thereby facilitating the prediction of its distal goal which, in turn, enables successful coordination.