When is ostensive communication used for joint action?

Joint actions typically require that information relevant for performing a task together is available to the interaction partners. In some situations, such information is perceptually retrievable and salient enough for co-actors to simply use it. In other situations, the relevant information needs to be actively shared among co-actors, e.g., by making it more perceptually salient or indicating it by means of a conventional signal. Here we consider a third case, where the information is not perceptually available and cannot be communicated by conventional means. How do joint action partners coordinate in such situations? We propose that co-actors resort to ostensive communication, that is, they draw attention to the fact that they intend to communicate some specific information. Two experiments tested the proposed role of ostensive communication for joint action. In a non-verbal joint building task, the category membership of different objects was known to only one person in a dyad, who needed to inform the partner which object type to use. In line with our hypothesis, most participants highlighted a particular object category with an ostensive gesture (characterized by containing more submovements than a natural placing movement) to resolve perceptual ambiguity. We conclude that ostensive communication is especially useful for joint action in situations where task-relevant information is not available to all co-actors and where it cannot be perceptually highlighted or conventionally communicated.