Does the anticipation of compatible partner reactions facilitate action planning in joint tasks?

Observing another human's actions influences action planning, but what about merely anticipating them? In joint action settings where a partner's subsequent actions are a consequence of one's own actions, such contingent partner reactions can be regarded as action effects. Therefore, just like automatic effects they might facilitate those of a person's actions that overlap with them in relevant features. In Experiments 1 and 2, the spatial compatibility of contingent partner reactions was manipulated and compared with the influence of automatic effects. Experiment 1 used a simplistic scenario in which lateral keypress actions by the subject were responded to by mouse movements of a partner producing spatially compatible or incompatible visual effects. Experiment 2 transferred the paradigm to a more complex task in which subjects manually relocated virtual objects on a multi-touch display, and these or other objects were subsequently manipulated by the partner. In Experiment 1, compatible partner reactions speeded up subjects' preceding actions, whereas in Experiment 2 the influence was not statistically reliable. To test whether influences of partner reaction compatibility could be found in such naturalistic settings at all, Experiment 3 also used a multi-touch setting but varied temporal instead of spatial compatibility, which has several methodological advantages. This time, a compatibility effect emerged in subjects' movement initiation times, whereas contrast effects were found for movement durations. These findings indicate that the principles of ideomotor action control can be extended to joint action settings. At the same time, they also emphasize the importance of task features in determining whether our own behaviour is influenced by anticipations of another person's reactions.