Does perceived voluntariness of others’ actions induce vicarious sense of agency? Evidence from human-robot interaction

Sense of Agency (SoA) is the feeling of control over one's actions and outcomes. People can experience "vicarious" SoA towards other agents, either other humans or artificial agents such as robots. A commonly used measure of implicit SoA is the Intentional Binding (IB) effect, which is stronger when the action is voluntary, relative to involuntary. However, it remains unclear whether this is true also for vicarious SoA. Thus, in three experiments, participants performed an IB task alone and with another agent, namely with another human (Experiment 1) or with the humanoid robot iCub (Experiments 2 and 3). The co-agents' actions were presented as voluntary or involuntary- triggered by a mechanical device. Participants reported the time of occurrence of self-generated actions, as well as the other's human actions (Experiment 1), the robot actions (Experiment 2) or robot outcomes (Experiment 3). Experiment 1 showed that both self- and vicarious IB occurred only when the actions were voluntary. In Experiment 2, IB for self-actions occurred only when voluntary, but vicarious IB over iCub's actions occurred irrespective of whether the action was presented as "voluntary" or "involuntary". Experiment 3 showed that IB over tone outcomes occurred for self-generated and robot actions. Our findings suggest that voluntariness of actions plays a role in the emergence of the IB affect (and, by extension, of SoA) only if predictive processes are at play. They also indicate that vicarious IB for robots is based on postdictive processes, and this, perceived voluntariness of the robot actions does not modulate the vicarious IB effect for robots.