Different effects of social intention on movement kinematics when interacting with a human or virtual agent

The execution of object-directed motor actions is known to be influenced by the intention to interact with others. In this study, we tested whether the effects of social intention on the kinematics of object-directed actions depended on whether the task was performed in the presence of a human or a virtual confederate. In two experiments, participants had to grasp a glass and place it to a new position, with either a personal intention (to fill the glass themselves using a bottle) or a social one (to have the glass filled by the human confederate or the virtual agent using the bottle). Experiment 1 showed that the kinematics of the object-directed actions was modulated by the social intention but only when interacting with a human confederate. Experiment 2 showed that the effects of social intention on object-directed actions performance can be improved using feedback-based learning. Specifically, participants proved able to progressively adapt their motor performances as if they were expressing social intention to a virtual confederate as well. These findings emphasize the importance of the modulatory role of social intention on non-verbal motor behaviour, and enrich the understanding of the interaction with virtual agents. We studied the effect of social intention on object-directed actions while interacting with a human or a virtual agent. Results showed an effect of social intention on object-directed actions when interacting with a human confederate. No effect of social intention on object-directed actions was found instead when interacting with a virtual agent. Following a training, participants adapted their motor performances and expressed the social intention to the virtual agent. These findings underline the importance of considering social intention when implementing interactions with virtual agents.