Reading out bodily cues to predict interactions

Successful motor coordination in social interactions needs the swift interpretation of others’ actions, yet the pivotal social cues guiding these interactions remain incompletely understood. We conducted three experiments focusing on the execution (Action Execution task) and observation (Action Obseration 1 and 2) of a basic motor act—grasping an object—leveraging ecological stimuli. In the Action Execution task, we compared actions executed with individualistic (grasp to place) or social (grasp to pass) intentions, employing hand kinematics analyses. Grasping with a social intention exhibited slower, more precise execution and distinct hand placement on the object. Video recordings of actions from this first study were used as stimuli for the two subsequent action observation experiments in which two different cohorts of participants had to guess the actions’ outcome by watching the reach-to-grasp phase. In Action Observation 1, participants could only see the arm movement. A classification analysis revealed reaching speed and hand positioning on the grasped object as a major predictor of participants’ responses. Moreover, lower hand placement correlated with perceiving actions as socially directed. This novel finding may be linked to social affordance processing, reflecting the space available for potential interaction. In Action Observation 2, the person in the video was wholly visible, including the face. Using eye-tracking, we identified the face as the primarily attended feature. Our results unveil potential markers of social intentions in motor acts related to goal-oriented hand-object interactions and facial signals, emphasizing individuals’ reliance on multiple bodily cues for action predictions in naturalistic interactions.