Causal Action: A Fundamental Constraint on Perception and Inference About Body Movements

The human body navigates the environment via locomotory movements that leverage gravity and limb biomechanics to propel the body in a particular direction. This process creates a causal link between limb movements and whole-body translation. However, it is unknown whether humans use this causal relation as a constraint in perception and inference with body movements. In the present study, participants rated actions of other individuals as more natural when limb movements (as a cause) occurred before body displacements (as an effect) than when limb movements temporally lagged behind body displacements. This causal expectation for human body movements not only affected perceptual impressions regarding the naturalness of observed actions but also guided the interpretation of motion cues within a more generalized causal context. We interpret these results within a framework of causality as evidence that the constraint of causal action plays an important role in perception and inference with body movements.