Causal actions enhance perception of continuous body movements

Our experience of motion depends not only on spatiotemporal features of stimuli, but also on our recognition of seemingly higher-level properties, as when we see an actor's body movements as goal-directed. Here, we examined how the perception of social causation in human actions guides the perceptual interpolation of motion in the observation of body movements. Natural human-object interactions were recorded for videos in which a person prepared to catch a ball thrown by another person. We manipulated the number of image frames between key postures to yield a short clip with different frame rates, and asked participants to judge whether the catcher's action showed smooth movements or sudden changes. In the causal condition, the catcher faced toward the ball and the thrower to preserve an intention-based causal relation between the ball's movement and the catcher's action in which the former causes the catcher's intention to act. In the non-causal condition, the catcher performed the same movements to raise their hands to catch a ball, except that they faced away from the ball, creating the impression of either a psychic reaction or coincidental non-goal-directed behavior, which makes movements of the ball appear to be an implausible cause of the catcher's intention to act. Across four experiments, we found that humans were more likely to judge the catcher's body movements to be continuous in the causal condition than in the non-causal condition. The effect was maintained as long as the intention-based causal relation was present, even when only part of the chain of causal events was observed. These findings indicate that intention-based cause-effect relations in human actions guide perceptual interpolation of body movements.