Abstract
We see the world as continuous: given stationary frames as inputs, we perceive smooth movements of objects. The perceptual construction of smooth movements depends not only on low-level spatiotemporal features, but also high-level knowledge. We examined the role of causality in guiding perceptual interpolation of motion in human actions. We recorded 6 videos of human-object interactions (a thrower directing a ball to a catcher). Stimulus onset asynchrony (SOA) was manipulated for the 200-ms period in which the catcher prepared to receive the ball. Short SOAs consisted of 6 frames (33.3 ms/frame); long SOAs consisted of 2 frames (100 ms/frame). Participants were asked to judge whether the catcher’s action showed smooth body movements or sudden changes. In Experiment 1, only the ball movement and catcher’s action were shown, with the thrower occluded. In the causal action condition the catcher was facing the ball (so the ball movement would be interpreted as causing the catcher to move his body in preparation). In the non-causal condition, the catcher was facing away from the ball. The movement of the ball was identical in the two conditions. Participants were more likely to judge the catcher’s movements to be continuous in the causal condition. The illusory percept of smooth motion was strongest when the SOA was long. In Experiment 2 the displays showed body movements of both the thrower and catcher, but occluded the ball movement (thus removing the direct cause in a causal chain of thrower-ball-catcher). Facing direction of the catcher was varied in the same manner as in Experiment 1. Once again, the catcher’s motion was judged to be continuous more often in the causal condition. The findings indicate that causality guides interpolation of body movements, thereby completing the history of the observed action despite gaps in the sensory information.
Acknowledgement: NSF grant BCS-1655300