Categorizing coordination from the perception of joint actions.

The ability to perceive others' actions and coordinate our own body movements accordingly is essential for humans to interact with the social world. However, it is still unclear how the visual system achieves the remarkable feat of identifying temporally coordinated joint actions between individuals. Specifically, do humans rely on certain visual features of coordinated movements to facilitate the detection of meaningful interactivity? To address this question, participants viewed short video sequences of two actors performing different joint actions, such as handshakes, high fives, etc. Temporal misalignments were introduced to shift one actor's movements forward or backward in time relative to the partner actor. Participants rated the degree of interactivity for the temporally shifted joint actions. The impact of temporal offsets on human interactivity ratings varied for different types of joint actions. Based on human rating distributions, we used a probabilistic cluster model to infer latent categories, each revealing shared characteristics of coordinated movements among sets of joint actions. Further analysis on the clustered structure suggested that global motion synchrony, spatial proximity between actors, and highly salient moments of interpersonal coordination are critical features that impact judgments of interactivity.