Abstract: PURPOSE: Over the course of evolution, animals have developed particular motion patterns, including relative motion (i.e., biological motion) and common motion (i.e., animate motion). Meanwhile, humans and non-human animals have developed the ability to identify these patterns, called animacy perception. However, since previous studies mainly used synthetic paths, it remains unclear what the nature of animate motion and animacy perception is and the relationship between them.
METHODS: We proposed a new method to obtain actual animate motion by extracting the motion of the objects’ centroid (i.e., the center of gravity) using visual tracking algorithms, and we built a dataset containing both actual animate (birds’) and inanimate (drones’) motion. Then, we systematically compared the difference between animate and inanimate motion in both motion time-domain and frequency-domain features. Moreover, we evaluated how these motion features induced the animacy perception of human observers through psychophysical experiments. In each experiment, observers were asked to determine the animacy of a moving dot that moved along the centroid's trajectory in each trial.
RESULTS: We found that compared to drones, birds generally fly faster, and their centroid motion changes more dramatically in terms of both speed magnitude (acceleration) and moving direction (angular speed). More interestingly, we found that the trajectory fluctuations in birds' trajectories reflected the periodic wing flapping of birds. In the human behavior experiments, we found that animacy perception positively correlates with four motion features: speed, acceleration, angular speed, and trajectory fluctuations. Using stepwise regression analysis, we found that fluctuations and acceleration played a more important role than speed, while angular speed's effect was insignificant.
CONCLUSIONS: The defining features of animate motion are acceleration and angular speed since the flexibility of living objects is higher than that of non-living objects. For flying animals, trajectory fluctuations might also be a defining feature. The positive relationship between animacy perception and key features of animate motion indicates that the human brain is sensitive to the critical features of animate motion. We speculate that the brain might have specialized neurons and neural circuits in the brain to process them.

Key words: animate motion, animacy perception, trajectory fluctuations, acceleration, angular speed