Abstract:

The perception of animacy from motion cues, or perceiving a moving entity as being alive, is critical to human survival and serves as a precursor to social interaction. As a central element of life, motion provides rich information about the behavior of living organisms. In recent years, there has been a growing body of research on how people perceive animacy from diverse motion cues. Here, we review the empirical findings and discuss the possible cognitive and neural mechanisms underlying animacy perception elicited by motion cues.

In general, motion cues that trigger animacy perception can be classified into two types: movements with biological motion patterns and those conveying intention, each having distinctive features essential for animacy perception. Specifically, for natural or simulated biological motion, local limb movements, rhythmic motion, and self-propelled motion (including motions that violate gravity) are key factors influencing the perception of animacy. For biological or non-biological movements that signify intention, flexibility and effectiveness of goal-directed motion patterns and some specific spatiotemporal relationships within interactive motions can lead to animacy perception.

Several cognitive accounts have been proposed to explain the abovementioned phenomena from the perspectives of visual information processing or social cognition. Among the information processing accounts, the life detector hypothesis proposes that there is a visual filter tuned to the internal local motion characteristics of terrestrial vertebrate organisms in the human brain for detecting ‘life signals’, and the energy violation hypothesis emphasizes that the overall external movement of the abstract incarnation of a living being, such as self-propelled motion, gives the impression that a moving entity has internal energy source and thus being animate. In the social cognition views, the intention hypothesis suggests that animacy attributions require intentional attributions and that animacy is perceived whenever the observer infers that an object's motion exhibits an intentional mental state, and the rational action principle holds that reasonable and effective actions under the constraints of the current scenario are interpreted as initiated by a living entity. These theoretical accounts explain how a specific aspect of motion can elicit animacy perception. However, a systematic framework is needed to describe how the two types of motion cues, which often exist simultaneously in reality, work together to cause animacy perception, and to quantitatively evaluate the relative roles of these motion cues in the perception of animacy.

Recent research on the brain mechanisms of animacy perception suggests that it may engage multiple cortical and sub-cortical brain regions with different functionalities. We tentatively hypothesize that the brain network in which dynamic signals induce animacy perception may include the following components: sub-cortical structures such as the superior colliculus (SC) and ventrolateral nucleus (VLN) extract key dynamic features expressing basic movement patterns of living organisms; cortical regions such as the fusiform gyrus (FG), medial prefrontal cortex (mPFC), and temporoparietal junction (TPJ) process multiple levels of intentional information that provide the basis for a higher level of animacy assessments; and the posterior superior temporal sulcus (pSTS) and intraparietal sulcus (IPS) play an integrated role in processing both types of motion signals, with their activation intensity reflecting the degree of perceived animacy.

Future studies should examine the distinct roles of the two types of motion cues in animacy perception and their interactive mechanisms from cognitive, behavioral genetic, and neural aspects. Meanwhile, the organization and connection of the brain network for animacy perception from motion cues and the exact function of each node in this network remain to be illuminated. In addition, exploring the computational principle of animacy perception from motion cues and treating the deficits in such ability as a potential marker of social cognitive disorders would help promote its application in artificial intelligence and clinical situations.

Key words: animacy perception, biological motion, life detection, goal-directed motion, intention understanding