Animacy perception is essential for human survival and reproduction and provides a prelude to social interaction. The unique movement patterns of living organisms, also known as biological motion (BM), are a critical source of animacy information. However, how humans perceive animacy from the motion cues in BM remains largely unexplored. In this thesis, we presented two studies aiming to address this issue.
Study 1 focused on the fundamental question regarding the neural basis of human perception of animacy from the kinematic cues in walking, the most basic form of BM in terrestrial animals, using the visual adaptation paradigm. In five experiments, we found that preexposure to animate BM (i.e., walking) and the less animate motion counterpart induced significant adaptation aftereffects on animacy perception. This effect persisted after adaptation to feet movements carrying diagnostic local kinematic cues but not after viewing the static form of BM, indicating there are neuronal populations dedicated to animacy perception from BM based on motion signals. Moreover, adapting to the movement of pigeons could bias animacy perception for human motions, revealing that the neural representation of animacy from BM stimuli can transfer across species. These results suggest that animacy perception of BM involves a neural mechanism driven by local foot motion signals and responsive to cross-species kinematic cues, supporting the existence of an inherent `life detector' in the human brain.
In Study 2, we further explored which kinematic features of the walking patterns influence animacy perception. We conducted three experiments to examine the role of three kinematic features (i.e., acceleration in limb movements, opponent motion of contralateral limbs, and articulation of joints) in the perception of animacy, using a 2-alternative-forced-choice (2AFC) task or an animacy rating task combined with the Bradley-Terry data analysis model. Our results suggest that all of these features have significant effects on human perception of animacy, with an interaction between the effects of limb opponent motion and articulation features. We used machine learning model to further analyze the importance of these three factors and their sub-features to animacy perception. We found that the influence of articulation motion and opponent motion of contralateral limbs is stronger than acceleration in limb movements, and some specific dynamic characteristics of limb movement (such as the phase difference between horizontal and vertical opponent motion) play a key role in animacy perception.
These findings provide compelling evidence for the existence of a `life detection' mechanism in the human brain based on some key kinematic features prevalent in the motion of terrestrial species. Moreover, several kinematic features, including acceleration, limb opponent motion, and articulation, are crucial to inducing the perception of animacy from BM.
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