Seeing life in motion: Animacy perception across species revealed by adaptation

Humans and other animals can readily perceive animacy from biological motion (BM) - the distinctive movement patterns of living entities. However, how the brain extracts animacy from BM remains largely unclear. The current study investigated this issue in human participants using visual adaptation, a non-invasive tool for probing neural mechanisms encoding specific stimulus properties. Results showed that prolonged exposure to intact human walkers, compared to non-BM adaptors, biased the perception of subsequent walking stimuli toward less animate. This adaptation aftereffect persisted following adaptation to local foot motions carrying diagnostic kinematic cues, but was absent after exposure to static body forms, indicating the involvement of neuronal populations encoding animacy from BM based on motion signals. Moreover, adapting to pigeon movements also biased animacy perception for human motions, revealing a shared mechanism for encoding animacy from cross-species BM signals. These results support the existence of an inherent "life detection" system in the human brain, attuned to local foot motions and kinematic features prevalent in vertebrate movements, which may lay a foundation for perceiving life from motion across terrestrial species.