Adaptation of Perceived Animacy from Biological Motion: Evidence for a "Life Motion Detector"

Humans can readily perceive animacy from biological motion (BM) - the distinctive movement patterns of living entities. However, how the human brain extracts animacy information from BM remains largely unclear. The current study investigated this issue using visual adaptation, a non-invasive tool for revealing neural mechanisms underlying the selective processing of specific properties. Results showed that prolonged exposure to intact human walkers, compared to non-BM adaptors, biased the perception of subsequent walking stimuli towards 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. Public significance statementsDiscerning whether a moving entity is animate is crucial for survival and for generating appropriate responses. This study explored how the human brain perceives animacy based on a prevalent movement pattern among vertebrates, namely, walking. Leveraging a non-invasive visual adaptation paradigm, five experiments demonstrated that pre-exposure to intact human walking, feet-only walking, and even intact pigeon walking motion sequences - but not static human forms - elicited significant adaptation aftereffects in animacy perception of human walkers. These findings support the existence of neural substrates encoding perceived animacy based on biological motion signals, particularly those carried by local foot motions and cross-species kinematic features. Such mechanisms may be integral to an inherent life detection system in the human brain.