Abstract: PURPOSE: Humans can readily perceive animacy from the unique movement patterns of living creatures, known as biological motion (BM). Researchers have proposed that there is an evolutionarily ancient mechanism in the visual system tuned to the local motion of terrestrial vertebrates to serve as a ‘life detector’. Nevertheless, evidence for this hypothesis mainly came from the direction discrimination task that did not directly assess the perception of ‘life’, and how the brain encodes perceived animacy from BM cues remains unclear.
METHODS: Here, we investigated these issues using the animacy rating task with the visual adaptation paradigm. Repeated adaptation to a stimulus with a given feature, e.g., being more or less animate, will cause the deviation of the perception of the following stimuli to the opposite direction, e.g., being less or more animate. This adaptation aftereffect is considered to result from weakened neuronal responses specific to the tested property of the adaptor, thus providing a non-invasive way to reflect the activity of neuronal populations through behavioral performances. Observers rated perceived animacy for a series of morphed motion stimuli that spanned a continuum between natural human walking and non-BM after adapting to different BM cues (Exp. 1: intact human motion; Exp. 2: feet motion; Exp. 3: static human form; Exp. 4: pigeon motion) and the non-BM controls. The rating scores were fit to a psychometric function, and the adaptation aftereffect was assessed using the shifts in the point of subjective equality (PSE) between the adapting conditions in each experiment.
RESULTS: We found that preexposure to intact human BM and non-BM stimuli 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.
CONCLUSIONS: These results suggest that perceiving animacy from BM involves a neural mechanism driven by local foot motion signals and responsive to cross-species kinematic cues, supporting the existence of a ‘life detector’ tuned to animate motion in the human brain.

Key words: Animacy perception, biological motion, visual adaptation, aftereffects