Recognition of biological motion plays a crucial role in human survival and development. The human perceptual system is highly sensitive to information conveyed by biological motion, such as direction, gender, identity, and emotion. Among other things, the directional information contained in biological motion is of great biological and social significance. The facing orientation of a person largely suggests the person's potential intentions, determining whether we need to prepare for a response. The perception of other living entities’ motion direction is even a matter of life and death. The current study probed whether the directional information of biological motion bear specialized processing in human perceptual system.
In Study One, we proposed that due to its great social significance, the orientation perception of biological motion would be influenced by its social relevance. We adopted bistable biological motion stimuli. By modulating three factors critically pertinent to social interaction, the distance, the size, and the speed, we found that the more relevant the stimulus was to social interaction, the more likely people were to perceive it as facing them. This leads to the conclusion that the relevance to social interaction modulates the bistable biological motion perception. Since people facing us enjoy a higher priority in social interactions, facing percepts may receive preferential processing. In Study Two, we speculated because of the great evolutionary pressure to perceive other living entities’ motion direction, the processing for the motion direction of biological motion is more sensitive compared with non-biological motion. We found that the biological motion figure walking at the same spatial position could induce a classical motion illusion, the flash-lag effect, in the absence of actual translation. Local motion was sufficient to trigger this effect, indicating a role of motion signal independent of the global human form. Non-biological motion, on the other hand, cannot yield this effect. Depriving the critical biological property of biological motion would also eliminate the effect. These results suggest that certain movement templates may be inherent in the perceptual system, enabling the perceptual system to perceive the motion direction even without perceiving the overall translation. The present study argues in two aspects for the specialized processing for the directional information of biological motion and provides new evidence for the encoding, storage, and decoding mechanism of biological motion.