Facial Skin Blood Flow Enhances the Human Likeness of Artificial Agents

Recent studies have shown that implementing explicit social cues, such as gaze, facial expressions, and gestures, in artificial agents can improve impressions of these agents. However, humans may also use implicit physiological cues, such as facial coloration and cardiac information, in social perception. The present study examined whether subtle skin color changes reflecting pulse signals enhance the perceived human likeness of artificial agents, and whether this effect depends on agent type, signal type, observers interoceptive sensibility, and their awareness of the skin color changes. Participants observed morphed face stimuli created from artificial agents and human faces and judged whether each stimulus appeared human-like or robot-like. In Experiment 1, skin color changes based on human-derived pulse wave signals enhanced perceived human likeness for a highly human-like agent, but not for a less human-like agent. In Experiment 2, perceived human likeness was enhanced not only by pulse-based skin color changes but also by sinusoidal skin color changes matched to the pulse wave signal in terms of mean amplitude and number of peaks. In addition, participants with higher scores on some subscales of the Multidimensional Assessment of Interoceptive Awareness (MAIA), a subjective measure of interoceptive sensibility, tended to notice the skin color changes. However, neither observers interoceptive sensibility nor their awareness of skin color changes directly explained the enhancement of perceived human likeness induced by skin color changes. These results suggest that subtle skin color changes reflecting pulse wave information may function as implicit dynamic cues signaling embodiment or biologicalness in artificial agents, thereby contributing to perceived human likeness.