The Effect of Robot's Facial Features on Users' Perception: Evidence from Subjective, Pupillometry, and Electroencephalography Measures

The present study aimed to investigate how facial features, including facial ratio, eye shape, and mouth presence, influence users' perceptions of social robots in terms of anthropomorphism, trustworthiness, and overall impressions. Using electroencephalogram (EEG) and eye-tracking technologies, we conducted a 2 (face ratio) x 2 (eye shape) x 2 (with or without mouth) full factorial experiment designed within-subject. EEG signals and pupil diameters were recorded while participants viewed images of robots with different facial design features. The results showed that the shape of robot's eyes significantly influenced users' perceptions, with round eyes being associated with higher ratings of anthropomorphism, trustworthiness, and overall positivity. Robots with lower facial width-to-height ratios (fWHR) induced smaller average pupil diameters in users than those with high fWHR. Additionally, robots with lower fWHR were perceived as more anthropomorphic, trustworthy and users produced higher theta rhythm when the eye shape was round but did not when it was rectangular. The presence of a mouth led to higher ratings of anthropomorphism rather than trustworthiness and other measures. Overall, the findings highlight the importance of facial features in shaping users' perceptions of social robots and provide practical implications for designing social robots with anthropomorphic facial features.