Abstract
It has been reported that observer’s pupil response synchronizes to that of observed person. However, most of the previous researches used the stimulus with unnatural artificial pupil changes, such as stationary or linearly dilating/contracting pupil image so that the characteristics of temporal factor in pupil synchronization, which seems important in natural pupil oscillation, is still unknown. Furthermore, the effect of pupil synchronization on mental state has not been addressed directly. In this research, in order to understand the temporal characteristics and role of pupil synchronization in natural situation, we investigated the frequency selectivity in pupil synchronization and its effect on face impression. In Exp.1, we presented image of human eyes with pupil diameter changing sinusoidally and measured observer’s pupil response. Results showed that when viewing the pupil oscillation with relatively high frequency, power of the frequency component in observer’s pupil oscillation increased significantly. On the other hand, when viewing the pupil oscillation with relatively low frequency, such a correspondence in the frequencies was not observed. In Exp.2, we presented image of human face with pupil diameter synchronized to that of observer with the same or opposite amplitude and asked the observer to evaluate favorable impression of the face image. Results showed that favorable impression tended to increase when viewing the pupil synchronized with the same amplitude of observer’s pupil. These results indicate that pupil synchronization occurs in specific frequencies and affects the impression of face image and suggest that pupil synchronization has important roles in natural human-to-human communication scene.