抄録
In the realm of nonverbal communication, few things are as expressive as the face. A complex combination of features, the human face can shift and change to produce a staggering array of different messages. The eyes may be the most important to conveying these. The pupils of humans dilate in connection to our interest in a subject and dilate more in response to arousing stimuli such as angry faces. Recent research has shown that the pupil size in images of faces modulates the observer's own pupil dilation. Externally, the human eye is ideally set up to communicate. The black spot of the pupil is surrounded by the colorful iris, and set against the white backdrop of the sclera. Such contrast and color make the eyes distinctively expressive, and facilitate the observation of gaze and pupil dilation. In non-human primates such as chimpanzees, the eye coloration and facial coloration around the eyes is different than in humans. In the current experiment we investigate whether pupil dilation in response to conspecifics is uniquely human or whether it is an evolutionary older mechanism. To this extent we presented the eye region of male and female chimpanzees to both species (6 chimpanzees and 14 humans) while measuring their pupil dilation. We manipulated the pupils in the stimuli so that in one condition they were increasing in size over 4 seconds, and in the other condition decreasing. The results show that the pupil of the human participants dilated mostly in response to human male increasing versus decreasing pupils whereas chimpanzees' pupil dilated mostly in response to male chimpanzee increasing versus decreasing pupils. Both species' pupils dilated mostly when observing increasing male conspecific pupils which may be most evolutionary adaptive to detect and/or respond to.
