Eye Pupil is Constricted to Attractive Images and Dilated to Salient Ones at Different Time Windows

Classic literature suggests that pupils dilate when viewing attractive and emotionally arousing pictures. However, the images used in previous studies may be confounded by their rarity or novelty, which involves cognitive evaluation in addition to mere attractiveness. The current study addressed the issue by presenting images with matched salience or attractiveness while orthogonally controlling the other evaluation aspect. The images were selected based on a large-scale online experiment in which 166 participants evaluated 942 grayscale images from the International Affective Picture System. These images were then submitted to a pupillometry experiment, in which 30 participants viewed each image for eight seconds while their pupillary responses were recorded. Results showed that the initial pupil constriction response was more prominent when viewing attractive images. By contrast, late pupil dilation was larger for salient and attractive images. The results reveal the complexity of the dynamic pupil response trace, presumably underlaid by different mechanisms.