Hsin-I Liao¹, Yuta Suzuki¹, Shigeki Nakauchi²; ¹NTT Communication Science Laboratories, NTT Corporation, Japan, ²Department of Computer Science and Engineering, Toyohashi University of Technology, Japan

The classic literature indicates that the pupil is dilated when seeing attractive and emotionally arousing pictures, such as nude male and female portraits (Hess, 1965; Hess & Polt, 1960), presumably via sympathetic nervous system activation. By contrast, Liao et al. (2020) showed that the pupil is constricted when seeing attractive faces, presumably reflecting a positive loop between seeing and liking (preferring), as pupil constriction improves visual acuity. Emotionally arousing images with high intrinsic salience might surprise experimental participants, in addition to merely attracting them, thus affecting pupil response differently from images that are simply preferred. The current study aimed to explain these seemingly contradictory findings by investigating how pupil response is affected by the salience and attractive aspects of an image. We selected 942 images from the International Affective Picture System (IAPS) and converted them into grayscale with an equal mean luminance. We first conducted an online experiment in which 166 participants rated the salience and attractiveness of each image. Based on the rating results, we identified three sets of images with matched average salience or attractiveness while orthogonally controlling the other rating aspect. We then conducted a pupillometry experiment where another 30 participants viewed each image for eight seconds and rated its salience and attractiveness while their pupillary response was recorded. Results showed that initial pupil constriction at around the 1 s interval was more prominent for the highly attractive images, consistent with Liao et al. (2020). By contrast, late pupil dilation (after 2 s) was larger for high-salience than low-salience images. The results reveal an early pupil constriction response to attractiveness and a late pupil dilation response to salience, presumably underlaid by different mechanisms.