Eye Preference for Emotional Stimuli in Sichuan Snub-Nosed Monkeys

This study presents the first evidence of effects of applying both positive and negative stimuli simultaneously on visual laterality in Old World monkeys. Thirteen captive individuals of Sichuan snub-nosed monkeys (Rhinopithecus roxellana) were chosen as focal subjects in the monocular box task. In total, 4 emotional categories (the preferred, the novel, the neutral, and the fearful) of visual stimuli were applied, and eye preference was recorded when individuals looked at each stimulus through an observation hole in the box. We found evidence of visual laterality at the individual level, but not at the group level for each stimulus. For the preferred stimulus, 9 individuals showed significant right-eye preference while 4 individuals showed significant left-eye preference. For the other 3 stimuli, 7 individuals displayed significant right-eye preference while 6 individuals displayed significant left-eye preference. Totally, 11 of 13 individuals showed consistency in the visual laterality direction (7 right-eye preference and 4 left-eye preference) across the 4 stimuli. The remaining 2 individuals displayed right-eye preference for the preferred stimulus while they showed left-eye preference for the other 3 stimuli. There was no significant difference among various stimuli regarding the direction of visual laterality. However, there was a significant difference in the strength of visual laterality among various stimulus categories. The strength of visual laterality for the preferred stimulus was significantly lower than that for the other 3 stimuli. The strength of visual laterality for the fearful stimulus was significantly higher than that for the novel stimulus and the neutral stimulus. Furthermore, the looking duration for the preferred stimulus was significantly higher than that for the other 3 stimuli. The looking duration for the novel stimulus was significantly higher than that for the neutral stimulus and the fearful stimulus. The looking duration for the neutral stimulus was significantly higher than that for the fearful stimulus. Our findings indicate emotional valence of stimuli significantly influence eye looking duration and the strength of visual laterality but not for the direction of visual laterality in this species. Taken together, emotional valence of stimuli plays an important role in the eye use of R. roxellana.