Applying an eye tracking technique to gibbons: First study using scanpath measurements for visual stimuli

Compared to the abundance of research on cognition in various nonhuman primate species, studies of gibbons -- often called "the small apes" -- remain limited, despite the importance of gibbons for understanding evolutionary processes in humans and other apes. Over the past decade, eye tracking techniques have been established in chimpanzees and other nonhuman primates using the free-participation method, which requires no physical restraint of the subjects. We investigated the feasibility of using the same method to record visual scanpaths in gibbons. We attempted to measure the eye movements of three adult gibbons while they spontaneously viewed images, with no prior fixation training. Calibration was successful in all three individuals, with errors of less than one degree. In total, 24 stimuli were used, with landscape and nonhuman primate face photographs presented on one-quarter of the screen, to test the prediction that gibbons would change their viewing time depending on image category. All three gibbons viewed the images for longer than the background, and primate face images for longer than landscapes. These results are consistent with previous findings in other primate species that faces attract more attention than non-face stimuli, suggesting that this effect is common across primates. This study demonstrates the feasibility of using eye tracking with gibbons. Further studies on gibbon visual exploration and cognition may enhance our understanding of the phylogenetic origins of hominid intelligence as well as the unique evolution of gibbons.