Species difference in the timing of gaze movement between chimpanzees and humans
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How do humans and their closest relatives, chimpanzees, differ in their fundamental abilities for seeing the visual world? In this study, we directly compared the gaze movements of humans and the closest species, chimpanzees, using an eye-tracking system. During free viewing of a naturalistic scene, chimpanzees made more fixations per second (up to four) than did humans (up to three). This species difference was independent of the semantic variability of the presented scenes. The gap–overlap paradigm revealed that, rather than resulting from the sensitivity to the peripherally presented stimuli per se, the species difference reflected the particular strategy each species employed to solve the rivalry between central (fixated) and peripheral stimuli in their visual fields. Finally, when presented with a movie in which small images successively appeared/disappeared at random positions at the chosen presentation rate, chimpanzees tracked those images at the point of fixation for a longer time than did humans, outperforming humans in their speed of scanning. Our results suggest that chimpanzees and humans differ quantitatively in their visual strategies involving the timing of gaze movement. We discuss the functional reasons for each species’ employing such specific strategies.
KeywordsChimpanzee Eye-tracking Fixation duration Gap–overlap
This research received financial support from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Japan Grants-in-Aid for Scientific Research (nos. 16002001, 19300091, 20002001, 212299) and the JSPS/MEXT global COE programs (D07 and A06). We thank Drs T. Matsuzawa, S. B. Hrdy, I. Adachi, S. Hirata, and Y. Hattori for their help and invaluable comments. We also thank the Centre for Human Evolution Modelling Research at the Primate Research Institute for the daily care of the chimpanzees.
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