Abstract
This study offers a new method for examining the bodily, manual, and eye movements of a chimpanzee at the micro-level. A female chimpanzee wore a lightweight head-mounted eye tracker (60 Hz) on her head while engaging in daily interactions with the human experimenter. The eye tracker recorded her eye movements accurately while the chimpanzee freely moved her head, hands, and body. Three video cameras recorded the bodily and manual movements of the chimpanzee from multiple angles. We examined how the chimpanzee viewed the experimenter in this interactive setting and how the eye movements were related to the ongoing interactive contexts and actions. We prepared two experimentally defined contexts in each session: a face-to-face greeting phase upon the appearance of the experimenter in the experimental room, and a subsequent face-to-face task phase that included manual gestures and fruit rewards. Overall, the general viewing pattern of the chimpanzee, measured in terms of duration of individual fixations, length of individual saccades, and total viewing duration of the experimenter’s face/body, was very similar to that observed in previous eye-tracking studies that used non-interactive situations, despite the differences in the experimental settings. However, the chimpanzee viewed the experimenter and the scene objects differently depending on the ongoing context and actions. The chimpanzee viewed the experimenter’s face and body during the greeting phase, but viewed the experimenter’s face and hands as well as the fruit reward during the task phase. These differences can be explained by the differential bodily/manual actions produced by the chimpanzee and the experimenter during each experimental phase (i.e., greeting gestures, task cueing). Additionally, the chimpanzee’s viewing pattern varied depending on the identity of the experimenter (i.e., the chimpanzee’s prior experience with the experimenter). These methods and results offer new possibilities for examining the natural gaze behavior of chimpanzees.
Highlights
Human and nonhuman primates rely primarily on vision to retrieve information from the outside world
We examined whether our data on general patterns of eye movements were comparable to the results reported in previous studies relying on table-mounted eye tracking [8,23]
Our method enabled us to record Pan’s eye movements over the course of 3 minutes while Pan engaged in her usual interactions with Experimenter 2 (E2); for example, Pan engaged in overt greeting gestures when E2 appeared, and she performed the task actions and occasionally requested the reward after the task began (Fig. 3 and Table 1)
Summary
Human and nonhuman primates rely primarily on vision to retrieve information from the outside world. In nonhuman primates such as macaques, a magnetic search coil method is commonly used for eye tracking [2,3,4,5,6,7] This method requires the coil to be implanted on the eye surface of the subjects and the heads of the subjects to be firmly fixed in place by a chin rest or a bite bar. A recent study solved this problem using a video-based, table-mounted eye tracker, allowing eye tracking without head restraints [8] This eye tracker uses wideangle camera lenses to search for both corneal and pupil reflections from the eyes and compensates for head movements (indicated by the corneal reflection) when calculating eye movement (indicated by the pupil reflection). This same method is commonly used in human infants [9] and, more recently, in dogs [10,11]
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