How chimpanzees (Pan troglodytes) perform in a modified emotional Stroop task
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The emotional Stroop task is an experimental paradigm developed to study the relationship between emotion and cognition. Human participants required to identify the color of words typically respond more slowly to negative than to neutral words (emotional Stroop effect). Here we investigated whether chimpanzees (Pan troglodytes) would show a comparable effect. Using a touch screen, eight chimpanzees were trained to choose between two simultaneously presented stimuli based on color (two identical images with differently colored frames). In Experiment 1, the images within the color frames were shapes that were either of the same color as the surrounding frame or of the alternative color. Subjects made fewer errors and responded faster when shapes were of the same color as the frame surrounding them than when they were not, evidencing that embedded images affected target selection. Experiment 2, a modified version of the emotional Stroop task, presented subjects with four different categories of novel images: three categories of pictures of humans (veterinarian, caretaker, and stranger), and control stimuli showing a white square. Because visits by the veterinarian that include anaesthetization can be stressful for subjects, we expected impaired performance in trials presenting images of the veterinarian. For the first session, we found correct responses to be indeed slower in trials of this category. This effect was more pronounced for subjects whose last anaesthetization experience was more recent, indicating that emotional valence caused the slowdown. We propose our modified emotional Stroop task as a simple method to explore which emotional stimuli affect cognitive performance in nonhuman primates.
KeywordsChimpanzee Emotional Stroop Great apes Attentional bias Cognitive bias
The authors wish to thank the staff at Leipzig Zoo, particularly the zoo veterinarian and the chimpanzee caretakers, for their various contributions to stimulus preparation and data collection. We thank Thurston Cleveland Hicks and Fabrizio Maffessoni for their contributions to stimulus preparation. We thank Alexander Weiss for providing a German version of the Hominoid Personality Questionnaire. We thank Daniel Geissler, Stefan Leideritz, Johannes Grossmann, and Sarah Peoples for providing personality ratings of the chimpanzees.
Compliance with ethical standards
Animal husbandry and research comply with the “EAZA Minimum Standards for the Accommodation and Care of Animals in Zoos and Aquaria,” the “WAZA Ethical Guidelines for the Conduct of Research on Animals by Zoos and Aquariums,” and the “Guidelines for the Treatment of Animals in Behavioral Research and Teaching” of the Association for the Study of Animal Behavior (ASAB).
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