Visual perception of natural objects in chimpanzees (Pan troglodytes)

Masayuki Tanaka

Kyoto University, Japan

Human categorization is characterized by similarity between items from the same category and dissimilarity between the items from different categories. Humans also distinguish items from the same category although they perceive similarity between the items. I investigated whether chimpanzees perceive and categorize natural objects in the same way as humans. In Experiment 1, four chimpanzees were trained to match a picture of the same item as a sample among four comparison stimuli. The stimuli were color photographs of items from four categories (flower, tree, weed, grounds). The photographs of the sample and the correct comparison differed each other. After training, test trials were inserted among training trials. In test trials, comparison stimuli were four items from four different categories, and involved no photograph of the same item as the sample. In test trials, all subjects responded to the items from the same categories as the sample significantly frequently than the chance level. The results suggest that the chimpanzees perceive similarity between items from the same natural category though they can distinguish them each other. Experiment 2 investigated how much the chimpanzees could perceive similarity between items from the same category. The same subjects were trained to choose flowers among 12 photographs of items from four categories. The stimuli were those used in Experiment 1. After training, the subjects were tested whether they could choose flowers among novel stimuli. All subjects chose significantly more the photographs of novel flowers than chance level. In addition, one chimpanzee could choose flowers among pictures, in which some visual information were reduced from the photographs. The results suggest that chimpanzees could perceive similarity between various flower items and might have a simple representation of "flower" concept.

Tanaka, M. (1995). Object sorting in chimpanzees (Pan troglodytes): Classification based on physical identity, complementarity, and familiarity. Journal of Comparative Psychology, 109, 151-161.

Tanaka, M. (1996). Information integration about object-object relationships by chimpanzees (Pan troglodytes). Journal of Comparative Psychology, 110, 323-335.

Tanaka, M. (1997). Formation of categories based on functions in a chimpanzee (Pan troglodytes). Japanese Psychological Research, 39, 212- 225.

Tanaka, Masayuki

Section of Language and Intelligence,

Primate Research Institute, Kyoto University

Inuyama, Aichi 484-8506, Japan

mtanaka@pri.kyoto-u.ac.jp