Visual categorization and prototype effects in pigeons

Masako Jitsumori

Chiba University, Japan

A comparative approach on categorization in humans and non-human animals may contribute to our understanding of evolutionary and ecological significance of concept formation. It has been shown that pigeons and monkeys could learn to discriminate polymorphous stimuli, i.e. stimuli for which no single feature is either necessary or sufficient to determine category membership (e.g., Jitsumori, 1993; 1994). Categorization by animals is thus not an all-or-none process determined by the presence or absence of common feature(s). Category membership is continuous; some exemplars are more typical members of a category than others. A prototype effect, which was initially reported in the human literature, is recently demonstrated in pigeons (e.g. Jitsumori, 1996), by using artificial stimuli with the prototypes designed to represent central tendencies of categories. These studies have used artificial stimuli, because it is often required to precisely control the stimuli and to create physical prototypes from the exemplars used for training. Based on the finding in Jitsumori & Yoshihara (1997) that pigeons used multiple facial features to discriminate human faces, we have examined prototype effects in pigeons by using human faces, i.e., naturalistic visual stimuli, created by the morphing technique. A physical prototype created by averaging training exemplars is assumed to correspond to a prototype abstracted by categorization training. With the categories constructed to mimic family resemblance of natural categories, pigeons showed clear evidence of prototype effects (Makino & Jitsumori, in submission; Makino, in preparation). Pigeons trained to discriminate typical and atypical exemplars of a category, showed multiple prototypes of the sets of atypical exemplars, each of which were perceptually disparate to one another (Jitsumori & Matsukawa, in preparation). The prototype effects shown by pigeons in these studies are not attributable to peak-shift effects or additive integration of features used for training, but well explained by abstraction of central tendencies of categories.

Jitsumori, M. (1993). Category discrimination of artificial polymorphous stimuli based on feature learning. Journal of Experimental Psychology: Animal Behavior Processes, 19, 244-254.

Jitsumori, M. (1994). Discrimination of artificial polymorphous categories by rhesus monkeys (Macaca mulatta). Quarterly Journal of Experimental Psychology, 47B, 371-386.

Jitsumori, M. (1996). A prototype effect and categorization of artificial polymorphous stimuli in pigeons. Journal of Experimental Psychology: Animal Behavior Processes, 22,405-419.

Jitsumori, M., & Yoshihara, M. (1997). Categorical discrimination of human facial expression by pigeons: A test of the linear feature model. Quarterly Journal of Experimental Psychology, 50B, 253-268.

Jitsumori, Masako

Department of Cognitive and Information Sciences, Chiba University

1-33 Yayoi-cho, Inage-ku, Chiba, Chiba, 263-8522, Japan

masako@cogsci.l.chiba-u.ac.jp