Categorization and formation of equivalence classes in animals: Studies in Japan on the background of contemporary developments

Recent studies on categorization and concept learning in nonhuman animals are described, sketching studies in Japan on the background of contemporary developments of the research field. The topics included are (1) similarity-based perceptual concepts (open-ended categories, categorical coherence, synthetic approach, prototype effects, levels of categorization); (2) functional equivalence between category members, and (3) non-similarity-based associative concepts (Sidman equivalence, mediated equivalence, emergent differential sample behavior, emergent matching by exclusion, integration of equivalence relations). The review suggests that studies in this field are continuing to contribute in the understanding of animal cognitive behavior and also to the contemporary developments of comparative psychology. The issues left open for future research are illustrated.