Stimulus equivalence in nonhuman animals

Abstract

The purpose of this paper is to review the researches on stimulus equivalence in nonhuman animals, based on the formulation by Sidman. Among the behavioral properties that he proposed as essential elements of stimulus equivalence, “reflexivity” has been confirmed in various species, and the evidence of “transitivity” has also been obtained in pigeons. However, “symmetry” seems much more difficult to be formed, and specifically, “equivalence” was shown only in a sea lion. On the other hand, stimulus equivalence also has been examined using procedures other than traditional conditional discrimination, or by many-to-one (MTO) matching-to-sample and Pavlovian conditioning. Recently, positive evidence was obtained in budgerigars, based on the interpretation that behavior, in addition to stimulus, should be included as a member of an equivalence class. These new approaches in research have suggested that equivalence may not be a behavioral ability that enables us to use language, but a basic cognitive function common to other animals. This proposition, indicating that equivalence is apt for comparative study, implies that the conventional research methods should be reexamined.