Functional equivalence and class expansion in rats using olfactory stimuli.

Simple discrimination reversal procedures have been successful in demonstrating functional equivalence classes in animals. The current study tested whether class expansion could be demonstrated in rats following the formation of functional equivalence classes. Olfactory stimuli were assigned to two arbitrary sets, and rats were trained on a successive simple discrimination task to respond to members of only one set at a time. When discriminated responding emerged, the reinforcement contingencies were reversed. After repeated reversals, probe sessions demonstrated functional equivalence classes in 14 of 15 rats across three experiments. Subsequently, the reversal procedure was used to train functional equivalence between one exemplar from each established class and two novel stimuli. Tests for class expansion, conducted between stimuli in the same set but without a history of training in the same session, were mixed. Experiment 1, which began expansion training after six-member classes were formed, did not provide clear evidence for class expansion. In Experiments 2 and 3, where expansion training began with smaller classes, class expansion was observed in six of eight rats. Class expansion is a property shared with human equivalence classes, suggesting that the discrimination reversal procedure provides a promising strategy for continuing research on equivalence in animals.