Repeated reversals of concurrent olfactory discriminations in rats: A search for functional equivalence.

Equivalence class formation has been difficult to demonstrate in nonhumans, but one method that has been successful is a simple discrimination procedure in which contingencies associated with two sets of arbitrary discriminative stimuli are repeatedly reversed. Pigeons and sea lions shift responding after encountering the newly-reversed contingency with only a few set members, showing evidence of functional equivalence. We used this strategy to determine whether similar findings would occur in rats using olfactory stimuli. Rats were trained to nose-poke in the presence of six stimuli arbitrarily designated as members of the positive set; responses to the six members of the negative set were not reinforced. When discriminative performance was established, contingencies associated with each set were reversed and re-reversed each time subjects met a performance criterion. All subjects successfully acquired the concurrent simple discriminations and were exposed to between 12 and 60 reversals, but none showed clear evidence of functional class formation until a final procedure in which the stimulus sets that had been in place were arbitrarily rearranged. Acquisition with these new stimulus sets was impaired, showing that class membership generated by the original stimulus sets interfered with learning the new ones, thus providing evidence of functional equivalence.