Functional equivalence and class expansion in rats using olfactory stimuli
Repeated rule-flip training with smells builds new equivalence classes and limited expansion in rats.
01Research in Context
What this study did
Richardson and team worked with rats instead of children. They used smells, not pictures.
The rats learned to dig in cups for food when one smell meant reward and another meant no food. Then the rules flipped. The reward smell became the no-food smell and vice versa.
After many flips, most rats acted as if new, untrained smells belonged to the same group as the trained smells.
What they found
Fourteen out of fifteen rats formed smell groups, or functional equivalence classes.
When the scientists started with tiny groups and then added new smells, six out of eight rats still treated the new smells like the old ones. Class expansion happened without extra training.
How this fits with other research
Peña et al. (2006) and Galizio et al. (2018) already showed rats can match identical smells. Richardson adds a cheaper method: just flip the rules again and again.
Cerutti et al. (2004) warned that shared stimulus functions slow equivalence learning. Richardson’s rats prove that functions created through reversals can still build classes. The earlier study looked at pre-existing functions; this one builds new ones.
Davison et al. (1984) formed classes in children without naming. Richardson shows the same emergent performance in rats, pushing non-verbal equivalence research into a simpler species.
Why it matters
You now have an animal model that creates equivalence without fancy matching-to-sample gear. If you study basic processes, reversal training with smells is fast, cheap, and repeatable. The rat data also back up your clinical hunch: once a learner has a small class, new items can join with minimal extra work. Try starting with tiny sets and let expansion do the heavy lifting.
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02At a glance
03Original abstract
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.
Journal of the Experimental Analysis of Behavior, 2025 · doi:10.1002/jeab.70021