Functional equivalence classes in rats: Repeated olfactory discrimination reversals and delayed stimulus probes
Repeated reversals with olfactory cues give rats robust functional equivalence classes, offering a new animal model to screen equivalence-based teaching tactics.
01Research in Context
What this study did
Galizio et al. (2023) trained rats to tell smells apart. When the scent changed, the rats had to switch which lever they pressed. The team kept flipping the rules to see if the rats would form "functional equivalence classes."
They added surprise probe trials with delays up to 10 seconds. If the rats treated the new smells like the old ones, that showed the smells had become part of the same class.
What they found
The rats learned to group the smells. After several reversals, they treated new smells the same as the original ones, even after a short wait.
Tweaking the order and timing made the effect stronger. The animals showed clear transfer of function — a lab sign of equivalence.
How this fits with other research
Prichard et al. (2015) first showed rats can match identical smells, but the rats failed on symmetry tests. Galizio adds repeated reversals and now the same species forms full functional classes. The earlier study missed equivalence because it stopped at identity training.
Mason et al. (2025) pushed further. They taught rats a left vs. right response, then checked if untrained smells in the same class would control that response. Only two of six rats held the transfer steady. Galizio’s stricter reversal method may build stronger classes than the looser transfer test Mason used.
Duker et al. (1991) showed that reward and penalty functions travel through equivalence classes with humans. Galizio’s rat work mirrors this idea: once the smells are equivalent, their functions swap together after a reversal.
Why it matters
You now have a rodent model for equivalence. That means faster, cheaper tests of new teaching tactics before you try them with kids. Try building classes with quick reversals instead of long identity-only drills. Watch whether the learner transfers responses to new items after a short delay — if they do, the class is solid. Use this check in your next conditional-discrimination program to be sure stimulus relations are tight before moving on.
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02At a glance
03Original abstract
The simultaneous matching-to-sample procedures that are widely used to study stimulus equivalence in human participants have generally been unsuccessful in animals. However, functional equivalence classes have been demonstrated in pigeons and sea lions using a concurrent repeated reversal discrimination procedure. In this procedure, responding to one set of stimuli is reinforced but responding to a different set is not and the set associated with reinforcement is changed with multiple reversals during the experiment. The experiments reported here were designed to assess whether functional equivalence classes could be demonstrated in rats using similar techniques. Rats were initially trained with two sets of olfactory stimuli (six odors/set). Following many reversals, probe reversal sessions were conducted in which rats were exposed to a subset of the members of each set and, later in the session, the withheld stimuli were introduced. Responding to these delayed probe trials in accord with the reversed contingencies constituted transfer of function. There was some evidence of transfer in Experiment 1, but the effects were relatively weak and variable. Experiment 2 introduced procedural changes and found strong evidence of transfer of function consistent with the formation of functional equivalence classes. These procedures offer a promising strategy to study symbolic behavior in rodents.
Journal of the Experimental Analysis of Behavior, 2023 · doi:10.1002/jeab.827