Emergent identity but not symmetry following successive olfactory discrimination training in rats.
Rats learned to match identical smells but could not flip the relation, showing symmetry is not guaranteed even when identity is strong.
01Research in Context
What this study did
Prichard et al. (2015) taught rats to sniff out matching smells. One scent came first. Then two new scents followed. The rats had to nose-touch the one that matched the first smell.
Next the team tested for symmetry. They reversed the order. Now the choice scents came first. The rats had to pick the original scent last. Pigeons pass this test. The rats did not.
What they found
The rats aced identity matching. They picked the correct smell almost every time. But when the order flipped, they acted like they had never seen the task before.
Symmetry failed even though the training was identical to studies where pigeons show symmetry. The skill stayed stuck in one direction only.
How this fits with other research
Galizio et al. (2023) later got rats to show equivalence, but they used a different game. Instead of simple matching, they kept reversing the rules. After many reversals, the rats treated new smells as if they belonged to the same family. Same species, same nose, new procedure—new outcome.
Mason et al. (2025) found fragile transfer in rats. Five of six animals first copied a left/right response to untrained smells, but only two kept doing it. Together these papers say rats can build equivalence, yet the effect is brittle and depends on how you train.
Cordova et al. (1993) showed humans with intellectual disability easily pass identity matching with pictures. Rats can also master identity with smells, so the basic skill spans both species and senses.
Why it matters
If you run equivalence lessons, do not assume symmetry will pop out for every learner. Test both directions. Try varied training such as reversals or delayed probes before you call a program a failure. Species, sense, and procedure all shape what emerges.
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02At a glance
03Original abstract
The search for symmetry in nonhuman subjects has been successful in recent studies in pigeons (e.g., Urcuioli, 2008). The key to these successes has been the use of successive discrimination procedures and combined training on identity, as well as arbitrary, baseline relations. The present study was an effort to extend the findings and theoretical analysis developed by Urcuioli and his colleagues to rats using olfactory rather than visual stimuli. Experiment 1 was a systematic replication of Urcuioli's (2008) demonstration of symmetry in pigeons. Rats were exposed to unreinforced symmetry probes following training with two arbitrary and four identity conditional discriminations. Response rates on symmetry probe trials were low and provided little evidence for emergent symmetry in any of the seven rats tested. In Experiment 2, a separate group of six rats was trained on four identity relations and was then exposed to probe trials with four novel odor stimuli. Response rates were high on identity probe trials, and low on nonmatching probe trials. The similar patterns of responding on baseline and probe trials that were shown by most rats provided a demonstration of generalized identity matching. These findings suggest that the development of stimulus control topographies in rats with olfactory stimuli may differ from those that emerge in pigeons with visual stimuli. Urcuioli's (2008) theory has been highly successful in predicting conditions necessary for stimulus class formation in pigeons, but may not be sufficient to fully understand determinants of emergent behaviors in other nonhuman species.
Journal of the experimental analysis of behavior, 2015 · doi:10.1002/jeab.169