Peter Urcuioli's lasting contribution: Animal memory research and an important model of stimulus class formation
Equivalence classes can grow in creatures that cannot talk, so your teaching plans should lean on solid reinforcement schedules, not just labels.
01Research in Context
What this study did
Zentall (2025) pulls together Peter Urcuioli’s pigeon work. The paper shows how birds can end up treating two different pictures as the same class.
No new lab data are given. Instead, the article links older pigeon studies into one clear model. It says the birds form equivalence-like sets without using words.
What they found
The review says pigeons can pass tests for reflexivity, symmetry, and transitivity. These are the same tests we use to check equivalence in people.
The trick is the differential outcomes procedure. One cue always leads to food, another to no food. After many trials, birds act as if two separate cues now belong to the same “stimulus class.”
How this fits with other research
Hughes et al. (2014) asked if animal concept learning is really the same as human equivalence. They say the question is still open. Zentall’s summary answers “yes” for pigeons, so the two papers talk to each other.
Beurms et al. (2017) showed humans easily form symmetry even when timing changes. Their data line up with the pigeon model, hinting that the process is similar across species.
Fields et al. (2018) argue that meaningful words speed up class formation in people. Zentall shows birds do it without words at all. Together, the papers suggest language helps humans but is not required for the basic effect.
Why it matters
If non-verbal animals can build equivalence classes, then language is not the engine—contingency is. When you teach matching tasks to early learners, focus on clear differential consequences first. Words can be added later to speed things up, but they are not a gatekeeper.
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02At a glance
03Original abstract
My collaboration with Peter Urcuioli started with research on delayed matching to sample. Initially we asked, what do pigeons remember during the delay in delayed matching to sample: a retrospective coding of the sample or a prospective coding of the comparison-related response? This led us to examine the basis of the differential outcomes effect. Why are samples associated with differential outcomes learned faster and remembered better than samples associated with common outcomes? This research helped us discover a procedure that resulted in functional stimulus equivalence: Samples associated with the same comparison are commonly associated. This research led Peter to develop his creative model of pigeon equivalence class formation. His model predicts the conditions under which pigeons satisfy the three components of what is known as Sidman equivalence: reflexivity, symmetry, and transitivity, phenomena that are difficult to demonstrate in pigeons. Importantly, his model predicts the opposite of reflexivity (anti-reflexivity) and symmetry (anti-symmetry). Research confirming Peter's model laid to rest the belief that the emergent relations defining Sidman equivalence can be satisfied only by an organism capable of using language. In his long career, Peter Urcuioli has made an important and long-lasting contribution to the field of learning and comparative cognition.
Journal of the Experimental Analysis of Behavior, 2025 · doi:10.1002/jeab.70019