Peter Urcuioli's lasting contribution: Animal memory research and an important model of stimulus class formation.

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.