Fig. 2.
A comparison of the steps required to produce symmetry relations in a typical experiment with human participants (top) and in the Urcuioli procedures with pigeons (below). Uppercase letters A and B refer to the nominal stimuli and lowercase s and c refer to the sample and comparison positions respectively. Arrows point from sample to comparison stimuli. Solid arrows represent explicitly trained relations, whereas broken arrows represent emergent relations. Note that symmetry emerges in humans after training a single relation (top), but that three relations are trained in the Urcuioli procedure. The emergent Bs ➔ Ac relation is thus an example of a two-node (As and Bc) transitivity/equivalence relations in pigeons