| Sumario: | Two experiments explored how order of training, familiar pictures, and abstract stimuli that acquired discriminative functions influenced equivalence class formation. In Experiment 1, three 3-node, 5-member equivalence classes (A → B → C → D → E) were established using a variation of the simultaneous protocol that involved the serialized training of the AB, BC, CD, and DE baseline relations in that order, after which the emergence of all derived relations was tested on a concurrent basis. Classes were formed by 70 % of participants when the A-stimuli were familiar pictures and were members of the first-trained AB relations, while the B-E stimuli were abstract shapes. Classes were formed by 40 % of participants when the E-stimuli were familiar pictures and were members of the last-trained DE relations, while the A-D stimuli were abstract shapes. In two matching conditions, abstract stimuli that had acquired discriminative functions (Ss) prior to class formation were substituted for the A and E stimuli, and classes were formed by 20 % of participants in each condition. Further, greater enhancement effects were obtained using pictures than abstract stimuli that have acquired discriminative functions when these stimuli were the A members of the classes, while very little differences in yield were observed when they were the E members of the classes. Thus, likelihood of class formation could have been influenced by the position of a meaningful stimulus or an S in the structure of a class (i.e., A or E) and/or their order of introduction during training (first or last). For more than 92 % of the participants (37 of 40), a final sorting task produced the same outcomes as the derived relations tests. Experiment 2 isolated the effects of these variables by the concurrent establishment of the baselines with the A stimuli as pictures or as Ss. No participants formed classes in either condition. Thus, class enhancement by a meaningful stimulus depended on its inclusion in the first trained baseline relation and not its placement in the class structure.
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