Thirty college students attempted to form three 3-node 5-member equivalence classes under the simultaneous protocol. After concurrent training of AB, BC, CD, and DE relations, all probes used to assess the emergence of symmetrical, transitive, and equivalence relations were presented for two test blocks. When the A-E stimuli were all abstract shapes, none of 10 participants formed classes. When the A, B, D, and E stimuli were abstract shapes and the C stimuli were meaningful pictures, 8 of 10 participants formed classes. This high yield may reflect the expansion of existing classes that consist of the associates of the meaningful stimuli, rather than the formation of the ABCDE classes, per se. When the A-E stimuli were abstract shapes and the C stimuli became S(D)s prior to class formation, 5 out of 10 participants formed classes. Thus, the discriminative functions served by the meaningful stimuli can account for some of the enhancement of class formation produced by the inclusion of a meaningful stimulus as a class member. A sorting task, which provided a secondary measure of class formation, indicated the formation of all three classes when the emergent relations probes indicated the same outcome. In contrast, the sorting test indicated "partial" class formation when the emergent relations test indicated no class formation. Finally, the effects of nodal distance on the relatedness of stimuli in the equivalence classes were not influenced by the functions served by the C stimuli in the equivalence classes.

The inclusion of a meaningful stimulus in a set of abstract stimuli enhances the likelihood of forming an equivalence class with the set. Class enhancement effects can be due to the discriminative, conditional discriminative, and class-based behavioral functions served by the meaningful stimulus. This experiment determined whether acquisition of an identity conditional discriminative function by an abstract stimulus enhances the formation of an equivalence class of which it is a member along with other abstract stimuli. Forty adults attempted to form 3 three-node five-member equivalence classes (A→B→C→D→E) using the simultaneous protocol. In the PIC group, the C stimuli were pictures and the A, B, D, and E stimuli were abstract shapes. In the ABS group, all of the stimuli were abstract shapes. In the Id-S-MTS (identity simultaneous matching-to-sample) and Id-6sD-MTS (identity 6 s delayed matching-to-sample) groups, prior to class formation, identity conditional discriminations were formed with the C stimuli using simultaneous or 6 s delayed matching-to-sample procedures, respectively. Classes were formed by 80 and 60 % of participants in the PIC and delayed identity groups, and by 0 and 10 % of participants with no prior training (ABS group) or after forming identity relations on a simultaneous basis. These outcomes were confirmed with post class formation sorting tests. Thus, a portion of the class enhancing effects of meaningful stimuli can be attributed to their presumed delayed identity conditional discriminative function. Adventitious coding or mediating behavior during identity training might have influenced acquisition of baseline relations and likelihood of class formation.

Undergraduates in six groups of 10 attempted to form three 3-node 5-member equivalence classes (A → B → C → D → E) under the simultaneous protocol. In five of six groups, all stimuli were abstract shapes; in the PIC group, C stimuli were pictures with the remainder being abstract shapes. Before class formation, participants in the Identity-S and Identity-D groups were given preliminary training to form identity conditional discriminations with the C stimuli using simultaneous and 6 s delayed matching-to-sample procedures, respectively. In the Arbitrary-S and Arbitrary-D groups, before class formation, arbitrary conditional discriminations were formed between C and X stimuli using simultaneous and 6 s delayed matching-to-sample procedures, respectively. With no preliminary training, classes in the PIC and ABS groups were formed by 80% and 0% of participants, respectively. After preliminary training, class formation (yield) increased with delay, regardless of relational type. For each of the two delays, yield was slightly greater after forming arbitrary- instead of identity-relations. Yield was greatest, however, when a class contained a meaningful stimulus (PIC). During failed class formation, probes produced experimenter-defined relations, participant-defined relations, and unsystematic responding; delay, but not the relation type in preliminary training influenced relational and indeterminate responding. These results suggest how meaningful stimuli enhance equivalence class formation.

In the present study, equivalence class formation was influenced by the temporal point of inclusion of a meaningful stimulus when baseline relations were serially or sequentially trained, and much less so by the location of the meaningful stimulus in the nodal structure of the class. In Experiment 1, participants attempted to form three 3-node, 5-member classes (A→B→C→D→E) under the simultaneous protocol. After serially training the baseline relations AB, BC, CD, and DE, in that order, the emergence of all emergent relations was tested concurrently. In the A-as-PIC condition, Awas meaningful stimulus and B to E were meaningless stimulus, and 60 % of the participants formed classes. In addition, classes were formed by 40 %, 70 %, 40 %, and 20 % of the participants in the B-as-PIC, C-as-PIC, D-as-PIC, and E-as-PIC groups, respectively. Thus, the likelihood of class formation could have been influenced by the location of a meaningful stimulus in the class structure and/or by its order of introduction during training. In Experiment 2, we controlled for any effect of order of introduction by the concurrent training of all of the baseline relations. Regardless of the location of the meaningful stimulus, 0-20 % of participants formed classes. Thus, the temporal order of introducing a meaningful stimulus was the primary modulator of the class-enhancing property of meaningful stimuli, and not the location of the meaningful stimulus in the class structure.

The present experiment showed that a simple discriminative function acquired by an abstract stimulus through simultaneous and/or successive discrimination training enhanced the formation of an equivalence class of which that stimulus was a member. College students attempted to form three equivalence classes composed of three nodes and five members (A→B→C→D→E), using the simultaneous protocol. In the PIC group, the C stimuli were pictures and the A, B, D, and E stimuli were abstract shapes. In the ABS group, all of the stimuli were abstract shapes. In the SIM + SUCC (simultaneous and successive) group, simple discriminations were formed with the C stimuli through both simultaneous and successive discrimination training before class formation. Finally, in the SIM-only and SUCC-only groups, prior to class formation, simple discriminations were established for the C stimuli with a simultaneous procedure and a successive procedure, respectively. Equivalence classes were formed by 80% and 70% of the participants in the PIC and SIM + SUCC groups respectively, by 30% in the SUCC-only group, and by 10% apiece in the ABS and SIM-only groups. Thus, pretraining of combined simultaneous and successive discriminations enhanced class formation, as did the inclusion of a meaningful stimulus in a class. The isolated effect of forming successive discriminations was more influential than that of forming simultaneous discriminations. The establishment of both discriminations together produced an enhancement greater than the sum of the two procedures alone. Finally, a sorting test documented the maintenance of the classes formed during the simultaneous protocol. These results also provide a stimulus control-function account of the class-enhancing effects of meaningful stimuli.