Stimulus Relatedness in Equivalence Classes: Interaction of Nodality and Contingency

Abstract

Stimuli in equivalence classes are functionally interchangeable, and thus suggest that the class members are equally related to each other. A growing number of experimental findings, however, have shown that under appropriate conditions of testing, the relational strength of the stimuli in an equivalence class may be determined by the nodal structure of the class and the number of nodes the separate the stimuli in a multinodal class. Data that support this view have been obtained (a) during the delayed emergence of classes, (b) during gradual transfer of functions after class formation, (c) from the reaction times displayed by derived relations that varied in nodal number during and after class formation, (d) by derived relations in the absence of class formation, (e) from retrospective reports of reinforcer presentations during derived relations probes, (f) from brain imaging studies that showed unique areas of cortical activation produced by post-class formation probes of baseline and derived relations, (g) within class preference tests that pitted relations of different nodal numbers, and (g) the results of response transfer tests that bifurcated multinodal equivalence classes in precise accordance with nodal structure. Taken together, the data suggest that the stimuli in an equivalence class acquire two functions: one class based, and the other nodally based. Performances reflective of each are under the discriminative control of test trials that had different formats. When the format signals the primacy of between-class variables, only one response option is available per class, and the stimuli in the class produce responding indicative of the interchangeability, and the equal relatedness of the stimuli in the class. When the trial format signal the primacy of within-class variables, at least two response options are available per class, and the stimuli and relations produce differential responding that are inverse functions of nodality which indicate the unequal relatedness of the stimuli in the class.