Stimulus equivalence: Effects of a default-response option on emergence of untrained stimulus relations

Default-response options, intended to measure uncertainty, sometimes are included in discrete-choice measurement in an attempt to clarify stimulus control of remaining response options. Recent experiments have used a default-response procedure to investigate emergent stimulus relations, but no study to date has compared effects of different default-response procedures on emergence of the untrained relations that define stimulus equivalence. Five groups of college undergraduates (each n = 16) completed a conditional discrimination training procedure to instate the stimulus relations prerequisite to three three-member equivalence classes; a training review intermingling all of the explicitly trained relations; and tests for emergent relations. The groups differed in terms of (a) presence versus absence of a "none" option during emergent relations tests and (b) the amount of experience with "catch trials" in which "none" was the correct selection. Stimulus equivalence was demonstrated in 94% of subjects in a control group who were trained and tested without the "none" response option and without catch trials and in 41% of subjects in the "none" groups. Among subjects in the "none" groups who failed to demonstrate equivalence initially, 95% did so when retested under control-group conditions. Across "none" groups, probability of equivalence class formation was positively correlated with amount of experimental experience with catch trials in preliminary training and equivalence testing. Among the emergent relations defining stimulus equivalence, reflexivity was most often precluded by the "none" option, although there was evidence of group differences in relation specificity. These results suggest that a default-response option can interfere with the formation of emergent relations, and that the effects are contextually sensitive. Although there may be advantages to employing default-response procedures in studies of emergent stimulus relations, the responses they control should be viewed as behavior under specific stimulus control rather than a generic expression of uncertainty.