Previous studies comparing groups of subjects have indicated differential probabilities of stimulus equivalence outcome as a function of training structures. One-to-Many (OTM) and Many-to-One (MTO)

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In most studies of delayed matching-to-sample (DMTS) and stimulus equivalence, the delay has remained fixed throughout a single experimental condition. We wanted to expand on the DMTS and stimulus equivalence literature by examining the effects of using titrating delays with different starting points during the establishment of conditional discriminations prerequisite for stimulus equivalence. In Experiment 1, a variation of a single-subject withdrawal design was used. Ten adults were exposed to one condition where the delay titrated between 0 and 3000 ms and another condition where the delay varied from 5000 to 8000 ms. Subsequently, participants were re-exposed to the condition they had first experienced. Results show that starting the titration of the delay at 5000 ms may have had a facilitatory effect on stimulus equivalence responding for some participants. For several participants, however, performance was stable throughout the experiment, apparently not affected by either the variations of the starting point of the titrating delay or by previous exposure to stimulus equivalence training and test procedures. In Experiment 2, we examined the effects of being exposed to the same stimulus equivalence procedure three times, again with adults as participants. Results show that such extended exposure had very limited effects on stimulus equivalence responding. This indicates that single-subject withdrawal designs may be an appropriate approach for studying stimulus equivalence.

Conditional discriminations established using a linear series training structure were shown in a previous study to be intact when unreinforced trials assessing such performance were presented interspersed among stimulus equivalence test trials. Experimenter-defined correct performance on these trials was seen both for participants responding in accordance with stimulus equivalence and for some participants who did not. The goal of the current study was to investigate the status of previously trained relations at the time of stimulus equivalence tests in further contexts. Experiment 1 investigated this following many-to-one conditional discrimination training with 20 adult participants. Experimenter-defined correct performance on probes for trained relations was observed for 16 out of 17 of participants who formed stimulus equivalence classes, as well as for all participants not responding in accordance with stimulus equivalence. In Experiment 2, 18 adult participants received linear series, many-to-one, or one-tomany conditional discrimination training and tests for stimulus equivalence with interspersed training trials. Subsequently, participants experienced three additional tests for derived and trained performance within a month of the original training. Continued testing and the passage of time had little effect on the stability of both learned and derived performance for participants initially responding in accordance with stimulus equivalence. Participants who failed to form stimulus equivalence classes in the initial test, but responded in line with the trained relations, displayed more varied performance over time. Results from both experiments indicate that the lack of stimulus equivalence performance is not commonly caused by a breakdown of trained relations at the time of the stimulus equivalence test.Establishing the prerequisite behavior for stimulus equivalence commonly involves training multiple conditional discriminations consisting of arbitrarily related stimuli using matching-to-sample (MTS) procedures. Conditional discriminations are defined by a discriminative stimulus, a comparison, being a behaviorally effective part of a three-term contingency only when a specific conditional stimulus, a sample, has been presented. In commonly used stimulus equivalence procedures, stable experimenter-defined correct performance in an MTS procedure establishing conditional discriminations is followed by a test for the emergence of derived performance having the properties of reflexivity, symmetry, and transitivity (Sidman and Tailby 1982). If performance according to these properties is observed, a stimulus equivalence class is considered established. Three common variations of MTS found in the stimulus equivalence literature are the linear series (LS), many-to-one (MTO), and one-to-many (OTM) training structures. In the LS structure, sets of conditional discriminations are linked by having the same stimuli appear as samples and comparisons on different trials in the MTS procedure; for example, by training s...