An update on the search for symmetry in nonhumans

Sidman et al.'s (1982) failure to find evidence for symmetry (bidirectional associations between stimuli) in monkeys and baboons set the stage for decades of work on emergent relations in nonhumans. They attributed the failure to the use of procedures that did not (1) promote stimulus control based on the relation between the sample and correct comparison and (2) reduce control by irrelevant stimulus features. Previous reviews of symmetry in nonhumans indicated that multiple exemplar training and successive matching might encourage appropriate stimulus control. This review examined 16 studies that investigated symmetry in 94 subjects, including pigeons, rats, capuchin monkeys, and baboons. Several studies used alternative training procedures to minimize sources of irrelevant stimulus control, and many combined multiple exemplar training with other procedural modifications. Symmetry was observed in approximately 30% of subjects. Studies that reported the strongest evidence for symmetry used successive matching-to-sample procedures that included training on both symbolic and identity relations, and studies finding mixed evidence employed alternative methods. These studies highlight the challenge in creating training procedures that promote symmetry and the need to assess the underlying sources of control on positive demonstrations.     

Stimulus control topographies and tests of symmetry in pigeons

Pigeons were tested for symmetry after A-B training under conditions designed to avoid problems that may prevent its emergence, namely the change of stimulus location in testing relative to training and the lack of requisite discrimination training. In Experiment 1, samples appeared in two locations during baseline training to minimize the impact of stimulus location. Experiments 2 and 3 included multiple-location training along with additional identity and symbolic matching training, respectively, to explicitly train all of the simultaneous and successive stimulus discriminations required for testing. Experiment 4 provided reinforcement for symmetrical matching relations with some stimulus sets (with multiple-location training) prior to symmetry testing with different sets. In all experiments, pigeons showed no evidence of symmetry despite the fact that baseline (A-B) matching transferred to novel locations. Additional tests for reflexivity (Experiment 2) yielded similar outcomes. These results indicate that the change in stimulus location is not the sole reason that pigeons do not show symmetry and increase the plausibility of arguments that symmetry and other indexes of stimulus equivalence may be beyond the capabilities of the pigeon.     

The development of emergent differential sample behavior in pigeons

Three experiments attempted to replicate Manabe, Kawashima, and Staddon's (1995) finding of emergent differential sample behavior in budgerigars that has been interpreted as evidence of functional equivalence class formation. In Experiments 1 and 2, pigeons initially learned two-sample/ two-alternative matching to sample in which comparison presentation was contingent on pecking one sample on a differential-reinforcement-of-low-rate (DRL) schedule and the other on a fixed-ratio (FR) schedule. Later, two new samples were added to the task. Comparison presentation on these trials occurred after the first sample peck following a predetermined interval (Experiment 1) or after completion of either the DRL or FR requirement, whichever occurred first (Experiment 2). Experiment 1 found no evidence for emergent spaced versus rapid responding to the new samples as they established conditional control over the familiar choices. By contrast, differential responding did emerge for some pigeons in Experiment 2, with responding to each new sample coinciding with the pattern explicitly conditioned to the original sample occasioning the same comparison choice. This emergent effect, however, disappeared for most pigeons with continued training. Experiment 3 systematically replicated Experiment 2 using differential peck location as the sample behavior. Differential location pecking emerged to the new samples for most pigeons and remained intact throughout training. Our findings demonstrate a viable pigeon analogue to the budgerigar emergent calling paradigm and are discussed in terms of equivalence- and non-equivalence-based processes.     

Failure to replicate the 'work ethic" effect in pigeons

We report six unsuccessful attempts to replicate the "work ethic" phenomenon reported by Clement, Feltus, Kaiser, and Zentall (2000). In Experiments 1-5, pigeons learned two simultaneous discriminations in which the S+ and S- stimuli were obtained by pecking an initial stimulus once or multiple (20 or 40) times. Subsequent preference tests between the S+ stimuli and between the S- stimuli mostly revealed indifference, on average, between the S+ from the multiple-peck (high-effort) trials and the S+ from the one-peck (low-effort) trials, and likewise between the two respective Sstimuli. Using a slightly different procedure that permitted assessment of the relative aversiveness of low versus high effort, Experiment 6 again revealed a pattern of indifference despite showing that pigeons took considerably longer to begin pecking on high- than on low-effort trials. Our findings call into question the reliability of the original findings and the sufficiency of the hypothesized within-trial contrast mechanism to produce them.     

Some tests of response membership in acquired equivalence classes

Pigeons were trained on many-to-one matching in which pairs of samples, each consisting of a visual stimulus and a distinctive pattern of center-key responding, occasioned the same reinforced comparison choice. Acquired equivalence between the visual and response samples then was evaluated by reinforcing new comparison choices to one set of samples, and examining generalization of these choices to the other samples. Three separate experiments found no evidence of such generalization, as indexed by performance on class-consistent versus class-inconsistent tests. Other tests showed that the pigeons' center-key response patterns during training had indeed served as a conditional cue for choice. These results do not support the hypothesis that different defined responses can become members of acquired equivalence classes.     

Membership of Defined Responses in Stimulus Classes

The Psychological Record - Sidman (2000) has suggested that in addition to conditional and discriminative stimuli, class-consistent defined responses can also become part of an equivalence class....     

Expansion of Sidman's theory: The inclusion of prompt stimuli in equivalence classes

Stimulus equivalence is defined as the ability to relate stimuli in novel ways after training in which not all of the stimuli had been directly linked to one another. Sidman (2000) suggested all elements of conditional discrimination training contingencies that result in equivalence potentially become class members. Research has demonstrated the inclusion of samples, comparisons, responses, and reinforcers in equivalence classes. Given the evidence that all elements of a conditional discrimination become part of the class, the purpose of this study was to determine if class-specific prompts would also enter into their relevant equivalence classes. Experiment 1 investigated the inclusion of prompts in an equivalence class using abstract stimuli with neurotypical students enrolled in higher education courses. Experiment 2 systematically replicated Experiment 1 using meaningful stimuli and individuals diagnosed with autism spectrum disorder. The results of both experiments demonstrated that class-specific prompts became part of equivalence classes with the other positive elements of the contingency. The results are discussed in terms of class expansion and the potential impact on equivalence-based instruction.     

Assessing the contributions of S-O and R-O associations to differential-outcome matching through outcome reversals

Pigeons were trained on symbolic matching with 2 samples, 2 pairs of comparisons, and different outcomes for the correct responses within each comparison pair. For one group, the 2 samples were also associated with different outcomes, whereas for another group, they were not. When the response-outcome (R-O) relations for one pair were subsequently reversed, the group trained with differential sample-outcome (S-O) associations was significantly disrupted in its performance on both reversed- and nonreversed-outcome trials. By contrast, the group trained with just differential R-O associations was disrupted only on reversed-outcome trials. These results were replicated when the outcomes on the initially nonreversed trials were then reversed. The findings indicate that differential S-O associations, when present, have a stronger influence on matching performances than differential R-O associations. They are also consistent with hierarchical and configural models of discriminative control.     

Transfer of pigeons' matching to sample to novel sample locations

This study examined the conditions under which conditional stimulus control by the sample stimuli in three-key matching-to-sample paradigms would generalize across the different possible sample locations. In Experiments 1 and 2, the samples appeared on the left and right side keys during initial training and then on the center key during testing. Transfer of pigeons' matching performances to the center-key samples was evident after both identity and symbolic matching training. In Experiment 3, pigeons trained on symbolic matching with two side-key samples or with a side-key and a center-key sample generally transferred their learned matching performances to those samples when they subsequently appeared in the remaining (novel) location. These results indicate that, when two-choice conditional discriminations are learned with more than one sample location, the visual characteristics of the sample per se predominantly come to control the pigeons' comparison choices. This finding encourages the use of the multiple-location training procedure as a way of reducing control by location, thus providing a more discriminating test of symmetry in animals.     

WHEN IS A FAILURE TO REPLICATE NOT A TYPE II ERROR?

Zentall and Singer (2007) challenge our conclusion that the work-ethic effect reported by Clement, Feltus, Kaiser, and Zentall (2000) may have been a Type I error by arguing that (a) the effect has been extensively replicated and (b) the amount of overtraining our pigeons received may not have been sufficient to produce it. We believe that our conclusion is warranted because (a) the original effect has not been replicated despite multiple attempts to do so and (b) the statement that more extended overtraining may be needed itself suggests that the original effect is not reliable.