Peter J. Urcuioli

Evidence for Common Coding in Many-To-One Matching: Retention, Intertrial Interference and Transfer

Deux experiences etudient chez le pigeon le codage des stimuli standards associes avec les stimuli de comparaison dans une epreuve dappariement

Behavioral and associative effects of differential outcomes in discrimination learning

The role of the reinforcer in instrumental discriminations has often been viewed as that of facilitating associative learning between a reinforced response and the discriminative stimulus that occasions it. The differential-outcome paradigm introduced by Trapold (1970), however, has provided compelling evidence that reinforcers are also part of what is learned in discrimination tasks. Specifically, when the availability of different reinforcing outcomes is signaled by different discriminative stimuli, the conditioned anticipation of those outcomes can provide another source of stimulus control over responding. This article reviews how such control develops and how it can be revealed, its impact on behavior, and different possible mechanisms that could mediate the behavioral effects. The main conclusion is that differential-outcome effects are almost entirely explicable in terms of the cue properties of outcome expectancies—namely, that conditioned expectancies acquire discriminative control just like any other discriminative or conditional stimulus in instrumental learning.

Common Coding in Pigeons Assessed Through Partial Versus Total Reversals of Many-to-One Conditional and Simple Discriminations

Common coding of stimuli was examined in pigeons in 3 experiments involving many-to-one mapping of lines and hues onto common events. The common events were shapes in Experiment 1 (involving delayed symbolic matching-to-sample) and food-no-food outcomes in Experiments 2 and 3 (involving simple discriminations). In Phase 2 of Experiments 1 and 2, the hue discriminations were reversed for Group Hue, the line discriminations were reversed for Group Line, and both discriminations were reversed for Group Hue-Line. Line reversals were learned faster by Group Hue-Line than by Group Line, but differences in reversal learning were not found with hues. In Experiment 3, both hue and line discriminations were repeatedly reversed until reversal transfer was stable. Relative to this baseline, significantly poorer performance was found on a line-only reversal. Overall, the results suggest that when a hue and a line are associated with a common event, both may be centrally represented as the hue.

Some relationships between outcome expectancies and sample stimuli in pigeons’ delayed matching

Two sets of experiments examined how differential outcomes affect conditional stimulus control by the samples in delayed matching-to-sample. Pigeons were initially trained on symbolic delayed matching with reinforcing outcomes that were either differential or nondiffereatial with respect to the samples. In one set of experiments, the outcome manipulation involved different (p = 1.0 vs. 0.2) versus the same (p = 0.6) probabilities of food; in the other, food and no-food outcomes were used. Following initial acquisition and mixed-delay tests, the matching procedure in each study was discontinued while the samples were nondifferentially reinforced with the same probability of food, or with food and no food, respectively. When later retested on delayed matching with those nondifferential outcomes, birds initially trained with different reinforcement probabilities matched at the same levels of accuracy as those trained with the same probability. By contrast, birds initially trained with food versus no-food outcomes showed lower levels of matching accuracy than their nondifferential controls. Subsequent transfer tests showed that matching performances by the differential birds in both studies had been originally cued in part by differential outcome expectancies. Apparently, the expectancies based upon different probabilities of food provided a source of conditional stimulus control that did not compete with the samples. By contrast, the expectation of food versus no food reduced (overshadowed) sample-stimulus control.

Interaction of sample dimension and sample-comparison mapping on pigeons’ performance of delayed conditional discriminations

Coding strategies developed in the acquisition of delayed conditional discriminations can be assessed by independently manipulating sample and comparison memory load. Two stimulus dimensions that can affect memory load were examined: Number of stimuli in the sample and comparison sets (two vs. four) was manipulated between groups in a 2×2 design, and discriminability of sample and comparison stimuli (hues vs. lines) was manipulated between counterbalancing subgroups and within subjects. The results indicated large effects of sample discriminability but not of comparison discriminability, evidence for retrospective coding. There was also a significant effect of number of stimuli in the comparison set (although only with hard-to-discriminate samples) but not of number of stimuli in the sample set, evidence for prospective coding. These findings suggest evidence for retrospective coding with easy-to-discriminate samples, independently of number of stimuli in the comparison set, and evidence for prospective coding with hard-to-discriminate samples.

4 Acquired equivalences and mediated generalization in pigeon's matching-to-sample

Publisher Summary This chapter discusses acquired equivalences in animal learning, particularly those instances reported in pigeons matching-to-sample (MTS). “Acquired equivalence” refers to the finding that very different stimuli can sometimes produce the same behavior even though one of them has no direct reinforced connection with that behavior. The attempt involves the resurrection of the explanatory ideas proposed by Hull that physically unrelated stimuli come to control the same behavior because they produce some common reaction that mediates the generalization of subsequent performances from one stimulus to another. The chapter explains this mediated generalization account of acquired equivalences. This chapter describes (after some general background material) three MTS tasks in which acquired sample equivalences have been demonstrated but which differ in the degree to which mediational processes are apparent: (1) acquired sample equivalences arising from an overt mediator, (2) acquired sample equivalences via common reinforcing outcomes, and (3) acquired sample equivalences from common comparison-stimulus associations.

On the role of differential sample behaviors in matching-to-sample.

Pigeons were trained on matching-to-sample (MTS) with differential sample-response requirements that were identical with respect to two pairs of sample stimuli but were either correlated or uncorrelated with correct choice. Experiment 1A showed that birds in the uncorrelated condition were slower to reach criterion levels of accuracy than birds in the correlated condition in spite of their equivalent sample discriminations. However, correlated birds were more disrupted in their matching performances than the uncorrelated birds when subsequently switched to nondifferential sample-response requirements (Experiment 1B). Experiment 2 showed that differential sample behaviors also generated higher levels of accuracy on delayed MTS when correlated with choice, and that accuracy in this condition did not differ as a function of whether the samples were hues or lines. Sample dimension did affect memory performance, on the other hand, in the uncorrelated condition. In Experiment 3, reversing differential sample-response requirements for one pair of samples substantially reduced matching accuracy in the correlated group but had almost no effect in the uncorrelated group. These findings demonstrate that differential sample behaviors directly control pigeons matching performances and also overshadow conditional stimulus control by the samples when these behaviors are predictive of correct choice. The facilitation in matching produced by differential sample behaviors apparently arises from the additional cue these behaviors provide, not because they enhance sample discriminability.

Memory codes in pigeon short-term memory: Effects of varying the number of sample and comparison stimuli ☆

Abstract In a delayed conditional discrimination task, pigeons can remember either some aspect of the conditional stimulus (i.e., they can code retrospectively) or some aspect of the stimulus to which they will respond at the end of the delay (i.e., they can code prospectively). To determine the nature of the memory code, we varied the number of possible sample stimuli (two or four) and the number of possible comparison stimuli (two or four) factorially across groups. Birds in all four groups were initially trained on a zero-delay, conditional discrimination with lines (vertical and horizontal) and/or shapes (circle and triangle), and were then tested with longer delays between sample offset and comparison onset. Acquisition of the conditional discrimination was affected by both the number of sample and comparison stimuli: birds were slower to reach criterion the greater the number in either stimulus set. During delay testing, however, only the number of comparisons affected performance. Overall, retention was poorer with four comparisons than with two. These data provide evidence for prospective coding in pigeon short-term memory.

Coding of hedonic and nonhedonic samples by pigeons in many-to-one delayed matching

In two experiments, pigeons were trained on many-to-one delayed matching in which samples of food and one hue were each associated with one shape comparison, and samples of no food and a different hue were each associated with a second shape comparison. When later tested with delays between sample and comparison stimuli, pigeons showed nonparallel delay functions, typically found with food and no-food samples (i.e., steeply declining food-sample delay functions, and relatively flat no-food-sample delay functions). Furthermore, the slopes of the hue-sample delay functions were similar to those on the food/no-food-sample trials. In Experiment 2, following many-toone delayed matching, when the hue samples were associated with new comparisons and then food and no-food samples replaced the hues, evidence was found for transfer of training indicative of the common coding of samples associated with the same comparison in original training. The transfer results suggest that the asymmetrical hue-sample functions resulted from the common coding of samples associated with the same comparison.

On the relationship between differential outcomes and differential sample responding in matching-to-sample.

Four experiments examined control over choice by differential sample responding in matching-to-sample with differential outcomes. In Experiment 1, pigeons initially learned to match with food versus no-food outcomes. Their performances later transferred to other samples to which responding versus not responding had been explicitly reinforced with a single outcome (food). In Experiment 2, pigeons initially learned to produce the comparisons by pecking one sample but not the other. Transfer was then observed to new samples associated with food versus no food (and thus often vs. seldomly pecked). Experiments 3 and 4 showed that transfer of matching required differential behavior to each sample set and did not depend on explicit conditioning of that behavior prior to acquisition. Together, these results show that differential sample behaviour provides a redundant cue for choice in differential outcome matching-to-sample.