Rats exhibit asymmetrical retention functions for hedonic and nonhedonic samples in many-to-one symbolic delayed matching to sample

Rats were initially trained in a symbolic delayed matching-to-sample task either to discriminate hedonic samples that consisted of food or no food or to discriminate tone samples that differed in frequency and location. The retention functions for both the hedonic and tone samples were asymmetric, with forgetting of the food sample or the high-frequency tone occurring more rapidly than forgetting of the no-food sample or the low-frequency tone. Next, many-to-one (MTO) training was given in which tone samples were added for the rats initially trained with hedonic samples, and hedonic samples were added for the rats initially trained with tone samples. For both groups, a food sample and a tone sample (tone-F) were associated with responding to one lever (e.g., stationary), and a no-food sample and a different tone sample (tone-NF) were associated with responding to the alternative lever (e.g., moving). During retention testing, we found equivalent forgetting for the food and no-food samples, but forgetting of the tone-F sample occurred more rapidly than forgetting of the tone-NF sample. This is the first MTO study to suggest that rats, like pigeons, may use hedonic samples as the basis for the common coding of nonhedonic samples in MTO delayed matching.     

Memory codes for temporal and nontemporal samples in many-to-one matching by pigeons

Pigeons were trained to match temporal (2 and 8 sec of keylight) and color (red and green) samples to vertical and horizontal comparison stimuli. In Experiment 1, samples that were associated with the same correct comparison stimulus displayed similar retention functions; and there was no significant choose-short effect following temporal samples. This finding was replicated in Phase 1 of Experiment 2 for birds maintained on the many-to-one mapping, and it was also obtained in birds that had been switched to a one-to-one mapping by changing the comparison stimuli following color samples. However, in Phase 2 of Experiment 2, when the one-to-one mapping was produced by changing the comparison stimuli following temporal samples, a significant choose-short effect was observed. In Experiment 3, intratrial interference tests gave evidence of temporal summation effects when either temporal presamples or color presamples preceded temporal targets. This occurred even though these interference tests followed delay tests that failed to reveal significant choose-short effects. The absence of significant choose-short effects in Experiment 1 and in Phase 1 of Experiment 2 indicates that temporal samples are not retrospectively and analogically coded when temporal and nontemporal samples are mapped onto the same set of comparisons The interference test results suggest that the temporal summation effect arises from nonmemorial properties of the timing system and is independent of the memory code being used     

Common coding by pigeons in a many-to-one delayed matching task as evidenced by facilitation and interference effects

Pigeons were first trained on many-to-one delayed matching in which pairs of hue and line-orientation samples were associated with individual comparison stimuli. They were then trained to match two of the original samples (either hues or line orientations) to new comparisons, after which 2-sec delays were inserted between the samples and comparisons. In testing, the remaining samples were presented as interpolated stimuli during the delays. When the interpolated stimulus had been associated with the same comparison as the sample in many-to-one matehing, performance was significantly more accurate than when it had been associated with a different comparison. This finding adds to the evidence that samples sharing common comparison associations are commonly coded.     

Mediated transfer testing provides evidence for common coding of duration and line samples in many-to-one matching in pigeons

A three-phase transfer design was used to determine whether pigeons use a single, common code to represent line and duration samples that are associated with the same comparison stimulus. In Phase 1, two sets of samples (two lines and two durations) were associated with either a single set of comparisons (Group MTO, many-to-one) or with different sets of comparisons (Group OTO, one-to-one). In Phase 2, one set of samples was associated with a new set of comparisons. In Phase 3 (transfer test), the alternate set of samples was substituted for the Phase 2 samples. Group MTO, but not Group OTO, demonstrated immediate transfer. It was concluded that associating a line and a duration sample with the same comparison stimulus results in representation of those samples by a single code.     

The role of test stimuli in matching to compound samples by pigeons

Two pigeons matched to sample in a three-key operant conditioning chamber. In Experiment I, two different kinds of samples were presented on the center key.Element samples were members of one of two sample sets — colors (a red or blue disk) or lines (a vertical or horizontal orientation of a set of white lines). These samples were followed by their respective sample sets on the side keys as comparison stimuli.Compound samples consisted of a set of lines superimposed on a colored disk. Following these samples, either sample set could appear as comparison stimuli. Matching to compound samples was less accurate than matching to element samples. One interpretation is that sharing of attention among elements of a compound sample weakened stimulus control by each element. A different interpretation is that an element sample controlled matching better because it was physically identical to a comparison stimulus whereas a compound sample was not. Experiments II–IV evaluated this “generalization decrement” alternative by testing element- vs. compound sample control with both element and compound comparison stimuli. Irrelevant elements were added to form compound comparison stimuli, some of which were physically identical to a preceding compound sample, but never identical to an element sample. In all experiments, the addition of irrelevant elements of comparison stimuli reduced sample control. However, the generalization decrement hypothesis failed to predict how differences in performance maintained by element and compound samples were affected by different tests of sample control. Matching accuracy appeared to be independently determined by the number of elements in a sample and whether irrelevant elements were present during tests of sample control.     

Coding of feature and no-feature events by pigeons performing a delayed conditional discrimination

Pigeons’ performance of a delayed conditional discrimination with presence versus absence of conditional (sample) stimuli was examined in two experiments. The pigeons showed steeper retention functions with feature (i.e., presence) samples (either food or yellow) than with no-feature (i.e., absence) samples (either no food or no yellow). These results suggest that pigeons code features and respond only by default to test stimuli (comparisons) associated with no features. In contrast, the overall superiority of performance on no-feature-sample trials compared with feature-sample trials in both the food/no-food- and yellow/no-yellow-sample tasks was reversed at a 0-sec delay in the food/no-food-sample group, but not in the yellow/non-yellow-sample group. This difference in results with hedonic versus nonhedonic samples suggests that the crossover in delay performance on food/no-food-sample trials is produced by the formation of backward associations between the food-associated comparison stimulus and the food sample.     

Some tests of the anticipatory mediated generalization model of acquired sample equivalence in pigeons’ many-to-one matching

Transfer-of-control tests typically show the development of acquired equivalence between samples occasioning the same comparison choice in pigeons’ many-to-one matching-to-sample. Specifically, when some of those samples are later explicitly trained to occasion new comparison choices, the remaining samples immediately exert control over the new choices as well. In the present experiments, we examined whether or not this transfer effect depends on the order in which the various sample-comparison relations in training are learned. One group of pigeons initially acquired 0-delay many-to-one matching with four samples and two comparisons, followed by 0-delay matching with two of those samples and two new comparisons. Another group of pigeons learned the two-sample matching task first, followed by many-to-one matching. When subsequently tested for their ability to match the remaining samples from many-to-one matching to the comparisons used in the two-sample task, both groups showed comparable levels of transfer. These findings challenge the view that common anticipatory processes ostensibly arising from the samples in many-to-one matching are necessary mediators for the subsequent transfer effects indicative of acquired sample equivalence.     

Differential-outcomes effect using hedonically nondifferential outcomes with delayed matching to sample by pigeons

When differential outcomes follow correct responses to each of two comparison stimuli in matching to sample, relative to the appropriate control condition, higher matching accuracy is typically found, especially when there is a delay between the sample and the comparison stimuli. In two experiments, we examined whether this differential-outcomes effect depends on using outcomes that differ in hedonic value (e.g., food vs. water). In Experiment 1, we found facilitated retention when a blue houselight followed correct responses to one comparison stimulus and a white houselight followed correct responses to the other, prior to nondifferential presentations of food. In Experiment 2, we found facilitated retention again when a blue houselight followed correct responses to one comparison stimulus and a tone followed correct responses to the other, prior to nondifferential presentations of food. The results of both experiments indicate that the differential-outcomes effect does not depend on a difference in hedonic value of the differential outcomes, and they suggest that outcome anticipations consisting of relatively arbitrary but differential stimulus representations can serve as cues for comparison choice.     

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.     

Delayed matching in pigeons with food and no-food samples: Further examination of backward associations

When pigeons are trained on a delayed conditional discrimination with presence versus absence samples and tested with delays, a bias to choose the comparison associated with the absence sample is observed with increasing delay. Additionally, when the samples consist of food versus no food, this trial-type performance difference is reversed on short-delay trials: a bias to choose the comparison associated with the presence sample develops with delay testing. This reversal in comparison bias at short delays has been attributed to a preference produced by backward associations between the hedonic samples and the nonhedonic choice stimuli. In the present experiment, we tested an alternative hypothesis, that the short-delay comparison bias is produced by proactive interference—in particular, from reinforcement obtained on the previous trial—by including a group trained with reinforcement on only half of the trials with a correct response. According to the proactive interference account, this group should have shown a smaller short-delay comparison bias than would the typical 100% reinforcement group. Instead, consistent with a backward-association interpretation, the magnitude of the short-delay comparison bias shown by the 50% group was significantly greater than that shown by the 100% group.     

Reinforcement expectancy and trial spacing effects in delayed matching-to-sample by pigeons

Four experiments assessed the role of reinforcement expectancies in the trial spacing effect obtained in delayed matching-to-sample by pigeons. In Experiment 1, a differential outcome (DO) group received reinforcement with a probability of 1.0 for correct comparison responses following one sample stimulus and a probability of 0.2 for correct comparison responses following the other sample stimulus. The nondifferential outcome (NDO) group received reinforcement with a probability of 0.6 for correct responses to either stimulus. While matching accuracy was higher for the DO group than for the NDO group, both groups showed an equivalent decline in accuracy as the intertriai interval (ITI) duration was decreased. However, within the DO group, ITI duration affected performance on low-probability-of-reinforcement trials but not on high-probability-of-reinforcement trials. In Experiment 2, delay interval (DI) duration was 5, 10, or 15 sec and accuracy was higher for the DO group than for the NDO group at all DI durations. In addition, accuracy decreased similarly on high- and low-probability-of-reinforcement trials for the DO group as DI was increased. In Experiment 3, all birds were studied under DO conditions and ITI duration was manipulated along with DI duration. At the short DI duration, decreasing ITI duration had a detrimental effect on low-probability-of-reinforcement trials but no effect on high-probability-of-reinforcement trials. At the long DI duration, decreasing ITI duration had detrimental effects on both types of trials. In Experiment 4, unsignaled ITI reinforcers disrupted accuracy when the DI was long and when the ITI was short. The applicability of scalar expectancy theory to these data is discussed.     

Control of pigeons’ matching and mismatching performance by instructional cues

Pigeons were trained on a two-stimulus-shape (a plus and a circle) complex conditional discrimination that required birds to match sample and comparison stimuli on some trials and to mismatch on other trials, depending on the level of chamber illumination (bright or dark). Following acquisition, the birds were transferred to a novel color (red and green) task. For half of the birds, the contingenties between levels of illumination and the match/mismatch response requirements were consistent with training (nonreversal condition). For the remaining birds, the contingencies between levels of illumination and match/mismatch response requirements were the opposite of those established in training (reversal condition). Birds in the nonreversal condition acquired the color match/mismatch task at a significantly faster rate than birds in the reversal condition. These results indicate that relation-based responding (generalized matching/ mismatching) is subject to discriminative control.     

Differential outcomes and many-to-one matching: Effects of correlation with correct choice

Pigeons trained on many-to-one matching-to-sample with different probabilities of reinforcement for correct choice acquired the task more rapidly and showed better working memory performance when those different probabilities were correlated with the correct comparisons than when they were uncorrelated with them. Furthermore, this advantage was maintained when all correct choices were subsequently reinforced at the same probability, even though the change to nondifferential outcomes produced a drop in accuracy in the (formerly) correlated group. When birds were later shifted from correlated to uncorrelated outcomes or vice versa, the original between-group differences were reversed or eliminated. These data suggest that differential outcomes will potentiate matching performance if they generate an expectancy cue that is also predictive of correct choice. In addition, different outcomes may enhance common coding of the samples in many-to-one matching to the extent that they too are correlated with the correct comparison alternatives.     

Interference of delayed matching to sample in pigeons: Effects of interpolation at different periods within a trial and stimulus similarity

Delayed matching-to-sample performance by pigeons was interfered with by displaying a monochromatic annulus around the center (sample) pecking key. The wavelength of the annulus and its point of interpolation within a trial were varied to determine possible differential effects on matching accuracy. Experiment 1 showed that delayed matching was most disrupted when the interference stimulus (570 nm, 630 nm, or achromatic white) appeared during the delay interval of a trial. Little if any disruption occurred when the interference stimulus was present during the sample and choice periods. The spectral relationship between the chromatic interference stimuli (570 and 630 nm) and the sample stimuli (570 and 630 nm) did not consistently influence the degree to which matching accuracy was affected in any interpolation condition. Experiment 2 found a similar pattern of within-trial effects when the interference stimulus was simply a change from a white achromatic annulus to a chromatic one. This finding indicates that illumination changes, such as the popular houselight variation, are not necessary to produce interference in delayed matching to sample. Even with illumination held constant, however, performance was not differentially sensitive to the similarity between interference and sample stimulus wavelengths. It is suggested that other experiments showing similarity effects in interference of delayed matching to sample were conducted in such a way that subjects confused the interfering stimuli with the samples.     

Matching-to-sample performance by pigeons trained with visual-duration compound samples

Pigeons were trained on duration matching-to-sample in which each of four combinations of signal type (red or white light) and duration (2 or 10 see) was mapped onto a different choice stimulus. Probe trials in Experiments 1 and 2 involved a successive presentation of two duration samples. In each experiment, birds tended to summate two durations when the same signal was presented twice, but not when two different signals appeared. These results contrast with those reported by Spetch and Sinha (1989), who found a summation effect with both same-signal and different-signal compounds. In Experiment 3, pigeons chose among two alternatives which were both associated with the duration of the sample but of which only one was also associated with the signal type of the sample. Pigeons systematically chose the stimulus that matched both sample duration and signal type. The implications of these findings are discussed in terms of transfer of training and coding of event duration.     

Delayed matching in the pigeon: Interference produced by the prior delayed matching trial

Pigeons’ delayed matching performance on Trial n was examined as a function of whether the correct and incorrect comparison stimuli from Trial n?1 were maintained in the same role on Trial n (positive transitions), were reversed in role on Trial n (negative transitions), or were absent on Trial n (neutral transitions). Relative to neutral transitions, positive transitions did not significantly facilitate performance. Negative transitions, however, produced significant proactive interference on Trial n, and the magnitude of proactive interference was greater when the Trial n retention interval was 1 sec than when it was 0 sec. As the intertriai interval increased from 2 to 10 sec, the amount of interference dissipated. The results suggest that a prior delayed matching trial can serve as a significant source of forgetting but not a significant source of facilitation on an immediately following delayed matching trial.     

An evaluation of the role of dual coding in mediating the effect of incorrectly cuing the comparison dimension in delayed matching in pigeons

Pigeons were trained on delayed matching-to-sample trials in which red and green sample stimuli were equally often followed by color comparisons and by line-orientation comparisons. The color samples were preceded and accompanied by cues (a triangle or a black dot) that signaled whether the comparisons on that trial would be colors or lines. Length of the retention interval was manipulated during testing, and probe trials were included on which the dimension of the comparison stimuli either was cued incorrectly or was not cued. Accuracy on incorrectly cued and on no-cue trials was less than that on correctly cued trials, and the magnitude of this effect was not influenced by the length of the retention interval. Accuracy on incorrectly cued and on no-cue trials was equivalent, and was greater than chance. The data are inconsistent with two dual-coding interpretations of the effects of incorrectly cuing the dimension of the comparison stimuli in which it is held that both retrospective and prospective sample coding occurs in this task.     

Generalization of visual matching and delayed matching by a California sea lion (Zalophus californianus)

Only a limited number of species have been found capable of generalized matching-to-sample (MTS) after exposure to relatively few training exemplars. We trained a juvenile, experimentally naive California sea lion (Zalophus californianus) in MTS, using a pair of three-dimensional objects as samples. Successful matching to a criterion of 90% correct or better over 2 successive sessions was attained in 12 sessions (269 trials and 70 errors). Two subsequent “partial” transfer tests, in which each of the two training objects was paired with a novel test object, and four additional transfer tests, all with novel objects, were presented following training. An 80% performance criterion over 2 successive sessions was reached, or closely approximated, in from 2 to 4 transfer sessions for all transfer tests; errors to criterion tended to be reduced across the successive novel transfer tests and were as few as five during the final two tests; and performance on the first 48 trials of the last two novel transfers was not significantly different from a near-ceiling level baseline performance measure. Neophobic responses of the sea lion to new objects precluded an unbiased evaluation of immediate (Trial 1) transfer. The sea lion’s short-term memory for sample objects was also measured. Matching performance was maintained at a level of 78% correct responses or better for delays through to 45 sec after removal of the sample object. At a 58-sec delay, the longest tested, performance declined to 69% correct responses. These retention levels are only somewhat below levels reported for dolphins and nonhuman primates tested on visual delayed MTS, but they are above levels typically reported for pigeon subjects.     

The effects of sodium pentobarbital on matching and oddity performance in pigeons

The effects of sodium pentobarbital on matching and oddity performance in pigeons were examined by employing a higher-order conditional discrimination paradigm. In this paradigm, the line orientation which was superimposed on all of the response keys signaled whether a response to the matching color or a response to the nonmatching color was correct. All pigeons had extensive previous training in this paradigm and were tested at each of three dosage levels: 5, 7.5, and 10 mg/kg. For all birds, a clear dose-related decrease in accuracy was observed; however, the effect was not differential for matching and oddity trials. Accuracy reductions were accompanied by an increase in position preference on both types of trials. The data are compatible with recent claims that physical identity of the sample and correct comparison stimulus need have no special status for pigeons.     

Emergent relations in pigeons following training with temporal samples

In two experiments, we investigated emergent conditional relations in pigeons using a symbolic matching-to-sample task with temporal stimuli as the samples and hues as the comparisons. Both experiments comprised three phases. In Phase I, pigeons learned to choose a red keylight (R) but not a green keylight (G) after a 1-s signal. They also learned to choose G but not R after a 4-s signal. In Phase II, correct responding consisted of choosing a blue keylight (B) after a 4-s signal and a yellow keylight (Y) after a 16-s signal. Comparisons G and B were both related to the same 4-s sample, whereas comparisons R and Y had no common sample. In Phase III, R and G were presented as samples, and B and Y were presented as the comparisons. The choice of B was correct following G, and the choice of Y was correct following R. If a relation between comparisons that shared a common sample were to emerge, then responding to B given G would be more likely than responding to Y given R. The results were generally consistent with this prediction, suggesting, for the first time in pigeons, the emergence of novel relations that involve temporal stimuli as nodal samples.