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.     

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.     

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.     

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.     

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.     

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.     

Flexible coding of temporal information by pigeons: Event durations as remember and forget cues for temporal samples

The ability of pigeons to use event durations as remember (R) and forget (F) cues for temporal samples was examined. Pigeons were required to indicate whether a houselight sample stimulus was short (2 sec) or long (6 sec) by pecking a red or a green comparison stimulus. After training with a constant 10-sec delay interval, temporal cues (illumination of the center key) were presented 2 sec after the offset of the temporal samples. For one group, a short (2-sec) temporal cue served as the R cue and a long (6-3ec) temporal cue served as the F cue. This was reversed for a second group of birds. During training, comparison stimuli were always presented following the temporal R cue, but never following the temporal F cue. Tests for the effectiveness of the temporal R and F cues showed that F cues were equally effective in reducing matching accuracy in both groups of birds. It was concluded that pigeons used the duration of the cue to determine whether or not to rehearse the memory code for the temporal 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.     

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.     

Memory for empty time intervals in pigeons

Pigeons were trained to discriminate short (2 sec) and long (8 sec) empty intervals that began each trial. In group consistent, onset of an empty interval was marked by a brief presentation of red keylight, and termination of the interval was marked by a brief presentation of green keylight. In group inconsistent, red and green served equally often as the first and second markers across trials. Testing revealed that, in group consistent, (1) birds were sensitive to the relation between marker color and marker type and (2) presentation of the second marker did not initiate timing a new interval. Testing also revealed a robust choose-long effect at delays longer than the training delay and indifference between the comparisons on no-sample trials. Both of the latter findings differ from those typically obtained when filled intervals are employed. It was concluded that pigeons process filled and empty intervals differently.     

Memory interval distribution effects in pigeons

Pigeons were trained with two concurrent delayed conditional discriminations that involved different distributions of memory intervals, namely 1 and 5 sec in the “short” and 5 and 10 sec in the “long” distribution. Memory for the initial stimulus was much better after 5 sec in the short distribution than in the long one. The memory functions between the intervals within each distribution were essentially flat. These findings were replicated in a second study that involved differential outcomes; this procedure enhanced memory generally within trials so that memory intervals of 1, 9, and 19 sec could be used, with 9 sec serving as the common memory interval. The findings are interpreted on the basis of the subject’s expectation of the likely duration of the upcoming memory interval and the associated delay of reinforcement.     

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.     

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.     

Memory for duration in pigeons: Dissociation of choose-short and temporal-summation effects

Five groups of pigeons were trained in a symbolic choice-matching feast involving short (2-sec) and long (10-sec) durations of houselight as samples. Four groups also received training with a second set of samples: line orientations or 2- and 10-sec presentations of keylight. The type of sample-to-comparison mapping varied across groups. Although only two of the five groups demonstrated a choose-short effect (a tendency to choose the comparison associated with a short sample at longer delays), all groups demonstrated temporal summation (a tendency to respond on the basis of the combined duration of two successively presented samples). Moreover, the magnitude of temporal summation was equivalent in groups that did and did not-demonstrate a choose-short effect. The results suggest that the processes underlying the perception of sample duration remain invariant across different sample-to-comparison mapping arrangements, but that the memory code used to retain temporal information varies.     

Alterations in the memory code for temporal events induced by differential outcome expectancies in pigeons

The effect of differential outcome expectancies on memory for temporal and nontemporal information was examined. Pigeons were trained to match short (2-sec) and long (8-sec) sample durations to red and green comparison stimuli, and vertical and horizontal lines to vertical and horizontal comparison stimuli. In Experiment 1, one differential outcome (DO) group received food for correct choices on short-sample trials, whereas another received food for correct choices on long-sample trials. On line-orientation trials, half of each DO group received food for correct responses following vertical samples, whereas the other half received food for correct responses following horizontal samples. Overall retention was greater in the DO groups than in a nondifferential (NDO) group that received either food or no food for correct responses on a random half of all trials. Furthermore, although the NDO group displayed a choose-short bias for temporal samples, both DO groups displayed equivalent biases to select the comparison stimulus associated with food. In Experiment 2, differential outcome expectancies were extinguished off-baseline. Subsequently, in the first nondifferential outcome test session, the. DO groups performed less, accurately than the NDO group. These findings indicate that temporal samples are not retrospectively and analogically coded when they are differentially associated with food and no food. Instead, they are remembered in terms of the corresponding outcome expectancies.     

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.     

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.     

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.