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.     

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     

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.     

Representation strength in pigeon short-term memory: Effect of delay training

An attempt was madeto manipulate the strength of internal stimulus representations by exposing pigeons to brief delays between sample offset and comparison onset in a delayed conditional discrimination. In Experiment 1, pigeons were first trained on delayed conditional discrimination with either short (0.5-sec) delays or no delays. When delays were increased by 2.0 sec, birds trained with a delay performed at a higher level than did birds trained with no delays. In Experiment 2, subjects were first trained on a delayed simple discrimination. Following a circle stimulus, responses to a white key were reinforced; however, following a dot stimulus, responses to the white key were not reinforced. The pigeons were then trained on a delayed conditional discrimination involving hue samples and line-orientation comparisons with differential outcomes. Choice of vertical following red yielded food; choice of horizontal following green yielded no food. Mixed delays were then introduced to birds in Group Delay, whereas birds in the control group received overtraining. When tested on a delayed simple discrimination with hue stimuli (red and green initial stimuli followed by white response stimulus), pigeons in Group Delay tended to perform at a higher level than did birds in the control group (i.e., although the birds in both groups responded more following red than following green, birds in Group Delay did this to a greater extent than did birds in the control group). Thus, experience with delays appears to strengthen stimulus representations established during training.     

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.     

Retardation and facilitation of matching acquisition by differential outcomes

Pigeons were trained on many-to-one matching-to-sample with food and no-food outcomes that were either differential or nondifferential with respect to the sample stimuli. In the differential condition, outcomes were correlated with the correct comparison-alternatives-for half of the subjects, and were uncorrelated with those alternatives for the remaining subjects. Relative to non-differential training, matching acquisition was facilitated in the correlated condition but retarded in the uncorrelated condition. These results clearly demonstrate that differential outcomes do not affect conditional discrimination learning merely by enhancing the discriminability or distinctiveness of the samples with which they are associated. Rather, they apparently give rise to another discriminative cue (viz., an outcome expectancy), which can either enhance or interfere with performance, depending on its predictive validity.     

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.     

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.     

Restricted processing of simultaneously presented brightness and pattern stimuli in pigeons

Performance during simultaneous matching-to-sample was assessed in pigeons presented with element and compound visual samples. In Experiment 1, pigeons were trained with a symbolic matching procedure, in which different pairs of colored comparison cues presented on side keys were mapped onto a bright or dim houselight as one pair of sample stimuli and onto vertical and horizontal lines on the center key as a second pair of sample stimuli. They were then tested with houselight-line compound samples. It was found that matching accuracy for lines was significantly diminished with compound samples relative to element samples. Conversely, house-light intensities were matched as well with compound samples as with element samples. In Experiment 2, a similar effect was found with pigeons that had been trained to match only line samples. In Experiment 3, it was discovered that sample duration had no influence on the matching deficit found with lines following compound samples in birds either trained or not trained to match houselight intensities. These results, taken in combination with recent findings from experiments with auditory-visual compounds, suggest a restricted processing account of pigeon processing of simultaneously presented stimuli from different sources.     

Prospective representation: The effects of varied mapping of sample stimuli to comparison stimuli and differential trial outcomes on pigeons’ working memory

The mapping of sample stimuli onto comparison stimuli and the nature of trial outcomes were factorially manipulated in a delayed matching-to-sample procedure. In the many-to-one condition (MTO), responding to a particular comparison was correct following several sample stimuli, whereas in the one-to-many condition (OTM), responding to several comparison stimuli was correct following a particular sample. Probabilities of reinforcement for correct responding to comparison stimuli were either differential (DO) or nondifferential (NDO). Four groups of pigeons were trained under four combinations of mapping and outcome conditions, MTO-DO, MTO-NDO, OTM-DO, and OTM-NDO. Testing at delay intervals of 1, 2, 4, and 8 sec revealed significant effects due to both the mapping variable and the differential outcomes variable. It was argued that the poorer performance obtained in the OTM condition was due to the differential prospective coding requirements placed on reference and working memory by this mapping. In the OTM conditions, a greater number of response codes had to be retrieved from reference memory and multiple response codes may have overburdened working memory, which has a limited capacity.     

The differential outcomes effect in pigeons is not reduced by eliminating response-outcome associations: Support for a two-process account

Animals perform two-choice conditional discriminations at a higher level if each of the two correct responses to the comparison stimuli is reinforced with a different outcome. According to the two-process view, this differential outcomes effect (DOE) results from the response-cuing function of expectancies generated by the conditional stimuli (i.e., samples). According to the shared-outcomes view, differential response-outcome associations contribute to the effect. In the present research, pigeons that were trained with differential outcomes associated with the samples, butnot with the comparisons, revealed a DOE during delay testing that was comparable to that obtained in a “true” differential-outcomes group. Thus, a two-process interpretation of the DOE was supported. In the second experiment, transfer testing with sample replacement confirmed these findings and, in addition, provided evidence that differential sample behaviors exerted some control over zero-delayed choice.     

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.     

Attenuation by contextual cues of retroactive interference of a conditional discrimination in rats

We investigated the effect of associating unique contextual cues with an interpolated learning task on retroactive interference in long-term memory. Rats were originally trained in a two-bar operant chamber with an auditory conditional discrimination stimulus. During interpolated learning, which occurred in either the original or a new context, some rats were trained on a probability learning task that did not include the auditory stimuli present during original learning. Subsequent retraining on the original conditional discrimination task in the original context showed that (1) significant retroactive interference occurs in rats, and (2) the presence of unique contextual cues during interpolated learning significantly reduces this interference. These results extend the conditions under which the susceptibility to retroactive interference can be altered by contextual cues.     

Animal memory: The contribution of generalization decrement to delayed conditional discrimination retention functions

Delayed conditional discriminations in which a sample indicates which comparison stimulus is correct have typically been used in working memory research with animals. Following acquisition with no (0-sec) delay between the offset of the sample and the onset of the comparison stimuli, delays of variable duration are introduced. The resulting retention functions are taken as a measure of memory. We suggest that, in addition to memory loss due to the delay, the comparison of matching accuracy at the 0-sec training delay with relatively novel test delays may produce a generalization decrement that varies as a function of increasing delay. We tested this hypothesis by training pigeons with a mixed delay procedure from the start and found that the retention functions for these pigeons were significantly shallower than those for a control group trained with 0-sec delays and tested with longer delays, and, although reduced in magnitude, the differences persisted for as many as 15 sessions. We propose that a measure of animals’ working memory can be obtained uninfluenced by a generalization decrement if they have received comparable training with all of the delays that are tested.     

Event duration memory: The effects of delay-interval illumination and instructional cuing

The effect of interference treatments on pigeons’ working memory for event duration was investigated, using a successive matching-to-sample procedure. In three experiments, birds were trained to match different keylight durations (2 or 6 sec) to different comparison colors (red or green) following delays of 0 to 12 sec. The interfering effect of delay-interval illumination and illumination change was assessed in Experiments 1 and 2. It was found that the absolute levels of houselight illumination influenced delayed matching accuracy. Birds trained with houselight illumination showed larger decrements in matching accuracy with increasing delays than did birds trained with darkened delay intervals. In addition, increases in delay-interval illumination relative to baseline produced greater interference with delayed matching accuracy than did decreases in houselight illumination relative to baseline. In Experiment 3, the effect of interpolated instructional cues to remember or forget was examined. As in other directed forgetting experiments employing conventional modality characteristics as the samples to be remembered, it was found that instructional cues to forget, presented during the delay interval, reduced matching accuracy compared to instructional cues to remember. It was concluded that these findings support models of temporal memory that assume temporal information is coded into categorical information onto some nontime dimension over models that assume temporal information is remembered amodally as specific time durations.     

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.     

Overshadowing in matching-to-sample: Reduction in sample-stimulus control by differential sample behaviors

Separate groups of pigeons were trained to high levels of accuracy on 0-delay matching-to-sample with sample-response requirements that were either differential or nondifferential with respect to the sample stimuli. Differential subjects produced the comparisons by completing a differential-reinforcement-of-low-rates-of-responding 3-sec (DRL 3″) requirement during one sample and a fixed-ratio (FR 10) requirement during the other. Nondifferential subjects produced the comparisons by completing the same schedule requirement (either DRL 3″ or FR 10) for both samples. Following acquisition to criterion, the DRL and/or FR sample-response requirements were replaced by a nondifferential single-peck (CRF) requirement in order to assess the degree to which the samples had acquired control over choice in each group. This change disrupted performance in all subjects, but the disruption was greater for the differential birds, which generally performed at lower levels of accuracy and required more sessions of retraining to reach criterion levels of accuracy than the nondifferential birds. Follow-up experiments revealed that comparison choices by the differential birds were primarily controlled by their DRL vs. FR sample-specific behaviors. The relatively poor performance of the differential group during testing with CRF requirements suggests that the cue arising from the birds’ differential sample behaviorshad also overshadowed the sample stimuli for conditional control over choice. The unique, and rather unusual, aspect of this overshadowing effect is that it occurred in spite of the fact that the overshadowed cue (that provided by the samples) was necessary for producing the cue that resulted in overshadowing (the differential sample behaviors). This finding has potentially important implications for the differential outcomes effect in conditional discrimination learning and for attentional processes in compound-cue situations in general.     

Errors in pigeons’ memory for number of events

In Experiment 1, pigeons were trained in a within-subjects design to discriminate sequences of light flashes (illumination of the feeder) that varied in number, but not in time (2f/4sec and 8f/4sec), and in time, but not in number (4f/2sec and 4f/8sec). Number samples required a response to one of two comparison dimensions (either color or line), whereas time samples required a response to the remaining comparison dimension. Delay testing revealed a significant choose-small bias following number samples and a significant choose-long bias following time samples. In Experiment 2, testing confirmed that in the absence of a sample, there was a bias to respond small to the number comparisons and long to the time comparisons. Additional tests indicated that the birds were discriminating time samples on the basis of the number of light flashes occurring during the last few seconds of the time samples, rather than on the basis of the total duration of the flash sequence. Consequently, the choose-long bias observed for time samples during delay testing was really a choose-small bias. In Experiment 3, the birds received baseline training with a 5-sec delay and were subsequently tested at shorter and longer delays. A choose-large bias occurred at delays shorter than the baseline training delay, whereas a choose-small bias was again observed at delays longer than the baseline delay. These findings provide additional empirical support for the conceptualizing of memory for number and time in terms of a common mechanism.     

Common coding in pigeons: Partial versus total reversals of one-to-many conditional discriminations

Common coding in pigeons was examined using a delayed conditional discrimination in which each sample stimulus was associated with two different comparison stimuli (one-to-many mapping). In Experiment 1, pigeons matched circle and dot samples to red and green hues and vertical and horizontal line orientations. In Experiment 2, the samples were red and green and the comparisons were vertical and horizontal spatial positions (up vs. down and left vs. right). Following acquisition to high levels of accuracy in each experiment, the associations between the samples and either both sets or only one set of comparisons were reversed. Pigeons learned the total reversals faster than the partial reversals. These results suggest that when different comparisons are associated with a common sample, they may become functionally equivalent.     

Value transfer in concurrent-schedule discriminations by pigeons

When pigeons are trained on a discrete-trial simultaneous discrimination, some of the value associated with the positive stimulus appears to transfer to the negative stimulus (Zentall & Sherburne, 1994). Pigeons preferred a negative stimulus that had been discriminated from an always-positive stimulus (S+) over a negative stimulus that had been discriminated from a sometimes-positive stimulus (S±). A very different finding (suggestive of transitivity of preference or contrast) was reported by Belke (1992). On concurrent probe tests of stimuli associated with equal variable interval (VI) schedules but originally trained in alternative concurrent pairs (one with a richer schedule, the other with a poorer schedule—VI 20 sec vs. VI 40 sec and VI 40 sec vs. VI 80 sec), the stimulus originally paired with the poorer schedule was preferred. But Belke’s results may have been obtained because the pigeons had been trained to peck the VI 40 sec paired with the poorer schedule and they had been trained not to peck the VI 40 sec paired with the richer schedule. In the present experiment, we avoided this bias by training pigeons on two concurrent schedules in which the tested stimuli both had been associated with the poorer schedule of the pair [A(VI 20 sec) vs. B(VI 80 sec) and C(VI 40 sec) vs. D(VI 80 sec)]. Evidence for value transfer was demonstrated when on probe trials pigeons preferred B over D.