Cocaine and food deprivation: effects on food-reinforced fixed-ratio performance in pigeons.

Key pecking by 6 pigeons was maintained by a fixed-ratio 30 schedule of food presentation while body weights were 80% of free-feeding weights. Acute administration of cocaine (0.3 to 13.0 mg/kg, i.m.) dose-dependently decreased response rates. Dose-effect curves were shifted to the right when 3 of the 6 pigeons were maintained at 70% of free-feeding weights and were shifted to the left when the other 3 pigeons were maintained at 90% of free-feeding weights. Then a dose of cocaine that initially decreased response rates by more than 95% of control rates was administered before each daily session. Comparable degrees of tolerance to these rate-decreasing effects developed in the two groups. The rate at which responding recovered was relatively rapid for pigeons in the 70% free-feeding-weight group and was slower for 2 of the 3 pigeons in the 90% free-feeding-weight group. When body weights were then increased from 70% to 80% or were decreased from 90% to 80% of free-feeding weight, performance was disrupted initially only for pigeons whose weight went from 70% to 80% of free feeding. In the present experiment the degree of deprivation may have indirectly influenced the degree of tolerance that developed to cocaine's response rate-decreasing effects because it directly influenced the dose chosen to be administered chronically. The degree of deprivation appeared to have a more direct influence on the rate at which tolerance developed.     

Free birds aren't fat: Weight gain in captured wild pigeons maintained under laboratory conditions

Nine feral pigeons, 5 from an urban setting and 4 from a rural setting, were captured and maintained for 42 days under free-feeding conditions comparable to those arranged for laboratory subjects. On average, birds increased their body weights by 17% over this period. The range of increase across birds was 9 to 30%. These findings suggest that the food deprivation arranged for laboratory pigeons, which is characteristically 80% of free-feeding weights, may in some sense be less severe than it first appears.     

Retrospective coding in pigeons' delayed matching-to-sample

In this study we examined how coding processes in pigeons' delayed matching-to-sample were affected by the stimuli to be remembered. In Experiment 1, two groups of pigeons initially learned 0-delay matching-to-sample with identical comparison stimuli (vertical and horizontal lines) but with different sample stimuli (red and green hues or vertical and horizontal lines). Longer delays were then introduced between sample offset and comparison onset to assess whether pigeons were prospectively coding the same events (viz., the correct line comparisons) or retrospectively coding different events (viz., their respective sample stimuli). The hue-sample group matched more accurately and showed a slower rate of forgetting than the line-sample group. In Experiment 2, pigeons were trained with either hues or lines as both sample and comparison stimuli, or with hue samples and line comparisons or vice versa. Subsequent delay tests revealed that the hue-sample groups remembered more accurately and generally showed slower rates of forgetting than the line-sample groups. Comparison dimension had little or no effect on performance. Together, these data suggest that pigeons retrospectively code the samples in delayed matching-to-sample.     

Stability of pigeon body weight under free-feeding conditions

Increases in regulatory oversight of animal research require verification of effects of standard practices. There are no formal guidelines for establishing free-feeding weights in adult pigeons. In the present study, pigeons were obtained from a commercial supplier, weighed upon arrival, and then held in quarantine for 7 days with free access to food. Subsequently, still with continuous access to food, they were weighed daily for 30 days. No significant changes in weights occurred over the 30-day period for male pigeons, indicating that seven days is sufficient for establishing a baseline body weight. A secondary finding of higher day-to-day variability in the weights of female pigeons may serve as a method of sexing pigeons.     

Cocaine's effects on food-reinforced pecking in pigeons depend on food-deprivation level.

Four pigeons deprived to 80% of their laboratory free-feeding weights pecked keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated strictly with 15-s timeouts separating them; each was presented six times. When rates of pecking were stable, 2 pigeons' weights were reduced to 70%, and the other 2 pigeons' weights were increased to 82.5% to 85% of free-feeding levels. Cocaine (1.0, 3.0, 5.6, and 10.0 mg/kg and saline) was administered 5 min prior to sessions. When each dose had been tested twice, pigeons' weights were adjusted to the level that they had not yet experienced, and cocaine was tested again. Cocaine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and under the fixed-interval schedule at high doses, and increased rates under the fixed-interval schedule at low low doses. Reductions in pecking rates occurred at lower doses under both schedules in 3 of 4 pigeons when they were less food deprived compared to when they were more food deprived. Low doses of cocaine increased low baseline rates of pecking in the initial portions of the fixed-interval schedules by a greater magnitude when pigeons were more food deprived. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of cocaine. The implications of these results for the mechanisms by which food deprivation increases cocaine self-administration and for the dependence of cocaine's effects on the baseline strength of operant behavior are discussed.     

Scaling pigeons' choice of feeds: bigger is better.

Preferences of hungry pigeons among 10 grains and pellets were analyzed using a Thurstone scaling procedure. The recovered scales were positively correlated with size of the feed. The correlations improved when the Thurstonian assumption of equal-sized discriminal dispersions (Case V) was replaced with the assumption of proportional-sized dispersions (Case VI), as entailed by Weber's law. The correlations weakened when the experiments were conducted with the pigeons close to their free-feeding weights, where the probability of sampling alternative grains increased. In the final experiment, exposure to a large pellet shifted the preferences between two smaller pellets.     

Effects of body weight on discriminative-stimulus control by phencyclidine in the pigeon.

Using a color-tracking procedure with responses reinforced under a second-order schedule, the discriminative-stimulus properties of phencyclidine were studied in pigeons maintained at 70%, 80%, or 90% of their free-feeding weights. The generalization curves for phencyclidine were similar at all three body weights. Generalization curves for pentobarbital, d-amphetamine, and saline were also unrelated to body weight. These data suggest that food deprivation may not influence the discriminative-stimulus properties of drugs in the way that it influences the reinforcing-stimulus properties of drugs. The reason may be that during discrimination training interoceptive stimuli resulting from food deprivation do not become conditioned to the stimulus properties of the drug, because the food-deprivation stimuli are paired equally often with the presence and absence of drug stimuli.     

Predictable long-delay matching-to-sample trials result in long-latency sample responding by pigeons

When a delayed matching-to-sample task does not involve short-delay trials interspersed among the long-delay trials, there is evidence that pigeons show long latencies in responding to the sample (sample observing response). They may even stop responding altogether. Two experiments were conducted to confirm the phenomenon and determine what aspect of the procedure is responsible for it. In Experiment 1 pigeons were trained on a delayed matching-to-sample task in which the delay remined constant throughout each session and was increased as each bird reached a performance criterion. The results indicated that sampleresponse latency increased in a regular fashion as delay increased. When the delay increased to 8.0 s, responding to the sample often stopped for long periods of time. In Experiment 2 pigeons were exposed to constant delay sessions (LONG), or mixed delay sessions with the delay length signaled (COR) or unsignaled (UNC). Sample-response latencies increased with increasing delay on long-delay trials for the LONG and the COR groups but not for the UNC group. Results supported the hypothesis that predictability of the long delay and not the loss of association between sample responses and reinforcement was responsible for the increased sample-response latency.     

Response stereotypy without automaticity: Not quite involuntary attention in the pigeon

Previous research has shown that when pigeons are required to peck each of two keys four times in any order for reinforcement, stereotyped response sequences develop, though they are not required by the task. However, despite this stereotypy, sequences are not automatized. They require sufficient attention that sequence performance is disrupted by a concurrent, delayed matching-to-sample task. In the present experiments, pigeons were required to execute response sequences and delayed matching-to-sample concurrently. Experiment 1 showed that the matching task disrupts sequence performance even when the matching sample stimuli do not have to be processed concurrently with sequence execution. The presence of sample stimuli during sequence execution seems to command attention even if the sample stimuli are also available for processing when sequences are not being executed. Experiment 2 used the directed forgetting technique to show that pigeons do seem able to ignore sample stimuli if they are given information that tells them they will not be required to remember those stimuli later. Together, the experiments suggest attentional mechanisms in the pigeon that are not quite involuntary.     

Motivational control of impulsive behavior interacts with choice opportunities

Impulsive behavior has been investigated through choice between a smaller/immediate reinforcer and a larger/delayed reinforcer, or through performance on a differential reinforcement of low rate (DRL) schedule. In the present study, we investigated a methodological divergence between these two procedures: in the former procedure, delay is a consequence of the subject's own choice, whereas in the later procedure, subjects are explicitly reinforced for delaying a response. In Experiment 1, 7 rats maintained at 80% of their free-feeding weights showed poorer efficiency of lever-pressing responses on a DRL 30-s schedule than when they were maintained at 90% of free-feeding weight. In Experiment 2, 16 rats were subjected to a concurrent chain schedule: the initial link was concurrent fixed ratio 1 fixed ratio 1, and each of these alternatives was followed by a short-DRL requirement with a one pellet reinforcer or a long-DRL requirement with a three pellet reinforcer. In one block of trials, rats were not allowed to choose between the two terminal links (forced-choice), whereas in the other block of trials rats were allowed to choose freely between the two terminal links (free-choice). Compared with rats maintained at 95% of their free-feeding weights, rats maintained at 80% of their free-feeding weights showed poorer efficiency in the terminal links’ DRL schedule performance (just as in Experiment 1), but this difference was shown only in the forced-choice blocks. These results indicate that motivational control of DRL schedule performance interacts with type of choice-making opportunity and highlight the direct comparison of motivational control of impulsive choice and DRL schedule performance.     

A matching-to-sample procedure for the study of visual perception and concept formation in infrahumans: A test in the pigeon

An apparatus suitable for conducting simultaneous or delayed matching-to-sample experiments with as many as eight comparison stimuli is described. Data are presented which indicate that pigeons are capable of performing such a task. It is concluded that the simultaneous matching-to-sample procedure with more than two comparison stimuli is an efficient means of obtaining information about discrimination capacity and that this procedure may be suitable for studying visual perception in infrahuman Ss.     

Food-deprivation level alters the effects of morphine on pigeons'' key pecking.

Four pigeons pecked response keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated separated by 15-s timeouts; each was presented six times. Pigeons were maintained at 70%, 85%, and greater than 90% of their free-feeding weights across experimental conditions. When response rates were stable, the effects of morphine (0.56 to 10.0 mg/kg) and saline were investigated. Morphine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and at high doses under the fixed-interval schedule. In some cases, low doses of morphine increased rates under the fixed-interval schedule. When pigeons were less food deprived, reductions in pecking rates occurred at lower doses under both schedules for 3 of 4 birds compared to when they were more food deprived. When pigeons were more food deprived, low doses of morphine increased rates of pecking in the initial portions of fixed intervals by a greater magnitude. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of morphine. These effects may share a common mechanism with increased locomotor activity produced by drugs and with increased drug self-administration under conditions of more severe food deprivation.     

Quantification of the effects of chlorpromazine on performance under delayed matching to sample in pigeons.

The effects of four doses of chlorpromazine (dose range 0.5 to 12.5 mg/kg) on performance under a delayed matching-to-sample procedure in pigeons was investigated, using the exponential model of memory (White, 1985). Performance was measured using a bias-free measure of discriminability, log d (Davison & Tustin, 1978), and negative exponential functions were fitted to individual-subject and group data at each dose level. A decrease in matching accuracy was found to be caused by an increase in the rate of forgetting, b, and a decrease in the initial discriminability, log d0. Changes in rate of forgetting and discriminability occurred at doses that had no statistically significant effect on response latency. The exponential model of memory accounted well for the data and provided a useful way of quantifying the effects of chlorpromazine on the processes involved in delayed matching-to-sample performance.     

Empirical validation of a procedure to correct position and stimulus biases in matching-to-sample

The development of position and stimulus biases often occurs during initial training on matching-to-sample tasks. Furthermore, without intervention, these biases can be maintained via intermittent reinforcement provided by matching-to-sample contingencies. The present study evaluated the effectiveness of a correction procedure designed to eliminate both position and stimulus biases. Following key-peck training, a group of 6 pigeons had extended exposure to matching-to-sample contingencies without a correction procedure, a group of 4 pigeons was briefly exposed to a simultaneous matching-to-sample procedure to assess biases prior to exposure to the correction procedure, and a group of 5 pigeons was exposed directly to the correction procedure. The correction procedure arranged that every time an incorrect match was made, the trial configuration was repeated on the subsequent trial until a correct match was made. Extended exposure to matching-to-sample contingencies without a correction procedure was associated with reduced biases eventually for most subjects, but rapid development of near-perfect accuracy and bias-free performance was observed upon the implementation of the correction procedure regardless of the type of bias. Bias-free performance was maintained following subsequent exposure to a zero-delay MTS procedure.     

The Effects of Hippocampal and Area Parahippocampalis Lesions in Pigeons: I. Delayed Matching to Sample

Four experiments were conducted to determine the effects of bilateral damage to the hippocampus and area parahippocampalis (Hp-APH) on visual memory in pigeons using the delayed matching-to-sample (DMS) procedure. In Experiment 1, we generated visual retention gradients with delays of 0, 1.5, 3, 6, and 12 sec both preoperatively and postoperatively in three pigeons with considerable preoperative visual DMS experience. Bilateral Hp-APH lesions had no effect whatsoever on visual retention. In Experiment 2, we examined the effects of Hp-APH lesions on both the acquisition of a visual DMS task with a 0-sec delay, and the subsequent retention performance with delays of 0, 3, 6, 12, and 24 sec. There was no difference between unoperated control pigeons and Hp-APH pigeons either in terms of the number of sessions required to learn the visual DMS task or in terms of their subsequent visual retention performance levels. In Experiments 3 and 4, we examined whether Hp-APH pigeons might be more sensitive than control pigeons to the effects of proactive interference (by reducing the duration of the intertrial interval) and retroactive interference (by introducing delay-interval illumination). Although reducing the duration of the intertrial interval and increasing the level of delay-interval illumination both resulted in lower performance levels on the visual DMS task, there was no indication that the Hp-APH pigeons were any more affected by the changes in interference levels than were unoperated control pigeons. These findings support the view that the Hp-APH in pigeons plays little role in the processing and retention of purely visual information.     

Neural correlates of a default response in a delayed go/no-go task

Working memory, the ability to temporarily retain task-relevant information across a delay, is frequently investigated using delayed matching-to-sample (DMTS) or delayed Go/No-Go tasks (DGNG). In DMTS tasks, sample cues instruct the animal which type of response has to be executed at the end of a delay. Typically, performance decreases with increasing delay duration, indicating that working memory fades across a delay. However, no such performance decrease has been found when the sample cues exist of present vs. absent stimuli, suggesting that pigeons do not rely on working memory, but seem to respond by default in those trials. We trained 3 pigeons in a DGNG task and found a similar default response pattern: The diverging slopes of the retention functions on correct Go and No-Go trials suggested that pigeons by default omitted their response following No-Go stimuli, but actively retained task-relevant information across the delay for successful responses on Go trials. We conducted single-cell recordings in the avian nidopallium caudolaterale, a structure comparable to the mammalian prefrontal cortex. On Go trials, many neurons displayed sustained elevated activity during the delay preceding the response, replicating previous findings and suggesting that task-relevant information was neurally represented and maintained across the delay. However, the same units did not show enhanced delay activity preceding correct response suppressions in No-Go trials. This activation-inactivation pattern presumably constitutes a neural correlate of the default response strategy observed in the DGNG task.     

Memory without awareness: pigeons do not show metamemory in delayed matching to sample

Metamemory, the ability to report on memory strength, is clearly established in rhesus macaques (Macaca mulatta) by converging evidence from several paradigms. In contrast, A. Inman and S. J. Shettleworth (1999) found no conclusive evidence of metamemory in pigeons. The authors studied pigeons further in 3 paradigms, with multiple tests of metamemory in each. Pigeons encountered a safe alternative to a matching-to-sample test either before (Experiment 2) or concurrently with (Experiment 3) the matching test. Choices of the safe option did not vary consistently with matching accuracy or change in trials with omitted samples in the way predicted for an animal with metamemory. In Experiment 4, confidence ratings following completion of the matching test also did not vary consistently as predicted by metamemory.     

Reversing the course of forgetting

Forgetting functions were generated for pigeons in a delayed matching-to-sample task, in which accuracy decreased with increasing retention-interval duration. In baseline training with dark retention intervals, accuracy was high overall. Illumination of the experimental chamber by a houselight during the retention interval impaired performance accuracy by increasing the rate of forgetting. In novel conditions, the houselight was lit at the beginning of a retention interval and then turned off partway through the retention interval. Accuracy was low at the beginning of the retention interval and then increased later in the interval. Thus the course of forgetting was reversed. Such a dissociation of forgetting from the passage of time is consistent with an interference account in which attention or stimulus control switches between the remembering task and extraneous events.     

How special is sameness for pigeons and people?

Because of the importance of the sense of sameness for psychological science and because of the tenuous support for this notion in pigeons' matching-to-sample behavior, we experimentally explored the possibly special status of sameness for pigeons. Using photographs from three different natural categories (dogs, fish, and flowers) in a three-alternative matching-to-sample design, we obtained a reliable sameness advantage for pigeons only when the number of correct sample-comparison combinations could have contributed to a sameness advantage; otherwise, no sameness advantage emerged. However, human participants exhibited an immediate and dramatic sameness advantage under essentially the same training and testing conditions as had been given to pigeons. At least under these experimental circumstances, humans exhibit a sameness advantage that far eclipses that of pigeons.     

Psychophysics of remembering

We present a new model of remembering in the context of conditional discrimination. For procedures such as delayed matching to sample, the effect of the sample stimuli at the time of remembering is represented by a pair of Thurstonian (normal) distributions of effective stimulus values. The critical assumption of the model is that, based on prior experience, each effective stimulus value is associated with a ratio of reinforcers obtained for previous correct choices of the comparison stimuli. That ratio determines the choice that is made on the basis of the matching law. The standard deviations of the distributions are assumed to increase with increasing retention-interval duration, and the distance between their means is assumed to be a function of other factors that influence overall difficulty of the discrimination. It is a behavioral model in that choice is determined by its reinforcement history. The model predicts that the biasing effects of the reinforcer differential increase with decreasing discriminability and with increasing retention-interval duration. Data from several conditions using a delayed matching-to-sample procedure with pigeons support the predictions.