Serial position effects in social transmission of food preference: Retention/demonstration intervals

The effects of retention and demonstration intervals on serial position were evaluated in two experiments with Long-Evans rats. A list of 3 demonstrators that had eaten one of three flavored foods was presented to naive observers. In Experiment 1, there were four groups, three groups with a retention interval compared with one group with a zero retention interval or no retention interval. In Experiment 2, the demonstration interval was reduced. Intervals of 15, 5, 2, and 1 min were used. In Experiment 1, primacy decreased gradually in the four groups as the retention interval was increased in duration. In Experiment 2, primacy also decreased gradually, and recency occurred with the 1-min demonstrator interval. The increase in the duration of the retention interval reduced primacy. The reduction in the duration of the demonstration interval decreased primacy and produced recency.     

Primacy,recency, and the variability of data in studies of animals’ working memory

The performance measures in many experiments on animal memory are expected to have an underlying binomial distribution, with additional variance contributed, for example, by between- subject differences. This paper examines whether the data from published studies of serial position effects (primacy and recency) in animals’ working memory conform to that expectation. In most cases, the variance, when it can be estimated, is consistent with those statistical assumptions, but in certain studies, it is significantly smaller than expected. This is usually a sign of faulty procedure or analysis, and possible causes are discussed. The conclusion is that much of the evidence for primacy in animals is unsatisfactory, on statistical or other methodological grounds. The analytic approach outlined here might usefully be applied to detect potential problems with other experiments of a similar type, especially when manually operated apparatus is employed, and to improve their statistical power.     

Rats’ memory for serially presented flavors: Effects of interstimulus interval and generalization decrement

In four experiments, the effect of sequential exposure to a series of five novel flavors on the subsequent neophobic response of water-deprived rats to those flavors when they were presented simultaneously was examined. After a list-test interval of 30 min and a list-interstimulus interval of 10 sec, the rats generally consumed more of the first and last flavors presented in the initial sequence. This finding was taken to reflect the existence of primacy and recency effects. Experiment 1 provided evidence that successive contamination can occur between flavors in the initial list, making subsequent recognition of later flavors in the list more difficult. However, this effect was overcome by presentation of water between each flavor during the list exposure. Experiments 2 and 4 showed that primacy was not a necessary result of successive contamination in this procedure, by demonstrating that increasing the interstimulus interval between list items decreased the size of the primacy effect. This result suggests that rats’ memory for serially presented items may be controlled by mechanisms different from those typically implicated in the human verbal memory literature. In Experiment 3, the question of whether the testing procedure adopted here could have introduced sources of artifactually produced serialposition effects was explored, but no such influence was found.     

Primacy effects in animal memory and human nonverbal memory

Evidence for primacy effects in animals’ list memory is accumulating, despite assertions that these primacy effects may be list-initiation-response artifacts (D. Gaffan, 1983; E. Gaffan, 1992). This evidence comes from list-memory experiments with pigeons and monkeys in which primacy changed with retention interval, experiments with monkeys in which primacy correlated with list length, and experiments with monkeys in which there were no list-initiation responses. Furthermore, there is growing evidence that animal memory is similar to, at least, the nonverbal part of human memory. This evidence comes from human nonverbal-memory experiments in which primacy changed with retention interval (similar to animals) when using kaleidoscope or snowflake stimuli, and similar experiments in which the verbal/nonverbal component was manipulated. Conditions conducive for obtaining primacy effects are discussed.     

Intraproblem retention during learning-set acquisition in bluejays (Cyanocitta cristata)

Three experiments were conducted examining short-term retention within individual object-discrimination learning-set (ODLS) problems. In Experiment I, it was found that intraproblem retention decreased during ODLS acquisition. Experiment II demonstrated that this phenomenon was not due to simple recency of experience with other ODLS problems. Experiment III demonstrated that retention was not influenced by the number of trials per problem or number of problems per session. These results were interpreted as supporting a conditional discrimination model of ODLS acquisition in bluejays.

     

The role of experimenter-odor cues in the performance of object-memory tasks by rats

Two experiments assessed whether odors left on stimulus objects by experimenters who handle them might confound the interpretation of ostensibly visually guided object-memory tasks for rats. In Experiment 1, rats were able to discriminate the relative recency with which an experimenter touched two otherwise identical objects (intertouch interval = 4 sec), presumably on the basis of an odorintensity discrimination. However, after the rats mastered the odor discrimination with no delay between when the second of the two stimulus objects was last touched by the experimenter and when the rats were permitted to attempt the discrimination, their performance dropped to chance levels when this delay was increased to 15 sec. In Experiment 2, rats were trained in two slightly different ways to perform a delayed-nonmatching-to-sample (DNMS) task, one that involved systematic differences in the temporal order in which the experimenter handled the sample and novel stimulus objects and one that did not. There were no significant differences in the rate at which rats mastered the DNMS task with these two procedures, and the performance of rats that were trained according to the former procedure was unaffected when they were switched to the latter procedure. Moreover, rats required considerably fewer trials to master the DNMS task than the rats in Experiment 1 required to master the odor discrimination. These findings demonstrate that, under certain circumstances, rats can discriminate the relative recency with which two objects are handled by an experimenter, but that this ability contributes little to their performance of conventional object-based DNMS tasks.     

Recognition memory for lists of visual stimuli in monkeys and humans

In Experiment 1, short-term memory for lists of visual stimuli was studied in four squirrel monkeys (Saimiri sciureus). A delayed-matching procedure was used in which a subject was presented with lists containing one, three, or six stimulus patterns, and memory for serial positions was probed by requiring the subject to choose between a list item and a nonlist item. The rate of item presentation was varied, as was the delay between the final item on a list and the retention test. In Experiment 2, the same procedures were used to compare recognition memory in four monkeys and four humans. Although differences in the levels and shapes of the serial-position curves appeared between species, both monkeys and humans showed primacy and recency effects. The presentation time of stimuli had a negligible effect on performance in both monkeys and humans, whereas delay significantly affected human retention but not monkey retention.     

Effect of N-R transitions during partial reinforcement pretraining on subsequent resistance to discrimination

Three groups of 12 rats received 25 pretraining trials to each future discriminandum employed in a subsequent differential brightness conditioning problem. Groups NR and RN received partial reinforcement (PRF) pretraining either with or without, respectively, transitions from nonrewarded to rewarded trials (N-R transitions). Group CRF received consistent reinforcement during pretraining. A fourth group (n=12), Group NP, received no pretraining. During discrimination learning, one-half of the rats in each group received all their daily S+ trials preceding their daily S? trials (+? sequence); the remainder of the rats received an intermixed sequence of trials to S+ and S? (+?+ sequence). Discrimination learning was faster under the +? sequence than under the +?+ condition, and discrimination learning was retarded in Group NR relative to the other three groups, which did not differ from one another, under both the +? and +?+ discrimination sequence conditions. The results are discussed with Reference to previous experiments demonstrating N-R transition effects on discrimination learning, a theoretical extension of sequential theory to discrimination learning, and the effects of nondifferential reinforcement prior to discrimination learning on learned irrelevance.     

Impact of brief or extended extinction of a taste aversion on inhibitory associations: Evidence from summation,retardation, and preference tests

In five conditioned taste aversion experiments with rats, summation, retardation, and preference tests were used to assess the effects of extinguishing a conditioned saccharin aversion for three or nine trials. In Experiment 1, a summation test showed that saccharin aversion extinguished over nine trials reduced the aversion to a merely conditioned flavor (vinegar), whereas three saccharin extinction trials did not subsequently influence the vinegar aversion. Experiment 2 clarified that result, with unpaired controls equated on flavor exposure prior to testing; the results with those controls suggested that the flavor extinguished for nine trials produced generalization decrement during testing. In Experiment 3, the saccharin aversion reconditioned slowly after nine extinction trials, but not after three. Those results suggested the development of latent inhibition after more than three extinction trials. Preference tests comparing saccharin consumption with a concurrently available fluid (water in Experiment 4, saline in Experiment 5) showed that the preference for saccharin was greater after nine extinction trials than after three. However, saccharin preference after nine extinction trials was not greater, as compared with that for either latent inhibition controls (Experiments 4 and 5) or a control given equated exposures to saccharin and trained to drink saline at a high rate prior to testing (Experiment 5). Concerns about whether conditioned inhibition has been demonstrated in any flavor aversion procedure are discussed. Our findings help explain both successes and failures in demonstrating postextinction conditioned response recovery effects reported in the conditioned taste aversion literature, and they can be explained using a memory interference account.     

A comparison of resistance to satiation and resistance to extinction

In two experiments, resistance to satiation was compared with resistance to extinction. In Experiment 1, rats given initial trials in a straight-alley runway while satiated failed to show increased resistance to satiation in a later test phase. This negative finding contrasts with the increased resistance to extinction usually found following initial nonrewarded trials in a straight alley. In Experiment 2, rats were extinguished or were run while satiated following deprived acquisition, and then were either shifted to the other condition or maintained under the same condition. A greater response decrement was produced by extinction than by satiation, both when current performance was examined and when the persistent effect of satiation or extinction on later performance was examined. These results show that there are important dissimilarities in the effects of satiation and extinction, dissimilarities that suggest that extinction is more nonrewarding or aversive than satiation. It seems likely that extinction involves processes (such as frustration, arousal of aversive motivation, and conditioned inhibition) not involved in satiation, which account for the greater response decrement in extinction as compared with satiation.     

A bowed serial-position function in habituation of sequential stimuli

The effect of serial position was examined in a study of habituation of the vasoconstrictive orienting response in rabbits. In a procedure analogous to a recognition memory test, subjects received multiple exposures to an invariant list of stimuli before their response was assessed to an isolated item that had consistently occupied either the first, middle, or last position in the list. A reliably bowed serial-position function was obtained, with the least habituation (poorest retention) occurring for the item that had occupied the middle position in the list. The finding of both a primacy and a recency effect parallels the most general finding in studies of human memory and encourages a compatible account of habituation in infrahuman subjects. The result was discussed in terms of the opportunities for differential rehearsal of list items and contextual cues at different positions in the list.     

Serial position effects in social transmission of food preference

The serial position effect in Long-Evans rats was evaluated in two experiments. In Experiment 1, the effect in a group for which an interstimulus interval (ISI) was introduced between items in a list of demonstrators was compared with that in a group without an ISI. With an ISI, a recency effect was produced. In Experiment 2, a serial position effect group was compared with four groups in which either the distinctiveness or the context of the middle item was changed, relative to the items on either side of it. A von Restorff effect was produced when a rat from a different strain was used as a demonstrator in Position 2. The results for Experiment 1 are discussed in relation to interstimulus effects in monkeys and those for Experiment 2 with respect to changes in the physical properties of middle items.     

Serial order anticipation learning in rats: Memory for multiple hedonic events and their order

Many characteristics of a series of discrete independent hedonic events may be remembered by rats in terms of, for example, how many events were rewarded and how many were nonre-warded. Such memory for multiple hedonic events, which has been shown to be a potent factor controlling instrumental responding, was examined here in five investigations employing serial anticipation learning in a runway. It was found that the ability of rats to remember the hedonic events reward and nonreward is highly developed, accurate, and quite resistant to forgetting and interference. Rats not only remembered a rewarded event and a nonrewarded event, but they also remembered the order in which the two events occurred. Rats remembered how many nonrewarded events there had been accurately enough to suggest that they were using some form of a counting mechanism. Rats exhibited little forgetting of eight prior discrete hedonic events, one rewarded followed by seven nonrewarded, even when these occurred over an interval of 20 min and involved considerable potential interference. In the serial learning situation employed here, marked primacy effects were obtained, earlier nonrewarded trials in a series being better anticipated than later ones. The primacy effect was found to depend upon the type of series employed. By assuming that stimulus generalizations occur between the multiple hedonic events remembered by rats, all anticipatory learning obtained here could be explained in considerable detail.     

Extinction of a saccharin—lithium association: Assessment by consumption and taste reactivity

Extinction of a conditioned palatability shift preceded extinction of conditioned taste avoidance whether rats were tested using a within-subjects design or a between-subjects design. In each of two experiments, consumption of 0.1% saccharin was paired with either 20 ml/kg of 0.15 M LiCl or equivolume physiological saline on a single trial. In Experiment 1, on each of 10 extinction trials, rats were given a taste reactivity test immediately prior to a consumption test. In Experiment 2, half of the rats were extinguished by taste reactivity testing and half of the rats were extinguished by a consumption test on each of 10 extinction trials. In both experiments, the aversive reactions of gaping and passive dripping were extinguished in a single trial and the suppression of ingestive reactions was extinguished in 2 trials; however, extinction of taste avoidance required 4–5 trials. These results suggest that rats continue to avoid a lithium-paired flavor even when they do not have an aversion to the taste.     

Comparing cognition by integrating concept learning,proactive interference,and list memory

This article describes an approach for training a variety of species to learn the abstract concept of same/different, which in turn forms the basis for testing proactive interference and list memory. The stimulus set for concept-learning training was progressively doubled from 8, 16, 32, 64, 128 . . . to 1,024 different pictures with novel-stimulus transfer following learning. All species fully learned the same/different abstract concept: capuchin and rhesus monkeys learned more readily than pigeons; nutcrackers and magpies were at least equivalent to monkeys and transferred somewhat better following initial training sets. A similar task using the 1,024-picture set plus delays was used to test proactive interference on occasional trials. Pigeons revealed greater interference with 10-s than with 1-s delays, whereas delay time had no effect on rhesus monkeys, suggesting that the monkeys’ interference was event based. This same single-item same/different task was expanded to a 4-item list memory task to test animal list memory. Humans were tested similarly with lists of kaleidoscope pictures. Delays between the list and test were manipulated, resulting in strong initial recency effects (i.e., strong 4th-item memory) at short delays and changing to a strong primacy effect (i.e., strong 1st-item memory) at long delays (pigeons 0-s to 10-s delays; monkeys 0-s to 30-s delays; humans 0-s to 100-s delays). Results and findings are discussed in terms of these species’ cognition and memory comparisons, evolutionary implications, and future directions for testing other species in these synergistically related tasks.     

Effect of prior nonreward on subsequent incentive growth during brief acquisition

Four experiments compared runway extinction or hurdle-jumping from nonreward performance following brief (10 trials) continuous or partial reinforcement acquisition. Some of the partial groups received all nonrewarded trials prior to any rewards. The major findings were that (l) rats receiving all nonrewarded experiences prior to rewarded ones were more persistent during extinction than continuously rewarded subjects; (2) rats receiving nonrewarded placements prior to rewarded ones in one compartment of a two-compartment box, failed to learn a hurdle-jumping response to escape nonreward, whereas rats not receiving the initial nonrewards did learn the escape response; (3) increasing the number of rewarded placements following initial nonrewarded ones offset the effect noted in (2). The results, which are discussed in the context of a frustration analysis of the small-trials partial reinforcement effect, suggest that incentive growth over rewarded trials is retarded when the rewards have been preceded by nonrewards. The similarity of these results to those investigating the phenomenon of latent inhibition is apparent, and possible mechanisms responsible for the present results are suggested in current theoretical accounts of latent inhibition.     

Superlatent inhibition and spontaneous recovery: Differential effects of pre- and postconditioning CS-alone presentations after long delays in different contexts

In two pairs of three-stage conditioned taste aversion experiments, we examined the effects of delay interval (1 or 21 days) between the second and third stages, and of context in which the animals spent the delay (same as or different from the context of the other stages) on latent inhibition (LI) and spontaneous recovery following extinction. In the LI experiments (Experiments 1A and 1B), the first stage comprised nonreinforced presentations to saccharin or to water. In the second stage, rats were conditioned by saccharin paired with LiCl. In the extinction experiments (Experiments 2A and 2B), the order of the stages was reversed. For all experiments, Stage 3, the test stage, consisted of three presentations of saccharin alone. There was a super-LI effect in the saccharin-preexposed group that spent the 21- day delay in the different context (Experiment 1A). When the delay was spent in the same context, there was no difference in the amount of LI between the short- and long-delay groups (Experiment 1B). Conversely, there was a spontaneous recovery effect in the long-delay/same-context group (Experiment 2B), but not in the long-delay/different-context group (Experiment 2A). The pattern of results, incompatible with current explanations of delay-induced changes in memory performance, was interpreted in terms of an interaction between the delay conditions (same or different delay context), which modulate the extinction of previously acquired context-CS-nothing associations (during CS-alone presentations), and primacy effects.     

Reward sequence and reinforcement level as determinants of S− behavior in differential conditioning

In two differential conditioning experiments, groups of 10 rats each differed with respect to average reward and schedule of reward received in S+. Nonreward (N) occurred on all S? trials. In both experiments, extinction of responding to S? (resistance to discrimination) was extensively regulated by reward sequence and was largely independent of average reward. In Experiment 1, resistance to discrimination was a function of transitions from N to rewarded (R) trials (N-R transitions). In Experiment 2, resistance to discrimination was increased by large reward on the R trial of N-R transitions and decreased by large reward on the R trial of R-N transitions. These schedule effects on resistance to discrimination parallel the effects of comparable schedules on resistance to extinction following partial reinforcement. The results are discussed in terms of sequential theory, reinforcement level theory, and their implications for various schedule manipulations that have previously shown S? behavior to be inversely related to average reward in S+.     

Further evidence that Norway rats do not socially transmit learned aversions to toxic baits

Three experiments were undertaken to examine the effects of interactions with demonstrator rats made ill by injection of lithium chloride (LiCl) on the later food choices of their observers. We found that (1) observer rats that had been taught an aversion to an unfamiliar diet exhibited a substantial reduction of that aversion after interacting with poisoned demonstrators that had eaten the diet to which the observers had learned an aversion, (2) exposure of an observer rat to poisoned demonstrator rats that had eaten a diet interfered with later acquisition by the observer of an aversion to the diet that the poisoned demonstrators had eaten, and (3) after interacting with poisoned demonstrators that had eaten one of two diets, observers that ate both diets and were then made ill formed an aversion to whichever diet their respective, poisoned demonstrators had not eaten. The present experiments, like previous studies both in our laboratory and elsewhere, failed to provide any evidence that naive observer rats will learn to avoid a food as a result of interacting with demonstrator rats that had eaten the food and exhibit symptoms of toxicosis. To the contrary, observer rats in the present experiments exhibited an enhanced preference for foods eaten by sick demonstrators.     

Spontaneous recovery after extinction of a conditioned taste aversion

Four experiments examined whether or not spontaneous recovery could occur after extinction in the conditioned taste-aversion paradigm. After three extinction trials, spontaneous recovery was obtained over an 18-day retention interval (Experiments 1, 2, and 3). The effect was not due to changes in the unconditioned preference for saccharin over the retention interval (Experiment 2) or to an increase in a nonextinguished aversion over time, as indicated by tests with both the original, nonextinguished aversion (Experiment 1) and with a weaker one (Experiment 3). Spontaneous recovery was not obtained when extinction was overtrained (eight trials) and a 49-day retention interval was used (Experiment 4). However, saccharin intake at asymptote reached the level of baseline water intake, and not the highly preferred level shown by never-conditioned controls. Results of all four experiments suggest that extinction does not return an averted taste to the status of an unconditioned one.