Memory,serial anticipation pattern learning,and transfer in rats

Learning & Behavior - Three experiments tested hypotheses about whether rats respond appropriately to, or track, an orderly series of reward magnitudes terminating in nonreward by encoding the...     

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.     

Element discriminability as a determinant of serial-pattern learning

Hulse and Dorsky found that rats were better able to track (run slowly to) 0 food pellets in a strongly monotonic (decreasing) serial pattern (14-7-3-1-0 food pellets) than in either a weakly monotonic one (14-5-5-1-0) or a nonmonotonic one (14-1-3-7-0). These findings were seen as incompatible with associative approaches based on animal experiments. Instead, they were taken to be consistent with cognitive theories of human behavior that relate pattern difficulty to formally defined structural complexity. In Experiment 1, tracking was found to be poorer with a strongly monotonie series (15-10-5-0) than with either of two weakly monotonic series (15-15-0-0 or 14-14-2-0), and in Experiment 2 a nonmonotonic series (1-29-0) produced better tracking than a strongly monotonic one (20-10-0). Although these results are not necessarily incompatible with the structural complexity view, they do suggest that “element discriminability” is a factor in serial-pattern learning. They are, therefore, compatible with a memory approach that views tracking as a form of discrimination learning.     

Serial conditioning as a function of parametric variations of an interfood clock

Two experiments examined the effects of differences in the parameters of a clocked interfood interval on the obtained distribution of responding to the stimuli in that interval. In the first experiment, a 3×3 factorial design assessed the effects of the number of components and the durations of those components. Food was presented irrespective of behavior following 5, 10, or 20 discrete stimuli across durations of 3, 6, or 12 sec each. Responding began at the approximate midpoint of the interval. More responding occurred in earlier portions of the last half of the interval as the number and the durations of individual stimuli decreased or as the overall interfood interval decreased. The second experiment, also a 3×3 factorial design, manipulated the probability and duration of food presentation following a 60-sec trial containing 10 discrete stimuli. A 2.0-, 3.5-, or 5.0-sec food presentation followed 100%, 25%, or 10% of the trials. Responding again began at the approximate midpoint of the interval. More responding occurred in earlier portions of the last half of the interval only when both food duration and the proportion of reinforced trials increased. Both experiments therefore showed that the onset of responding occurs at the approximate midpoint of clocked interfood intervals in spite of a wide variety of CS and US manipulations.     

Serial pattern learning by rats: Transfer of a formally defined stimulus relationship and the significance of nonreinforcement

Two experiments with rats tested independent predictions from cognitive theories of serial pattern learning. The animals learned to anticipate, as measured by running times in a straight alley, different quantities of food pellets organized into formally defined, five-element serial patterns. In Experiment 1, for some animals the patterns were all formally structured according to a monotonic “less than” relationship in which any quantity was always less than its predecessor. For others, no consistent formal rule was applied. Results of a transfer test with a new pattern showed positive transfer if the formal structure of the new pattern was identical to that used initially, but negative transfer if the new pattern was formally different. In Experiment 2, two groups learned the monotonic patterns 18-10-6-3-1 or 18-10-6-3-0 food pellets, while two others learned the nonmonotonic patterns 18-3-6-10-1 or 10-3-6-10-0 food pellets. We asked if the difference in value of the terminal element, 1 or 0 food pellets, would affect the facility with which the patterns were learned. The results showed that learning rate and the qualitative response to the elements of the monotonie and nonmonotonic patterns were independent of the value of the terminal element. Both experiments lend additional support to the utility of using cognitive models of human serial-pattern learning for an analysis of the sequential behavior of nonhuman animals.     

Serial discrimination reversal learning in pigeons as a function of intertrial interval and delay of reinforcement

Pigeons learned a series of reversals of a simultaneous red-green visual discrimination. Delay of reinforcement (0 vs. 2 sec) and intertrial interval (ITI; 4 vs. 40 sec) were varied across blocks of reversals. Learning was faster with 0-sec than with 2-sec delays for both ITI values and faster with 4-sec ITIs than with 40-sec ITIs for both delays. Improvement in learning across successive reversals was evident throughout the experiment, furthermore, even after more than 120 reversals. The potent effects of small differences in reinforcement delay provide evidence for associative accounts and appear to be incompatible with accounts of choice that attempt to encompass the effects of temporal parameters in terms of animals’ timing of temporal intervals.     

Serial pattern learning of temporal intervals

The problem was to determine how rats adjust the times of their lever responses to repeating sequences of interfood intervals. In Experiment 1, rats were trained on an interval schedule of reinforcement with a 12-element Fleshler-Hoffman series with a mean of 60 sec; the order was as follows: ascending, random with repetition, random with replacement, random without replacement. In Experiment 2, rats were trained with a 10-element ascending or descending series (from 2⁰ to 2⁹ sec), and in a ramp procedure in which these intervals increased and then decreased repeatedly. In the ascending, descending, and ramp conditions (but not in the random conditions), postreinforcement pause (PRP) was a function of the interval. PRP was most highly correlated with an interval later in the series. Theories of conditioning and timing based on the averaging of past experience must be modified to account for such anticipatory behavior.     

Behavioral heat intake as a function of reward duration in rats

Animals working for heat in a cold environment increase responding when reward duration is reduced, but in many instances the increase in responding is not sufficient to prevent a decrease in the amount of heat obtained. Eight experiments were conducted to investigate the reason why rats’ barpress responses for heat are less efficient at short reward durations. The results show that a ceiling effect is not involved and also that improper response-topography or differences in whole-body heat absorptivity or in response effort cannot explain the phenomenon. Evidence was found for the hypothesis that response rate does not increase sufficiently at short reward durations because many short rewards produce a greater afferent signal than do few long rewards. This inequality seems to be caused by characteristics of the stimuli that result in a greater relative change in skin temperature (ΔT) or in the rate of change of skin temperature (δT/δt) for short-duration rewards.     

Habituation and recovery of orienting in rats as a function of stimulus significance

Three experiments measured orienting of rats to stimuli varying in intrinsic significance. In the first, rats showed greater reactivity to a recording of a conspecific’s distress squeal than to a simulated mimic squeal when the stimuli were presented at 100 dB but not at 80 dB, and orienting to all stimuli habituated rapidly with repeated stimulus exposures. Experiment 2 showed that, following several stimulus exposures, recovery of orienting after rest periods of 1 and 7 days was a function of the stimulus: Orienting to the distress squeal presented at 100 dB recovered more rapidly than did orienting to the mimic squeal at 100 dB or to either the mimic or the distress squeal presented at 80 dB. Experiment 3 showed that habituation to the most significant stimulus, the distress squeal presented at 100 dB, was retarded at long (24-h) interstimulus intervals, presumably a consequence of less stimulus-to-stimulus transfer of habituation with this stimulus. The results are discussed in terms of biological constraints on habituation of orienting and on recovery of orienting following habituation.     

Preference for food and water in rats as a function of delay of reward

In Experiment I, rats which were both hungry and thirsty were given a choice between a food reward and a water reward. The animals preferred food to water when the reward was delivered immediately, but preferred water to food when a 30-sec delay was imposed in the goalbox before the reward was received. Experiment II replicated the results of the first experiment and showed, in addition, that when the delay was imposed in a separate delay chamber devoid of differential goalbox cues, subjects preferred food to water, similar to the immediate group. The results were discussed in terms of an incentive value process and a competing response hypothesis.     

Serial learning and transfer in rats: Effects of changes in stimulus-stimulus associations,pattern structure,and serial position information

Four groups of rats (n=6 per group) were trained in a runway on a serial learning task. Groups were treated identically in Phase 1, receiving two daily presentations of a five-element series consisting of decreasing numbers of .045-g food pellets over successive runs, for example, 14-7-3-1-0. All groups learned to anticipate, and run slowly to, the terminal 0-pellet element, behavior that has been attributed to learning of a less-than rule, stimulus-stimulus (S-S) associations, and knowledge of the serial position of items. In Phase 2, subjects were transferred to one of four test series: 20-14-7-3-0,20-7-3-14-0, 20-14-7-3-1-0, or 20-7-14-3-1-0. Anticipation was disrupted on the first two series, which maintained the integrity of serial position information and in the first case the less-than rule, but eliminated the terminal portion of the associative chain. Anticipation was unimpaired by transfer to either of the last two series. These series maintained the integrity of the terminal 3-1-0 portion of the associative chain but presented altered information about serial position, and in the last case also altered the less-than rule. The results, which supported the memory-discrimination model of rat serial learning, are discussed with reference to related transfer experiments in human serial learning.     

Amnesia induced by hypothermia as a function of treatment-test interval and recooling in rats

In Experiment I, a repeated tests procedure was employed to assess hypothermia-induced amnesia of a footshock experience. Rats tested 4 h after training treatment showed no memory loss, but amnesia was present at 24 h. Although recovery of memory was obtained when the same animals were cooled 2 h prior to a 50-h test, repeated testing also tended to attenuate amnesia. In Experiment II, independent groups were tested at 6 or 50 h after training treatment. Again, memory of the footshock was present at the short, but not at the long, interval. Recooling shortly prior to the 50-h test eliminated amnesia. Experiment III indicated that the return of memory produced by recooling did not persist if testing was delayed. These findings suggested that hypothermia may function as an important contextual cue for memory retrieval.     

Sex differences in rats’ stationary exploration as a function of stimulus and environmental novelty

Stationary visual scanning and sniffing of novel stimuli were measured using a head-poke response to supplement existing equivocal data on sex differences in exploration. With a short exposure to a novel apparatus, males and females showed a similar frequency of exploratory bouts, but the bouts were longer in females, showing that the sex difference is independent of a simple activity difference. With longer exposure, the difference was reversed, with males “compensating” for their initially lower level. Following familiarization to the apparatus, the sexes did not differ in exploration of an introduced novel stimulus. These results are consistent with males being more susceptible to inhibitory influences, such as disturbance and extreme novelty. Differences in within-trial changes in the level of exploration of a novel stimulus in novel and familiar environments were also observed.     

Performance in differential instrumental conditioning as a function of the pattern of partial S+ reward

A total of 46 rats, distributed across two experiments, received differential instrumental conditioning trials in a nonchoice brightness-discrimination apparatus. In each experiment, groups differed with respect to the pattern of rewarded and nonrewarded S+ trials. Response in S? was never reinforced. The results of both experiments indicated that the pattern of S+ reward events influences S? response levels in a fashion consistent with expectations derived from stimulus-specificity theory.     

Difficult serial anticipation learning in rats: Rule-encoding vs. memory

Rats, trained in a runway, were asked to anticipate, while running slowly, the last two events in repeating series of .045-g food pellets. The series were either weakly monotonic (14, 5, 5, 1, and then 0 pellets/run) or nonmonotonic (5-5-14-1-0). While the terminal 0-pellet event was better anticipated in the weakly monotonic series, the reverse was the case for the next-to-last 1-pellet event. These findings were expected from a memory-discrimination learning hypothesis of serial learning, which suggests that the memory of one event in a series can be used to signal the next event. However, the better anticipation of the 1-pellet event by the nonmonotonic group was inconsistent with the recently stated rule-encoding position of Hulse (1980). According to that view, difficult series of the sort employed in the present investigation are learned by encoding the rule structure of the series, with events in the series with the simple rule structure (the weakly monotonic series in this investigation) being better anticipated than events in the series with the complex rule structure.     

Word Reading Fluency as a Serial Naming Task

Word list reading fluency is theoretically expected to depend on single word reading speed. Yet the correlation between the two diminishes with increasing fluency, while fluency remains strongly correlated to serial digit naming. We hypothesized that multi-element sequence processing is an important component of fluency. We used confirmatory factor analyses with serial and discrete naming tasks with matched items, including digits, dice, objects, number words, and words, performed by about 100 Greek children in each of Grades 1, 3, and 5. Separable serial and discrete factors emerged across grades, consistent with distinct skill dimensions. Loadings were greater for serial than discrete, suggesting that discrete processing does not fully determine serial processing. Average serial performance differed more than discrete between grades, consistent with improvement beyond single-item speed. Serial word reading aligned increasingly with the serial factor at higher grades. Thus, word reading fluency is gradually dominated by skill in simultaneously processing multiple successive items through different stages (termed “cascading”), beyond automatization of individual words.     

Formal structure and pattern length in serial pattern learning by rats

When rats learn to anticipate a sequence of stimulus events, such as a serial pattern of different food quantities, they are sensitive to the rule-based formal structure relating the magnitude of successive stimuli. Earlier research has shown that if formal structure is simple (e.g., if a single “less than” rule relates the size of each successive quantity), patterns are learned faster than if formal structure is complex (e.g., if two or more rules such as “less than” and “greater than” relate successive pattern quantities). Two experiments tested the hypothesis that pattern length modulates the role of pattern complexity. We predicted that pattern length and pattern complexity interact in determining pattern difficulty. That is to say, long complex patterns should be learned more slowly than short complex patterns. However, long simple patterns should be learned faster than short simple patterns. In Experiment 1, rats ran a straight runway to receive repeated sequences of food quantities. The long-monotonic group received a formally simple 18-10-6-3-1-0 pattern, in which each number represents a quantity of food pellets. The long-nonmonotonic group received a formally complex 10-1-3-6-18-0 pattern. Similarly, the short-monotonic and short-nonmonotonic groups received 18-1-0 and 1-18-0 patterns. Pattern tracking—fast and slow running in anticipation of large and small quantities of food, respectively—was taken as an index of pattern learning. In Experiment 2, comparable patterns were used, but rats leverpressed in a discrete-trial procedure; response latencies measured pattern tracking. In both experiments, rats learned formally simple patterns faster than they did formally complex patterns. In Experiments 1 and 2, but less clearly in Experiment 2, the predicted interaction was obtained. The results support and generalize the idea that rats encode and use some representation of the formal rule structure of serial patterns as they learn them.     

Retention of environmental habituation in rats as a function of the environment during the retention interval

The time course of female rats’ habituation to familiar test environments was measured with a resident-intruder paradigm. A resident was permitted 0, 2, 5, 10, or 30 min of environmental adaptation prior to introduction of an intruder, and a dominance test was then conducted in which both rats were allowed to compete for drops of water. The degree of dominance which the resident attained was a function of the duration of adaptation, reaching a maximum after 5 min. The retention of this habituation was investigated as a function of the type of retention environment and the duration of the retention interval. Retention intervals of 5 or 25 min in unfamiliar environments produced significant loss of habituation, while the same intervals in familiar environments did not produce habituation loss. The degree of similarity between habituated and retention environments did not appear to be an important variable affecting retention. When the retention interval was extended to 25 h in a familiar retention environment, no environmental habituation was retained.