Pigeons’ short-term memory for temporal and visual stimuli in delayed matching-to-sample

In the present experiment, we compared directly pigeons’ short-term memory of temporal and visual stimuli in a delayed matching-to-sample task. The sample stimuli consisted of red and green lights presented for 5 and 30 sec, followed by a retention interval and blue and yellow comparisons. For subjects in the visual group, duration was irrelevant and the color of the sample was the conditional cue. For animals in the temporal group, color was irrelevant and duration of the sample was the conditional stimulus. The results showed that acquisition of the matching task was faster and accuracy was higher in the visual than in the temporal group. More importantly, memory of either sample generally declined at a similar rate when the duration of the retention interval was increased and when the intertrial interval was reduced. Taken together, the results indicate that with 1–8-sec retention intervals, short-term memory for temporal stimuli is similar to that found with color-visual samples. The findings are discussed in terms of retrospective and prospective processing.     

Remembering as discrimination in delayed matching to sample: Discriminability and bias

Task difficulty in delayed matching-to-sample tasks (DMTS) is increased by increasing the length of a retention interval. When tasks become more difficult, choice behavior becomes more susceptible to bias produced by unequal reinforcer ratios. Delaying reinforcement from choice behavior also increases both task difficulty and the biasing effect of unequal reinforcer probability. Six pigeons completed nine DMTS conditions with retention intervals of 0, 2, 4, 6, and 8 sec, in which reinforcer delays of 0, 2, and 4 sec were combined with ratios of reinforcer probabilities of .5/.5, .2/.8, and .8/.2 for correct red and green responses. Discriminability (logd) decreased with both increasing retention interval duration and increasing reinforcer delay. Sensitivity to reinforcement, the tendency for ratios of choice responses to follow unequal reinforcer probabilities, also increased as a function of both increasing retention interval and increasing reinforcer delay. The result is consistent with the view that remembering in DMTS tasks is a discriminated operant in which increasing task difficulty increases sensitivity to reinforcement.     

Pigeons exhibit higher accuracy for chosen memory tests than for forced memory tests in duration matching-to-sample

Following training to match 2- and 8-sec durations of feederlight to red and green comparisons with a 0-sec baseline delay, pigeons were allowed to choose to take a memory test or to escape the memory test. The effects of sample omission, increases in retention interval, and variation in trial spacing on selection of the escape option and accuracy were studied. During initial testing, escaping the test did not increase as the task became more difficult, and there was no difference in accuracy between chosen and forced memory tests. However, with extended training, accuracy for chosen tests was significantly greater than for forced tests. In addition, two pigeons exhibited higher accuracy on chosen tests than on forced tests at the short retention interval and greater escape rates at the long retention interval. These results have not been obtained in previous studies with pigeons when the choice to take the test or to escape the test is given before test stimuli are presented. It appears that task-specific methodological factors may determine whether a particular species will exhibit the two behavioral effects that were initially proposed as potentially indicative of metacognition.     

In search of consolidation of short-term memory in nonhuman animals

Wixted (Annual Review of Psychology, 55, 235 – 269, 2004) has argued that forgetting is due to consolidation failure. Previous research with humans and nonhuman animals has reported evidence for consolidation in intermediate or long-term memory (LTM). The present study examines whether consolidation occurs in short-term memory in pigeons. Delayed matching-to-sample accuracy was reduced when retroactive interference (an extraneous task in Experiment 1 or houselight illumination in Experiment 2) was interpolated in the retention interval. Accuracy was not greater, however, when interference occurred at the end of the retention interval, as compared with when it occurred at the beginning. That is, there was no evidence for consolidation in short-term memory for pigeons. We did find, however, the beginning–end effect originally reported by Roberts and Grant (Journal of Experimental Psychology: Animal Behavior Processes, 4, 219–236, 1978) and the recovery from forgetting reported by White and Brown (Journal of the Experimental Analysis of Behavior, 96, 177–189, 2011). The results are discussed in relation to temporal distinctiveness theory as an alternative to consolidation.     

Pretest cuing after forgetting of a food-motivated maze task in rats: Synergistic action of context and reinforcement

It is now well known in animal studies that spontaneous forgetting, as well as performance decrements resulting from other sources, can be alleviated by means of pretest treatments. Several previous experiments have shown that reliable forgetting is observed after a 25-day training-to-test interval on a relatively complex maze. This forgetting can be alleviated by pretest exposure to background stimuli in the experimental room. The effects of this treatment can be modulated by varying either the duration of the treatment or the length of the cuing-test interval. The purpose of the experiment presented here was twofold: (1) to demonstrate the multidimensionality of memories in our paradigm by comparing the effects of different reminder treatments, namely pretest access to the reinforcer and contextual cuing, and (2) to test the general hypothesis, formulated by Spear (1978, p. 418), that “the elicitation and retrieval of the target attribute of a memory depend on the arousal of a sufficientnumber or kind of the remaining attributes of this memory.” The data showed that pretest food reinforcement had no significant effect on retention performance; that pretest contextual cuing did, in some precise conditions, alleviate forgetting; and that some combinations of context and reinforcement had what amounted to a synergistic action on retention performance.     

A comparison of the short-term memory performances of pigeons and jackdaws

Two experiments employed a delayed conditional discrimination procedure in which half the trials began with the presentation of food and half with no food; following a retention interval, subjects were presented with a choice between red and green keys, a response to one of which was reinforced according to whether the trial had started with food or no food. In Experiment 1, after 38 training sessions during which the retention interval was gradually increased, pigeons performed at a moderate level with intervals of 5 to 7.5 sec. A final test produced a steep forgetting function for food trials, but not for no-food trials; performance was unaffected by the duration of the intertriai interval (10 or 40 sec). Experiment 2 used the delayed conditional discrimination procedure to compare short-term memory in jackdaws (Corvus monedulus) with that in pigeons. Although the performance of the jackdaws was below that of the pigeons at the start of training, they showed more rapid learning over long delays, and, in the final test, a shallower forgetting function for food trials than that shown by pigeons. The results suggested superior short-term memory in jackdaws, which may help to explain the better performance of corvids in general when compared with that of pigeons in certain complex learning tasks.     

On metaknowledge in the pigeon: An organism’s knowledge about its own behavior

In two experiments, pigeons pecked side keys in a discrete-trials setting in which shorter and longer runs of successive pecks on the left key before a switch to the right key occasionally produced, after a brief retention interval, a short-term memory probe for the most recent run length. In Experiment 1, a probe involved red and green side keys. A peck to a green (red) key was reinforced if the previous run length was shorter (longer). The dependent variable was the probability of a peck to the correct color. In Experiment 2, a probe involved an autoshaping procedure in which a response-noncontingent reinforcer was delivered after a 5-sec presentation of a green (red) center key if the previous run had been a shorter (longer) one. A reinforcer was not delivered when a red key followed a shorter pattern or a green key followed a longer pattern. The production of runs conformed to many previous molecular data on the way the local temporal patterning of behavior adapts to, that is, displays knowledge of, a reinforcement contingency. The probe results showed that a pigeon can report which of two run lengths it recently has emitted. Thus, a pigeon can, in a sense, describe its own adaptive behavior. Since the adaptive behavioral patterning on the center key may be said to represent knowledge, and since the probe behavior is a self-characterization or self-report by the organism about this knowledge, the probe behavior may be said to represent knowledge about knowledge, or metaknowledge. The data extend previous work on metaknowledge in the pigeon to a third type of adaptive temporal pattern of behavior, that is, run length (instead of response duration and interresponse time), and provide a second type of probe procedure, that is, autoshaping, by means of which a nonverbal organism can be asked what it knows about what it is doing to adapt to an environmental contingency.     

Rat spatial memory: Resistance to retroactive interference at long retention intervals

An attempt was made to disrupt memory for spatial information by interpolating a task of high similarity to the to-be-remembered task during a long retention interval. Rats were trained in an 8-arm maze in which choosing each arm without repetition was the optional strategy. A 4-h delay was imposed between the 4th and 5th choices. At various times during the retention interval, the rats ran a second identical maze located in another room. No evidence of retroactive interference was observed. In the second experiment, the rat was required to remember the interpolated spatial task during the retention test. This was accomplished by allowing the rat to make four choices in the first maze and then, after a variable period of time, four choices in the second maze. Four hours after exposure to each maze, retention was tested. Choice accuracy on the retention tests was high and equivalent on both mazes. Requiring the rat to remember which arms it had visited in a second maze did not impair memory for the first maze. These results demonstrate that rats can segregate spatial memories established in different contexts with considerable proficiency.     

Control of delayed matching-to-sample performance using directed forgetting techniques

Pigeons acquired a successive delayed matching-to-sample task at a delay interval of 4 sec. Instructional stimuli were interpolated in the delay interval signaling the occurrence (R-cue) or nonoccurrence (F-cue) of comparison stimuli, a procedure modeled after the directed forgetting techniques commonly used in human memory studies. Accuracy on probe trials (in which comparison stimuli were presented following F-cues) was reduced relative to performance on standard training trials in which R-cues signaling the occurrence of comparison stimuli appeared in the same temporal location. The extent of the reduction in accuracy depended on the temporal location of the F-cues, the reduction being greater when the cue was more remote from the comparison stimuli. Examination of retention interval keypecking revealed a strong correlation between matching performance and retention interval responding.     

Spatial memory in rats: Resistance to retroactive interference

Treatments that interfere with animals’ short-term retention (e.g., in delayed matching-to-sample) were studied using a spatial memory task. Rats performed in an eight-arm radial maze in which choosing each arm without repetition was the optimal behavior. Performances were interrupted between fourth and fifth choices for a delay of 15 sec to 2 min. A variety of events occurring during the delay interval did not disrupt memories for prior choices (as assessed by the accuracy of postdelay choices). The ineffective treatments included variations in visual and auditory environments, removal from the maze, food consumed during the delay, a distinctive odor added to the maze, or combinations of these manipulations. Additionally, performance on another spatial task (a four-arm maze) during the delay between Choices 4 and 5 did not interfere with performance in the eight-arm maze. These findings suggest that rats’ memories for spatial locations are immune to retroactive interference, at least within the range of conditions reported, and that the rat can successfully segregate memories for spatial locations established in different contexts.     

Pigeons’ spatial memory: V. Proactive interference in the delayed matching of key location paradigm occurs only under restricted conditions

In the delayed matching of key location procedure, pigeons must remember the location of the sample key in order to choose correctly between two comparison keys. The deleterious effect of short intertrial intervals on key location matching found in previous studies suggested that pigeons’ short-term spatial memory is affected by proactive interference. However, because a reward expectancy mechanism may account for the intertriai interval effect, additional research aimed at demonstrating proactive interference was warranted. In Experiment 1, matching accuracy did not decline from early to late trials within a session, a finding inconsistent with a proactive interference effect. In Experiment 2, evidence suggestive of proactive interference was found: Matching was more accurate when the locations that served as distractors and as samples were chosen from different sets. However, this effect could have been due to differences in task difficulty, and the results of the two subsequent experiments provided no evidence of proactive interference. In Experiment 3, the distractor on Trialn was either the location that had served as the sample on Trialn ? 1 or one that had been a sample on earlier trials. Matching accuracy was not inferior on the former type of trial. In Experiment 4, the stimuli that served as samples and distractors were taken from sets containing 2, 3, 5, or 9 locations. Matching accuracy was no worse, actually slightly better, with smaller memory set sizes. Overall, these findings suggested that pigeons’ memory for spatial location may be immune to proactive interference. However, when, in Experiment 5, an intratrial manipulation was used, clear evidence of proactive interference was found: Matching accuracy was considerably lower when the sample was preceded by the distractor for that trial than when it was preceded by the sample or by nothing. Possible reasons why interference was produced by intratrial but not intertrial manipulations are discussed, as are implications of these data for models of pigeons’ short-term spatial memory.     

Arousal and short-term memory: Effects of caffeine and trial spacing on delayed-alternation performance

Two experiments tested the prediction of Kesner’s (1973) parallel memory-stores theory that arousal reduces retention in short-term memory. Using rats as subjects, the effect of caffeine on delayed-alternation performance in a spatial discrimination was investigated. Trial spacing was also manipulated, both alone and in combination with the drug variable. The results showed that, with massed trials, caffeine facilitated alternation at the short delay and inhibited it at the longer delays tested. Using spaced trials, caffeine decreased alternation at all delays. Spaced trials produced overall superior performance as compared with massed trials. This pattern of results is consistent with the view that caffeine reduces short-term retention but, paradoxically, can also increase performance under massed trials by decreasing proactive interference (i.e., retention) from earlier trials.     

Memory for a stimulus versus anticipation of a response: Contrasting effects of proactive interference in two delayed comparison tasks

Pigeons were trained to perform two delayed comparison tasks that differed only in the temporal placement of the retention interval relative to the sample and comparison stimuli. In one task (conditional delayed response—CDR), the subject could determine the correct response prior to the retention interval. In the second task (delayed conditional response—DCR), the subject was required to compare stimuli presented before and after the retention interval. Although overt mediational responding was not observed in the CDR condition, retention was, nevertheless, greater than in the DCR condition. Furthermore, this difference was amplified at short intertrial intervals. Finally, retention in the DCR, but not the CDR, condition was disrupted by proactive interference (PI) from previous sample stimuli. The results suggest that memory is less closely tied to the sample stimulus when the stimuli necessary to determine the correct response are presented prior to the retention interval (CDR and other delayed response tasks) than when the subject must compare stimuli across the retention interval (DCR, or delayed matching-to-sample, tasks). This difference may lead directly, or indirectly through an interaction with PI, to task differences in retention.     

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.     

Attenuation of the CS-preexposure effect after a retention interval in preweanling rats

The effects of flavor preexposure and retention interval were assessed in 6- and 12-day-old rats. Conditioned aversions to a flavor appeared at both ages. The conditioning of the younger pups was unaffected by conditioned stimulus (CS) preexposure and was not evident after a 10-day retention interval. For the 12-day-old rats, preexposure to either the flavor CS or a different flavor attenuated aversion strength when the rats were tested soon after conditioning. Other 12-day-old rats that were tested 10 days after conditioning also expressed substantial aversions, but with a retention interval of this length, the aversions were equivalent for animals preexposed to the CS and those not preexposed before conditioning. This loss of the CS-preexposure effect over a long interval, which has also been observed in adult rats, identifies the locus of this effect as postacquisition and perhaps at the stage of memory retrieval.     

Similarities between human and animal spatial memory: Item and order information

Human subjects, sitting at the center of a circle of eight lights, were tested on analogues of radial-maze item-recognition (Roberts & Smythe, 1979) and order-recognition (Kesner & Novak, 1982) tasks. Subjects in the item-recognition condition saw a list of seven lights, and then the nonlist (eighth) light was tested against the first, fourth, or seventh light from the list. The subjects were required to point toward the nonlist light. Subjects in the order-recognition condition saw a series of eight lights, followed by a test of the first and second, fourth and fifth, or seventh and eighth serial positions. They were asked to point toward the light with the earlier serial position. Subjects’ item-recognition serial-position curves exhibited a recency effect with a 0-sec retention interval (Experiments 1 and 2), and were U-shaped (Experiment 1) or flat (Experiment 2) with a 30-sec retention interval. Subjects’ order-recognition serial-position curves were U-shaped at both retention intervals. Subjects’ reported mnemonics were, generally, unrelated to their choice accuracy. The results suggest analogous memory processes in animals and humans.     

Stimulus salience and asymmetric forgetting in the pigeon

Pigeons were trained using a symbolic delayed matching-to-sample procedure involving bright versus dim houselight samples. We hypothesized that when sample stimuli differ in salience, increasing the size of the retention interval will affect performance on trials initiated by the more salient sample only. In agreement with this prediction, accuracy following the dim sample did not decline as the retention interval increased, whereas accuracy following the bright sample declined to well below 50% correct. In a second experiment, the less salient (dim) sample from Experiment 1 was arranged as the more salient sample in a sample/no-sample procedure. Accuracy on dim sample trials then declined to well below 50% correct as the retention interval increased, whereas accuracy on no-sample trials remained constant. The results suggest that when sample stimuli differ in salience, pigeons may transform the nominal discrimination task into a detection task in which they respond on the basis of the presence or absence of the more salient sample.     

Auditory short-term memory in the budgerigar (Melopsittacus undulatus)

We trained two budgerigars (Melopsittacus undulatus) with operant techniques in a delayed matching-to-sample task using pairs of acoustic stimuli. These stimuli included simple pure tones, complex, species-specific vocalizations, and tone-vocalization combinations. The birds were then tested with different retention intervals. The budgerigars’ short-term memory was similar for complex, species-specific vocalizations and for simple pure tones. By contrast, they showed significantly better short-term memory when tested with two sounds drawn from different acoustic categories.     

The effect of crying on long-term memory in infancy

The influence of crying on infants' long-term memory for a learned response was investigated in 3 experiments. In each, infants were trained to move a crib mobile containing 10 identical objects by means of kicking and were then exposed to a reinforcer containing only 2 of these components. This shift in component numerosity produced crying in 53% of the infants. Infants who cried in response to the reward shift evidenced no retention of the contingency 1 week later (Experiment 1) but did have excellent retention at 1 day (Experiment 2). In Experiment 3, a brief reactivation treatment alleviated forgetting at 3 weeks regardless of the presence of crying in response to the change in mobiles. An unexpected recency effect characterized the efficacy of the reactivation treatment. The results indicate that crying in response to the violation of a reward-expectation habit functions as an amnesic agent to produce accelerated forgetting.     

Does contextual change determine long-term forgetting?

A series of experiments were run to test the hypothesis that “spontaneous forgetting” could result from subtle contextual changes. The first experiment demonstrated that when Sprague-Dawley male rats are trained in a runway alley with a food reinforcer, retention performance is dramatically affected by a change in the pattern of the walls of the training apparatus when testing takes place 1, 3, or 5 days following training and not after 1 week. Experiment 2 demonstrated that this performance deficit cannot be alleviated by any of the three selected cuing treatments, whereas “spontaneous forgetting” (resulting from a training-to-test interval of 14 days) can be. These data indicate that the detrimental effect of contextual change reduces over time, and that such an effect cannot be interpreted in terms of retrieval failure. These results lend strong support to the argument that the disruptive contextual change effect is basically different from the disruptive effect that results from an extended training-to-test interval.