Ontogeny of early event memory: II. Encoding and retrieval by 2- and 3-month-olds

The ability of 2-month-old infants to discriminate changes in a 5-object crib mobile following a retention interval of 24 hr was assessed using the mobile conjugate reinforcement paradigm. Infants were trained in 3 daily 15-min sessions to produce mobile movement by footkicking. Twenty-four hr later, independent groups received generalization tests with mobiles containing 1–5 novel objects substituted into their original training mobile. A control group was tested with the original training mobile. These findings were compared with findings of 2 previous studies involving identical procedures with 3-month-olds, reanalyzed for measures of individual performance over successive test minutes. Although, in absolute terms, 2-month-olds had a flat generalization gradient relative to 3-month-olds, when each infant's kick rate during the generalization test was expressed relative to that infant's kick rate before, and at the end of, training, it was found that the generalization gradients of 2- and 3-month-olds were indistinguishable. The relative response measures indicated a surprising degree of specificity by both age groups: Test mobiles containing more than 1 novel object did not cue retrieval, but test mobiles containing no more than 1 new object yielded perfect retention and complete generalization. These data indicate that infants as young as 2 months are capable of encoding and maintaining a representation of the specific details of their training context for at least 24 hr and, after that delay, can perform fine discriminations based on the discrepancy between their test context and that representation.     

Reinstatement Effects on Retention at 3 Months of Age

In three experiments with 56 3-month-olds, we examined the effect of different numbers of reinstatements (reminders) on long-term retention. Infants learned to move a crib mobile by kicking and subsequently received one, two, or three reinstatements. Each reinstatement was a partial training episode one-sixth the duration of original training. Presenting a single reinstatement when the memory was inactive failed to recover it 1 day later (Experiment 1), but increasing the number of reinstatements when the memory was active to two (Experiment 2) and three (Experiment 3) progressively protracted retention. Although 3-month-olds typically remember for less than 6 days, after three reinstatements they still exhibited retention 6 weeks after training. Untrained controls who received an identical regimen of reinstatements exhibited no retention. These results demonstrate that periodic reinstatements can maintain young infants' retention over long delays and that the state of the memory at the time of reinstatement is critical to its effectiveness.     

Substituting new details for old? Effects of delaying postevent information on infant memory

How that which we remember is selectively distorted by new information was studied in 3-monthold infants who learned to move a particular crib mobile by operant foot kicking. Infants who were passively exposed to a novel mobile 1, 2, or 3 days later subsequently treated the novel mobile as if they had actually been trained with it. Also, after the longest exposure delay, they no longer recognized the original mobile. Likewise, when the novel mobile was exposed after the longest delay, it could prime the forgotten training memory in a reactivation paradigm, but the original mobile no longer could. These data reveal that what we remember about an event is selectively distorted by what we encounter later. Moreover, the later in the retention interval we encounter new postevent information, the greater is its impact on retention.     

The effects of distributed training on retention of operant conditioning in human infants

Long-term retention of operant footkicking acquired in the mobile conjugate reinforcement paradigm was assessed as a function of the distribution of training time. In the first study, 3-month-old infants were trained for either one 18-min session or for two 9-min or three 6-min sessions separated by 24-hr intervals. All infants exhibited retention during a test administered immediately after training, but only those trained in a single session continued to perform the conditioned response during cued-recall tests 7 or 14 days later. Infants trained in three sessions showed no evidence of remembering the contingency even after a week. A warm-up decrement, seen in the day-to-day performance of infants in the distributed conditions, was eliminated in the second study by the interpolation of a nonreinforcement period at the outset of daily sessions. This procedure also enhanced long-term retention such that infants trained in three 6-min session now remembered the contingency for 14 days and did not differ from infants who had received a single 18-min session. Whether distributed training facilitates or impairs long-term retention appears to depend on the opportunity for infants to acquire a sufficient number (or kind) of effective retrieval cues during original learning.     

Verbal cues facilitate memory retrieval during infancy

In three experiments, 18-month-olds were tested in a deferred imitation paradigm. Some infants received verbal information during the demonstration and at the time of the test (full narration), and some did not (empty narration). When tested after a 4-week delay, infants given full narration exhibited superior retention relative to infants given empty narration (Experiment 1). This retention advantage appears to be due to the effects of verbal cues at the time of memory retrieval. There was no effect of verbal cues that were presented only at the time of original encoding (Experiment 2A). Furthermore, infants who received verbal cues only at the time of retrieval exhibited superior retention relative to infants who received verbal cues only at the time of original encoding (Experiment 2B). These findings demonstrate that verbal cues can enhance memory retrieval by participants who are not yet fluent speakers themselves.     

Forming contextual categories in infancy

The role of context in categorization was examined in four experiments with 3-month-olds. In all experiments infants learned to kick their feet to activate a mobile on 3 successive days. Infants were trained with a different mobile exemplar during each daily session. Categorization of a physically dissimilar novel object (Butterfly) was assessed 1 to 14 days later. In Experiments 1A and 1B, infants were trained and tested in a highly distinctive context. They included Butterfly in the mobile category after 1 and 7, but not after 14, days (Experiment 1A). This latter result was due to a categorization failure because infants did respond to another novel exemplar of the original training series after 14 days (Experiment 1B). In Experiment 2A, infants who were passively exposed to Butterfly for 3 minutes in the distinctive context at the end of category training responded to Butterfly 1 day later when it was encountered “out of context.” This result was also obtained in Experiment 2B when the distinctive context was present only during the final training session. Taken together, these findings demonstrate that categorization of a novel object is influenced by the context present when the object is initially encountered and by previous encounters with that object in the category context. Clearly, infants are capable of contextual categorization very early in the first year of life.     

Infants' eyewitness testimony: Effects of postevent information on a prior memory representation

In eyewitness testimony research, postevent information impair retention of the original event and increases the probability that interpolated information will be identified as part:of the original event. The present experiments studied these effects with 3-month-olds. Infants learned to kick to move a particular crib mobile and then were briefly exposed to information about a novel mobile. The novel postevent information impaired recognition of the-original mobile when it immediately followed training but not when it was delayed by 1 day. Like adults, infants treated the postevent information as part of the original training event, continuing to do so for at least 2 weeks. We propose that postevent information displaces conflicting information coactive with it in primary memory and creates a new, updated memory token of the event. Once the new token leaves primary memory, however, it is protected; only a copy can be retrieved and modified in the future.     

The effects of a target–stimulus reminder on performance in a novel object recognition task

The effects of original training-stimulus pre-test reminders were examined in a novel object recognition (NOR) task. NOR is a task that examines memory for complex stimuli, and is driven by the rats’ tendency to spend significantly more time exploring novel objects over those previously experienced. In this task, a delay is imposed between a training experience during which the animal is allowed to investigate a set of identical objects, and a later test exposure where the animal encounters one of the original objects and a novel object with which it has had no previous experience. Experiment 1 demonstrated that performance at 24 h is significantly worse than at an immediate delay (1 min). In the second experiment, it was demonstrated that neither a 10-s nor a 30-s reminder treatment, in the absence of training, resulted in a level of preference for novelty, a measure of memory for the original object, that was significantly greater than chance. Experiment 3 illustrated significant performance effects of a 30-s training stimulus reminder administered 15 min prior to test with a 24-h retention interval. The final experiment illustrated that the additional 30-s of object exposure is effective in enhancing performance only if it occurs shortly prior to test. Animals receiving the additional 30-s immediately following training did not experience such beneficial effects. It was concluded, based upon these results, that pre-test training-stimulus reminders in this task produce effects similar to those seen in more traditional tasks of learning and memory.     

Epinephrine modulates long-term retention of an aversively motivated discrimination

These experiments examined the effects of post-training epinephrine (Epi) on retention of an aversively motivated discrimination task. Male CFW mice were trained to escape from footshock by entering one of two alleys of a Y-maze. On a 24-h retention test (six trials) the correct alley was reversed. The findings of Experiment 1 indicate that errors on the discrimination reversal varied directly with number of trials (criterion of 0, 3, or 6 successive correct choices) on the original training. These findings indicate that errors on discrimination reversal training provide a sensitive index of retention of the original training. In Experiment 2, mice were trained to a criterion of three successive correct choices and were given post-training injections of saline or Epi (0.1, 0.3, or 1.0 mg/kg ip). On a 24-h discrimination reversal test mice given the low doses of Epi made more errors than did saline controls while mice given the high dose made fewer errors. In Experiment 3, mice trained as in Exp 2 received post-training saline or Epi (0.3 or 1.0 mg/kg) and were tested for retention either 1 week or 1 month later. At each retention interval, performance was comparable to that found with a 24-h retention interval. The findings provide additional evidence that post-training Epi produces long-lasting dose-dependent modulating effects on memory storage.     

Reinstatement versus reactivation effects on active memory in infants

Reinstatement and reactivation are procedurally different reminder paradigms used with infants and children, but most developmental psychologists do not distinguish between them. In 4 experiments with 102 three-month-olds, we asked if they differ functionally as well. Independent groups of infants received either a reactivation or a reinstatement reminder 3 days after training, when the memory is active, but its specific details have been forgotten. In Experiment 1, we measured retention after increasing delays until infants forgot altogether. A single reinstatement protracted retention twice as long after training as a single reactivation. In Experiments 2-4, whether the reminder was the original training stimulus or a novel one differentially affected the duration and specificity of memory in the 2 procedures as well. These data demonstrate that the distinction between reinstatement and reactivation is not artificial. In addition to differing procedurally, reinstatement and reactivation differ functionally, with different memory-preserving effects.     

Memory reactivation in the second year of life

Memory reactivation has not been systematically studied with infants older than 1 year. Currently, three experiments examined the effects of a reactivation treatment (priming) on retention throughout the second postnatal year. Fifteen- and 18-month-olds learned an operant train task, forgot it, received a 2-min or 10-s prime, and later were tested for retention. Although the longer prime was effective for 15-month-olds, 18-month-olds required the shorter prime (Experiment 1). The memory was reactivated after delays two (18 months) and three (15 months) times longer than infants originally remembered it (Experiment 2). The reactivated memory was forgotten as fast as the original memory after the 2-min prime and twice as fast after the 10-s prime (Experiment 3). The fact that reactivation changes quantitatively but not qualitatively throughout infancy suggests that the same mechanism mediates it at all ages. These findings have major implications for the impact of early experience on cognitive development.     

An assessment of the role of handling cues in "spontaneous recovery" after extinction

Three experiments examined the assertion that presession handling cues that accompany training with reinforcement might account for spontaneous recovery when they reoccur following extinction. In Experiment 1, after extensive training on a variable-interval schedule, key pecking in pigeons was extinguished following either normal or distinctively different handling and transportational cues. Those cues resulted in enhanced spontaneous recovery 24 hr later when normal cues were reinstated. In Experiment 2, however, subjects tested following the normal handling cues showed no more spontaneous recovery than did subjects that spent the entire extinction-test interval in the experimental chambers and thus were tested without handling cues altogether. In Experiment 3, a group whose test for recovery began 10 min after being placed in the chambers yielded as much spontaneous recovery as did a group tested normally. Furthermore, a group for which extinction began at mid-session and for which handling therefore could not be a discriminative cue for extinction showed no more spontaneous recovery than did the other two groups. Handling cues thus contributed to spontaneous recovery only after explicit discrimination training, as provided in Experiment 1.     

Roles of function, reminding, and variability in categorization by 3-month-old infants

The roles of function, reminding, and exemplar variability in categorization of a physically dissimilar object were studied with 3-month-old infants trained to move a crib mobile by kicking. Performance on a transfer test with a motionless novel object provided evidence of categorization. In Experiments 1 and 2, infants, like adults, initially categorized novel objects on the basis of physical appearance, but only if trained with multiple exemplars, after delays of 1 and 7 days. In Experiment 3, prior knowledge of an object's functional properties overrode physical dissimilarity as the basis for categorization and enabled reminding of the classification response 2 weeks later. In Experiment 4, postevent contingency information overrode physical and functional properties as the basis for categorization. These findings indicate that expectations and goals influence infants' category decisions and raise the possibility that infants of 3 months respond by analogy.     

Infants' memory processing of a serial list: list length effects

Six-month-olds, trained with a three-mobile serial list, exhibit a primacy effect 24 h later. In three experiments, we demonstrated that increasing list length impairs their memory for serial order. In all experiments, 6-month-olds were trained with a five-mobile list. In Experiment 1, infants failed to exhibit a primacy effect on a 24-h delayed recognition test, recognizing mobiles from all serial positions. In Experiment 2, infants did exhibit a primacy effect on a reactivation (priming) test, suggesting that they may originally have encoded serial-order information. Experiment 3 confirmed that serial-order information was represented in infants' training memory. After the reactivation treatment, infants were precued with one list member and tested for recognition of another. When precues specified valid order information, infants recognized test mobiles from the later serial positions. The memory dissociation for serial order on delayed recognition and reactivation tests adds to the growing evidence that young infants possess two functionally distinct memory systems.     

Positive behavioral contrast in 3-month-old infants on multiple conjugate reinforcement schedules.

Positive behavioral contrast was assessed in two experiments with young infants using multiple conjugate reinforcement schedules. Reinforcement was produced by footkicks which activated the objects of an overhead crib mobile in a manner proportional to the vigor and rate of responding. Distinctive color/pattern cues on the sides of the objects served as discriminative stimuli for components of the multiple schedule. In Experiment 1, infants were trained with one cue (S+) only before insertion of S+ into a multiple schedule with an extinction component. A control group received S+ throughout all sessions. In Experiment 2, a multiple schedule was introduced at the outset, and responses in both components were reinforced before the introduction of extinction in the second component. In a final phase, reinforcement was reintroduced into the second component. Positive behavioral contrast occurred in both experiments. Response reduction in the extinction component was seen only in individual relative response curves. In both experiments, negative emotional behaviors accompanied the extinction component, and in Experiment 1, cooing accompanied presentations of S+.     

Basolateral amygdala lesions do not prevent memory of context-footshock training

The present studies examined the effects of basolateral amygdala (BLA) lesions induced prior to or after context-footshock training on 48-h memory, using several retention measures. In experiment 1, male Sprague-Dawley rats with bilateral BLA lesions (NMDA, 12.5 mg/mL, 0.2 μL) were given footshock training in one compartment of a two-compartment alley. Rats were habituated to the alley and 24 h later were given two footshocks in the shock compartment. Retention was tested 48 h later, using latency to enter the shock compartment and time spent freezing as measures of memory. Two days later, they were tested again and received a footshock on each re-entry of the shock compartment prior to remaining in the safe compartment for 200 consecutive seconds. The BLA lesions did not block retention as assessed by freezing or number of re-entries of the shock compartment. In experiment 2, no prior habituation was given, and only one footshock was used for the training. BLA lesions did not block retention, as indicated by latencies to enter the shock compartment on a 48-h test or by number of entries of the shock compartment. Experiment 3 examined the effects of the GABA_A agonist muscimol infused into the BLA prior to the 48-h retention test. The muscimol infusions decreased retention test entrance latencies but did not block retention as assessed by the number of subsequent entries of the shock compartment. These findings provide additional evidence that an intact BLA is not required for the acquisition or retention of context-footshock training.     

Factors affecting conditional discrimination learning by pigeons

In Experiment 1 (within subjects) and Experiment 2 (between subjects) it was shown that the sequential training of pigeons on a color discrimination and then on its reversal, each in a different floor-tilt/texture context, failed to produce conditional control of discriminative performance by those contexts. Daily alternation between the two problems (with correlated contexts) was successful, however. In each of these experiments conditional control was better reflected in generalization test performance in extinction than during sessions of training with reinforcement.     

A parametric variation of delayed reinforcement in infants.

This study is an exploration of the parameters of delayed reinforcement with 6 infants (2 to 6 months old) in two experiments using single-subject repeated-reversal designs. In Experiment 1, unsignaled 3-s delayed reinforcement was used to increase infant vocalization rate when compared to a differential-reinforcement-of-other-than-vocalization condition and a yoked, no-contingency comparison condition. In Experiment 2, unsignaled 5-s delayed reinforcement was used to increase infant vocalization rate when compared to an alternating-treatments comparison condition. The alternating-treatments comparison consisted of 3-min components of differential reinforcement of other behavior and 3-min components of a nontreatment baseline. Successful conditioning was obtained in both experiments. These results contrast with those of previous infancy researchers who did not obtained conditioning with delays of 3 s and who attributed their findings to the limitations of the infant's memory capacity. We present an alternative conceptual framework and methodology for the analysis of delayed reinforcement in infants.     

Failure to find evidence of stimulus generalization within pictorial categories in pigeons

Pigeons' key pecks were reinforced in the presence of pictures from one of two categories, cats or cars. A single picture associated with reinforcement was used in Experiment 1, and 20 pictures from the same category were associated with reinforcement in Experiment 2. Pigeons then were presented with novel test pictures from the training category and from the other, previously unseen, category. During Session 1 of testing, pigeons pecked no more often at pictures from the reinforced category than at pictures from the previously unseen category. When pigeons were trained with pictures associated with reinforcement or its absence from different categories in Experiment 3, differential responding to novel pictures from different categories appeared during Session 1. These findings argue against a process of automatic stimulus generalization within natural categories and in favor of the position that category distinctions are not made until members of at least two categories are compared with one another.     

Acquisition and retention of a letter-detection skill

In two experiments, we examined the acquisition and retention of a letter-detection skill with a consistent-mapping procedure. In Experiment 1, subjects were trained from 0 to 4 sessions at detecting the letter H in displays containing random letters, and retesting occurred after a 1-month delay. Performance improved and in some cases became more automatic, and the performance level was maintained over the retention interval. When tested with a prose passage, the high error rate on the word THE was eliminated after training and after the retention interval, regardless of the amount of training. In Experiment 2, two subjects were given 12 sessions of training followed by a retention test 6 months later. For 1 subject there was also a retention test 15 months after acquisition. Performance improved dramatically with training, and substantial but not complete automaticity was achieved. Performance on the retention tests was close to the final acquisition level. The surprising lack of forgetting in this study was contrasted with the substantial forgetting typically found in studies of verbal learning.