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.     

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.     

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.     

Contextual determinants of retrieval in three-month-old infants

Three-month old infants learned to kick to produce movement in an overhead crib mobile on a conjugate reinforcement (FRI) schedule. All infants were trained in the presence of one of two distinctive crib bumpers. In Experiment 1, infants receiving cued-recall tests in the presence of the original training bumper remembered the conditioned association 1 week later, but those tested in the presence of a different bumper did not. Two weeks later, neither training group evidenced retention of the contingency. In Experiment 2, infants who received a 3-min exposure (“a reactivation treatment”) to the original training bumper 24 h prior to the 2-week retention test exhibited excellent long-term retention, whereas those exposed to a different bumper as a reminder did not, irrepective of whether it was identical to the test bumper or not. These findings demonstrate that the context alone, even though it was never exclusively paired with reinforcement, can act as a retrieval cue.     

Repeated reminders inrease the speed of memory retrieval by 3‐month‐old infants

In two experiments, we examined the effect of repeated reminder treatments on the speed of memory retrieval by 3‐month‐old human infants. Infants were trained for two consecutive days to kick their feet to produce movement in an overhead mobile. Infants in the one‐reminder condition received a 3 min reminder treatment 13 days after the conclusion of training. Infants in the two‐reminder condition received one 3 min reminder treatment 6 days after the conclusion of training and a second reminder treatment 7 days later (i.e. 13 days following the conclusion of training). Infants in the no‐reminder control condition were not exposed to the reminder prior to the long‐term retention test. In the absence of a reminder treatment, infants exhibited complete forgetting during the long‐term test. Infants exposed to one reminder exhibited retention when tested 24 h after their only reminder, but not when tested earlier. Infants exposed to two reminder treatments, on the other hand, exhibited retention when tested 1, 4 or 24 h after their second reminder treatment. We conclude that the opportunity to retrieve the memory on a prior occasion facilitated subsequent memory retrieval.     

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.     

An Examination of the Selection of Criterion Scores for the Study of Motor Learning and Retention

150 Ss were given 50 practice trials on the stabilometer over a period of 4 days (20 trials on Day 1 followed by a 1-day rest, 10 trials on Day 2, followed by a 7-day rest, 10 trials on Day 3 followed by a 14-day rest, and 10 trials on Day 4). A series of learning scores reflecting improvement in performance over the 4 days and retention scores reflecting loss of performance between days were obtained. The reliability of individual differences in these scores were calculated using the odd-even method and by progressively increasing the number of trials included in the measure of initial and final performance level. On the basis of the findings obtained it was concluded that using 4 – 6 trials in initial and final ability levels results in a difference (criterion) score which combines relatively high reliability with a relatively high estimate of the amount of change in performance.     

Olfactory context and memory retrieval in 3-month-old infants

Three experiments examineed whether olfactory stimuli can produce context-dependent effects in infant memory. In eash, 3-month-old infants learned to kick to control the movement of an overhead modile in the presence of an ambient odor. Retention was assessed 1, 3, or 5 days later. During the retention test, the olfactory context was either the same odor, a different odor, or “no odor.” At 1 day, infants exhibited retention when tested in the presence of the same odor. Infants in the no odor condition exhibited partial retention, whereas memory retrieval was completely disrupted for infants tested in the presence of the different odor. After the 3- and 5-day intervals, all groups showed forgetting. These results were not consistent with recent studies of 3-month-old infants trained and tested in the presence of a distinctive visual and auditory context. The findings suggest that olfactory stimuli are a silent feature of the environment for 3-month-old infants, but that they function as context cues in a way that differs from other sensory modalities.     

Interaction between catecholaminergic and opioid systems in an active avoidance task

Male NMRI mice were given intravenous injections of the noradrenergic neurotoxin DSP4 or the vehicle 24 to 72 h prior behavioral testing. Animals were given 2 days of training on a one-way active avoidance task. Naloxone was given in one of three doses prior to training on Day 1 and Day 2 or prior to training on Day 1 only (saline was given prior to training on Day 2). There was a dose-dependent impairment of acquisition by naloxone in the vehicle-pretreated groups; 10 mg/kg naloxone produced a significant impairment of acquisition. Naloxone also modulated retention (Day 2) performance of the active avoidance task. For vehicle-pretreated mice, 1 mg/kg naloxone facilitated and 10 mg/kg naloxone-impaired performance on Day 2. DSP4 alone produced an impairment of acquisition of this task but had no effect on retention; Day 2 scores were slightly higher in the DSP4-pretreated group than in the vehicle-pretreated group. Naloxone produced somewhat different effects in DSP4-pretreated animals than in vehicle-pretreated animals. Naloxone (1 mg/kg) ameliorated the DSP4-induced impairment of acquisition; 10 mg/kg naloxone did not significantly alter the acquisition performance of this group. For the DSP4-pretreated mice that received naloxone before training on both days, the dose-response characteristics for retention scores were similar to those of vehicle-pretreated mice; 1 mg/kg naloxone was the facilitatory dose. However, for DSP4-treated mice that received naloxone before training on Day 1 only, there was a shift to the right in the effective facilitatory dose of naloxone. For these animals, 10 mg/kg naloxone but not 1 mg/kg naloxone significantly enhanced retention performance. We discuss these results in the context of a possible state-dependent modulation by naloxone in the DSP4-treated animals.     

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.     

Binocular vision and spatial perception in 4- and 5-month-old infants

Four experiments investigated the relation between the development of binocular vision and infant spatial perception. Experiments 1 and 2 compared monocular and binocular depth perception in 4- and 5-month-old infants. Infants in both age groups reached more consistently for the nearer of two objects under binocular viewing conditions than under monocular viewing conditions. Experiments 3 and 4 investigated whether the superiority of binocular depth perception in 4-month-olds is related to the development of sensitivity to binocular disparity. Under binocular viewing conditions in Experiment 3, infants identified as disparity-sensitive reached more consistently for the nearer object than did infants identified as disparity-insensitive. The two groups' performances did not differ under monocular viewing conditions. These results suggest that, binocularly, the disparity-sensitive infants perceived the objects' distances more accurately than did the disparity-insensitive infants. In Experiment 4, infants were habituated to an object, then presented with the same object and a novel object that differed only in size. Disparity-sensitive infants showed size constancy by recovering from habituation when viewing the novel object. Disparity-insensitive infants did not show clear evidence of size constancy. These findings suggest that the development of sensitivity to binocular disparity is accompanied by a substantial increase in the accuracy of infant spatial perception.     

Social feeding reverses learned flavor aversions in spotted hyenas (Crocuta crocuta).

Spotted hyenas are highly social carnivores with competitive feeding habits. It was hypothesized that social feeding might be capable of reversing an established flavor aversion. The dominant hyena of each of 4 captive social groups was given a novel food mixed with LiCl on Day 1; aversion to this food was demonstrated on Day 2. On Day 3, the dominant hyena and 3 or 4 members of her group were offered the food simultaneously. Three of the 4 dominant hyenas ate substantial quantities of the food during this group feeding, and all 4 hyenas ate the following day when offered the food alone. In contrast, when averted to a different novel food, the hyenas refused that food on 3 consecutive days. In this species social factors, such as those operating during group feeding, are sufficiently powerful to attenuate, immediately and dramatically, a strong LiCl-induced aversion.     

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.     

Exploration and Selection of Intralimb Coordination Patterns in 3-Month-Old Infants

Through the exploration of their own capacities and the selection of adaptive responses, infants learn new motor solutions. Using a conjugate reinforcement mobile procedure, previous researchers have repeatedly shown that infants increase their leg kick frequency to control a mobile that is connected to their ankles. That traditional experimental design allows multiple motor solutions to the task and therefore provides limited information about the infants' capacity to explore and select specific motor solutions. The author designed a new experimental procedure to study infants' capacity to discover and adopt specific motor solutions. The new, constraining mobile reinforcement procedure requires a specific motor response and therefore the development of a more finely tuned perception—action map than has previously been experimentally demonstrated. To gain reinforcement from the mobile, infants had to produce a coordinated hip and knee extension within the same leg. The results from the 13 infant participants showed that they were capable of increasing their frequency of coordinated movements to make the mobile move. Those results suggest that infants at the age of 89–106 days are sensitive to intralimb coordination task requirements and are capable of mapping their own limb dynamics to the environmental information.     

Contextual interference effects in sequence learning for young and older adults

Practice of different tasks in a random order induces better retention than practicing them in a blocked order, a phenomenon known as the contextual interference (CI) effect. Our purpose was to investigate whether the CI effect exists in sequence learning, such that practicing different sequences in a random order will result in better learning of sequences than practicing them in blocks, and whether this effect is affected by aging. Subjects practiced a serial reaction time task where a set of three 4-element sequences were arranged in blocks or in a random order on 2 successive days. Subjects were divided into 4 groups based on a 2-GROUP (young or old) by 2-ORDER (random or blocked practice) between-subject design. Three days after practice (Day 5), subjects were tested with practiced and novel sequences to evaluate sequence-specific learning. The results replicate the CI effect in sequence learning in both young and older adults. Older adults retained sequences better when trained in a random condition than in a blocked condition, although the random condition incurs greater task switching costs in older adults during practice. Our study underscores the distinction between age-related effects on learning vs. performance, and offers practical implications for enhancing skill learning in older adults.     

Repeated Play Reduces Video Games’ Ability to Elicit Guilt: Evidence from a Longitudinal Experiment

Statements supported mostly by correlational and cross-sectional studies suggest that playing violent video games can cause emotional desensitization. A longitudinal experiment examined a) whether repeated violent game play leads to emotional desensitization and b) whether desensitization generalizes to other play and real-life experiences. Participants played alternative versions of the same violent game for the first four days; on these days, the character role was varied between-subjects to be moral (United Nations soldier) or immoral (terrorist soldier). On Day 5, all participants played a novel game as a terrorist. Results indicate two things. First, habituation occurs over repeated game play: Repeated exposure decreased the ability of the original game to elicit guilt. Second, the decreased ability to elicit guilt can generalize to other game-play experiences: Guilt elicited by the novel game on Day 5 was reduced for the immoral character condition compared to the moral character condition. The current study provides causal, longitudinal evidence regarding the potential for video game play to lead to emotional desensitization with regard to future video game-play experiences.     

The effect of a salient odor context on memory retrieval in young infants

Three-month-old infants were trained to move a mobile in the presence of a coconut or cherry odor (context). One or 5 days later, the infants were tested for retrieval in the presence of either the same odor, the alternate odor, or no odor. Infants tested with the training odor displayed retention at both intervals; retention was not seen at either interval in the alternate odor or no odor conditions. These data suggest that the odor combines with the mobile to form a compound-stimulus representation of the learned task whose presence after both short (1 day) and long (5 days) intervals is a necessary retrieval cue.     

Reconsolidation of episodic memories: a subtle reminder triggers integration of new information

Recent demonstrations of "reconsolidation" suggest that memories can be modified when they are reactivated. Reconsolidation has been observed in human procedural memory and in implicit memory in infants. This study asks whether episodic memory undergoes reconsolidation. College students learned a list of objects on Day 1. On Day 2, they received a reminder or not, and then learned a second list. Memory for List 1 was tested immediately on Day 2 (Experiment 2) or on Day 3 (Experiment 1). Although the reminder did not moderate the number of items recalled from List 1 on either day, subjects who received a reminder incorrectly intermixed items from the second list when recalling List 1 on Day 3. Experiment 2 showed that this effect does not occur immediately and thus is time-dependent. The reminder did not affect memory for List 2 on Day 3 (Experiment 3), demonstrating that modification occurred only for the original memory (List 1). The study demonstrates the crucial role of reminders for the modification of episodic memory, that reconsolidation of episodic memory is time-dependent, and, in contrast to previous reconsolidation findings, that reconsolidation is also a constructive process, one that supports the incorporation of new information in memory.     

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.