Independent paths in the development of infant learning and forgetting

Sleep problems and recall memory in children with Down syndrome and typically developing controls

BACKGROUND

Research conducted with typically developing (TD) infants and children generally indicates that better habitual sleep and sleep after learning are related to enhanced memory. Less is known, however, about associations between sleep and recall memory in children with Down syndrome (DS).

AIMS

The present study was conducted to determine whether parent-reported sleep problems were differentially associated with encoding, 1-month delayed recall memory, and forgetting over time in children with DS and those who were TD.

METHODS AND PROCEDURES

Ten children with DS (mean age = 33 months, 5 days) and 10 TD children (mean age = 21 months, 6 days) participated in a two-session study. At each session, recall memory was assessed using an elicited imitation paradigm. Immediate imitation was permitted at the first session as an index of encoding, and delayed recall was assessed 1 month later. In addition, parents provided demographic information and reported on child sleep problems.

OUTCOMES AND RESULTS

Although parents did not report more frequent sleep problems for children with DS relative to TD children, regression-based moderation analyses revealed that more frequent sleep problems were associated with increased forgetting of individual target actions and their order by children with DS. Evidence of moderation was not found when examining encoding or delayed recall.

CONCLUSIONS AND IMPLICATIONS

Although group differences were not found when considering parent-reported sleep problems, more frequent sleep problems were positively associated with increased forgetting by children with DS relative to those who were TD. Although future experimental work is needed to determine causality, these results suggest that improved sleep in children with DS might reduce forgetting, ultimately improving long-term recall memory.

Age- and performance-related differences in encoding during early childhood: insights from event-related potentials

Previous studies have found that children show rapid and significant improvements in their ability to remember individual items and the contextual details that surround these items (i.e., episodic memory) during early childhood. Encoding processes have been suggested to contribute to the development of episodic memory; however, few studies have investigated encoding processes. The goal of the current study was to examine age- and performance-related effects on encoding in children between 4 and 8 years of age using event-related potentials (ERPs). Results revealed effects of both age and performance on encoding, as indexed by the ERPs response. However, the nature of these effects differed between subsequent recognition and subsequent recollection, as well as for the two ERP components (i.e., Nc and LSW) examined. These findings are important as they contribute empirical evidence that encoding processes show developmental change across early childhood. In addition, these findings highlight the importance of controlling for performance differences in future studies examining developmental changes in episodic memory.

Developments in Declarative Memory

The second year of life is marked by changes in the robustness of recall memory. Both retrieval and storage processes have been implicated as the major source of age-related improvements in recall. Children 13 to 20 months of age were matched for levels of learning of laboratory events (thereby eliminating encoding as a source of developmental difference) and tested for recall after delays as long as 6 months. In Experiment 1, 16-month-olds evidenced less loss of information and more relearning than 13-month-olds. In Experiment 2, 20-month-olds evidenced less loss of information and more relearning than 16-month-olds. Patterns of performance across test trials and in relearning implicate a decline in susceptibility to storage failure as the primary source of the observed developmental trend.

Development of episodic and autobiographical memory: The importance of remembering forgetting

Some memories of the events of our lives have a long shelf-life-they remain accessible to recollection even after long delays. Yet many other of our experiences are forgotten, sometimes very soon after they take place. In spite of the prevalence of forgetting, theories of the development of episodic and autobiographical memory largely ignore it as a potential source of variance in explanation of age-related variability in long-term recall. They focus instead on what may be viewed as positive developmental changes, that is, changes that result in improvements in the quality of memory representations that are formed. The purpose of this review is to highlight the role of forgetting as an important variable in understanding the development of episodic and autobiographical memory. Forgetting processes are implicated as a source of variability in long-term recall due to the protracted course of development of the neural substrate responsible for transformation of fleeting experiences into memory traces that can be integrated into long-term stores and retrieved at later points in time. It is logical to assume that while the substrate is developing, neural processing is relatively inefficient and ineffective, resulting in loss of information from memory (i.e., forgetting). For this reason, focus on developmental increases in the quality of representations of past events and experiences will tell only a part of the story of how memory develops. A more complete account is afforded when we also consider changes in forgetting.

Interconnected growing self-organizing maps for auditory and semantic acquisition modeling

Based on the incremental nature of knowledge acquisition, in this study we propose a growing self-organizing neural network approach for modeling the acquisition of auditory and semantic categories. We introduce an Interconnected Growing Self-Organizing Maps (I-GSOM) algorithm, which takes associations between auditory information and semantic information into consideration, in this paper. Direct phonetic–semantic association is simulated in order to model the language acquisition in early phases, such as the babbling and imitation stages, in which no phonological representations exist. Based on the I-GSOM algorithm, we conducted experiments using paired acoustic and semantic training data. We use a cyclical reinforcing and reviewing training procedure to model the teaching and learning process between children and their communication partners. A reinforcing-by-link training procedure and a link-forgetting procedure are introduced to model the acquisition of associative relations between auditory and semantic information. Experimental results indicate that (1) I-GSOM has good ability to learn auditory and semantic categories presented within the training data; (2) clear auditory and semantic boundaries can be found in the network representation; (3) cyclical reinforcing and reviewing training leads to a detailed categorization as well as to a detailed clustering, while keeping the clusters that have already been learned and the network structure that has already been developed stable; and (4) reinforcing-by-link training leads to well-perceived auditory–semantic associations. Our I-GSOM model suggests that it is important to associate auditory information with semantic information during language acquisition. Despite its high level of abstraction, our I-GSOM approach can be interpreted as a biologically-inspired neurocomputational model.

Beyond initial encoding: measures of the post-encoding status of memory traces predict long-term recall during infancy

The first years of life are witness to rapid changes in long-term recall ability. In the current research we contributed to an explanation of the changes by testing the absolute and relative contributions to long-term recall of encoding and post-encoding processes. Using elicited imitation, we sampled the status of 16-, 20-, and 24-month-old infants' memory representations at various time points after experience of events. In Experiment 1, infants were tested immediately, 1 week after encoding, and again after 1 month. The measure of 1-week trace status was a unique predictor of 1-month delayed recall. In Experiment 2, infants were tested immediately, 15 min, 48 h, and 2 weeks after encoding and again 1 month later. The measures of 15-min and 48-h trace strength contributed unique variance in 1-month delayed recall. The findings highlight the need to consider post-encoding processes in explanations of variability in long-term memory during infancy.

Explaining variance in long-term recall in 3- and 4-year-old children: the importance of post-encoding processes

Long-term recall is influenced by what originally was encoded as well as by the efficacy of retrieval processes. The possible explanatory role of post-encoding processes by which initially labile memory traces are stabilized and integrated into long-term memory (i.e., consolidated) has received relatively less research attention. In the current research, we examined 3- and 4-year-old children's recall of multi-step event sequences immediately after seeing them modeled as a measure of encoding, 1 week later as a measure of the status of the memory trace post-encoding, and 1 month later as an assessment of long-term recall. We tested recall of events with three different levels of internal structure and with three different levels of support for retrieval. Measures of the post-encoding status of the memory trace explained significant variance in long-term recall when they were the sole predictors of performance, and they contributed unique variance in long-term recall even after accounting for the variance associated with encoding. The results imply that a complete explanation of forgetting during childhood must include not only roles for encoding and retrieval processes but also roles for post-encoding processes.

Social cues at encoding affect memory in 4-month-old infants

Available evidence suggests that infants use adults' social cues for learning by the second half of the first year of life. However, little is known about the short-term or long-term effects of joint attention interactions on learning and memory in younger infants. In the present study, 4-month-old infants were familiarized with visually presented objects in either of two conditions that differed in the degree of joint attention (high vs. low). Brain activity in response to familiar and novel objects was assessed immediately after the familiarization phase (immediate recognition), and following a 1-week delay (delayed recognition). The latency of the Nc component differentiated between recognition of old versus new objects. Pb amplitude and latency were affected by joint attention in delayed recognition. Moreover, the frequency of infant gaze to the experimenter during familiarization differed between the two experimental groups and modulated the Pb response. Results show that joint attention affects the mechanisms of long-term retention in 4-month-old infants. We conclude that joint attention helps children at this young age to recognize the relevance of learned items.

Equal Learning Does Not Result in Equal Remembering: The Importance of Post-Encoding Processes

Explanations of variability in long-term recall typically appeal to encoding and/or retrieval processes. However, for well over a century, it has been apparent that for memory traces to be stored successfully, they must undergo a post-encoding process of stabilization and integration. Variability in post-encoding processes is thus a potential source of age-related and individual variance in long-term recall. We examined post-encoding variability in each of two experiments. In each experiment, 20-month-old infants were exposed to novel three-step sequences in each of three encoding conditions: watch only, imitate, and learn to criterion. They were tested for recall after 15 min (as a measure of the success of encoding) and either weeks (1, 2, or 3: Experiment 1) or days (1, 2, or 4: Experiment 2) later. In each experiment, differential relative levels of performance among the conditions were observed at the two tests. The results implicate post-encoding processes are a source of variance in long-term recall.

Cognitive training during infancy and adolescence accelerates adult associative learning: critical impact of age, stimulus contingency and training intensity

A growing body of evidence supports the hypothesis that juvenile cognitive training shapes neural networks and behavior, and thereby determines the adult's capacity for learning and memory. In particular, we have shown that infant rats, even though they do not develop an active avoidance strategy in a two-way active avoidance task, show as adults accelerated learning in the same learning task. This indicates that a memory trace was formed in the infant rats, which most likely is recruited during adult training. To identify the learning conditions, which are essential prerequisites to form this memory trace in infancy or adolescence, we investigated the critical impact of: (i) age, (ii) CS-UCS contingency, and (iii) pre-training intensity on this facilitating effect. We observed: (i) an age-dependent improvement of avoidance learning, (ii) that the beneficial impact of infant or adolescent pre-training on adult learning increases with the age at pre-training, (iii) that CS-UCS contingency during infant pre-training was most efficient to accelerate adult learning, (iv) that pre-training intensity (i.e. number of pre-training trials) was positively correlated with the pre-training induced acceleration of adult learning, and (v) that infant rats, compared to adolescent rats, need a higher training intensity to show learning improvement as adults. These results indicate that infant rats develop a goal-oriented escape strategy, which during adult training is replaced by an avoidance strategy, facilitated by the recruitment of the CS-UCS association, which has been learned during infant training. Based on these results the future challenge will be to identify the specific contribution of prefronto-limbic circuits in infant and adult learning in relation to their functional maturation.

Constructing a past in infancy: a neuro-developmental account

The ability to recall the past is fundamental yet until relatively recently, infants were assumed to lack the capacity. Contrary to this perspective, non-verbal tests indicate that developments in recall are well underway by late in the first year of life; by the end of the second year, long-term recall is reliable and robust. New research combining electrophysiological and behavioral measures is identifying the loci of age-related changes: they are attributed to more effective and efficient encoding, consolidation and storage processes associated with developments in the temporal-cortical network that subserves recall. The emerging framework, which applies to episodic and autobiographical memory, highlights the essential developmental continuities in memory from infancy onwards and sheds new light on the phenomenon of childhood amnesia.

Electrophysiological Indexes of Encoding and Behavioral Indexes of Recall: Examining Relations and Developmental Change Late in the First Year of Life

Long-term memory undergoes pronounced development in the latter part of the 1st year. This research combines electrophysiological (event-related potential [ERP]) and behavioral (deferred imitation) measures of encoding and recall, respectively, in an examination of age-related changes in and relations between encoding and recall during this time. In a short-term longitudinal study, infants were exposed to different multistep sequences at 9 and at 10 months. In both phases, they were tested for immediate recognition of the events via ERPs (as an index of encoding), and for recall of them 1 month later. At both ages, infants encoded the events; encoding was more robust at 10 months than at 9 months. After the 1-month delay, infants failed to recall the events experienced at 9 months, but evidenced recall of the events experienced at 10 months. In spite of developmental differences in encoding and recall over this period, indexes of encoding at 9 months were correlated with measures of recall of events experienced at 10 months and tested 1 month later.

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.

The Role of Conceptual Recoding in Reducing Children's Retroactive Interference

Reductions in children's retroactive interference were examined with conceptual recoding. Children learned two 10-item lists of toys; items on the 2nd list could also be classified as vehicles. Some children were not told about this 2nd category, whereas others were told either at the end of acquisition or just prior to the retention test 24 hr later. The results showed that (a) children benefited from the recoding instruction, (b) younger but not older children failed to benefit from the recoding manipulation when it occurred just prior to the retention test, and (c) recoding reduced retroactive interference primarily through affecting storage processes. These results provide new evidence concerning the importance of making information distinctive in storage in children's retention.

Infants' learning, memory, and generalization of learning for bimodal events

Study 1 investigated whether infants 3 and 7 months of age show differential learning of and memory for sight-sound pairs depending on whether or not temporal synchrony was present; memory was assessed after a 10-min and 1-week interval. Study 2 examined whether 7-month-olds show generalization of learning when they encounter novel bimodal events that are similar (changes in size, orientation, or color, and spectral sound properties) to the sight-sound pairs learned 1 week earlier based on temporal synchrony. For Study 1, infants received a familiarization phase followed by a paired-comparison preference procedure to assess for learning of the sight-sound pairs. One week later a memory test was given. Results confirmed that 7-month-olds had no difficulty learning auditory-visual pairings regardless of whether or not events were temporally synchronous, and they remembered these 10 min and 1 week later. In contrast, 3-month-olds showed poorer learning of sight-sound associations in the no-synchrony than synchrony conditions, and memory for sight-sound pairs 1 week later was shown only for the synchrony conditions. Results for Study 2 revealed generalization of learning of bimodal pairings under all stimulus conditions after a 1-week interval at 7 months of age. Implications of these findings for development of intersensory knowledge are discussed.

Reexposure breeds recall: effects of experience on 9-month-olds' ordered recall

On the basis of three experiences, 9-month-olds recall specific events after one month. We tested whether multiple experiences are necessary, or merely facilitative, of 9-month-olds' long-term recall. In two experiments, using deferred imitation, infants were exposed to multi-step sequences either one time, two times, or three times prior to a one-month delay. In Experiment 1, regardless of the number of experiences, infants did not demonstrate recall. In Experiment 2, infants were tested on and reexposed to the events after one week; recall was tested again one month later. Performance after 1 week in Experiment 2 was greater than performance after one month in Experiment 1. Moreover, presumably as a function of reexposure after one week, infants recalled the individual actions of the events one month later. Infants in the three-experience condition also evidenced ordered recall after one month. Implications for the developmental status of the neural substrate supporting long-term recall are discussed.

Distinctiveness effects in children's long-term retention

In 3 experiments, kindergarten and second-grade children's retention was examined in the context of 2 distinctiveness manipulations, namely, the von Restorff and bizarre imagery paradigms. Specifically, children learned lists of pictures (Experiments 1a and 1b) or interactive images (Experiment 2) and were asked to recall them 3 weeks later. In Experiments 1a and 1b, distinctiveness was manipulated perceptually (changing colors) and conceptually (changing categories or switching to a numeral), whereas in Experiment 2, distinctiveness concerned the interaction (common or bizarre) between the referents. The results showed that (a) older children retained more information than younger children, (b) younger but not older children failed to benefit from numerically distinct information, and (c) distinctiveness in other domains facilitated children's retention. These results highlight the importance of distinctive information in children's retention.

When distinctiveness fails, false memories prevail

The argument advanced in this article is that false memories can arise because of processes that normally affect forgetting, namely, the decline of distinctiveness and the rise of retroactive interference. Specifically, when the distinctiveness of a trace relative to the background of other traces diminishes, the potential for interference among like traces increases. To the extent that memories lose their distinctive properties, including the source of the memory, such memories may become confused with events that are supposed to be recalled as actually having occurred. This idea is elaborated in the context of studies of the effects of distinctiveness on reducing retroactive interference in children's long-term retention. It is concluded that advances in understanding false memories and the role distinctiveness might play in reducing such misrememberings is contingent on the development of additional formal modeling approaches like the one presented in the lead paper by Brainerd and Reyna (1998, this issue).

The ebb and flow of infant attentional preferences: evidence for long-term recognition memory in 3-month-olds

Using paired-comparisons, 3-month-olds' (n = 148) recognition of dynamic visual events was investigated after retention intervals of 1 minute, 1 day, and 1 and 3 months (Experiment 1) and 1 minute, 1 day, and 1 week (Experiment 2). Participants were either tested at each retention interval (Multiple Tests) or tested at one interval (Single Test). The proportion of total looking time to the novel event and the length of the longest look to novel and familiar events in the first 15 s of the retention test revealed significant novelty preferences at 1 minute and 1 day and a null preference at 1 week for Multiple- and Single-Test groups. At 1 month, Multiple- (Proportion of Total Looking Time and Longest Look) and Single-Test groups (Longest Look only) preferred the familiar event. The 3-month test revealed a familiarity preference (both measures) for Single- and a null preference for Multiple-Tests groups. This changing pattern of attentional preferences is consistent with models of infant recognition memory in which novelty, familiarity, and null preferences are considered conjointly and hypothesized to reflect the accessibility of novel and familiar event representations in memory.