Minimum duration of reactivation at 3 months of age.

Briefly exposing subjects to an isolated component of an event after they have forgotten can reactivate their memory of it, leading to renewed retention on an ensuing test. In two experiments with forty-eight 3-month-old infants, we asked what minimum duration of a reactivation treatment could recover their forgotten memory of an operant mobile task and whether the minimum duration was affected by how long the memory was forgotten. In Experiment 1, the minimum duration for reactivating the memory 1 week after forgetting was 120 s-substantially longer than the minimum duration required for reactivation at 6 months after the same relative delay. In Experiment 2, the minimum effective duration for reactivation increased linearly with the time since forgetting, from 7.5 s after 1 day to 180 s after 3 weeks. This study reveals that the duration of an effective memory prime is directly related to age and to memory accessibility.

Visual-preference and operant measures of infant memory.

In three experiments with sixty 3- and 6-month-olds, we examined whether operant and visual-preference measures of retention are equivalent. Infants learned to move a mobile by kicking and then received a paired-comparison test with the familiar (training) mobile and a novel one. Kicking above baseline was the direct measure of retention, and longer looking at the novel mobile was the visual-preference or inferred measure. Retention was tested 1 day after training (Experiment 1) or reactivation (Experiment 2) and immediately or 4 days after training (Experiment 3). Despite differences in age and retention interval, infants consistently exhibited retention on the operant measure, but not on the visual-preference measure. These data reveal that the two measures of retention are not equivalent. When expectations are associated with a particular stimulus, infants indicate retention directly and robustly, rarely looking longer at a novel test stimulus. This result is not limited to operant situations.

Mediated imitation in 6-month-olds: remembering by association.

In two experiments with 72 6-month-olds, we examined whether associating an imitation task with an operant task affects infants' memory for either task. In Experiment 1, infants who imitated target actions that were modeled for 60 s on a hand puppet remembered them for only 1 day. We hypothesized that if infants associated the puppet imitation task with a longer-remembered operant task, then they might remember it longer too. In Experiment 2, infants learned to press a lever to activate a miniature train-a task 6-month-olds remember for 2 weeks-and saw the target actions modeled immediately afterward. These infants successfully imitated for up to 2 weeks, but only if the train memory was retrieved first. A follow-up experiment revealed that the learned association was bidirectional. This is the first demonstration of mediated imitation in 6-month-olds across two very different paradigms and reveals that associations are an important means of protracting memories.

Infants' long-term memory for a serial list: recognition and reactivation.

Serial lists contain information about item identity and item order. Using a task designed for nonverbal animals, we previously found that 3- and 6-month-olds exhibited a primacy effect after 24 hr, remembering both item identity and item order. Presently, we examined their memory of list information after longer delays. In Experiment 1, the serial-position curve reverted to a U-shape after 1 week at both ages, revealing that the common practice of attributing primacy and recency effects to long- and short-term memory, respectively, is flawed. In Experiment 2, a precuing procedure confirmed that 6-month-olds' memory still contained order information after 1 week, but 3-month-olds' reactivated memory contained none. Experiments 3A and 3B confirmed that increasing the complexity of information that was learned shortened the delay after which it could be retrieved. Testing infants after delays longer than have previously been used with animals or human adults sheds new light on an old phenomenon.

Information pick-up by infants: what is it, and how can we tell?

Young infants spend the better portion of their 1st year merely looking around, and what they notice can have critical cognitive consequences. Although looking measures are commonly used to predict what information infants pick up about objects and/or their relationship to other objects, these predictions are often ambiguous. Ultimately, that knowledge can be unambiguously revealed only by giving infants an opportunity to directly act on or use it.

A dissociation in infants' memory for stimulus size: evidence for the early development of multiple memory systems.

Adults' memory performance on recognition (explicit memory) tests is sensitive to stimulus size, but their performance on priming (implicit memory) tests is not. This memory dissociation is taken as evidence for two, functionally distinct memory systems. Young infants, however, are thought to possess only a single representational system that supports implicit memory; the system that supports explicit memory is thought not to mature before 8-9 months of age. In two experiments with 54 infants, we asked if 3-month-olds exhibit a memory dissociation for stimulus size on recognition and priming tests. All infants learned to move a mobile displaying +s of a given size. In Experiment 1, infants recognized +s in the original size but not 33% smaller or larger. In Experiment 2, +s were effective memory primes in a reactivation task, irrespective of size. The finding that young infants exhibit a memory dissociation for stimulus size adds to growing evidence that two memory systems are functional from early in development.

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.

Decreases in the response latency to priming over the first year of life.

We previously reported that the latency of responding to a memory prime in a reactivation procedure decreases between 3 and 6 months of age. The present study extended this analysis through the first year of life. In this study, 6-, 9-, and 12-month-olds learned an operant task. One week after they had forgotten it, infants were exposed to a component of the original event as a memory prime and were tested after different delays for evidence of retention. Although the interval between the original event and priming increased linearly with age-from 3 weeks at 6 months to 9 weeks at 12 months, the latency of responding after priming decreased linearly with age-from 1 hr at 6 months to 0-1 s at 12 months. The latency of responding after priming was not task-specific; at 6 months, it was identical in two different tasks. These results provide additional evidence that priming in reactivation studies with infants is the same automatic, perceptual identification phenomenon as repetition priming in studies with adults.

Retroactive interference in 3-month-old infants.

Three experiments assessed the effect of the similarity and timing of interpolated information on retroactive interference in human infants. After 3-month-olds learned to move a mobile (the cue) in a distinctive context by kicking, they were exposed to novel stimuli and were tested 24 hr later for recognition of their training stimuli. In Experiment 1, the interpolated cue, context, or both were novel. Retroactive interference occurred unless the interpolated and training stimuli shared no components. In Experiments 2 and 3, the timing of exposure to the interpolated context or cue, respectively, varied. A novel context impaired recognition after exposure delays up to 2 hr, whereas a novel cue impaired recognition after exposure delays up to 40 min. The finding that the cue is less vulnerable to retroactive interference than the context suggests that it is processed more rapidly. These experiments reveal that retroactive interference in infants depends on both the similarity and timing of the interpolated information.

Long-term maintenance of infant memory.

The present experiments with human infants asked whether periodic nonverbal reminders could maintain a memory established at 2 months of age over a substantial period of development. In Experiment 1, a reactivation reminder recovered infants' forgotten memory after 3 weeks, but a reinstatement reminder did not. In Experiment 2, 2-month-olds received a reminder every 3 weeks through 6(1/2) months of age and a final test at 7(1/4) months of age. A preliminary retention test preceded each reminder; which type of reminder (reinstatement or reactivation) infants received depended on performance during this test. Infants exhibited significant retention 4(1/2) months later, and most remembered 5(1/4) months later, when infants outgrew the task. Untrained controls exhibited no retention after any delay. These data confirm that periodic reminders can maintain early memories over significant periods of development and challenge popular claims that preverbal human infants cannot maintain memories over the long term because of neural immaturity or an inability to rehearse experiences by talking about them.

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.

An expanding training series protracts retention for 3-month-old infants.

This research examined whether an expanding training series protracts retention for infants as it does for children and adults. In three sessions spanning an 8-day period, 3-month-olds learned to move a crib mobile by kicking. Intersession intervals were either constant (1 or 4 days) or progressively expanding (average ISI = 4 days). The expanding-series group exhibited significant retention on a delayed recognition test 3 weeks after training was over, but the two constant-series groups exhibited none. Although the 1-day constant-series group remembered after 1 week, the 4-day constant-series group did not. Surprisingly, a reactivation treatment administered 4 weeks after training was over was ineffective whether infants were trained, reminded, and tested in a distinctive context or not. These results demonstrate that the retention advantage afforded by programming training sessions in an expanding series extends to infants and suggest that the upper limit on reactivation is timed from initial encoding and not from the point of forgetting.

Developmental changes in the specificity of memory over the first year of life.

In two experiments with 260 infants between 2 and 12 months of age, we examined how differences between the conditions of encoding and retrieval affect retention. Initially, 9- and 12-month-olds were tested with a different cue (Experiment 1) or in a different context (Experiment 2) after delays spanning their respective forgetting functions. These data were then combined with corresponding data previously collected from 2-to 6-month-olds trained and tested in an equivalent task. The resulting analyses revealed that the specificity constraints on memory retrieval become progressively looser at the extremes of the forgetting function with age. With increasing age, retention was less affected by cue changes after shorter absolute delays and, except at 6 months, by context changes after longer absolute delays. This pattern dovetails with evidence of decreasing specificity in the retrieval cues required for deferred imitation during infants' 2nd year and reveals that the memory abilities of older children evolve gradually from early in infancy.

Perceptual constraints on infant memory retrieval.

In three experiments with 78 3-month-olds, we asked what determines whether or not a stimulus will pop out and cue retrieval from long-term memory. All infants were trained with mobiles displaying either Qs (feature-present stimuli) or Os (feature-absent stimuli) and were tested 24 h later. When the diagonal line of the Q bisected its rim, feature-absent stimuli controlled retrieval in tests with homogeneous displays, and stimulus novelty controlled retrieval in tests with pop-out displays. A follow-up experiment revealed that the similarity between Q and O determined whether or not Q popped out: When its tail projected externally from the rim, Q popped out and cued retrieval, but O did not (search asymmetry). When its tail projected internally from the rim, however, 3-month-olds failed to discriminate Q from O (the externality effect). These data reveal that target-distractor similarity constrains whether or not a feature-present stimulus will pop out and cue retrieval.

Levels-of-processing effects in infant memory?

In 3 experiments, we manipulated 3-month-olds' attention to different components of a training display and assessed the effect on retention of those components. Attention was manipulated via a pop-out display (one target amidst 6 distractors) that enhances selective attention to the target relative to the distractors, and retention was assessed with displays composed entirely of targets or distractors. In Experiment 1, infants recognized both target and distractors after 1 day, confirming that both are initially encoded at some level. In Experiment 2, infants recognized a target L and distractor Ls after delays longer and shorter, respectively, than infants trained with Ls in a homogeneous display. Experiment 3 replicated the preceding pattern of results with a + stimulus. Thus, increasing or decreasing attention to an item during encoding produces a corresponding increase or decrease in its memorability. This finding is consistent with a levels of processing account and is inconsistent with accounts that deny a capacity for explicit memory to prelinguistic infants.

The timing of food availability affects growth in chicks.

Three experiments challenged the ability of domestic chicks to grow normally by differentially restricting when and for how long food was available. In Experiments 1 and 2, food was available for six 1-hr, three 2-hr, two 3-hr, or one 6-hr (a.m., p.m.) periods/day over the first 3 posthatch weeks. Control groups received continuous access to food. In Experiment 3, different amounts of light surrounded the 6-hr feeding period. In Experiments 2 and 3, chicks composed their own diets from separate sources high in protein or carbohydrate. Except for the single 6-hr meal preceding dark, large meals at other times of day impaired growth--primarily because chicks consumed insufficient dietary protein and ate less earlier in the light phase. We conclude that both the amount eaten and the proportion of the diet consumed as protein at given times of the day are phylogenetically acquired strategies that fit the omnivorous, diurnal chicken to its niche, independent of its momentary requirements, and appear early in development.

The ontogeny of long-term memory over the first year-and-a-half of life.

This research documents the development of long-term memory in human infants from 2 months through the end of the first year-and-a-half of life. In the initial study phase, we trained 6- to 18-month-old human infants in an operant task and tested them after increasing delays until they exhibited no retention for 2 successive weeks. In the second phase, their data were combined with data previously obtained from 2- to 6-month-olds in an equivalent task. The resulting function revealed that the duration of retention increases monotonically between 2 and 18 months of age. This increase was not due to age differences in original learning. This is the first systematic analysis of the course of long-term memory across an extended period of infant development that is based on standardized parameters of training and testing. It provides a reference function against which measures of retention from infants of different ages that are obtained in different memory tasks with different parameters can be meaningfully compared.

Behavioral thermoregulation in chicks: the best nest.

The ability of prehomeothermic chicks to thermoregulate behaviorally was studied in chicks with continuous access to heated nests, running wheels, and separate sources of high and low protein. In Experiment 1, cold-reared groups with heated or unheated transparent nests ate the same amount and selected the same dietary fractions, but chicks with heated nests ran less and grew faster. Despite this, groups maintained normal body temperatures. In Experiment 2, chicks were cold- or warm-reared with heated or unheated painted nests, or no nests. Cold-reared chicks with heated nests spent most of their time in them. They selected diets containing a higher protein:carbohydrate ratio than cold-reared chicks with unheated nests but ate less, thereby consuming less absolute protein and growing more slowly. Despite differences in growth, intake, and dietary choice, all chicks maintained normal body temperatures. These data reveal that behavioral thermoregulation has a privileged status for chicks over the first 3 weeks of life. Prehomeothermic chicks exercise complex and effective solutions to energetic challenges when offered behavioral options that simulate those available in nature.

Dissociation between features and feature relations in infant memory: effects of memory load.

Four experiments examined the effects of the number of features and feature relations on learning and long-term memory in infants. Three-month-olds learned to activate a mobile composed of either two or three kinds of blocks that differed in color, the figures displayed on them, and the figures' colors. Twenty-four hours later, infants trained with two objects discriminated feature recombinations but those trained with three objects did not. Even infants trained with three objects discriminated novel features, however, indicating that they remembered the individual features but not the relations among them. A subsequent experiment revealed that this dissociation between features and relations was induced by differential accessibility to memory rather than an encoding failure. We conclude that the size of the memory load selectively constrains infants' long-term memory for relational information. These results suggest that in infancy, as in adulthood, features and relations are psychologically distinct and that memory organization parallels the organization of perceptual processing.

Dissociations in infant memory: rethinking the development of implicit and explicit memory.

Extending the Jacksonian principle of the hierarchical development and dissolution of function to the development and dissolution of memory, researchers have concluded that implicit (procedural) memory is a primitive system, functional shortly after birth, that processes information automatically, whereas explicit (declarative) memory matures late in the 1st year and mediates the conscious recollection of a prior event. Support for a developmental hierarchy has only been inferred from the memory performance of adults with amnesia on priming and recognition-recall tests in response to manipulations of different independent variables. This article reviews evidence that very young infants exhibit memory dissociations like those exhibited by adults with normal memory on analogous memory tests in response to manipulations of the same independent variables. These data demonstrate that implicit and explicit memory follow the same developmental timetable and challenge the utility of conscious recollection as the defining characteristic of explicit memory.