Harlene Hayne

Reactivation of infant memory: implications for cognitive development.

Publisher Summary Studies have shown that procedures that tap long-term memory yield a picture of infant memory radically different from that provided by paradigms involving measures of visual attention. Not only can 2- to 3-month-old infants recognize a specific cue, but they also can remember its predictive significance. In addition, their long-term memories are highly specific. Whether they remember or not on any given occasion depends upon the context, both proximal and distal, in which the retrieval cue is encountered. However, infants memories are hierarchically organized. They forget specific details of the proximal context more rapidly than its general features; as this occurs, they increasingly exploit distal contextual cues. Distal contextual information sharpens their discrimination of the test situation after increasingly longer delays, thereby protecting the original memory against retrieval in an inappropriate context.

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

Abstract 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 infants kick rate during the generalization test was expressed relative to that infants 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.

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 objects 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.

Infant memory for place information

The present studies were designed to examine the role of place cues in memory retrieval during early infancy. Three-month-old infants were trained to move a mobile by kicking Two weeks later, memory retrieval was disrupted if they were reminded in a location or place different from where they had been trained, but not if they were reminded in the same place (Experiment 1A). The same result was obtained even though highly salient cues in their immediate visual surround remained unchanged during reminding (Experiments 1B and 1C). No disruption was seen, however, when retrieval was cued in a different place after only 1 day (Experiment 2). These findings unequivocally demonstrate that infants as young as 3 months encode incidental information about the place where an event occurs and suggest that early memories are buffered against retrieval in potentially inappropriate contexts over the long term.

The time-window hypothesis: Implications for categorization and memory modification***

In four experiments, 100 three-month-old infants acquired a functional alphanumeric category in which they learned to kick to move a yellow block mobile on which the category cues were displayed. Following training, infants were passively and briefly (3 min) exposed to functional information that a novel and highly physically dissimilar object (butterfly) shared with the training exemplars. If the exposure occurred at any point within a 4-day time window of training, the novel object was integrated with the prior memory of category training. It could cue retrieval of the training memory both on a transfer test 24 hours later and in a reactivation paradigm 3 weeks later. Moreover, when the object was exposed at the end of the time window, its retention was significantly prolonged, but at the expense of the original category members. We propose that each succeeding retrieval progressively broadens the time window within which new (or old) information can retrieve the memory or category concept. By this mechanism, new information can be integrated with old over relatively long intervals.

Forming Contextual Categories in Infancy.

Abstract 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.

Ambient temperature effects on energetic relations in growing chicks

The effects of different ambient temperature conditions on the diet selection, intake, growth, body temperature, and activity of immature domestic chicks were assessed in two experiments. In Experiment 1, the ambient temperature either remained warm during both the light and dark phases of the photoperiod, as is characteristic in laboratory settings, or was warm during the light phase and cold during the dark phase. The latter condition reflects the daily temperature pattern in natural settings. Chicks exposed to low nocturnal ambient temperatures had lower body temperatures in both phases of the photoperiod, were less active, ate more, selected a higher percentage of carbohydrate in their diets, and grew faster but were less feed-efficient than warm-reared controls. In Experiment 2, the ambient temperature was either cool in both phases of the photoperiod or cool in the light phase and warm in the dark phase. Chicks reared continuously in the cold had lower body temperatures, selected a high-carbohydrate diet, and grew faster, but both rearing groups were relatively inactive. These results show that an animals body temperature, diet composition, food intake, feed efficiency, and activity reflect its 24-hr energy requirements and are a part of a general strategy of maximizing energy income and minimizing energy expenditure in response to energetic challenges to growth.

Diet selection by chicks

Seven experiments with 324 chicks tested their ability to select a nutritionally adequate diet from separate sources of purified casein and various supplements, including gelatin (a source of two amino acids), a gelatin-creatine mixture (a source of three amino acids), and fiber (nonnutritive bulk). Nonselecting controls consumed the basal purified-casein diet or a supplemented purified-casein diet. Chicks in all selection conditions composed diets that yielded normal intake, normal body temperature, normal activity, and the maximum growth possible for their intake. They also selected components in the same percentages as in premixed diets. In all instances, their selection was nonrandom and regulated. What chicks included in their diet depended on what else was available. Although the specific percentage taken from each dietary component varied across different selection alternatives, these differences affected neither intake nor growth. Selection, per se, incurred a caloric cost. Chicks selecting from fractions of a corn-and-soy diet offset this cost by increasing intake compensatorily, but chicks with a purified-casein fraction did not, suggesting that some unspecified property of casein placed a ceiling on its intake. These findings unequivocally demonstrate that immature chicks not only can self-select nutritionally adequate diets, but can do so with unexpected precision by exploiting different but equally successful strategies.

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.

Learning and retention of conditioned aversions by freely feeding chicks

The present experiments assessed poison-based aversion learning and retention in freely feeding and drinking domestic chicks whose drinking water was colored blue and adulterated with LiCl for a 24-hr period. The amount of LiCl self-administered by 11-day-old chicks and their subsequent avoidance of unadulterated water of the same color was examined. The results of four experiments demonstrated that chicks self-administered large and often lethal doses of the LiCl solution. Chicks subsequently avoided blue water during two-bottle preference tests administered 3 to 7 days but not 14 days after exposure. These data indicate that neophobia alone is insufficient to prevent nondeprived chicks from ingesting large quantities of a toxin during their initial encounter with it. The lack of long-term retention in the present experiments indicates that naturally occurring aversions based on visual and illness cues, while effective in the short term, may not be a major factor in the choices made by freely feeding and drinking chicks over the long term.