Seven-month-old infants chunk items in memory

Although working memory has a highly constrained capacity limit of three or four items, both adults and toddlers can increase the total amount of stored information by "chunking" object representations in memory. To examine the developmental origins of chunking, we used a violation-of-expectation procedure to ask whether 7-month-old infants, whose working memory capacity is still maturing, also can chunk items in memory. In Experiment 1, we found that in the absence of chunking cues, infants failed to remember three identical hidden objects. In Experiments 2 and 3, we found that infants successfully remembered three hidden objects when provided with overlapping spatial and featural chunking cues. In Experiment 4, we found that infants did not chunk when provided with either spatial or featural chunking cues alone. Finally, in Experiment 5, we found that infants also failed to chunk when spatial and featural cues specified different chunks (i.e., were pitted against each other). Taken together, these results suggest that chunking is available before working memory capacity has matured but still may undergo important development over the first year of life.     

Infants recognize counting as numerically relevant

Children do not understand the meanings of count words like “two” and “three” until the preschool years. But even before knowing the meanings of these individual words, might they recognize that counting is “about” the dimension of number? Here in five experiments, we asked whether infants already associate counting with quantities. We measured 14‐ and 18‐month olds’ ability to remember different numbers of hidden objects that either were or were not counted by an experimenter before hiding. As in previous research, we found that infants failed to differentiate four hidden objects from two when the objects were not counted—suggesting an upper limit on the number of individual objects they could represent in working memory. However, infants succeeded when the objects were simply counted aloud before hiding. We found that counting also helped infants differentiate four hidden objects from six (a 2:3 ratio), but not three hidden objects from four (a 3:4 ratio), suggesting that counting helped infants represent the arrays’ approximate cardinalities. Hence counting directs infants’ attention to numerical aspects of the world, showing that they recognize counting as numerically relevant years before acquiring the meanings of number words.     

Parallel non-verbal enumeration is constrained by a set-based limit

Adults can represent approximate numbers of items independently of language. This approximate number system can discriminate and compare entities as varied as dots, sounds, or actions. But can multiple different types of entities be enumerated in parallel and stored as independent numerosities? Subjects who were prevented from verbally counting watched an experimenter hide sequences of objects in two locations. The number of object types, which contrasted in category membership, color, shape, and texture, varied from 1 to 5, and object types were completely temporally intermixed. Subjects were then asked how many objects of each type were in each location. In three experiments, subjects successfully enumerated the objects of each type in each location when 1-3 types were presented, but failed with 4 or 5 types, regardless of the total number of objects seen. Thus, adults can perform simultaneous enumeration of multiple sets that unfold in temporally intermixed fashion, but are limited to 3 such sets at a time. Furthermore, they perform these parallel enumerations in the absence of training or instruction, and can do so for sets of objects that are hidden in distinct locations. The convergence of this 3-set capacity limit with the 3-item capacity limit widely observed in studies of working memory suggests that each enumeration requires a single slot in memory, and that storage in memory is required before enumeration can occur.     

Infants’ coding of location in continuous space

The ability to code location in continuous space is fundamental to spatial behavior. Existing evidence indicates a robust ability for such coding by 12 months, but systematic evidence on earlier origins is lacking. A series of studies investigated 5-month-olds’ ability to code the location of an object hidden in a sandbox, using a looking-time paradigm. In Experiment 1, after familiarization with a hiding-and-finding sequence at one location, infants looked longer at an object being disclosed from a location 12 inches (30 cm) away than at an object emerging from the hiding location, showing they were able to code location in continuous space. In Experiment 2, infants reacted with greater looking when objects emerged from locations 8 inches (20 cm) away from the hiding location, showing that location coding was more finely grained than could be inferred based on the first study. In Experiment 3, infants were familiarized with an object shown in hiding-and-finding sequences at two different locations. Infants looked longer at objects emerging 12 inches (30 cm) away from the most recent hiding location than to emergence from the other location, showing that infants could code location even when events had previously occurred at each location. In Experiment 4, after familiarization with two objects with different shapes, colors, and sounding characteristics, shown in hiding-and-finding sequences in two locations, infants reacted to location violations as they had in Experiment 3. However, they did not react to object violations, that is, events in which the wrong object emerged from a hiding location. Experiment 5 also found no effect of object violation, even when the infants initially saw the two objects side by side. Spatiotemporal characteristics may play a more central role in early object individuation than they do later, although further study is required.     

On the limits of infants' quantification of small object arrays

Recent work suggests that infants rely on mechanisms of object-based attention and short-term memory to represent small numbers of objects. Such work shows that infants discriminate arrays containing 1, 2, or 3 objects, but fail with arrays greater than 3 [Feigenson, L., & Carey, S. (2003). Tracking individuals via object-files: Evidence from infants' manual search. Developmental Science, 6, 568-584; Feigenson, L., Carey, S., & Hauser, M. (2002). The representations underlying infants' choice of more: Object files versus analog magnitudes. Psychological Science, 13(2), 150-156]. However, little is known about how infants represent arrays exceeding the 3-item limit of parallel representation. We explored possible formats by which infants might represent a 4-object array. Experiment 1 used a manual search paradigm to show that infants successfully discriminated between arrays of 1 vs. 2, 2 vs. 3, and 1 vs. 3 objects. However, infants failed to discriminate 1 vs. 4 despite the highly discriminable ratio, providing the strongest evidence to date for object-file representations underlying performance in this task. Experiment 2 replicated this dramatic failure to discriminate 1 from 4 in a second paradigm, a cracker choice task. We then showed that infants in the choice task succeeded at choosing the larger quantity with 0 vs. 4 crackers and with 1 small vs. 4 large crackers. These results suggest that while infants failed to represent 4 as “exactly 4”, “approximately 4”, “3”, or as even as “a plurality”, they did represent information about the array, including the existence of a cracker or cracker-material and the size of the individual objects in the array.     

Early declarative memory for location

The abilities of 7.5-month-old infants to recall the location of hidden objects after delays averaging 90 seconds were investigated in three experiments. Various kinds of events were introduced during the delays in order to examine the stability of early location memory. Recall, as shown by reaching towards the correct location, was most clearly found when the infants were allowed either to remain seated facing the hiding locations (Expt 1) or were turned around and immediately re-seated (Expt 2) during the delays. In both experiments, the infants’ attention was diverted from the hiding places, but during all or most of the delay the infants were facing the locations. Recall was dampened when infants were removed from the immediate location of the hiding and engaged in other activities such as looking at a picture during the delay (Expt 3). Further analyses indicated an effect for age that coincides with other research on location memory: evidence for recall was more clearly found for the older, but not the younger, 7-month-olds.     

Infants hierarchically organize memory representations

Throughout development, working memory is subject to capacity limits that severely constrain short‐term storage. However, adults can massively expand the total amount of remembered information by grouping items into chunks. Although infants also have been shown to chunk objects in memory, little is known regarding the limits of this ability. In particular, it remains unknown whether infants can create more complex memory hierarchies, binding representations of chunks into still larger chunks in recursive fashion. Here we tested the limits of early chunking, first measuring the number of items infants can bind into a single chunk and the number of chunks infants can maintain concurrently, and then, critically, whether infants can embed chunked representations into larger units. We tested 14‐month‐old infants' memory for hidden objects using a manual search task in which we manipulated memory load (the number of objects infants saw hidden) and the chunking cues provided. We found that infants are limited in the number of items they can chunk and in the number of chunks they can remember. However, we also found that infants can bind representations of chunks into ‘superchunks’. These results suggest that hierarchically organizing information strongly affects working memory, starting in infancy.     

Tracking individuals via object-files: evidence from infants’ manual search

In two experiments, a manual search task explored 12- to 14-month-old infants’ representations of small sets of objects. In this paradigm, patterns of searching revealed the number of objects infants represented as hidden in an opaque box. In Experiment 1, we obtained the set-size signature of object-file representations: infants succeeded at representing precisely 1, precisely 2, and precisely 3 objects in the box, but failed at representing 4 (or even that 4 is greater than 2). In Experiment 2, we showed that infants’ expectations about the contents of the box were based on number of individual objects, and not on a continuous property such as total object volume. These findings support the hypothesis that infants maintained representations of individuals, that object-files were the underlying means of representing these individuals, and that object-file models can be compared via one-to-one correspondence to establish numerical equivalence.     

Infants' search for visible objects: implications for the interpretation of early search errors

This study examined the error patterns of 9-month-old infants searching for hidden objects and objects that were visible within a container. Although errors occurred in both conditions, there were important differences between them. When the object was hidden, infants showed significant perseveration in that they searched more often at the object's previous hiding place than at a control location. When the object was visible, however, they made fewer errors and the errors they did make were as likely to be to the control location as to the previous hiding place. These results suggest that infants' errors in searching for a visible object reflect lapses of attention rather than systematic misunderstandings of objects or space and so are not incompatible with an information-processing account of early search.     

Individuation of pairs of objects in infancy

Looking-time studies examined whether 11-month-old infants can individuate two pairs of objects using only shape information. In order to test individuation, the object pairs were presented sequentially. Infants were familiarized either with the sequential pairs, disk-triangle/disk-triangle (XY/XY), whose shapes differed within but not across pairs, or with the sequential pairs, disk-disk/triangle-triangle (XX/YY), whose shapes differed across but not within pairs. The XY/XY presentation looked to adults like a single pair of objects presented repeatedly, whereas the XX/YY presentation looked like different pairs of objects. Following familiarization to these displays, infants were given a series of test trials in which the screen was removed, revealing two pairs of objects in one of two outcomes, XYXY or XXYY. On the first test trial, infants familiarized with the identical pairs (XY/XY) apparently expected a single pair to be revealed because they looked longer than infants familiarized with the distinct pairs (XX/YY). Infants who had seen the distinct pairs apparently expected a double pair outcome. A second experiment showed outcomes of a single XY pair. This outcome is unexpected for XX/YY-familiarized infants but expected for XY/XY-familiarized infants, the reverse of Experiment 1. This time looking times were longer for XX/YY infants. Eleven-month-olds appear to be able to represent not just individual objects but also pairs of objects. These results suggest that if they can group the objects into sets, infants may be able to track more objects than their numerosity limit or available working memory slots would normally allow. We suggest possible small exact numerosity representations that would allow tracking of such sets.     

Developmental Differences in Preferences for Using Color,Size, and Location Information to Disambiguate Hiding Places

We conducted four experiments to examine developmental differences in preferences for using color, size, and location information to disambiguate hiding places. Three- and 4-year-olds and adults described how to find a miniature mouse that was hidden in one of two highly similar small objects in a dollhouse. In Experiment 1, the hiding places could be disambiguated by either color or location. Three-year-olds preferred color to location whereas adults preferred location to color information. Four-year-olds showed no preferences. In Experiment 2, the hiding places could be disambiguated by either size or location. Four-year-olds preferred size to location information whereas adults preferred location to size information. Three-year-olds showed no preferences. In Experiment 3, the hiding places could be disambiguated by either color or size information. Adults preferred size to color information, but 3- and 4-year-olds showed no preference for either type of information. Experiment 4 revealed that when only location information was available for disambiguating the hiding places, 4-year-olds referred to disambiguating location information on a significantly greater percentage of trials than did 3-year-olds. Discussion focuses on the role of relational complexity and pragmatic knowledge in producing preferences for disambiguating information in spatial communication tasks.     

THIS ARTICLE HAS BEEN RETRACTED: Enumeration of objects and substances in non‐human primates: experiments with brown lemurs (Eulemur fulvus)

Both human infants and adult non‐human primates share the capacity to track small numbers of objects across time and occlusion. The question now facing developmental and comparative psychologists is whether similar mechanisms give rise to this capacity across the two populations. Here, we explore whether non‐human primates’ object tracking abilities are subject to the same constraints as those of human infants. In particular, we examine whether one primate species, the brown lemur (Eulemur fulvus), also fails to represent and enumerate objects when they behave non‐rigidly or non‐cohesively. We presented lemurs with a series of expectancy violation studies involving simple 1 + 1 addition events in which we varied the entities to be enumerated. Like infants, lemurs successfully enumerated the two objects when those objects were rigid, cohesive individuals, but failed to enumerate similar‐looking non‐rigid piles of sand. In contrast to human infants, however, lemurs successfully enumerated non‐cohesive objects that broke into multiple pieces. These results are discussed in light of recent theories about object processing in human infants and adults.     

Objects are individuals but stuff doesn't count: perceived rigidity and cohesiveness influence infants' representations of small groups of discrete entities

Young infants construct models of the world composed of objects tracked through time and occlusion. To date little is known about the degree to which these models are sensitive to the material make-up of the represented individuals. Two experiments probed 8-month-olds' ability to represent different kinds of entities: rigid, cohesive objects, flexible, cohesive objects, and non-rigid, non-cohesive portions of sand. In Experiment 1, infants represented an array of two rigid, cohesive objects hidden behind a single screen, but failed to represent hidden arrays of two flexible objects or two portions of sand. In Experiment 2, entities were hidden behind two screens instead of one, thereby reducing the information processing demands of the task. In that case, infants succeeded in representing arrays of both types of object stimuli, but again failed to represent the portions of sand. It is argued that (1) the processes by which infants individuate and track entities are sensitive to material kind, (2) rigid cohesive objects occupy a privileged status in this system, and (3) early knowledge about objects and substances has a quantificational aspect.     

Dissociation between small and large numerosities in newborn infants

In the first year of life, infants possess two cognitive systems encoding numerical information: one for processing the numerosity of sets of 4 or more items, and the second for tracking up to 3 objects in parallel. While a previous study showed the former system to be already present a few hours after birth, it is unknown whether the latter system is functional at this age. Here, we adapt the auditory‐visual matching paradigm that previously revealed sensitivity to large numerosities to test sensitivity to numerosities spanning the range from 2 to 12. Across studies, newborns discriminated pairs of large numerosities in a 3:1 ratio, even when the smaller numerosity was 3 (3 vs. 9). In contrast, newborn infants failed to discriminate pairs including the numerosity 2, even at the same ratio (2 vs. 6). These findings mirror the dissociation that has been reported with older infants, albeit with a discontinuity situated between numerosities 2 and 3. Two alternative explanations are compatible with our results: either newborn infants have a separate system for processing small sets, and the capacity of this system is limited to 2 objects; or newborn infants possess only one system to represent numerosities, and this system either is not functional or is extremely imprecise when it is applied to small numerosities.     

Patterns of 4-month-old infant responses to hidden silent and sounding people and objects

Responses of 4-month-old infants to hidden people and objects were investigated with equated task demands. Twenty-one 4-month-old infants were administered a combined task, in which they were shown a sounding stimulus that continued to sound after hiding, an auditory task, in which sound was the only source of information about the position of the object in space, and a vision task, in which a silent stimulus was shown to the infants prior to hiding. Five infant behaviours were coded: reaching, gazing, body movements, vocalizations and smiles. The infants reached significantly more for hidden objects than for people, to whom they vocalized instead. They further smiled, and moved their bodies more towards their invisible mother than to the other stimuli. Thus infants responded differentially to people and objects whether the stimuli were soundless (so that there was no cue to their presence) or not. This suggested that infants appreciated (a) that an object had been hidden; (b) this object was either animate or inanimate; and (c) different procedures were appropriate for the retrieval of, or for interacting with animate and inanimate objects. Discussion centres on the underlying representational system that allows for such appreciation.     

学习材料组块方式对相似词长时记忆的影响

已有关于材料相似性影响短时记忆的研究提示, 不相似材料组块相比于相似材料组块可能促进记忆。为验证该假设, 该研究采用学习-测查范式, 通过4个实验考察了学习材料组块方式对相似词长时记忆的影响及机制。结果发现：1)与相似词组块相比, 不相似词组块促进了相似词记忆; 2)不相似词组块的促进效应是通过增强相似词表共同词根的记忆而实现的; 3)不相似词组块的促进效应可能依赖于语音相似性。该结果说明不相似词组块可能是促进相似词汇记忆的有效途径之一。     

What’s the object of object working memory in infancy? Unraveling ‘what’ and ‘how many’

Infants have a bandwidth-limited object working memory (WM) that can both individuate and identify objects in a scene, (answering ‘how many?’ or ‘what?’, respectively). Studies of infants’ WM for objects have typically looked for limits on either ‘how many’ or ‘what’, yielding different estimates of infant capacity. Infants can keep track of about three individuals (regardless of identity), but appear to be much more limited in the number of specific identities they can recall. Why are the limits on ‘how many’ and ‘what’ different? Are the limits entirely separate, do they interact, or are they simply two different aspects of the same underlying limit?We sought to unravel these limits in a series of experiments which tested 9- and 12-month-olds’ WM for object identities under varying degrees of difficulty. In a violation-of-expectation looking-time task, we hid objects one at a time behind separate screens, and then probed infants’ WM for the shape identity of the penultimate object in the sequence. We manipulated the difficulty of the task by varying both the number of objects in hiding locations and the number of means by which infants could detect a shape change to the probed object. We found that 9-month-olds’ WM for identities was limited by the number of hiding locations: when the probed object was one of two objects hidden (one in each of two locations), 9-month-olds succeeded, and they did so even though they were given only one means to detect the change. However, when the probed object was one of three objects hidden (one in each of three locations), they failed, even when they were given two means to detect the shape change. Twelve-month-olds, by contrast, succeeded at the most difficult task level.Results show that WM for ‘how many’ and for ‘what’ are not entirely separate. Individuated objects are tracked relatively cheaply. Maintaining bindings between indexed objects and identifying featural information incurs a greater attentional/memory cost. This cost reduces with development. We conclude that infant WM supports a small number of featureless object representations that index the current locations of objects. These can have featural information bound to them, but only at substantial cost.     

Infant perseveration and implications for object permanence theories: A PDP model of the AB task

From the earliest ages at which infants search for hidden objects, they make the AB error, searching perseveratively at previous rather than current hiding locations (Piaget, 1954). This paper presents a parallel distributed processing (PDP) model that instantiates an explicit set of processing mechanisms to account for a large and diverse set of data on infants’ AB errors. The model demonstrates how basic processes–the formation of latent memory traces and their interaction with developing active memory traces–can provide a unifying framework for understanding why and when infants perseverate. Novel predictions from the model are discussed, together with its challenges for theories that posit a concept of object permanence in the first year of life.     

Reasoning about a hidden object after a delay: evidence for robust representations in 5-month-old infants

The present research examined two alternative interpretations of violation-of-expectation findings that young infants can represent hidden objects. One interpretation is that, when watching an event in which an object becomes hidden behind another object, infants form a prediction about the event's outcome while both objects are still visible, and then check whether this prediction was accurate. The other interpretation is that infants' initial representations of hidden objects are weak and short-lived and as such sufficient for success in most violation-of-expectation tasks (as objects are typically hidden for only a few seconds at a time), but not more challenging tasks. Five-month-old infants succeeded in reasoning about the interaction of a visible and a hidden object even though (1) the two objects were never simultaneously visible, and (2) a 3- or 4-min delay preceded the test trials. These results provide evidence for robust representations of hidden objects in young infants.     

What do infants remember when they forget? Location and identity in 6-month-olds' memory for objects

What does an infant remember about a forgotten object? Although at age 6 months, infants can keep track of up to three hidden objects, they can remember the featural identity of only one. When infants forget the identity of an object, do they forget the object entirely, or do they retain an inkling of it? In a looking-time study, we familiarized 6-month-olds with a disk and a triangle placed on opposite sides of a stage. During test trials, we hid the objects one at a time behind different screens, and after hiding the second object, we removed the screen where the first object had been hidden. Infants then saw the expected object, the unexpected other object, or the empty stage. Bayes factor analysis showed that although the infants did not notice when the object changed shape, they were surprised when it vanished. This finding indicates that infants can represent an object without its features.