Mechanisms of Categorization in Infancy

This article presents a connectionist model of correlation‐based categorization by 10‐month‐old infants (Younger, 1985). Simple autoencoder networks were exposed to the same stimuli used to test 10‐month‐olds. The familiarization regime was kept as close as possible to that used with the infants. The performance of the model matched that of the infants. Both infants and networks used covariation information (when available) to segregate items into separate categories. The model provides a mechanistic account of category learning within a test session. It demonstrates how categorization arises as the product of an inextricable interaction between the subject (the infant) and the environment (the stimuli). The computational characteristics of both subject and environment must be considered in conjunction to understand the observed behaviors.     

A Comparison of Infants' Categorization in Paired and Successive Presentation Familiarization Tasks

Three experiments directly compared infants' categorization in variations of the visual familiarization task. In each experiment, 4‐ or 6‐month‐old infants were familiarized with a collection of dogs or cats and then their response to novel dogs and cats was assessed. In Experiment 1, 4‐month‐old infants responded to the exclusive distinction of dogs or cats when tested in a paired‐comparison task. In Experiments 2 and 3, 6‐month‐old infants, but not 4‐month‐old infants, responded to this same distinction in a successive presentation task, even when the amount of familiarization was equated to that of the paired comparison task. Therefore, familiarization with a particular set of stimuli does not induce infants to respond to a single category but rather they respond to different categories depending on features of the task.     

The Cat is out of the Bag: The Joint Influence of Previous Experience and Looking Behavior on Infant Categorization

We examined the effect of 4‐month‐old infants' previous experience with dogs, cats, or both and their online looking behavior on their learning of the adult‐defined category of cat in a visual familiarization task. Four‐month‐old infants' (N = 123) learning in the laboratory was jointly determined by whether or not they had experience with pets at home and how much they shifted their gaze back and forth between the stimuli during familiarization. Specifically, only infants with pets at home who also exhibited high levels of switching during familiarization remembered the individual cat exemplars or formed a summary representation of those cats. These results are consistent with recent theorizing about the processes of how infants' categorical representations are formed, and provide new understanding into how infants' categorization unfolds over time.     

Object Representation in Infants' Coordination of Manipulative Force

This study examined infants' use of object knowledge for scaling the manipulative force of object‐directed actions. Infants 9, 12, and 15 months of age were outfitted with motion‐analysis sensors on their arms and then presented with stimulus objects to examine individually over a series of familiarization trials. Two stimulus objects were used in the familiarization phase, and were identical in size, shape, and material, but different in color and weight. Following familiarization, two test objects that had been hidden from view were presented. The test objects were identical in appearance to the familiarization objects, but their color‐weight correspondence was reversed. Infants' actions on the test objects revealed selective, differential preparation for the specific weights experienced during familiarization. Because the objects were equivalent in their visual affordances for action, the differential preparation and coordination of manipulative force was based on knowledge acquired during the familiarization phase. Infants are capable of utilizing object representations to coordinate manipulative force in object‐directed actions.     

Object Function and Categorization in Infancy: Two Mechanisms of Facilitation

Does function facilitate categorization by focusing infants' attention generally on all commonalities among objects or specifically on functionally relevant properties? After familiarization to a novel category, 18‐month‐olds selected another category member from a pair of previously unseen test objects. In Experiments 1 and 2, infants chose globally similar over functionally similar and novel test objects. Functionally similar and novel test objects were chosen equally. These data suggest that function facilitates categorization through a general attention‐enhancing mechanism. However, when functions were more uniquely and transparently tied to object properties in Experiments 3 and 4, infants chose functionally similar over novel test objects. Globally and functionally similar test objects were chosen equally. Therefore, a specific attention‐enhancing mechanism also sometimes supports categorization.     

Are You an Animal Too? US and Malawian Infants' Categorization of Plastic and Wooden Animal Replicas

In this study, we examined the effects of infant country and exemplar material on 24 US and 22 Malawian (African) 15‐month‐olds' categorization of animals versus vehicles. Following familiarization with either plastic or wooden animal replicas, infants were tested with objects of both materials in a standard object‐examining task. Both US and Malawian infants demonstrated category formation regardless of the material of the animal replicas. In addition, infants extended a category of plastic animals to novel wooden animals, but did not extend a category of wooden animals to novel plastic animals. These findings document a uniform impact of stimuli characteristics on infant object categorization despite differences in infant cultural background and toy animal experience. In addition, they show that, in some cases, infants can generalize their categorization of animals from one type of replica to another.     

Contributions of attentional style and previous experience to 4-month-old infants' categorization

We examined how infants' categorization is jointly influenced by previous experience and how much they shift their gaze back-and-forth between stimuli. Extending previous findings reported by Kovack-Lesh, Horst, and Oakes (2008), we found that 4-month-old infants' (N = 122) learning of the exclusive category of cats was related to whether they had cats at home and how much they shifted attention between two available stimuli during familiarization. Individual differences in attention assessed in an unrelated task were not related to their categorization. Thus, infants' learning is multiply influenced by past experience and on-line attentional style.     

Global‐Before‐Basic Object Categorization in Connectionist Networks and 2‐Month‐Old Infants

A 3‐layered backpropagation connectionist network, configured as an autoassociator, learned to form global (e.g., mammal) before basic‐level (e.g., cat) category representations from perceptual input. To test the predicted global‐to‐basic order of category learning of the network, 2‐month‐olds were administered the familiarization/novelty‐preference procedure and examined for representation of global and basic‐level categories. Infants formed a global category representation for mammals that excluded furniture but not a basic‐level representation for cats that excluded elephants, rabbits, or dogs. The empirical results are consistent with the global‐to‐basic learning sequence observed in the network simulations.     

Infants' Looking at Possible and Impossible Screen Rotations: The Role of Familiarization

The effects of familiarization and age were examined using Baillargeon's rotating screen paradigm. In Condition A, 4‐month‐olds exposed to 7 180° familiarization trials looked significantly longer at the 180° test events than the 112° test events; there was a familiarity preference. In Condition B, which consisted of 12 instead of 7 180° familiarization trials, the 4‐month‐olds looked significantly longer at the 112° test events than the 180° test events; there was a novelty preference. In Condition C, which was similar to Condition A except that there were 112° familiarization trials, the infants showed a familiarity preference. Thus, 4‐month‐olds experiencing 7 familiarization trials exhibited a familiarity preference, and those experiencing 12 familiarization trials showed a novelty preference. In Condition D, 6‐month‐olds experienced 7 180° familiarization trials; there was no preferential looking to either familiar (180°) test events or novel (112°) test events. Therefore, looking behavior during the test trials was a function of the type of familiarization experience and age and not necessarily an inferred violation of physics.     

Infant Preferences for Structural and Prosodic Properties of Infant‐Directed Speech in the Second Year of Life

While a large literature discusses young infants' preference for an infant‐directed speaking style, few studies have explored preferences after the first year. The present work compares infants' preference for two different properties of IDS speech: prosodic changes (primarily pitch and pitch variability) and structural properties (utterance length; lexical repetition). We found that both 12‐ and 16‐month‐old infants continued to prefer listening to speech with the prosodic properties of IDS, but neither age showed any preference for speech with the lexical repetition and short utterances typical of IDS.     

Olfactory Familiarization and Discrimination in Preterm and Full‐Term Newborns

The purpose of this study was to observe olfactory detection and discrimination in preterm and full‐term newborns. Infants were familiarized for 10 trials with either vanillin or anethole. On each trial, a cotton swab perfumed with one of the odors was slowly moved in front of the baby's nose for 10 sec. For half of the preterm and full‐term infants, a new odor was presented after the last familiarization trial (experimental groups). For the other half, the same odor as during familiarization was presented (control groups). Facial and head movements for both populations and heart rates for preterm infants were recorded before, during, and after odor presentation. Preterm infants reacted to the scents by increasing facial actions and heart rate but not head movements. Full‐term infants increased facial and head movements. Neither population showed a clear behavioral habituation pattern, but full‐term newborns had a significantly reduced facial reactivity on the last familiarization trial compared to preterm infants. Preterm newborns did, however, show cardiac habituation on the last familiarization trial. Preterm and full‐term infants presented with a new odor after familiarization increased responding compared to infants presented with the same odor, indicating their ability to discriminate between 2 odors. Infants' reactivity and discrimination to odors indicate preterm and full‐term newborns' ability to be attuned to their olfactory environment.     

Categorical Representation of Facial Expressions in the Infant Brain

Categorical perception, demonstrated as reduced discrimination of within‐category relative to between‐category differences in stimuli, has been found in a variety of perceptual domains in adults. To examine the development of categorical perception in the domain of facial expression processing, we used behavioral and event‐related potential (ERP) methods to assess discrimination of within‐category (happy‐happy) and between‐category (happy‐sad) differences in facial expressions in 7‐month‐old infants. Data from a visual paired‐comparison test and recordings of attention‐sensitive ERPs showed no discrimination of facial expressions in the within‐category condition, whereas reliable discrimination was observed in the between‐category condition. The results also showed that face‐sensitive ERPs over occipital‐temporal scalp (P400) were attenuated in the within‐category condition relative to the between‐category condition, suggesting a potential neural basis for the reduced within‐category sensitivity. Together, these results suggest that the neural systems underlying categorical representation of facial expressions emerge during the early stages of postnatal development, before acquisition of language.     

Newborns' Perceptual Categorization for Closed and Open Geometric Forms

This study explored newborns' ability to perceive perceptual similarities between different exemplars of 2 broad classes of simple shapes: closed and open geometric forms. Three experiments were carried out using a visual paired‐comparison task. Evidence showed that, after familiarization either to closed‐shaped or to open‐shaped forms, newborns manifested a novelty preference for a novel‐category rather than for a familiar‐category exemplar (Experiment 1). This result could not be explained either as a consequence of the newborns' inability to discriminate between instances of the same category of simple geometric forms (Experiment 2), or as a consequence of a spontaneous preference for the novel‐category exemplars (Experiment 3). Overall, findings revealed that newborns are able to form broad categories of distinguishable geometric shapes by relying on the shapes' perceptual similarity.     

Individual Differences in Infants' Recognition of Briefly Presented Visual Stimuli

Infants' recognition memory has been shown to be related to individual differences in look duration and level of heart period variability. This study examined the effect of individual differences in these 2 measures on infants' recognition of briefly presented visual stimuli using a paired‐comparison recognition‐memory paradigm. A sample of 35 full‐term infants was studied longitudinally at 14, 20, and 26 weeks of age. Recognition memory for briefly presented stimuli was tested in 6 experimental conditions, with delays corresponding to different heart‐rate‐defined phases of attention. The 20‐and 26‐week‐old infants, and infants with high levels of heart period variability, generally showed more evidence of recognition memory for briefly presented visual stimuli. Greater evidence of recognition memory was observed when stimuli were presented during sustained attention. Infants with more mature baseline physiological responses show greater evidence of recognition memory, and stimulus and procedural factors may be more important for the study of individual differences in infant visual attention than has previously been suggested.     

Infants' Listening Preferences: Baby Talk or Happy Talk?

The most robust finding on infants' listening preferences has been widely characterized as a preference for baby talk (BT) over adult‐directed speech (ADS). Although prosodic modifications characteristic of BT also convey positive affect, differences in affect across BT and ADS speech registers have not been controlled in previous studies. This set of experiments sought to elucidate the basis for 6‐month‐olds' listening preference by independently manipulating affect and speech register. When affect was held constant, no preference for any speech register was observed. Moreover, when ADS stimuli presented more positive affect than BT stimuli, infants' preferences followed the positive affect. Higher and more variable pitch was neither necessary nor sufficient for determining infants' preferences, although pitch characteristics may modulate affect‐based preferences. The BT preference is thus attributable to a more general preference for speech that imparts relatively positive affect, a preference perhaps ascribable to a preexisting general‐purpose mechanism opportunistically exploited by language.     

The Influence of Labels,Non‐Labeling Sounds,and Source of Auditory Input on 9‐ and 15‐Month‐Olds' Object Categorization

This experiment examines the joint influence of auditory and social cues on infants' basic‐level and global categorization. Nine‐ and fifteen‐month‐olds were familiarized to a series of category exemplars in an object‐examining task. Objects were introduced with a labeling phrase, a non‐labeling sound, or no sound, and auditory input was presented orally by the experimenter or played on a hidden voice recorder. Novel objects from the familiarized category and a contrasting category were then presented. Results of analyses performed on novelty preference scores indicated that infants demonstrated basic‐level categorization in all conditions. However, infants at both age levels only demonstrated global categorization when labeling phrases were introduced. In addition, labels led to global categorization in 9‐month‐olds regardless of the source of those labels; however, labels only led to global categorization in 15‐month‐olds when the labels were presented orally by the experimenter.     

A Comparison of Visual Familiarization and Object‐Examining Measures of Categorization in 9‐Month‐Old Infants

Three studies were conducted to determine whether differential patterns of categorization observed in studies using visual familiarization and object‐examining measures hold up as procedural confounds are eliminated. In Experiment 1, we attempted as direct a comparison as possible between visual and object‐examining measures of categorization. Consistent with previous reports, 9‐month‐old infants distinguished a basic‐level contrast (dog–horse) in the visual task, but not in the examining task. Experiment 2 was designed to reduce levels of nonexploratory activity in an examining task; 9‐month‐olds again failed to distinguish categories of dogs and horses. In Experiment 3, we adopted a paired‐comparison test format in the object‐examining task. Infants did display a novel category preference under paired testing conditions. The results suggest that the different patterns of categorization often seen in looking and touching tasks are a reflection, not of different categorization processes, but of the differential sensitivity of the tasks.     

Relations Among Spontaneous Preferences,Familiarized Preferences,and Novelty Effects: Measurements With Forced‐Choice Techniques

We here describe a discrete trial, forced‐choice, combined spontaneous preference and novelty preference technique. In this technique, spontaneous preferences and familiarized (postfamiliarization) preferences are measured with the same stimulus pairs under closely parallel conditions. A variety of systematic stimulus variations were used in 16‐week‐old infants to explore the interrelations among spontaneous preferences, familiarized preferences, and familiarization (novelty) effects. Infants were exposed to pairs of 10° red and blue disks of varying colorimetric purity generated on a video monitor. Pairs of disks were identified for which spontaneous preferences were balanced at about 50–50 or unbalanced at about 75–25, and the magnitudes of familiarized preferences were determined. When spontaneous preferences were balanced at 50–50, novelty effects increased with increasing chromatic separation between the 2 stimuli, showing the independence of these variables. When spontaneous preferences were unbalanced, novelty effects were asymmetrical, being large after familiarization to the spontaneously preferred stimulus, but small or nonexistent after familiarization to the spontaneously nonpreferred stimulus. The potential uses of combined spontaneous preference and novelty preference techniques are discussed.     

Prosodic levelling during language shift: Okinawan approximations of Japanese pitch‐accent1

This article demonstrates how the language death process potentially contributes to the emergence of a new dialect. A sociolinguistic survey of the prosodic phonology of Japanese spoken in Yaeyama, Okinawa shows that the obsolescence process of Okinawan has resulted in a new dialect of Japanese. First, Okinawan is undergoing processes typical of language death, including increased variability and the elimination of phonological contrasts at the prosodic level. These processes are also occurring in the incoming language, Japanese. Second, these processes have influenced the acquisition of the prosody of Standard Japanese, resulting in a new prosodic system. Third, a positive attitude towards the local variant of Japanese is prevalent among the younger members of the community, particularly the working‐class males. These results suggest that a local prosodic system is emerging, resulting in a new dialect of Japanese.     

Six‐month‐old infants recognize phrases in song and speech

Infants exploit acoustic boundaries to perceptually organize phrases in speech. This prosodic parsing ability is well‐attested and is a cornerstone to the development of speech perception and grammar. However, infants also receive linguistic input in child songs. This study provides evidence that infants parse songs into meaningful phrasal units and replicates previous research for speech. Six‐month‐old Dutch infants (n = 80) were tested in the song or speech modality in the head‐turn preference procedure. First, infants were familiarized to two versions of the same word sequence: One version represented a well‐formed unit, and the other contained a phrase boundary halfway through. At test, infants were presented two passages, each containing one version of the familiarized sequence. The results for speech replicated the previously observed preference for the passage containing the well‐formed sequence, but only in a more fine‐grained analysis. The preference for well‐formed phrases was also observed in the song modality, indicating that infants recognize phrase structure in song. There were acoustic differences between stimuli of the current and previous studies, suggesting that infants are flexible in their processing of boundary cues while also providing a possible explanation for differences in effect sizes.