Consistent responses of human mothers to prelinguistic infants: the effect of prelinguistic repertoire size

Curiosity constructs communicative competence through social feedback loops

One of the most important challenges for a developing infant is learning how best to allocate their attention and forage for information in the midst of a great deal of novel stimulation. We propose that infants of altricial species solve this challenge by learning selectively from events that are contingent on their immature behavior, such as babbling. Such a contingency filter would focus attention and learning on the behavior of social partners, because social behavior reliably fits infants' sensitivity to contingency. In this way a contingent response by a caregiver to an immature behavior becomes a source of learnable information - feedback - to the infant. Social interactions with responsive caregivers afford infants opportunities to explore the impacts of their immature behavior on their environment, which facilitates the development of socially guided learning. Furthermore, contingent interactions are opportunities to make and test predictions about the efficacy of their social behaviors and those of others. In this chapter, we will use prelinguistic vocal learning to exemplify how infants use their developing vocal abilities to elicit learnable information about language from their social partners. Specifically, we review how caregivers' contingent responses to babbling create information that facilitates infant vocal learning and drives the development of communication. Infants play an active role in this process, as their developing predictions about the consequences of their actions serve to further refine their allocation of attention and drive increases in the maturity of their vocal behavior.

Learning how to learn from social feedback: The origins of early vocal development

Infants' prelinguistic vocalizations reliably organize vocal turn-taking with social partners, creating opportunities for learning to produce the sound patterns of the ambient language. This social feedback loop supporting early vocal learning is well-documented, but its developmental origins have yet to be addressed. When do infants learn that their non-cry vocalizations influence others? To test developmental changes in infant vocal learning, we assessed the vocalizations of 2- and 5-month-old infants in a still-face interaction with an unfamiliar adult. During the still-face, infants who have learned the social efficacy of vocalizing increase their babbling rate. In addition, to assess the expectations for social responsiveness that infants build from their everyday experience, we recorded caregiver responsiveness to their infants' vocalizations during unstructured play. During the still-face, only 5-month-old infants showed an increase in vocalizing (a vocal extinction burst) indicating that they had learned to expect adult responses to their vocalizations. Caregiver responsiveness predicted the magnitude of the vocal extinction burst for 5-month-olds. Because 5-month-olds show a vocal extinction burst with unfamiliar adults, they must have generalized the social efficacy of their vocalizations beyond their familiar caregiver. Caregiver responsiveness to infant vocalizations during unstructured play was similar for 2- and 5-month-olds. Infants thus learn the social efficacy of their vocalizations between 2 and 5 months of age. During this time, infants build associations between their own non-cry sounds and the reactions of adults, which allows learning of the instrumental value of vocalizing.

The many functions of vocal learning

The capacity to learn novel vocalizations has evolved convergently in a wide range of species. Courtship songs of male birds or whales are often treated as prototypical examples, implying a sexually selected context for the evolution of this ability. However, functions of learned vocalizations in different species are far more diverse than courtship, spanning a range of socio-positive contexts from individual identification, social cohesion, or advertising pair bonds, as well as agonistic contexts such as territorial defence, deceptive alarm calling or luring prey. Here, we survey the diverse usages and proposed functions of learned novel signals, to build a framework for considering the evolution of vocal learning capacities that extends beyond sexual selection. For each function that can be identified for learned signals, we provide examples of species using unlearned signals to accomplish the same goals. We use such comparisons to generate hypotheses concerning when vocal learning is adaptive, given a particular suite of socio-ecological traits. Finally, we identify areas of uncertainty where improved understanding would allow us to better test these hypotheses. Considering the broad range of potential functions of vocal learning will yield a richer appreciation of its evolution than a narrow focus on a few prototypical species. This article is part of the theme issue 'Vocal learning in animals and humans'.

Filling in the gaps: Acoustic gradation increases in the vocal ontogeny of chimpanzees (Pan troglodytes)

Human vocal ontogeny is considered to be a process whereby a large repertoire of discrete sounds seemingly emerges from a smaller number of acoustically graded vocalizations. While adult chimpanzee vocal behavior is highly graded, its developmental trajectory is poorly understood. In the present study, we therefore examined the size and structure of the chimpanzee vocal repertoire at different stages of ontogeny. Audio recordings were collected on infant (N = 13) and juvenile (N = 13) semi-wild chimpanzees at Chimfunshi Wildlife Orphanage, Zambia, using focal and ad libitum sampling. All observed call types were acoustically measured. These were predominantly grunts, whimpers, laughs, screams, hoos, and barks and squeaks. A range of spectral and temporal acoustic parameters were extracted, and fuzzy c-means clustering was used to quantify the size and structure of the repertoire. The infant and juvenile vocal repertoires were both best described with the same number of clusters. However, compared to infants, juvenile call clusters were less distinct from one another and could be extracted only when a low level of overlap between call clusters was permitted. Moreover, the acoustic overlap between call clusters was significantly higher for juveniles. Overall, this pattern shows greater acoustic overlap in juvenile vocalizations compared to infants, suggesting a trend toward increased acoustic gradation in chimpanzee vocal ontogeny. This may imply in contrast to humans, chimpanzees become increasingly proficient in using graded signals effectively rather than developing a larger repertoire of more discrete sounds in ontogeny.

Toward a speech-motor account of the effect of Age of Pacifier Withdrawal

Recently published works suggest that prolonged pacifier use affects abstract word processing later in life. Using the pacifier beyond age 3 years affects the conceptual relations used to define the meaning of words at age 6 years. Similarly, when semantically categorizing a set of abstract, concrete and emotional words, children aged 8 years who used the pacifier for a longer period of time were slower to process abstract stimuli, but not concrete and emotional ones. The children of these studies had typical development and no diagnosis of cognitive or linguistic disorders. These results, although correlational, suggest a possible relationship between extended use of a pacifier and the development of language skills, particularly for abstract words. The first goal of this theoretical work is to outline current evidence suggesting an association between prolonged pacifier use and the processing of abstract words. The second goal is to propose an account of the effect of Age of Pacifier Withdrawal (APW) within the DIVA neurocomputational model of speech development and production (Guenther & Vladusich, 2013). Using the pacifier during social interaction for a longer period might impede the processing of proprioceptive information and speech-motor programs (i.e., by limiting the co-articulation of speech, it could inhibit the building and consolidation of speech-motor articulatory gestures) as well as auditory input (because the child receives inaccurate input about his/her own speech). Thus, it seems useful to explore the auditory speech representation of children who use a pacifier beyond 3 years of age. We suggest that after the first year of life pacifier use should be impeded during social interaction and limited to use at bed time.

Adult responses to infant prelinguistic vocalizations are associated with infant vocabulary: A home observation study

This study used LENA recording devices to capture infants' home language environments and examine how qualitative differences in adult responding to infant vocalizations related to infant vocabulary. Infant-directed speech and infant vocalizations were coded in samples taken from daylong home audio recordings of 13-month-old infants. Infant speech-related vocalizations were identified and coded as either canonical or non-canonical. Infant-directed adult speech was identified and classified into different pragmatic types. Multiple regressions examined the relation between adult responsiveness, imitating, recasting, and expanding and infant canonical and non-canonical vocalizations with caregiver-reported infant receptive and productive vocabulary. An interaction between adult like-sound responding (i.e., the total number of imitations, recasts, and expansions) and infant canonical vocalizations indicated that infants who produced more canonical vocalizations and received more adult like-sound responses had higher productive vocabularies. When sequences were analyzed, infant canonical vocalizations that preceded and followed adult recasts and expansions were positively associated with infant productive vocabulary. These findings provide insights into how infant-adult vocal exchanges are related to early vocabulary development.

Development of adaptive communication skills in infants of blind parents

A fundamental question about the development of communication behavior in early life is how infants acquire adaptive communication behavior that is well-suited to their individual social environment, and how the experience of parent-child communication affects this development. The current study investigated how infants develop communication skills when their parents are visually impaired and cannot see their infants’ eye gaze. We analyzed 6-min video recordings of naturalistic interaction between 14 sighted infants of blind parents (SIBP) with (a) their blind parent, and (b) a sighted experimenter. Data coded from these interactions were compared with those from 28 age-matched sighted infants of sighted parents (controls). Each infant completed two visits, at 6–10 months and 12–16 months of age. Within each interaction sample, we coded the function (initiation or response) and form (face gaze, vocalization, or action) of each infant communication behavior. When interacting with their parents, SIBP made relatively more communicative responses than initiations, and used more face gaze and fewer actions to communicate, than did controls. When interacting with a sighted experimenter, by contrast, SIBP made slightly (but significantly) more communicative initiations than controls, but otherwise used similar forms of communication. The differential communication behavior by infants of blind versus sighted parents was already apparent by 6–10 months of age, and was specific to communication with the parent. These results highlight the flexibility in the early development of human communication behavior, which enables infants to optimize their communicative bids and methods to their unique social environment.

The social functions of babbling: acoustic and contextual characteristics that facilitate maternal responsiveness

What is the social function of babbling? An important function of prelinguistic vocalizing may be to elicit parental behavior in ways that facilitate the infant's own learning about speech and language. Infants use parental feedback to their babbling to learn new vocal forms, but the microstructure of parental responses to babbling has not been studied. To enable precise manipulation of the proximal infant cues that may influence maternal behavior, we used a playback paradigm to assess mothers' responsiveness to prerecorded audiovisual clips of unfamiliar infants' noncry prelinguistic vocalizations and actions. Acoustic characteristics and directedness of vocalizations were manipulated to test their efficacy in structuring social interactions. We also compared maternal responsiveness in the playback paradigm and in free play with their own infants. Maternal patterns of reactions to babbling were stable across both tasks. In the playback task, we found specific vocal cues, such as the degree of resonance and the transition timing of consonant-vowel syllables, predicted contingent maternal responding. Vocalizations directed at objects also facilitated increased responsiveness. The responses mothers exhibited, such as sensitive speech and vocal imitation, are known to facilitate vocal learning and development. Infants, by influencing the behavior of their caregivers with their babbling, create social interactions that facilitate their own communicative development.

Early life manipulations of vasopressin-family peptides alter vocal learning

Vocal learning from social partners is crucial for the successful development of communication in a wide range of species. Social interactions organize attention and enhance motivation to learn species-typical behaviour. However, the neurobiological mechanisms connecting social motivation and vocal learning are unknown. Using zebra finches (Taeniopygia guttata), a ubiquitous model for vocal learning, we show that manipulations of nonapeptide hormones in the vasopressin family (arginine vasotocin, AVT) early in development can promote or disrupt both song and social motivation. Young male zebra finches, like human infants, are socially gregarious and require interactive feedback from adult tutors to learn mature vocal forms. To investigate the role of social motivational mechanisms in song learning, in two studies, we injected hatchling males with AVT or Manning compound (MC, a nonapeptide receptor antagonist) on days 2-8 post-hatching and recorded song at maturity. In both studies, MC males produced a worse match to tutor song than controls. In study 2, which experimentally controlled for tutor and genetic factors, AVT males also learned song significantly better compared with controls. Furthermore, song similarity correlated with several measures of social motivation throughout development. These findings provide the first evidence that nonapeptides are critical to the development of vocal learning.

Caregiver communication to the child as moderator and mediator of genes for language

Human language appears to be unique among natural communication systems, and such uniqueness impinges on both nature and nurture. Human babies are endowed with cognitive abilities that predispose them to learn language, and this process cannot operate in an impoverished environment. To be effectively complete the acquisition of human language in human children requires highly socialised forms of learning, scaffolded over years of prolonged and intense caretaker-child interactions. How genes and environment operate in shaping language is unknown. These two components have traditionally been considered as independent, and often pitted against each other in terms of the nature versus nurture debate. This perspective article considers how innate abilities and experience might instead work together. In particular, it envisages potential scenarios for research, in which early caregiver verbal and non-verbal attachment practices may mediate or moderate the expression of human genetic systems for language.

Listeners can extract meaning from non-linguistic infant vocalisations cross-culturally

We present empirical evidence showing that the acoustic properties of non-linguistic vocalisations produced by human infants in different cultures can be used cross-culturally by listeners to make inferences about the infant’s current behaviour. We recorded natural infant vocalisations in Scotland and Uganda in five social contexts; declarative pointing, giving an object, requesting an action, protesting, and requesting food. Using a playback paradigm, we tested parents and non-parents, who either had regular or no experience with young children, from Scotland and Uganda in their ability to match infant vocalisations of both cultures to their respective production contexts. All participants performed above chance, regardless of prior experience with infants or cultural background, with only minor differences between participant groups. Results suggest that acoustic variations in non-linguistic infant vocalisations transmit broad classes of information to listeners, even in the absence of additional cues from gesture or context, and that these cues may reflect universal properties similar to the ‘referential’ information discovered in non-human primate vocalisations.

Multiple Coordination Patterns in Infant and Adult Vocalizations

The study of vocal coordination between infants and adults has led to important insights into the development of social, cognitive, emotional and linguistic abilities. We used an automatic system to identify vocalizations produced by infants and adults over the course of the day for fifteen infants studied longitudinally during the first two years of life. We measured three different types of vocal coordination: coincidence-based, rate-based, and cluster-based. Coincidence-based and rate-based coordination are established measures in the developmental literature. Cluster-based coordination is new and measures the strength of matching in the degree to which vocalization events occur in hierarchically nested clusters. We investigated whether various coordination patterns differ as a function of vocalization type, whether different coordination patterns provide unique information about the dynamics of vocal interaction, and how the various coordination patterns each relate to infant age. All vocal coordination patterns displayed greater coordination for infant speech-related vocalizations, adults adapted the hierarchical clustering of their vocalizations to match that of infants, and each of the three coordination patterns had unique associations with infant age. Altogether, our results indicate that vocal coordination between infants and adults is multifaceted, suggesting a complex relationship between vocal coordination and the development of vocal communication.

Language Development in the First Year of Life: What Deaf Children Might Be Missing Before Cochlear Implantation

OBJECTIVES

Language development is a multifaceted, dynamic process involving the discovery of complex patterns, and the refinement of native language competencies in the context of communicative interactions. This process is already advanced by the end of the first year of life for hearing children, but prelingually deaf children who initially lack a language model may miss critical experiences during this early window. The purpose of this review is twofold. First, we examine the published literature on language development during the first 12 months in typically developing children. Second, we use this literature to inform our understanding of the language outcomes of prelingually deaf children who receive cochlear implants (CIs), and therefore language input, either before or after the first year.

CONCLUSIONS

During the first 12 months, typically developing infants exhibit advances in speech segmentation, word learning, syntax acquisition, and communication, both verbal and nonverbal. Infants and their caregivers coconstruct a communication foundation during this time, supporting continued language growth. The language outcomes of hearing children are robustly predicted by their experiences and acquired competencies during the first year; yet these predictive links are absent among prelingually deaf infants lacking a language model (i.e., those without exposure to sign). For deaf infants who receive a CI, implantation timing is crucial. Children receiving CIs before 12 months frequently catch up with their typically developing peers, whereas those receiving CIs later do not. Explanations for the language difficulties of late-implanted children are discussed.

Mothers provide differential feedback to infants' prelinguistic sounds

Few studies have focused on mechanisms of developmental change during the prelinguistic period. The lack of focus on early vocal development is surprising given that maternal responsiveness to infants during the first two years has been found to influence later language development. In addition, in a variety of species, social feedback is essential for vocal development. Previous research demonstrated that maternal feedback to prelinguistic vocalizations influenced the production of more developmentally advanced vocalizations, suggesting that effects of maternal responsiveness on vocal development may start during the prelinguistic phase; however, because mothers were instructed how and when to respond to their infants' vocalizations, the timing and type of typical maternal feedback is unknown. In the present study, we analyzed unstructured play sessions for 10 mother–infant dyads to explore the relationship between prelinguistic vocal production and maternal responsiveness. Mothers responded contingently to prelinguistic vocalizations over 70% of the time. Mothers responded with more vocal responses compared to interactive responses (e.g., gazes, smiling, physical contact). Investigation of specific types of vocal responses revealed that mothers responded mainly with acknowledgments to both vowel-like sounds and consonant–vowel clusters. Mothers also showed differential responding to vocalizations that varied in quality. Mothers responded with play vocalizations to vowel-like vocalizations significantly more than to consonant–vowel clusters, whereas they responded with imitations to consonant–vowel clusters more than to vowel-like sounds. Mothers, therefore, appeared to regulate their contingent feedback relative to the speech-like quality of infants' vocalizations which may provide relevant stimulation to guide communicative development.

Learning to Produce Syllabic Speech Sounds via Reward-Modulated Neural Plasticity

At around 7 months of age, human infants begin to reliably produce well-formed syllables containing both consonants and vowels, a behavior called canonical babbling. Over subsequent months, the frequency of canonical babbling continues to increase. How the infant's nervous system supports the acquisition of this ability is unknown. Here we present a computational model that combines a spiking neural network, reinforcement-modulated spike-timing-dependent plasticity, and a human-like vocal tract to simulate the acquisition of canonical babbling. Like human infants, the model's frequency of canonical babbling gradually increases. The model is rewarded when it produces a sound that is more auditorily salient than sounds it has previously produced. This is consistent with data from human infants indicating that contingent adult responses shape infant behavior and with data from deaf and tracheostomized infants indicating that hearing, including hearing one's own vocalizations, is critical for canonical babbling development. Reward receipt increases the level of dopamine in the neural network. The neural network contains a reservoir with recurrent connections and two motor neuron groups, one agonist and one antagonist, which control the masseter and orbicularis oris muscles, promoting or inhibiting mouth closure. The model learns to increase the number of salient, syllabic sounds it produces by adjusting the base level of muscle activation and increasing their range of activity. Our results support the possibility that through dopamine-modulated spike-timing-dependent plasticity, the motor cortex learns to harness its natural oscillations in activity in order to produce syllabic sounds. It thus suggests that learning to produce rhythmic mouth movements for speech production may be supported by general cortical learning mechanisms. The model makes several testable predictions and has implications for our understanding not only of how syllabic vocalizations develop in infancy but also for our understanding of how they may have evolved.

Vocal patterns in infants with autism spectrum disorder: canonical babbling status and vocalization frequency

Canonical babbling is a critical milestone for speech development and is usually well in place by 10 months. The possibility that infants with autism spectrum disorder (ASD) show late onset of canonical babbling has so far eluded evaluation. Rate of vocalization or "volubility" has also been suggested as possibly aberrant in infants with ASD. We conducted a retrospective video study examining vocalizations of 37 infants at 9-12 and 15-18 months. Twenty-three of the 37 infants were later diagnosed with ASD and indeed produced low rates of canonical babbling and low volubility by comparison with the 14 typically developing infants. The study thus supports suggestions that very early vocal patterns may prove to be a useful component of early screening and diagnosis of ASD.

Effects of familiar contingencies on infants' vocal behavior in new communicative contexts

Recently, there has been a growing interest in understanding the mechanisms underlying vocal learning in songbirds and human infants. Research has demonstrated how contingent social feedback from social partners to immature vocalizations can play a role during vocal learning in both brown-headed cowbirds and prelinguistic infants. Contingencies in social interactions, particularly familiar contingencies, are important in developing preferences for social partners and shaping social exchanges Bigelow and Birch [1999]. Infant Behavior & Development 22:367-382]; however, little is known about how familiar contingencies that individuals experience during communicative exchanges play a role in new contexts. The current study examined differences in caregiver response patterns to infant vocal behavior and assessed how familiar contingencies influenced infant vocal behavior in novel communicative exchanges with caregivers. Infants were systematically exposed to high and low social feedback schedules during a play session. Results revealed the frequency of caregiver responsiveness to which infants were accustomed to affected infant vocal production during novel communicative situations. Infants with high responding caregivers vocalized with more mature vocalizations and used their vocalizations differently than infants with low responding caregivers during the high, but not low, response period. Specifically, infants with high responding caregivers directed more of their vocalizations at their caregiver and looked more at their caregiver after vocalizing, an indication of anticipating contingent responding. These results suggest that infants with high responding caregivers learned the association between vocalizing and contingent responses during the novel communicative interaction. This study demonstrates the need to understand how infants who experience a variety of contingencies in everyday interactions with caregivers carry over to other interactive situations.

Developmental process emerges from extended brain-body-behavior networks

Studies of brain connectivity have focused on two modes of networks: structural networks describing neuroanatomy and the intrinsic and evoked dependencies of functional networks at rest and during tasks. Each mode constrains and shapes the other across multiple timescales and each also shows age-related changes. Here we argue that understanding how brains change across development requires understanding the interplay between behavior and brain networks: changing bodies and activities modify the statistics of inputs to the brain; these changing inputs mold brain networks; and these networks, in turn, promote further change in behavior and input.

Effects of Parental Interaction on Infant Vocalization Rate, Variability and Vocal Type

Examination of infant vocalization patterns across interactive and noninteractive contexts may facilitate better understanding of early communication development. In the current study, with 24 infant-parent dyads, infant volubility increased significantly when parent interaction ceased (presenting a "still face," or SF) after a period of normal interaction ("face-to-face," or FF). Infant volubility continued at the higher rate than in FF when the parent re-engaged ("reunion," or RE). Additionally, during SF, the variability in volubility across infants decreased, suggesting the infants adopted relatively similar rates of vocalization to re-engage the parent. The pattern of increasing volubility in SF was seen across all of the most common speech-like vocal types of the first half-year of life (e.g., full vowels, quasivowels, squeals, growls). Parent and infant volubility levels were not significantly correlated. The findings suggest that by six months of age infants have learned that their vocalizations have social value and that changes in volubility can affect parental engagement.

Prespeech Vocalizations and the Emergence of Speech: A Study of 1005 Spanish Children

This study investigates 12 prespeech vocal behaviors which are taken to reflect children´s phonological, communicative and early symbolic development. It explores their development (onset, duration and extinction) and their relation to early lexical development. A structured parental questionnaire on prespeech vocalizations was developed, validated and used for the evaluation of 1005 Spanish children’s early vocal development (8–30 months). In parallel, the same children’s productive vocabulary was assessed using the vocabulary section of the European-Spanish MacArthur-Bates Communicative Development Inventories. Results highlight a global inverted U-shaped developmental pattern which emerges from the asynchronous development of the vocal behaviors examined, relating both their emergence and extinction to advances in linguistic development. Moreover, the protracted coexistence of prespeech vocalizations with early speech and their significant correlations with vocabulary size reveal a gradual transition into language. Overall results reinforce and extend previous findings on the development of prespeech vocalizations and establish their relevance as early indexes of linguistic development. Finally, positive evidence on the use of an assisted parental report method for reliably evaluating these developments is provided. Results are discussed within theoretical frameworks that conceive language as the emergent product of complex developmental processes.

Intonational patterns of nonverbal vocalizations in people with dementia

Nonverbal vocalizations in dementia are important clinically since they generally have been regarded as disruptive behavior that is disturbing. The aim of the present study is to describe the interactional pattern, including the prosodic package, of nonverbal vocalizations in a participant in a late stage of dementia. The acoustic analysis shows that the vocalizations do not differ significantly from the verbal utterances regarding mean fundamental frequency or pitch range. The mean fundamental frequency, F0, of the utterances from Anna was significantly higher than the mean F0 from the other elderly participants. The analysis demonstrates that there is a singing-like type of vocalizations that does not resemble the previously described patterns of nonverbal vocalizations. This pattern of the nonverbal vocalization does not resemble the intonation of Anna's verbal utterances. The other participants perceive Anna's vocalizations as potentially meaningful turns. Nonverbal vocalizations in clinical settings should be taken as communicative contributions.

The interplay between language, gesture, and affect during communicative transition: a dynamic systems approach

From a dynamic systems perspective, transition points in development are times of increased instability, during which behavioral patterns are susceptible to temporary decoupling. This study investigated the impact of the vocabulary spurt on existing patterns of communicative coordination. Eighteen typically developing infants were videotaped at home 1 month before, at, and after the vocabulary spurt. Infants were identified as spurters if they underwent a discrete phase transition in vocabulary development (marked by an inflection point), and compared with a group of nonspurters whose word-learning rates followed a trajectory of continuous change. Relative to surrounding sessions, there were significant reductions in overall coordination of communicative behaviors and in words produced in coordination at the vocabulary spurt session for infants who experienced more dramatic vocabulary growth. In contrast, nonspurters demonstrated little change across sessions. Findings underscore the importance of transitions as opportunities for observing processes of developmental change.

The value of vocalizing: five-month-old infants associate their own noncry vocalizations with responses from caregivers

The early noncry vocalizations of infants are salient social signals. Caregivers spontaneously respond to 30%-50% of these sounds, and their responsiveness to infants' prelinguistic noncry vocalizations facilitates the development of phonology and speech. Have infants learned that their vocalizations influence the behavior of social partners? If they have, infants should show an extinction burst in vocalizing when adults temporarily stop responding to infant vocalizations. Thirty-eight 5-month-olds were tested in the still-face paradigm with an unfamiliar adult. When the adult assumed a still face, infants showed an extinction burst. Thus, 5-month-olds have learned the social efficacy of their vocalizations on caregivers' behavior. Furthermore, the magnitude of 5-month infants' extinction bursts predicted their language comprehension at 13 months.

Social Feedback to Infants' Babbling Facilitates Rapid Phonological Learning

Infants' prelinguistic vocalizations are rarely considered relevant for communicative development. As a result, there are few studies of mechanisms underlying developmental changes in prelinguistic vocal production. Here we report the first evidence that caregivers'speech to babbling infants provides crucial, real-time guidance to the development of prelinguistic vocalizations. Mothers of 9.5-month-old infants were instructed to provide models of vocal production timed to be either contingent or noncontingent on their infants' babbling. Infants given contingent feedback rapidly restructured their babbling, incorporating phonological patterns from caregivers'speech, but infants given noncontingent feedback did not. The new vocalizations of the infants in the contingent condition shared phonological form but not phonetic content with their mothers' speech. Thus, prelinguistic infants learned new vocal forms by discovering phonological patterns in their mothers' contingent speech and then generalizing from these patterns.

The missing chapter: The interaction between behavioral and symbolic inheritance

A strength of Jablonka & Lamb's (J&L's) book lies in its accessible as well as thorough treatment of genetic and epigenetic inheritance. The authors also provide a stimulating framework integrating evolutionary research across disciplines. A weakness is its unsystematic treatment of the interaction between behavioral and symbolic inheritance, particularly in their discussion of language.

The relationship between reduplicated babble onset and laterality biases in infant rhythmic arm movements

This study examined changes in rhythmic arm shaking and laterality biases in infants observed longitudinally at three points: just prior to, at, and just following reduplicated babble onset. Infants (ranging in age from 4 to 9 months at babble onset) were videotaped at home as they played with two visually identical audible and silent rattles presented at midline for 1.5 min each. Rate of rattle shaking increased sharply from the pre-babble to the babble onset session; but there was no indication that this increase was specific to the right arm. This finding suggests that the link between babble onset and increased rhythmic arm activity may not be the product of language-specific mechanisms, but is rather part of a broader developmental process that is also perceptual and motor.

Vocal Expression of Emotions in Normally Hearing and Hearing-Impaired Infants

The vocalizations of seven normally hearing (NH) and seven severely hearing-impaired (HI) infants were compared to find out the influence of auditory feedback on preverbal utterances. It was tested whether there are general differences in vocalizations between NH and HI infants, and whether specific emotional states affect the vocal production of NH and HI infants in the same way. First, the acoustic structure of the three most common vocal types was analyzed; second, the composition of vocal sequences was examined. Vocal sequence composition turned out to be more affected by hearing impairment than the acoustic structure of single vocalizations. This result indicates that the acoustic structure of preverbal vocalizations is to a great extent predetermined, whereas the composition of vocal sequences is influenced by auditory input.

Social interaction shapes babbling: testing parallels between birdsong and speech

Birdsong is considered a model of human speech development at behavioral and neural levels. Few direct tests of the proposed analogs exist, however. Here we test a mechanism of phonological development in human infants that is based on social shaping, a selective learning process first documented in songbirds. By manipulating mothers' reactions to their 8-month-old infants' vocalizations, we demonstrate that phonological features of babbling are sensitive to nonimitative social stimulation. Contingent, but not noncontingent, maternal behavior facilitates more complex and mature vocal behavior. Changes in vocalizations persist after the manipulation. The data show that human infants use social feedback, facilitating immediate transitions in vocal behavior. Social interaction creates rapid shifts to developmentally more advanced sounds. These transitions mirror the normal development of speech, supporting the predictions of the avian social shaping model. These data provide strong support for a parallel in function between vocal precursors of songbirds and infants. Because imitation is usually considered the mechanism for vocal learning in both taxa, the findings introduce social shaping as a general process underlying the development of speech and song.