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

Abstract

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.

Introduction

The onset of reduplicated babbling (vocalizations consisting of syllable repetition, e.g., [bababa]) has long been considered to be a major milestone in early language development. Although infants begin to produce single syllables (e.g., Fagan, 2005, Oller, 2000) and frequently repeat single sounds (e.g., vowels; Fagan, 2005) prior to babble onset, reduplicated babble is considered an important developmental advance because it marks the first appearance of repeated strings of well-formed syllables; and syllables are the fundamental organizational unit of adult language. The emergence of syllabically organized vocalizations is therefore considered by many to be a crowning achievement of infant vocal development (see Oller, 2000, for extensive discussion).

While there is general agreement concerning the importance of reduplicated babble onset as a major developmental milestone, there is considerable controversy regarding the way in which it is conceptualized theoretically. Over the past 15 years, two contrasting views of reduplicated babble have emerged. These views—the motor stereotypy view and the linguistic view—differ sharply in their interpretation of the nature of reduplicated babble and its potential underlying mechanisms.

According to the motor stereotypy view, reduplicated babble is one among a family of rhythmically organized, stereotyped movements that are common in infant behavior (e.g., Kent, 1984). During the first year of life, infants frequently produce rhythmically organized, repeated movements of the limbs and whole body (see Thelen, 1979). These movements typically exhibit clear developmental trajectories, with a distinct time of onset, peak, and decline. They are performed when infants have some control over their limbs and the desire to act but are still working out problems of adapting behaviors in a flexible and stable manner to achieve an intentional goal. As such, they are closely correlated with relevant motor milestones (Thelen, 1979). For example, infants wave their arms before they reach and rock on all fours before they crawl; and recent work suggests that there is developmental continuity between these early movements and later-emerging skilled behavior (e.g., see Bhat and Galloway, 2006, Bhat et al., 2005).

MacNeilage and Davis (1993) have argued that reduplicated babble shares properties of rhythmic motor stereotypies because it begins fundamentally as a mandibular oscillation. In their view, mandibular oscillations, in combination with phonation and limited tongue control, form the basis for the repetitive patterns typical of early reduplicated babbling. Specifically, the repeated lowering and raising of the mandible results in a perceived contrast between consonants produced when the vocal tract is closed and vowels that are produced when the vocal tract is open. As infants gain control of the tongue and its position during mandibular cycles, they begin to widen the repertoire of syllabic patterns that occur in their vocalizations (MacNeilage and Davis, 2000, MacNeilage et al., 1999). In short, just as arm and leg rhythmicities are a product of the biomechanical properties of infant limbs under developing control, so too is reduplicated babble a product of the biomechanical properties of the mandible in combination with limited tongue control.

Evidence for the motor stereotypy view comes from studies indicating temporal coincidence between reduplicated babble onset and rhythmic motor activity, especially arm activity. Thus, for example, in longitudinal research, Thelen (1979) found a clear peak in frequency of rhythmic arm activity at around 28 weeks of age. This is the age at which many infants begin to produce reduplicated babble (e.g., Koopmans-van Beinum and van der Stelt, 1986, Oller and Eilers, 1988). More recent work has reported strong, positive correlations between ages of onset of reduplicated babble and rhythmic hand banging (e.g., Cobo-Lewis et al., 1996, Eilers et al., 1993).

Basing their interpretation on abundant evidence for the existence of a coupling of the vocal and motor systems¹ (particularly the manual system) from early in development, Iverson and Thelen (1999) have suggested that the close relationship between reduplicated babble onset and heightened upper limb rhythmicity is a product of the entrainment of coupled vocal–motor systems. According to their model, when a given behavior in one component of a coupled system is relatively stable and well-practiced, its activation naturally extends to the other component. When this happens, activity in the first system may “pull in” and entrain the activity of the complementary system. For example, when an infant is engaged in an intense bout of rhythmic limb activity, the level of activation in the motor system may “spill over” into the vocal system and entrain its activity, resulting in production of a co-occurring vocalization that may even be closely timed with the rhythmic limb movement (e.g., a string of reduplicated syllables, each articulating with a movement cycle in a bout of rhythmic arm banging; see Iverson & Fagan, 2004).

On this view, the onset of reduplicated babbling may be seen as a product of entrainment between the vocal and motor systems, specifically between rhythmically organized motor activity and vocalization. Infants have a long history of producing rhythmic manual movements prior to the onset of reduplicated babbling. These repetitive, rhythmically organized movements are very stable and well-practiced; and thus they should gradually entrain vocal activity, leading eventually to the production of the mandibular oscillations that comprise babbling. Thus, this behavioral coupling between vocalization and rhythmic arm activity that occurs in real time has implications for the emergence of reduplicated babble in developmental time; and this real time coupling may also have implications for the emergence of a linked gesture–speech system in developmental time. This is a topic to which we will return in the discussion.

Some investigators have taken issue with the notion that reduplicated babbling reflects the properties of a developing vocal–motor system, arguing instead that babbling is a systematic and fundamentally linguistic behavior (e.g., see Petitto, Holowka, Sergio, Levy, & Ostry, 2004, for a recent discussion). From this perspective, the mandibular cycles that comprise babbling reflect maximally contrasting syllabic units in the target language, and the production of babble reflects infants’ inherent sensitivity to the rhythmic and temporal properties of natural language. In other words, the fact that reduplicated babble contains fundamentally linguistic units (i.e., syllables) indicates that it is controlled by a set of mechanisms dedicated uniquely to language. The onset of babbling is triggered by exposure over developmental time to a patterned input to which infants are sensitive. The subsequent production of babble provides infants with opportunities to discover the particular set of sounds that is employed in their language.

Evidence for the claim that babbling reflects an emerging linguistic capacity comes from three main sources. First, there is a high degree of continuity between the babbling patterns developed by an individual child and the early word forms produced by that child (see Vihman, 1996, for a review). Second, delayed babble onset and/or less voluble babble productions are reliable predictors of delayed or disordered language development (e.g., Oller et al., 1998, Oller et al., 1999). Thus, for example, infants with Down syndrome are delayed in both onset of reduplicated babble and language development (Lynch et al., 1995); and the babble production of late talkers is characterized by small phonetic repertoires and simple syllable shapes (Stoel-Gammon, 1989).

A third piece of evidence, and one that is at issue in the present paper, is that although infant hand preferences are generally unstable and fluctuate throughout the first year (Corbetta & Thelen, 1999), infants appear to demonstrate a temporary right hand preference in manual activity at reduplicated babble onset. One of the first to report this phenomenon was Ramsay (1984). Presenting infants with toys that contained a salient feature (e.g., a button) providing auditory and visual feedback, Ramsay examined unimanual hand preferences in infants followed weekly from 5 months of age to 8 weeks following the onset of reduplicated babble. Toys were presented at midline, and the proportion of right-handed toy contacts (defined as attempts to activate the movable portion of the toy) were coded at each session. Reporting that infants generally first produced a significantly higher proportion of right- than left-handed toy contacts during the week of reduplicated babble onset, Ramsay (1984) suggested that synchrony between babble onset and emerging right hand preference might be indicative of a change in hemispheric specialization accompanying the emergence of reduplicated babble.

On the basis of this work and other research pointing to behavioral associations between right-handed activity and speaking in older children (e.g., Hiscock and Kinsbourne, 1978, Kinsbourne and McMurray, 1975) and left hemisphere control of both segmental speech gestures and movements of the preferred hand (Fried et al., 1991, Ojemann, 1984), Locke, Bekken, McMinn-Larson, and Wein (1995) took this argument a step further. They suggested that the appearance of a right hand bias at reduplicated babble onset indexes increasing brain specialization for language functions. To test this claim, Locke et al. (1995) designed a cross-sectional study in which they placed audible and silent rattles in the left and right hands of infants of different ages presumed to vary in the amount of time they had been producing reduplicated babble. In light of the well-documented temporal link between enhanced arm rhythmicity and reduplicated babble onset, they reasoned that if the onset of reduplicated syllable production and repetitive manual activity are controlled by a common mechanism in the left hemisphere, then there should be a disproportionate increase in right-handed rhythmic activity with the onset of babbling. In addition, since babbling is an audible behavior, they hypothesized that auditory feedback (the sound made by an audible vs. a silent rattle) would enhance repetitive right-hand activity at babble onset.

Reported results were generally consistent with the first prediction: rate of rattle shaking was relatively low among the youngest, pre-babbling infants and increased substantially, especially for the right hand, among infants who were presumed to be new babblers. Following babble onset, rate of rattle shaking declined somewhat (but remained above that for prebabblers) and the right hand advantage disappeared. With regard to the second prediction, infants tended to shake the audible more than the silent rattle, but this effect was not specific to babble onset. On the basis of these findings, Locke et al. (1995) concluded that “… repetitive right-hand activity increased at about the same age as repetitive vocal–motor activity, suggesting that babbling onset may reflect the maturation of control mechanisms in the left hemisphere” (p. 505).

Although Locke et al.’s (1995) results are intriguing in light of recent interest in language–motor links (e.g., Rizzolatti & Arbib, 1998), they are limited in several significant ways. First, the Locke et al. design was cross-sectional. Infants were divided into groups of prebabblers, new babblers, and experienced babblers based on parent report of whether or not the infant produced reduplicated babble and infant age. Since no information was available regarding babble onset for individual infants, length of time since the emergence of reduplicated babble was estimated on the basis of infant age in relation to data from the largest parametric study of babble onset then available (van der Stelt & Koopmans-van Beinum, 1986). Unfortunately, this procedure results in a confound between infant age and babbling status; and it is therefore unclear whether the observed increase in rate of shaking (especially right hand shaking) is specifically related to babble onset or simply a function of infant age. Second, because Locke et al. used a cross-sectional design, no information is available with regard to change over time in proportion of right-handed grasping and shaking. Third, no data are reported regarding the hand preferences of individual infants. Therefore, it is impossible to know how many recent babblers show the reported right hand preference effect.

The present study was designed as a partial replication and extension of Locke et al. (1995). Our goal was to use a longitudinal approach to gather evidence on the hypothesized relationship between onset of reduplicated babble and increase in right-handed manual rhythmicity. Through longitudinal observation employing a task generally comparable to that of Locke et al., we wished to pinpoint the onset of reduplicated babble for each infant and examine changes over time in grasping and rhythmic arm activity prior to, at, and following babble onset. With longitudinal data from infants roughly comparable to Locke et al.’s three critical groups (oldest prebabblers, recent babblers, and youngest experienced babblers), we could then track changes in proportions of right-handed grasping and shaking in individual infants. And because infants begin to babble at widely varying ages, we could disentangle the effects of infant age and babbling status.