47

3Language Processing and

Production in Infants and Toddlers

Barbara T. Conboy

Humans begin learning the patterns of their native language from their fi rst breaths of life. Gradual attunement to native language features and conceptual development over the fi rst year culminate in infants producing their fi rst words, a milestone eagerly anticipated and celebrated by caregivers. Remarkable development in other aspects of language also occurs over the fi rst few years, paving the way for the production and comprehension of longer units of language that allow young children to eff ectively communicate complex concepts with members of their communities. In most cases, language development occurs in a seemingly eff ortless manner whether a child is learning one or two native languages. Yet the language development of a small percentage of infants and toddlers lags behind that of their peers. Professionals interested in identifying language-learning disorders at an early age need to understand the range of behaviors considered typical of infants and toddlers. Innovative techniques developed over the past few decades have facilitated such an under-standing by allowing researchers to probe language perception, production, and processing at various points in development. Th is chapter summarizes what has been discovered from research, focusing on the fi rst 3 years.

Studies of groups of infants and toddlers acquiring more than one language have been scarce until recently. Given that most research has been with hearing infants, this chapter deals with the acquisition of spoken language and focuses on data from English–Spanish bilingual learners whenever possible; however, it also includes relevant research on bilingual infants and toddlers from other language communities and monolingual learn-ers. Together, the fi ndings of studies reviewed in this chapter suggest that the adaptable young brain develops ways of processing and producing language that are similar, but not identical, across monolingual and bilingual learning situations. General abilities present at birth allow infants to learn from whatever input is provided, and the learning process itself shapes the mechanisms that are used for further learning. Th e implications of such fi ndings for the early identifi cation and clinical management of bilingual infants and toddlers at risk for language-learning disorders are discussed.

48 Conboy

THE PRENATAL AND EARLY POSTNATAL PERIODS: BASIC MECHANISMS OF LANGUAGE LEARNING

Much of the basic sensory, perceptual, and cognitive machinery involved in processing spoken language is developed by the third trimester of gestation. Certain properties of speech, such as the fundamental frequency of a person’s voice and speech prosody (i.e., into-nation contours and stress patterns involving alternations in loudness and the durations of speech sounds), are detected by fetuses by 30 weeks gestational age and stored in memory (for a review, see Moon & Fifer, 2000). For example, fetal heart rates respond diff erently to the native (i.e., maternal) language versus a nonnative language and to the maternal voice versus an unfamiliar female voice (Kisilevsky et al., 2009). Aft er expectant mothers read aloud a nursery rhyme once each day during the last 4 weeks of their pregnancies, their fetuses’ heart rates responded diff erently when presented with an unfamiliar female voice reading that same passage versus an unfamiliar passage (DeCasper, Lecanuet, Busnel, Granier-Deferre, & Maugeais, 1994). Th e possibility that late-term fetal brains encode characteristics of speech and retain this information from the pre- to postnatal period is supported by research with newborn infants using techniques such as high-amplitude suck-ing (HAS), in which rates of nonnutritive sucks are compared as infants listen to diff erent stimuli. Newborns show diff erent sucking responses to their mothers’ voices than to unfa-miliar female voices (DeCasper & Fifer, 1980), to their native language than to a nonnative language (Byers-Heinlein, Burns, & Werker, 2010; Mehler et al., 1988; Moon, Cooper, & Fifer, 1993), and to a story read by their mothers during the last 6 weeks of pregnancy than to a novel story (DeCasper & Spence, 1986). Research conducted with preterm infants shortly aft er birth has also suggested that the neural mechanisms used for perceiving and forming memory traces of the acoustic properties of speech are available by 30–35 weeks conceptual age (Cheour et al., 1998). Fetuses may not hear all of the acoustic information in speech due to attenuation of higher frequency sounds in utero (see Moon & Fifer, 2000); however, the research suggests that they can process such information once exposed to it aft er birth.

Whether or not language learning begins in utero, it is clear that hearing infants begin life equipped to perceive fi ne-grained distinctions between speech sounds, an ability they will need for learning the forms of words. In a classic study in which HAS was used to test speech sound discrimination in 1- to 4-month-old monolingual English infants, changes in sucking rates were recorded when the stimulus sound changed to /ba/ from /pa/, indicating discrimination of these sounds (Eimas, Siqueland, Jusczyk, & Vigorito, 1971). Th ese results were initially interpreted as evidence that the ability to perceive speech sounds is innate. However, subsequent work has shown that such early speech sound discrimination abilities are not unique to humans and are not aligned with the phoneme categories of particu-lar languages, thus they likely derive from broad, innate auditory-perceptual abilities (for reviews, see Gervain & Mehler, 2010; Kuhl, 2004; see also the next section for an extensive discussion). Newborn infants prefer listening to speech over nonspeech sounds that have similar acoustic properties (Vouloumanos & Werker, 2007). Infants surrounded by English, Spanish, or any other language initially possess the same basic abilities to perceive speech sounds, but then they tune their perception based on their input. Moreover, though it was long believed that early exposure to diff erent languages aff ected only perception, not

Language Processing and Production 49

production, Mampe, Friederici, Christophe, and Wermke (2009) found that the cries of newborn infants correspond to the prosody of the maternal language. Infants whose mothers spoke French produced cries with primarily rising melody contours, and infants whose mothers spoke German produced cries with falling contours. Th us, some form of perception–production matching ability appears to be available at birth.

In addition to basic auditory-perceptual, memory, and imitative abilities; an inter-est in speech sounds; and sensitivity to the prosody of speech, infants start life with gen-eral computational abilities that may be used for detecting regularities in language (see Gervain & Mehler, 2010). One such ability, called statistical learning, allows infants to track the transitional probabilities of co-occurring elements in sequenced patterns, for exam-ple, the probability that a particular phoneme or syllable will follow another phoneme or syllable (Saff ran, Aslin, & Newport, 1996). Teinonen, Fellman, Näätänen, Alku, and Huotilainen (2009) tested the statistical learning of newborn infants using event-related potentials (ERPs), an electrophysiological technique that noninvasively measures the brain activity involved in sensory, perceptual, and cognitive processing and can be used with very young infants during awake or sleep states (for a review, see Friederici, 2005). In ERP research, electrodes placed on the scalp record activity produced by neurons in the cerebral cortex in response to particular stimuli, and epochs of this activity for each stimulus are averaged together, producing waveforms that can be analyzed with respect to their tim-ing, voltage amplitude, and distribution across the scalp. Teinonen et al. (2009) presented sleeping newborn infants with trisyllabic pseudowords played in a random order so that the probability that the last syllable of one pseudoword would be followed by the fi rst syl-lable of a diff erent pseudoword was always much lower than the within-word transitional probabilities of syllables. Infants’ ERPs were larger in amplitude to the fi rst syllable of each pseudoword compared to the medial and fi nal syllables, indicating that the newborn brain is sensitive to transitional probabilities that identify word onsets. However, there is no evi-dence that infants use transitional probabilities to recognize words in speech until later in infancy (Saff ran, 2001). Statistical learning has been documented for visual as well as audi-tory patterns (Kirkham, Slemmer, & Johnson, 2002) and in nonhuman animals (e.g., Toro & Trobalon, 2005), indicating it is a domain-general ability that can be used for various aspects of learning.

Although statistical learning is a powerful mechanism for identifying specifi c sequences of stimuli such as speech sounds within words, some aspects of language acqui-sition may require more abstract rule-like knowledge about how particular classes of ele-ments can be sequenced. It has been proposed that newborns possess an auditory primitive intelligence, which allows them to detect acoustic regularities and generate abstract rules to predict future auditory events (Carral et al., 2005). Carral and colleagues (2005) tested sleeping newborn infants using an ERP paradigm in which pairs of tones with ascend-ing frequency (i.e., the second tone had a higher frequency than the fi rst) occurred most of the time (the standard pattern) and pairs with descending frequency occurred rarely (the deviant pattern); the use of various tone pairs required infants to discriminate the patterns by forming an abstract rule rather than relying on absolute frequencies. Diff erent ERP amplitudes for the deviant compared to the standard pairs indicated that newborn infants represent abstract rules. Other evidence that newborns can learn rules comes from a functional near-infrared spectroscopy (fNIRS) study in which infants were tested on the

50 Conboy

ability to learn simple repetition-based structures (i.e., syllable sequences such as /mubaba/) that conform to a pattern (Gervain, Macagno, Cogoi, Peña, & Mehler, 2008). Gervain and colleagues (2008) found increased neural activity (i.e., changes in oxygentated/deoxygen-tated blood hemoglobin concentrations) in response to syllable sequences that conformed to the learned pattern compared to a novel pattern. Th ese fi ndings do not necessarily show that newborns form abstract linguistic rules (i.e., phonotactics, morphosyntax), but they do suggest that rule-learning abilities are present and available for use from birth.

Th e research discussed in this section indicates that some basic perceptual and cogni-tive mechanisms necessary for language learning are in place at birth. However, this does not mean that these mechanisms are mature, or functionally ready, at birth, nor does it mean that diff erent learning experiences—monolingualism, bilingualism, or culturally based language socialization practices—have no eff ect on the development of perceptual or cognitive systems. Th e cognitive abilities that support language learning develop within social communicative contexts, and evidence that they are shaped by those contexts is reviewed in the remainder of this chapter.

LANGUAGE DIFFERENTIATION IN BILINGUAL INFANTS

It must be true that infants learning two languages in infancy possess basic mechanisms for keeping those languages separate, given that they learn which phonological and gram-matical cues are relevant for each language. Th e ability to diff erentiate languages is present early in infancy. As noted in the previous section, one of the fi rst aspects of spoken lan-guage to which infants attend is prosody (i.e., the rhythm and melody of connected speech). Research has shown that infants respond diff erently to rhythmically distinct languages at or even prior to birth. For example, English is classifi ed as a stress-timed language because the duration between two stressed syllables is equal, and stressed syllables are longer in duration than unstressed syllables; Mandarin and Spanish are syllable-timed languages, in which stressed and unstressed syllables have roughly the same duration (see Gervain & Werker, 2008). Kisilevsky and colleagues (2009) found that 33- to 41-week-old fetuses whose mothers spoke English responded with diff erent heart rates to passages in English and Mandarin. Using HAS, Moon and colleagues (1993) found faster sucking rates for the maternal language in 2-day-old infants whose mothers spoke either English or Spanish, and Nazzi, Bertoncini, and Mehler (1998) showed that newborns can diff erentiate two unfamil-iar languages, but only when they are from diff erent rhythmic classes (see also Mehler et al., 1988).

Newborn infants exposed to two rhythmically distinct languages by a bilingual mother diff erentiate languages using the same basic perceptual and cognitive mechanisms as monolingual infants. Byers-Heinlein et al. (2010) used HAS with infants whose moth-ers spoke only English, both English and syllable-timed Tagalog, or English and Chinese (Cantonese or Mandarin) during their pregnancies. Th e bilingual and monolingual infants showed dishabituation (acceleration of sucking rate aft er deceleration from habituation) when there was a switch between English and Tagalog. Th e researchers also found a graded pattern in infants’ listening preferences for English versus Tagalog, supporting the view that infants use rhythm to tell languages apart. Monolingual English infants showed a strong preference for English over Tagalog; English–Tagalog bilingual infants had equally strong

Language Processing and Production 51

responses to both languages and a stronger response to Tagalog than monolingual infants; and Chinese–English bilingual infants showed a slight preference for English over Tagalog but a stronger response to Tagalog than monolingual English infants. Th is last fi nding was likely due to infants’ familiarity with the prosodic patterns of Cantonese and Mandarin, both of which are rhythmically similar to Tagalog. Th e results also extend the fi nding that newborn infants show a familiarity eff ect for a prosodic pattern they have heard pre- and/or postnatally when tested only a few days aft er birth.

Although rhythmic class is a powerful cue for language diff erentiation, infants learn-ing two rhythmically similar languages must rely on additional information. Bosch and Sebastián-Gallés (1997) investigated language diff erentiation in infants learning Catalan and Spanish. Although both are syllable-timed languages, they have some prosodic diff er-ences, including a greater prevalence of iambic (unstressed initial syllable; e.g., gi-RAFFE) versus trochaic (stressed initial syllable; e.g., RO-bot) words and the occurrence of reduced vowels (schwa) in unstressed syllables in Catalan but not Spanish. Th e researchers recorded how quickly infants shift ed their eye gaze toward a loudspeaker each time it began to play passages in either language. Monolingual 4-month-old infants learning either Catalan or Spanish oriented more quickly to their native language, whereas bilingual infants ori-ented equally quickly to both languages, even when one language was used more by their mothers. In a follow-up study using a diff erent testing procedure that relied on overall look-ing times rather than reaction times, 4-month-old Catalan–Spanish bilingual infants diff er-entiated their two languages (Bosch & Sebastián-Gallés, 2001). Th us, bilingual infants can detect switches from one of their languages to the other as young as 4 months, even when those languages are from the same rhythmic class. It is important to note that one testing method that was sensitive to language diff erentiation in monolingual infants was not useful for showing diff erentiation in bilingual infants the same age, but another testing method showed diff erentiation in both groups. Th ere are several examples in the literature in which a similar phenomenon has occurred.

In addition to auditory-perceptual abilities, infants possess visual abilities that may be used for learning aspects of spoken languages, and these skills may help bilingual infants separate their language input. A study that compared bilingual and monolingual infants of various ages provides intriguing evidence that bilingual infants may rely on visual cues to speech to a greater extent than monolingual infants, at least when the two languages they are learning have very diff erent prosodies. Weikum and colleagues (2007) presented Canadian infants from monolingual English, monolingual French, and bilingual English–French homes with videos of the same adults speaking English or French. Th e sound was turned off , and one video was played while infants watched; when the infants stopped watch-ing the video (habituated), the video was switched to the other clip of the same speaker, and recovery of looking time (dishabituation) to the language switch was measured. At 4 months, both bilingual and monolingual infants detected the language switch, but at 6 and 8 months, only bilingual infants succeeded. It is clear from this study and the other work on early language diff erentiation that infants possess various mechanisms they can use to keep their input languages perceptually separate, even when the two languages are rhythmically similar and even when both are spoken by the same caregivers. Th is means that having dif-ferent people speak to infants in each language (e.g., a one-parent, one-language strategy)

52 Conboy

is not necessary for infants to learn language-specifi c properties. In fact, such input is not common in most bilingual situations, and its occurrence does not consistently lead to the best outcomes (De Houwer, 2007).

ATTUNEMENT TO NATIVE PHONEME CATEGORIES IN THE FIRST YEAR OF LIFE

Language learners undergo a process that involves “tuning out” information in their input that is not relevant for learning the language as well as “tuning in” to information that is relevant. Th e limited research on this process in infants raised with two native languages shows some diff erences as well as many similarities between monolingual and bilingual fi rst language acquisition. In this section, relevant data on monolingual infants, and available data as well as current hypotheses regarding bilingual infants, are reviewed.

Among the fi rst linguistic features infants tune into are the phonetic properties of speech sounds such as vowels and consonants. For example, Spanish has 5 distinct vowel categories (monophthongs), and Standard American English has 12; each language also has diff erent consonant categories (see Chapter 15). Th us, infants learning English need to perceive acoustic distinctions between certain vowels that are irrelevant for infants learning Spanish, such as /i/ and /I/ in seat versus sit, and infants learning Spanish need to perceive other distinctions irrelevant for English learners, such as the trill /r/ and tap /ɾ/ phonemes that distinguish carro (car) and caro (expensive); infants learning both English and Span-ish bilingually need to learn when each set of distinctions applies. Infants must also ignore fundamental frequency and other acoustic aspects of interspeaker variability in order to learn from many diff erent speakers, and intraspeaker acoustic variations (e.g., in rate, loud-ness, pitch) that are not relevant for forming phoneme categories in their language. Some ability to ignore speaker diff erences when discriminating among speech sounds is present in newborns (Dehaene-Lambertz & Peña, 2001), but for the most part the ability to ignore acoustic variations that are not relevant for the native language develops gradually (for a review, see Kuhl, 2004). Researchers have used a variety of methods to determine when infants’ perception narrows to favor the native language. In addition to heart rate, HAS, ERP, and fNIRS methods, researchers have relied on infants’ looking behaviors to indicate discrimination of speech sounds. For example, Eilers and colleagues (Eilers, Gavin, & Oller, 1982; Eilers, Gavin, & Wilson, 1979) tested 6- to 8-month-old infants from English- and Spanish-speaking homes using a conditioned head turn task in which infants were trained to turn their heads toward an interesting toy when they detected a change in stimulus. Th e results showed that infants’ perception varied according to their language background. Using conditioned head turn with Canadian English-learning infants, Werker and Tees (1984) determined that discrimination of nonnative phonemes declines between 6 and 12 months; subsequent research has replicated this pattern across language communities using a variety of methods (see Kuhl, 2004, for a review). Several studies have also suggested that native-language perception improves throughout infancy (Cheour et al., 1998; Conboy et al., 2005; Kuhl et al., 2006; Rivera-Gaxiola, Silva-Pereyra, & Kuhl, 2005; Sundara, Polka, & Genesee, 2006; Tsao, Liu, & Kuhl, 2006).

A number of factors have been proposed to explain developmental changes in percep-tion of nonnative and native speech sounds. Monolingual English- and Spanish-learning

Language Processing and Production 53

infants both show ERP discriminatory responses to English and Spanish phoneme con-trasts at 7 and 11 months, but the eff ects for native and nonnative contrasts change with age, suggesting that language experience changes how the brain processes speech (Rivera-Gaxiola et al., 2007; Rivera-Gaxiola, Silva-Pereyra, & Kuhl, 2005; see also Cheour et al., 1998). Some nonnative speech sounds remain perceptible throughout life, possibly because of their acoustic salience and how they relate to native-language phoneme cat-egories (Best & McRoberts, 2003; Burnham, 1986; Werker & Curtin, 2005). For example, 11-month-old infants growing up in monolingual Spanish-speaking homes in central Mexico discriminated the acoustically salient voiceless aspirated [th] from voiceless unaspi-rated [t] on a conditioned head turn task, even though [th] does not occur in Spanish (Conboy, Jackson-Maldonado, & Kuhl, 2009). Characteristics of some native speech sounds also make them easier to perceive than others. Vowel categories are typically learned prior to consonants, but vowels that are more acoustically distinct from one another are easier to discriminate than are vowel pairs in close proximity, such as /e/ and /ε/ (Sebastián-Gallés & Bosch, 2009; see below), and native-language sounds that are acoustically salient are discriminated at younger ages than less salient ones (Narayan, Werker, & Beddor, 2010). Th e frequency of occurrence of phonemes in the native language also infl uences phoneme category formation (Anderson, Morgan, & White, 2003).

Th e exact relationships between speech sound perception and language development are not known, but negative correlations between native and nonnative consonant discrim-ination skills suggest that as infants become attuned to their native language, they tune out phonetic distinctions that are irrelevant for their native language (Kuhl et al., 2008). Bilingual and monolingual infants who have better native speech perception skills have larger native-language vocabularies than infants with worse native speech perception skills, whether vocabulary size is measured at the same age (Conboy et al., 2005, 2009) or at later ages (García-Sierra et al., 2011; Kuhl et al., 2008; Kuhl, Conboy, Padden, Nelson, & Pruitt, 2005; Tsao, Liu, & Kuhl, 2004). Monolingual infants who retain the ability to perceive nonnative speech sounds for a longer period of time have smaller native-language vocabu-laries in the second and third years than infants who tune out the same contrast (Kuhl et al., 2008; Rivera-Gaxiola, Klarman, Silva-Pereyra, & Kuhl, 2005). Moreover, cognitive control skills operating across domains of learning may assist infants in ignoring irrelevant cues while attending to those cues that are relevant for their native language. Monolin-gual infants at 8–11 months who tune out nonnative speech sounds have better cognitive abilities than those who continue to perceive nonnative contrasts (Conboy, Sommerville, & Kuhl, 2008; Lalonde & Werker, 1995). It is not known whether relationships between nonnative perception and other language and cognitive skills apply to bilingual infants. Th e process of learning diff erent phoneme categories across two languages may lead bilingual infants to maintain less “committed” perceptual systems (Kuhl, 2004), a form of “percep-tual wedge” that is advantageous for the bilingual case (Petitto, 2009). Studies with 7- and 8-month-old infants from monolingual and bilingual homes suggest that the experience of switching between diff erent sets of language cues in early bilingualism enhances domain-general cognitive control abilities (Ibañez, Pons, Costa, & Sebastián-Gallés, 2010; Kovács & Mehler, 2009a). Research has yet to link cognitive control abilities in bilingual infants to particular language-learning processes, such as the tuning out of irrelevant phonetic cues,

54 Conboy

but it is now clear that bilingual and monolingual infants develop nonequivalent language and cognitive processing systems (see Adesope, Lavin, Th ompson, & Ungerleider, 2010; Bialystok, 2009).

Several studies conducted with infants in Barcelona acquiring Catalan and/or Span-ish have focused primarily on how the distribution of sounds in the input aff ects bilingual versus monolingual perception. Catalan has three vowels not found in Spanish, including /ε/. Th e acoustic values of Spanish /e/ fall roughly halfway between those of Catalan /e/ and /ε/; thus, Bosch and Sebastián-Gallés (2003b) predicted that bilingual infants might perceptually group together the three vowels into a single category. Th e researchers com-pared infants’ auditory attention to minimal pair stimuli (tokens of [deði] and [dεð i]) by measuring how long the infants looked at a fl ashing light mounted above a speaker playing repetitions of the stimuli. Infants were familiarized with one set of stimuli, and, during the test phase, listening times were compared for tokens that matched the familiarized stimu-lus or switched to the other stimulus. Regardless of language background, 4-month-old infants showed longer looking times for the familiar versus the unfamiliar stimuli, indi-cating discrimination of the two vowels. As expected, 8-month-old monolingual Catalan infants discriminated the contrast, but monolingual Spanish infants, who do not hear /ε/ in their input, did not. An unexpected fi nding was that bilingual 8-month-olds did not discriminate the Catalan /e/ from /ε/, though their language input contained these vow-els. However, by 12 months, bilingual infants succeeded. Th e authors interpreted the dip in discrimination at 8 months as resulting from a temporary merging of the three vowel categories into one based on how bilingual infants hear the vowels in their input. In other words, it may be thought of as an appropriate response to properties of the bilingual input rather than a delay induced by bilingualism (for similar results with diff erent contrasts, see Bosch & Sebastián-Gallés, 2003a; Sebastián-Gallés & Bosch, 2009).

Th e many similarities between Catalan and Spanish may also aff ect infants’ ability to discriminate sounds that overlap in those two languages. Sundara and Scutellaro (2010) found that English–Spanish bilingual infants living in Los Angeles had no diffi culty dis-criminating English /e/ and /ε/ at 8 months, suggesting that when infants are learning two languages with quite diff erent prosody they may not perceptually merge members of close vowel categories. Burns, Yoshida, Hill, and Werker (2007) also found no evidence for a lag in discrimination or perceptual merging of acoustically close stop consonants in English–French Canadian bilingual infants. French stops have earlier voice onset times (VOTs) than English stops; the same sound, voiceless unaspirated [p], is perceived as /p/ by French speakers and as /b/ by English speakers. Th us, bilingual English–French infants need to develop diff erent phoneme category boundaries along the same VOT continuum for each of their languages. Burns et al. (2007) found that both monolingual English and bilingual 6- to 8-month-old infants discriminated the French /ba/-/pa/ contrast, but at later ages only the bilingual infants, who had exposure to French, discriminated those sounds. In contrast to the results of Bosch and Sebastián-Gallés (2003b), bilingual infants succeeded in discriminating both contrasts at all three ages (see also Sundara, Polka, & Molnar, 2008, for a similar result with a diff erent phonetic contrast). Finally, Albareda-Castellot, Pons, and Sebastián-Gallés (2011) found that 8-month-old bilingual Catalan–Spanish infants succeeded in discriminating /e/ and /ε/ when an anticipatory looking

Language Processing and Production 55

task was used rather than the familiarization-preference task used by Bosch and Sebastián-Gallés (2003b).

Th e results across studies with bilingual infants show that bilingual infants do not lag behind monolingual infants in phonetic perceptual development, though temporary perceptual merging in bilingual infants may occur when there is overlap between input cues. However, merging does not seem to occur for all close phoneme pairs nor for all bilingual infants. It is important to consider that there may be nothing uniquely “bilin-gual” about this pattern, because temporary perceptual merging has also been noted in 8-month-old monolingual English-learning Canadian infants tested on discrimination of the acoustically close English vowels /e/ and /I/ (Sabourin, Werker, Bosch, & Sebastián-Gallés, 2009). Rather, the ways that sounds are distributed in the input language could play a role: Catalan–Spanish infants are exposed to many cognate words across the languages that diff er by only a single vowel (e.g., /e/ versus /ε/), and monolingual infants learning the Western Canadian dialect of English hear many variants in the phonetic realization of /e/ in their input; in both cases, infants may be willing to accept more variability in their early phoneme categories (Sabourin et al., 2009). It is also important to note that, as is the case with many studies of infants and young children, diff erent testing conditions lead to dif-ferent results across studies. Researchers are increasingly using methods that minimize the cognitive demands on infants, such as the ERP technique described earlier. ERPs do not require an overt response and may refl ect diff erences in the neural activity linked to each language as well as the time course of processing. Shafer, Yu, and Datta (2011) used ERPs to test the discrimination skills of bilingual and monolingual infants and young children learning English and Spanish in New York City. Children were tested on the vowels /ε/ and /I/, which are phonemes in English but not Spanish. Across infants, there was evidence of discrimination in the ERPs, but these eff ects varied according to age and language experi-ence. Brain responses were diff erent in bilingual than in monolingual infants, suggest-ing that bilingual infants may recruit additional cognitive resources for processing speech contrasts.

Precisely how much and what types of language exposure infants need to extract the features of a language’s phoneme categories remains unknown. Studies of monolingual infants learning English, Japanese, and Mandarin have suggested that sociocultural fac-tors such as the characteristics of infant-directed speech may facilitate the formation of phoneme categories (Kuhl et al., 1997; Liu, Tsao, & Kuhl, 2003, 2007; Werker et al., 2007; see Kuhl, 2007, for a review). Yet not all adults interact with infants or use infant-directed speech in the same ways (Rogoff , 2003). For infants raised bilingually, diff erent amounts and types of input in each language typically result in uneven language learning (see Chapter 6 in this volume, for example). Th ese diff erences may occur very early in development and set the infant on diff erent growth trajectories for each language. Using ERPs, García-Sierra and colleagues (2011) investigated phoneme discrimination (English and Spanish /da/ versus /ta/, which vary in VOT; see the Burns et al., 2007, study described previously) in bilingual English–Spanish infants in San Antonio, Texas. At 10–12 months of age, the size of an ERP discriminatory eff ect for phoneme contrasts in each language was associated with relative amounts of English and Spanish input in infants’ homes, as reported by parents, and both variables were linked to later expressive vocabulary in each language.

56 Conboy

Studies of monolingual infants exposed to a second language naturalistically have shown that social contexts facilitate rapid phonetic learning. Kuhl, Tsao, and Liu (2003) provided 9- to 10-month-old infants from English-speaking homes with play ses-sions conducted in Mandarin or English and then tested the infants at 11 months on a Mandarin speech contrast using conditioned head turn. Infants who received the English sessions could not discriminate the contrast, but the infants exposed to Mandarin showed discrimination similar to Taiwanese Mandarin-learning infants. Moreover, infants who received exposure to Mandarin through DVDs, from the same speakers as the infants who attended the play sessions, did not show discrimination of the contrast. In a follow-up study, Conboy and Kuhl (2011) provided naturalistic exposure to Spanish to monolingual English-learning infants from 9.5–10.5 months. In this study, infants were tested using the same Spanish and English contrasts and ERP paradigm used in the García-Sierra et al. (2011), Rivera-Gaxiola, Silva-Pereyra, & Kuhl (2005) and Rivera-Gaxiola et al. (2007) studies described previously and were tested before and aft er the 12 sessions of exposure to Spanish. Prior to Spanish exposure, infants showed an ERP discriminatory eff ect (simi-lar to the eff ect noted for native-language perception in Rivera-Gaxiola et al., 2007) only for the English contrast; aft er the exposure, at 11 months, infants showed the eff ect for both the English and Spanish contrasts. An analysis of infants’ behaviors during the expo-sure sessions indicated that infants who showed more joint attention episodes with the Spanish tutors had larger ERP eff ects to Spanish aft er the exposure sessions (Conboy, Brooks, Meltzoff , & Kuhl, 2011). Together, these studies suggest that social engagement with speakers of a language is important for early phonetic learning. Th e results also show that even small amounts of naturalistic experience with a second language can aff ect infants’ speech perception.

Th e research reviewed here on how bilingual infants develop perceptual categories for phonemes in each language leads to the conclusion that linguistic input factors (absolute amounts of input in each language, the relative dominance of one language over the other in the input, and the specifi c ways in which particular speech sounds are distributed in the input to bilingual infants) play a role. Additional input factors such as the amounts of code-switching caregivers use with infants, the extent to which infants hear infant-directed speech in each respective language, and the extent to which infants hear bilingual speakers who use diff erent acoustic values for phonemes than monolingual speakers may also play a role, but these factors remain to be explored. Moreover, input factors aff ect perception diff erently depending on the maturation of infants’ neurophysiology and anatomy, and on other infant-level factors. Th e studies of infants acquiring a second language at 9–11 months described in this section suggest that social interaction factors—both the social context provided and infants’ emerging social-cognitive skills that allow them to jointly engage with adults providing language input—may infl uence phonetic learning.

In addition to perceiving phoneme categories, infants begin to produce language-specifi c speech sounds during the fi rst year. As noted previously, the eff ects of the native language on the prosody of infants’ cries have been reported. Segmental aspects of speech, such as distinctions between diff erent vowels and consonants, are acquired later (see Chapter 15). Infants’ vocalizations gradually come to refl ect experience with the native language(s) as well as maturation of the anatomical structures used for producing speech. A milestone achieved by typically developing hearing infants by 10 months, canonical

Language Processing and Production 57

babbling, is the production of well-formed syllables that contain phonetic elements from the infant’s native language. Oller, Eilers, Urbano, and Cobo-Lewis (1997) studied the emergence of canonical babbling in infants from monolingual English, monolingual Spanish, and bilingual English–Spanish homes. Using parent report and direct sampling, the researchers found no evidence of a lag in the onset of canonical babbling, nor any quan-titative diff erences in the proportion of well-formed syllables and vowel-like sounds, in bilingual compared to monolingual infants. Other studies have explored bilingual infants’ vocalizations to determine whether there is a single phonological pattern or separate patterns for each language. What is clear is that there is no one individual pattern to which all bilingual infants adhere, even those who have the same parents (e.g., Schnitzer & Krasinski, 1996). However, the language context in which babbling is recorded is likely to infl uence the patterns noted. Ward et al. (2009) recorded speech samples from 11- to 12-month-old infants from monolingual English-speaking homes who had previously received short-term exposure to Spanish through naturalistic play sessions (i.e., infants in the Conboy & Kuhl, 2011 study). Infants systematically used longer utterances and more multisyllabic units in the Spanish context than in the English context. Th us, even a small amount of exposure to a second language in infancy may have an impact on speech produc-tion as well as speech perception.

As with perception, the social nature of language input also aff ects infants’ early vocalizations. Goldstein and Schwade (2008) examined the babbling of 9.5-month-old English-learning infants whose mothers were instructed to provide models of vocal produc-tion contingent upon the infants’ vocalizations or in a noncontingent fashion. Th e infants given contingent feedback changed their vocalizations to match those of their mothers, whereas the infants who received noncontingent feedback did not. Diff erent social contexts for learning two languages could therefore lead to diff erent patterns of acquisition for each. Many infants and toddlers raised bilingually receive input in one language from a diff erent speaker than for their other language, and the extent to which speakers provide contin-gent feedback and other features of infant-directed speech could lead to unequal devel-opment in each language. Ramírez-Esparza, García-Sierra, and Kuhl (2010) found that 10- to 16-month-old infants raised with bilingual English–Spanish input or monolingual English input produced more babbling when their parents used infant-directed speech ver-sus adult-directed speech in their presence.

Th e research on early speech sound perception and production reviewed in this sec-tion shows that infants growing up with two fi rst languages constitute a unique but diverse group of learners. Th ere is no strong evidence that bilingualism per se leads to a delay in the acquisition of early milestones in speech perception or production, but diff erences brought on by the problem space bilingual learners face can be mistaken for delays. Diff erent pat-terns may be noted in one group of bilingual learners but not another because of linguistic properties of the two languages infants are learning and/or sociocultural factors.

EARLY LEXICAL DEVELOPMENT: SEGMENTATION, REPRESENTATIONS, MEANING, AND PROCESSING

During their fi rst year, infants begin to develop a lexicon in whatever language or languages to which they are exposed. Th e fi rst signs of word learning are noted in how infants respond to particular word forms that they have heard many times. Parents begin to become fairly

58 Conboy

reliable reporters of infants’ word recognition as early as 8 months (Fenson et al., 1993), but experiments show that infants recognize highly frequent words, such as their own names, by 4–6 months (Mandel, Jusczyk, & Pisoni, 1995; Tincoff & Jusczyk, 1999). Studies of receptive vocabulary size in large samples of infants, using parent-report inventories that contain checklists of common words (the MacArthur-Bates Communicative Develop-ment Inventories [CDI]), show large variability across English monolingual American infants (e.g., Fenson et al., 1993) and Spanish monolingual Mexican infants (e.g., Jackson-Maldonado et al., 2003). In infants growing up bilingually, similar ranges are seen when researchers use composite scores that take into account word knowledge distributed across both languages (e.g., Conboy & Th al, 2006; Marchman & Martínez-Sussmann, 2002; Pearson, Fernández, & Oller, 1993; see also Chapter 6). Individual diff erences in rates of lexical development are observed throughout the life span and are infl uenced by multiple factors, including amounts and types of input with each language as well as child-level fac-tors (Bornstein, Haynes, & Painter, 1998; Fenson et al., 1994; Hart & Risley, 1995; see also Chapter 6). Bornstein and colleagues (2006) found that individual diff erences in monolin-gual infants’ information-processing skills at 4 months predicted subsequent rates of devel-opment in cognition and language up to 4 years of age, even aft er other endogenous factors (such as children’s temperament) and exogenous factors (such as the home environment and maternal education levels) were statistically controlled. Variability in rates of word learning across bilingual infants would be expected to be as great, if not greater, than that in monolingual infants due to the varieties of ways bilingual infants receive input in each language and the linguistic properties of the two languages they are learning, in addition to the same factors that infl uence monolingual development.

Th e research reviewed in the fi rst two sections of this chapter has shown how infants may use domain-general cognitive abilities to extract phonetic features from language input. Similar abilities may be used to launch the word-learning process. Before infants can map word forms to meaning, they fi rst need to segment word forms from the speech stream, given that the majority of utterances directed to them occur as continuous speech rather than as isolated words (Brent & Siskind, 2001). Segmentation abilities in the latter part of the fi rst year predict later vocabulary skills (Newman, Bernstein Ratner, Jusczyk, Jusczyk, & Dow, 2006); thus, effi ciency at recognizing words in the speech stream as familiar units may be an important mechanism for facilitating subsequent learning of word meanings. By 8 months, infants have been shown to use bottom-up strategies, such as statistical learning, to detect word boundaries, pick out words from continuous speech, and map these words to meaningful referents (Graf Estes, Evans, Alibali, & Saff ran, 2007; Saff ran et al., 1996). By 6–8 months, infants have also been shown to use top-down strategies, such as identifying word onsets that are adjacent to words they already know (e.g., identifying new words that come directly aft er known words such as mommy and the; Bortfeld, Morgan, Golinkoff , & Rathbun, 2005; Shi & Lepage, 2008); however, infants younger than 1 year have few familiar, recognizable words to use for such top-down segmentation. How bilingual infants learn dif-ferent sets of statistical cues to word boundaries across each language remains to be shown, but work with adults suggests that statistical learning of two languages simultaneously is possible (Weiss, Gerfen, & Mitchel, 2009).

Th e use of transitional probabilities as cues to word boundaries can only take infants so far; thus, infants must also be able to use other cues for segmentation (Jusczyk, 1999).

Bilingual Language Development of ELLs 59

Infants may also use their sensitivity to prosodic features of languages for bottom-up word segmentation. English contains many stressed monosyllabic words, and the majority of English disyllabic words adhere to a strong–weak (trochaic) stress pattern in which the initial stressed syllable is louder and longer in duration than the unstressed syllable (e.g., MOM-my). Infants learning English show a listening preference for trochaic over iambic words by 6 months and segment trochaic words from the speech stream by 7.5 months, but fail to segment iambic words until 10.5 months (for reviews, see Jusczyk, 1999; Pons & Bosch, 2010). Given the preponderance of disyllabic trochaic and stressed monosyl-labic content words (nouns, verbs, and modifi ers) in English, treating stressed syllables as cues to word onsets would be a reliable strategy for picking out words in English, but this strategy would not work for all words or for all languages. Infants learning languages that do not have trochaic words, such as French, do not show a preference for trochaic words (Höhle, Bijeljac-Babic, Herold, Weissenborn, & Nazzi, 2009), and they segment disyllabic words with iambic patterns (i.e., fi nal syllable lengthening) by 8 months (Polka & Sundara, 2003; but see Nazzi, Iakimova, Bertoncini, Frédonie, & Alcantara, 2006). Polka and Sundara (2003) found that bilingual infants learning two rhythmically diff erent languages, English and French, could segment English as well as French disyllabic words by 8 months, indicat-ing that they use diff erent strategies for each language.

Th e properties of English and Spanish and data on other language groups lead to cer-tain predictions regarding how English–Spanish bilingual infants might use lexical stress to segment words from the speech stream. English–Spanish bilingual infants might adopt diff erent segmentation strategies for each language based on the input they hear, as was found with English–French infants (Polka & Sundara, 2003). Spanish, like French, is a syllable-timed language, but unlike French, roughly 60% of its disyllabic words are trochaic, and, like English but unlike French, Spanish uses stress patterns to contrast word meaning (e.g., the trochaic papa, pope, versus the iambic papá, dad; Pons & Bosch, 2010). However, unlike English, Spanish also has many iambic words, and there is no reduction of vowels in unstressed syllables to schwa. Pons and Bosch (2010) found that 9-month-old mono-lingual Spanish-learning infants in Barcelona preferred listening to CVC-CV (in which C is a consonant and V is a vowel) trochaic versus iambic patterns and to CV-CVC iambic versus trochaic patterns. Th ese preferences were expected because most Spanish CVC-CV words used with children are trochaic (e.g., lindo, pretty) and most CV-CVC words are iambic (e.g., calor, hot). Alternatively, English–Spanish bilingual infants might not be able to use lexical stress alone for segmenting disyllabic words in Spanish given the variability of stress patterns in words in Spanish—they might apply the English trochaic strategy to both languages or simply succeed in English segmentation but fail in Spanish segmentation until they are able to use additional cues. Preliminary data with monolingual and bilingual Catalan and Spanish-learning 8-month-old infants show that they are equally able to seg-ment monosyllabic words, suggesting no delay in this ability in bilingual infants; data on disyllabic words are not yet available (Bosch, Figueras, & Ramon-Casas, 2008). It is impor-tant to note that results from infants in Spain might not apply to infants learning American dialects of Spanish, particularly ones in which many trisyllabic diminutive forms are used with infants instead of their disyllabic counterparts (e.g., dedito, little fi nger), because such diff erences can change the weighting of lexical stress patterns and syllable structure cues in the input. Moreover, when bilingual infants hear many people code-switching, common in

60 Conboy

several English–Spanish bilingual communities in the United States, stress patterns in the input might diff er from those in other Spanish-speaking language communities.

As infants accrue experience with the particular phonetic features of their native language(s), they detect word boundaries using phonotactic rules governing which pho-neme sequences can occur in words in the language, and knowledge about allophonic vari-ations in how particular phonemes are realized phonetically depending on their position in a syllable or word. By 9 months, infants can quickly learn new phonotactic regularities and use these to segment words (Saff ran & Th iessen, 2003). Catalan–Spanish bilingual infants are sensitive to phonotactic constraints, even when they diff er across languages (Sebastián-Gallés & Bosch, 2002). Catalan, like English, allows some consonant clusters in word-fi nal position that Spanish does not allow. Bilingual 10-month-old infants listened longer to lists of pseudowords that were phonotactically legal for Catalan than to illegal pseudowords, and this preference was more pronounced in infants who heard more Catalan at home. It is important to note that there was no diff erence between Catalan monolingual and Catalan bilingual infants. Infants also learn how allophonic variations in words cue word onsets and off sets between 9 and 11 months (for reviews, see Gervain & Werker, 2008; Jusczyk, 1999). For example, by 10.5 months, monolingual English-learning infants can detect word boundaries using allophonic cues (e.g., the acoustic diff erences between nitrates and night rates; Jusczyk, Hohne, & Bauman, 1999). English voiceless stop consonants (i.e., /p/, /t/, /k/) are aspirated when in the initial position of a stressed syllable, but their unaspirated allophonic variants are used in other positions. In Spanish, voiced stop consonants (i.e., /b/, /d/, /g/) are used in phrase-initial position, following nasals, and following [l] in the case of /d/, but their spirantized allophones (i.e., [β], [ð], [γ]) are used in other phonetic contexts, though there are many variations across dialects (see Chapter 15). Whether and how Spanish-learning monolingual or English–Spanish bilingual infants use allophonic cues for segmenting words remains to be investigated.

Th e ability to recognize new words from connected speech using multiple cues and strategies is thus present by the end of the fi rst year. A recent study suggests that infants placed in a second language situation can learn to recognize words. Conboy and Kuhl (2010) recorded ERPs to words in 11-month-old infants from monolingual English-speaking homes who had previously received short-term laboratory-based exposure to Spanish through naturalistic play sessions from 9.5 to 10.5 months (i.e., infants in the Conboy & Kuhl, 2011, study). Diff erent neural responses were observed to words used during those sessions and to Spanish words the infants had never heard, suggesting that infants can segment new word forms and hold them in memory over several weeks, even when the words are presented in complex connected speech in another language that has diff erent statistical, rhythmic, phonotactic, and allophonic cues than the native language. Simultaneous bilingual infants would be expected to keep pace with monolinguals on segmentation and would perhaps develop unique abilities. For example, 12-month-old bilingual infants simultaneously learned two diff erent patterns for how syllables could be ordered in speech (i.e., AAB or ABA patterns, in which A represents one syllable and B another syllable); monolingual infants the same age succeeded in learning only one pattern (Kovács & Mehler, 2009b). Th is study did not directly address speech segmentation or the learning of phonotactic or allophonic rules, but the results suggest that bilingual infants become more fl exible learners of multiple speech patterns than monolingual infants. Th e

Language Processing and Production 61

experience of constantly being faced with competing sets of linguistic cues could sharpen bilingual infants’ ability to avoid interference, and could also facilitate further learning in those infants.

Vihman, Th ierry, Lum, Keren-Portnoy, and Martin (2007) proposed that word recog-nition skills may vary across bilingual and monolingual infants due to diff erent phonologi-cal patterns in the languages. Th ey tested infants from English and Welsh bilingual and monolingual homes at 9 to 12 months using an ERP familiar–unfamiliar word paradigm and a behavioral head turn preference task. Th e monolingual English-learning infants rec-ognized English trochaic words by 10 months, but the monolingual Welsh-learning infants did not recognize Welsh trochaic words at any point between 9 and 12 months. Th e bilin-gual infants showed an intermediate pattern, recognizing both English and Welsh words by 11 months. Th e authors proposed that experience with English leads infants to pay attention to word onsets because word-initial stress is phonetically realized as increased duration, intensity, and pitch of the vowel in that syllable. In contrast, Welsh, though also a stress-timed language with many trochaic words, phonetically realizes this accentual pat-tern with a shorter vowel in the stressed syllable and a longer consonant and vowel in the unstressed second syllable. It is important to note that this fi nding does not refl ect a delay induced by bilingualism, because bilingual infants outperformed monolingual Welsh infants. Infants learning both languages may pay greater attention to word-initial cues than infants learning only Welsh but less attention than infants learning only English. Th e results of Vihman et al.’ s study led to the prediction that bilingual infants learning English and another language with a diff erent stress pattern (e.g., Spanish) may show less attention to initial consonants in words from both languages than monolingual infants learning only English. However, it is possible that decreased attention to initial consonants and syllables in Welsh may be infl uenced by other linguistic factors that do not occur in Spanish. For example, Welsh has a grammatical property not found in English or Spanish in which the initial consonants of nouns change according to the nouns’ grammatical gender.

Th e possibility that word recognition is aff ected by circumstances unique to bilin-gualism has been supported by studies of infants as they process newly learned words. Werker and colleagues have proposed that, in infants just starting to build a vocabulary, the demands of learning the meaning of a new word and learning the phonological form of that word simultaneously compete for limited cognitive resources and initially result in phonologically underspecifi ed representations (e.g., Stager & Werker, 1997; see also Hallé & de Boysson-Bardies, 1996). Monolingual infants recognize mispronunciations of newly learned pseudowords that are phonetically close (e.g., dih for bih) at 17–18 months but not 14 months; younger infants detect mispronunciations (e.g., vaby for baby) when not required to link the word to meaning, when the word is very familiar, or when there is a greater phonetic distance between the correct and mispronounced word (for a review, see Werker, Byers-Heinlein, & Fennell, 2009). Studies of bilingual infants have revealed confl icting results that can, to some extent, be traced to diff erences in test paradigms. In two studies, children were tested using a switch task in which they were presented with pseudoword–picture pairings and aft er repeated exposures, the pairings were switched and the pseudoword was pronounced correctly or incorrectly. Fennell, Byers-Heinlein, and Werker (2007) found that English–French bilingual infants did not detect mispronuncia-tions of newly learned pseudowords even at 17 months, an age at which monolingual infants

62 Conboy

succeed. However, Mattock, Polka, Rvachew, and Krehm (2010) found that English–French bilingual infants the same age succeeded when words were presented in a bilingual mode (i.e., using both English and French pronunciations of the pseudowords), whereas mono-lingual infants from both language backgrounds failed, likely because they were not used to hearing words pronounced with the other language’s phonology. However, monolingual infants did succeed when tested in a monolingual mode.

Ramon-Casas, Swingley, Sebastián-Gallés, and Bosch (2009) suggest that bilingual children may learn to ignore mispronunciations in a language when they are raised with input from a parent who is not a native speaker of that language. Th ey tested children using a preferential looking paradigm in which two pictures were presented simultaneously with a spoken word that matched one of the two pictures. On some trials the target word was pronounced correctly, and on others the vowel was changed to one that contrasts meaning in Catalan only (e.g., /e/ to /ε/) or in both languages (e.g., /e/ to /i/). Spanish monolingual and Catalan–Spanish bilingual 18- to 28-month-old children treated word mispronuncia-tions that involved a Catalan contrast as acceptable variants of the target word, whereas monolingual Catalan children favored correctly pronounced Catalan words. However, when the mispronunciation involved a vowel change contrastive in both languages, Spanish monolingual and bilingual children showed a mispronunciation eff ect. Older bilingual children (31–55 months) favored correct pronunciations over mispronunciations that involved a Catalan vowel contrast, but only when they had more Catalan than Spanish exposure in the home. Th e authors suggested that because many bilingual infants in this community hear Catalan from native Spanish-speaking parents, they may hear less of a distinction between acoustically close Catalan vowels and learn to ignore such distinctions. Th e results of these studies raise an important point about receptive language testing with young children. If an examiner has limited profi ciency in the language used for testing, the results should be interpreted with caution, as mispronunciations may aff ect word rec-ognition. Yet, it may also be true that bilingual children adapt their processing systems to accommodate the phonological systems of both languages, especially when the language pairs have many cognates and in bilingual communities in which there is larger variability in pronunciation due to speakers of multiple dialects as well as nonnative speakers. Further research on these topics is needed.

During their second and third years, children encode the meanings of many new words as well as their phonological forms. Bilingual children learn words at similar rates as monolingual children, though their learning is distributed across two languages and single-language vocabulary sizes can be smaller than those of monolingual children the same age (for a review, see Chapter 6). It is important to realize that such patterns do not represent any sort of a delay in vocabulary development. Th ere is evidence that 20-month-old monolingual toddlers rapidly learn the meanings of novel words even when they are taught in foreign language sentence frames (Bijeljac-Babic, Nassurally, Havy, & Nazzi, 2009). Th us, there is no reason to believe that naturalistic exposure to a second language would hinder word learning. However, little is known about word-learning mechanisms in bilingual toddlers. Byers-Heinlein and Werker (2009) investigated whether toddlers learn-ing more than one language were more likely to associate novel words with novel objects rather than with objects for which they had already learned a name. Th is ability (known as

Language Processing and Production 63

disambiguation) was present in 17- to 18-month-old infants from monolingual homes and, to a lesser extent, infants from bilingual backgrounds. However, it was not noted in infants that age from trilingual backgrounds. Th ese results suggest that disambiguation processes are infl uenced by infants’ early language experiences. However, by 27–35 months, bilingual children use the disambiguation strategy at the same levels as monolingual children the same age (Frank & Poulin-Dubois, 2002). Th e adaptive ability of infants acquiring multiple languages to initially accept multiple labels for the same item to a greater extent than mono-lingual infants may explain how they keep pace with monolingual infants in learning new words, but this ability may change with increasing age and language experience as other skills become available to facilitate word learning.

Effi ciency in processing newly learned words also improves over the second and third years, and in bilingual toddlers, processing effi ciency is linked to vocabulary develop-ment in each language. Using a preferential looking paradigm, Marchman, Fernald, and Hurtado (2010) examined spoken word processing in 30-month-old children learning English and Spanish simultaneously in the San Francisco, California, area. Children were presented with two pictures and an auditory word in a sentence frame; their eye gazes to the picture that matched the word were measured. A previous study of monolingual Spanish-learning toddlers that used the same test procedure had shown that speed of word recognition at 18 and 24 months was linked to vocabulary size and to the quantity and quality of the language input children received (Hurtado, Marchman, & Fernald, 2008). In the bilingual sample studied by Marchman et al. (2010), MacArthur-Bates CDI scores indicated that some children had larger English expressive vocabularies and others had larger Spanish vocabularies, and these patterns were linked to relative amounts of exposure to each lan-guage. Eye movements showed that children processed words more rapidly in the language with the larger vocabulary size. Children who were faster at processing words in one lan-guage were not necessarily faster at processing words in their other language, indicating that processing effi ciency is linked to experience with individual words.

Word processing has also been studied using the ERP technique, which can detect the activation of diff erent groups of neurons used at various stages of word processing. Conboy and Mills (2006) used an ERP known–unknown word paradigm with 20-month-old toddlers who were raised with English and Spanish in San Diego, California. Th e ERPs to English and Spanish words that children were reported by their parents to know and that the children identifi ed on a picture-pointing task were compared to ERPs to unknown words in each language. Children’s expressive vocabulary sizes in each language were also measured using the MacArthur-Bates CDIs. Children with larger conceptual vocabulary sizes (a score that counted each concept for which the child knew a word in either language; see Chapter 6 for more detail on this topic) showed more effi cient processing in both lan-guages compared to children with smaller conceptual vocabularies, indicating that among children the same age, variation in vocabulary size is linked to how the brain processes words. In ERP studies, effi cient processing is typically defi ned as faster processing and spe-cialization of processing to more focal areas of the brain (Mills, Conboy, & Paton, 2005). Bilingual toddlers’ word processing was also associated with relative vocabulary sizes in each language, with earlier and more focal eff ects noted for the language with the larger vocabulary. Vocabulary size is only one indication of language experience, however; in this

64 Conboy

study, children’s vocabulary sizes in each language corresponded to parents’ reports about which language children heard the most. Th e diff erent patterns noted for each language of the same children show that language experience and learning are linked to how the brain processes words in each language above and beyond maturation.

Th e Conboy and Mills (2006) study partially replicated results from previous studies of monolingual English-learning toddlers the same age (e.g., Mills, Coff ey-Corina & Neville, 1997). Yet two important diff erences were seen in the ERP eff ects at a particular point in processing (200–600 milliseconds aft er each word was presented). First, in previous studies of monolingual 20-month-olds, ERP eff ects in this time range were limited to left temporal and parietal sites, but in the bilingual 20-month-olds, the eff ects for each language were broadly distributed across hemispheres, more closely resembling patterns noted in younger monolingual children with similar vocabulary sizes in each language. Th is is consistent with numerous studies showing that bilingual children typically know fewer words in each separate language than monolingual children because of the distributed nature of bilingual lexical development (see Chapter 6) and with previous reports that the effi ciency of process-ing is linked to vocabulary size (Mills et al., 1997). Second, for the bilingual children with larger conceptual vocabulary sizes, the eff ects were largest at right-anterior sites but only for words in the language with the larger vocabulary. Moreover, this right-anterior eff ect was only observed when children were tested in a mixed-language condition in which words alternated randomly between English and Spanish; it was not observed when a separate group of bilingual children was tested in a single-language condition, hearing blocks of only English or Spanish words at a time. Children tested on one language at a time also showed earlier eff ects than children tested in the mixed-language condition, indicating more effi -cient processing. Th ese results are consistent with studies of older bilingual children and adults that have shown it can take additional time and cognitive resources to access words when both languages are activated (see Chapter 5). Although ERPs do not provide precise information about the localization of brain activity, relative diff erences in eff ects indicate that distinct populations of neurons are activated, and refl ect diff erent cognitive resources being used for processing words across conditions. Th is fi nding needs to be replicated in a larger sample of children tested in both conditions, but the results suggest that the unique circumstances of bilingualism—activation of two competing language systems—give rise to slight variations in processing systems and that these are noted early in development. Preliminary evidence from a follow-up study with 2-year-olds from English–Spanish back-grounds in San Antonio, Texas, suggests that children with more advanced executive func-tion skills show more effi cient word processing when tested in a mixed-language condition than children with less advanced cognitive skills, indicating possible interactions between language and nonlinguistic cognitive processing (Conboy, Sommerville, Wicha, Romo, & Kuhl, 2011).

Th e studies described in this section suggest that at early ages bilingual children adapt their language processing skills to accommodate the unique demands of bilingual-ism, such as switching between languages. Additional evidence of word processing diff er-ences between bilingual and monolingual infants was provided by an ERP study of children acquiring English and Welsh in North Wales. Kuipers and Th ierry (2011) presented 2- and 3-year-old children with pictures of familiar objects and a word that either matched or

Language Processing and Production 65

did not match the picture in English (75% of the time) or in Welsh (25% of the time). Bilingual children rapidly detected the language switch (an early eff ect was noted in their ERPs within 200 milliseconds of hearing the word, similar to one reported in a study of bilingual adults). Monolingual English-learning children did not show this early eff ect but did show a later eff ect, indicating the use of diff erent processes. In other words, the bilin-gual children showed a unique language-change detection process that was not noted in monolingual children, who are not normally faced with two languages in their daily lives.

SUMMARY AND CLINICAL APPLICATIONS

Th e research reviewed in this chapter shows that bilingual and monolingual infants have similar developmental trajectories for certain skills but not others, and that when diff erences are noted, they refl ect the use of adaptive learning processes. Th e focus of early bilingual-ism research is moving away from questions about whether there are “defi cits” or “bene-fi ts” associated with bilingualism, and toward a more unifi ed, dynamic theory of how brain plasticity allows for adaptation to changing circumstances and requirements of learning. In spite of research fi ndings, there seems to be a persistent theme in both research and practice that considers monolingualism to be a standard against which other forms of fi rst language acquisition should be judged. Such a standard fails to value the unique aspects of early bilin-gual development. Based on available research fi ndings with typically developing infants and toddlers, speech-language pathologists and other professionals who work with bilingual infants and young children should consider the following points regarding practice.

1. Bilingual infants can perceptually separate their languages from birth. Clinicians do not need to advise parents to use a “one-parent, one-language strategy” because infants can use several diff erent cues to learn separate language systems.

2. Th ere is no such thing as a “prelinguistic infant.” From their fi rst moments of life, infants use low-level basic computational abilities to gradually build a language system, and in both monolingual and bilingual children these early domain-general abilities are continuous with later language acquisition. Infants provided with naturalistic lan-guage input from birth can further develop the basic learning mechanisms needed to acquire other aspects of language. Attention to these basic abilities may facilitate earlier detection of language-learning disorders.

3. Early bilingualism is characterized by large variability; there is no single bilingual pattern of development. Practitioners should be careful about generalizing the fi nd-ings from bilingual children from one community to bilingual children from another community. Th e unique linguistic properties of each language, typological distances between languages, and sociolinguistic diff erences in how languages are used with infants and young children can all infl uence language processing.

4. Children begin life with mechanisms for fi nding patterns in their input languages and develop strategies for language learning based on the statistical distributions of speech sounds, rhythmic patterns, and other properties of their languages. At young ages, infants can detect mispronunciations of words that they have already encoded in memory. Well-intentioned eff orts to conduct assessment and intervention in a child’s language may fall short when the practitioner is not highly profi cient in that language.

66 Conboy

When very young children are provided with language input that does not match the prosody or segmental features they are used to hearing, their ability to process such input may be compromised. Moreover, disruptions to the normal prosody and other phonological features of language may disrupt children’s ability to segment words from connected speech. One way out of this quandary is for clinicians who lack high levels of profi ciency in a client’s language or dialect to involve caregivers in assessment and intervention and to encourage caregivers to use the language in which they speak most naturally.

5. Practitioners should not underestimate the impact that small amounts of second lan-guage exposure can have on language perception and production. Research reviewed in this chapter has shown that infants as young as 9–10 months of age show changes in perception and in their vocalizations aft er as little as 5 hours of naturalistic exposure to a second language. When interviewing families about language use for the purpose of selecting assessment measures, practitioners should be careful to consider all forms of regular exposure to another language.

6. Social contexts and interactions are crucial for early language development. Practi-tioners must carefully consider the contexts in which they observe language skills in infants, consider the sources of language input, and obtain communication samples during socially interactive situations. When practitioners attempt to involve children’s caregivers in intervention, they should encourage caregivers to interact in ways that are socially and culturally appropriate, to provide the most naturalistic learning situation possible.

7. Processing effi ciency is as important for language functioning as knowledge of lan-guage forms. Research reviewed in this chapter has indicated that for bilingual infants and toddlers, processing effi ciency is linked to experience in each separate language. Clinical tools for measuring processing effi ciency could provide useful clinical infor-mation. Diff erences between typically developing infants and those with language disorders may be most noticeable when language processing (i.e., perception and speed of retrieval), rather than production, is examined and therefore may be invisible to practitioners who do not have access to reliable procedures for assessing children’s processing skills.

8. Children who have learned two languages from birth are native users of each language, but they are not monolingual users of either language because their bilingual language-learning situation has produced unique ways of processing language that are evident even in the earliest stages of development. Th ough there are many similarities between bilingual and monolingual development, the cognitive abilities that support the learn-ing of any particular language develop within sociocultural communicative contexts and are shaped by the demand for particular behaviors within those contexts. Th us, bilingual children should not be compared to monolingual standards, even during the earliest stages of language acquisition. Diff erences between bilingual and monolingual children refl ect appropriate responses to properties of the input rather than a delay induced by bilingualism. Practitioners should remember that the term delay connotes a defi cit view of bilingualism and is not an accurate way of describing the unique pat-terns associated with early bilingualism. In fact, the term could equally accurately be

Language Processing and Production 67

applied to monolingualism, given that some of the research reviewed in this chapter has shown that bilingual infants and toddlers outperform their monolingual peers on certain tasks. Over and over, testing paradigms have shown that bilingual infants can-not do certain things that monolingual infants can do at the same ages, but when other testing paradigms are used, bilinguals succeed and sometimes outperform their mono-lingual peers.

Unfortunately, few commercial tools both developed and normed with bilingual populations are available to clinicians. Clinicians working with English–Spanish bilingual infants and toddlers may use tools developed for monolingual infants and toddlers, such as the English and Spanish versions of the MacArthur-Bates CDI (Fenson et al., 2006; Jackson-Maldonado et al., 2003), but should do so cautiously. Although these inventories have not been normed on bilingual infants, studies have shown that they validly and reliably meas-ure early vocabulary size and other language skills in young bilingual children (Marchman & Martínez-Sussmann, 2002). Further research is needed to determine the long-term pre-dictive validity of such measures and norms for bilingual infants and toddlers.

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