As new language learners, young infants need to determine the possible sound forms in the ambient environment that entail lexical relevance. They must learn to ignore acoustic sound contrasts that do not carry meanings. This task may be more challenging for infants exposed to more than one language, which accounts for more than 50% of the world population (Grosjean, 2010), although how bilingual infants acquire language is largely derived from research studying monolingual infants. Similarities between monolingual and bilingual developmental trajectories can reveal fundamental learning mechanisms and highlight the nature of bilingual learning, whereas differences may reflect specific learning strategies and outcomes stemming from different learning environments. Tone languages consist of more than 60% of the world’s languages (Yip, 2002). The current study adds to our understanding of non-native tone-language learning and investigates the intersection of linguistic and lexical development by examining the learning of minimal pairs involving a tonal contrast across non-tone-language learning monolingual and bilingual infants in the second year after birth.

Infants have an astounding sensitivity to speech sounds in the ambient environment. As such, newborns discriminate between non-native languages through different rhythmic classes (Mehler et al., 1988; Nazzi et al., 1998a), pitch contours (Nazzi et al., 1998b), and lexical stress patterns (Sansavini et al., 1997). In the first year of life, infants tune in to their native sound inventories and tune out of non-native contrasts, a process known as perceptual attunement (Werker and Tees, 1984; Anderson et al., 2003; Kuhl et al., 2006; Watson et al., 2014). The language-specific attunement occurs around 8–12 months for consonants (Werker et al., 1981; Best et al., 1995) and around 6–8 months for vowels (Kuhl et al., 1992). By the end of the first year, infants’ sensitivity to non-native consonant and vowel contrasts greatly decreases. These perceptual patterns extend to adulthood (Tsushima et al., 1994; Tsao et al., 2000). As for the attunement of lexical tones, tone-language learning infants maintain and improve their tonal sensitivity (Harrison, 2000; Mattock and Burnham, 2006; Yeung et al., 2013; Shi et al., 2017a; Tsao, 2017). Meanwhile, non-tone-language learning infants’ sensitivity to tones greatly decreases at 9 months (Mattock and Burnham, 2006; Mattock et al., 2008; Liu and Kager, 2014; Cabrera et al., 2015; Shi et al., 2017b).

After perceptual attunement, listeners do not appear to totally lose sensitivity to tones. Instead, categorical perception (Hallé et al., 2004; Xu et al., 2006; Chen et al., 2015) and neuroimaging studies (Gandour et al., 2000; Kaan et al., 2008) suggest non-tone-language listeners are sensitive to tones, though perceiving them in an acoustic manner. In other words, non-tone-language listeners appear to demonstrate acoustic instead of linguistic processing of tones, determined by a number of factors across contexts, experiences, and modalities (Burnham et al., 2015a,b). A tonal sensitivity rebound has been found in non-tone-language learning infants in the second year after birth, resulting in a U-shaped tonal perceptual trajectory (Liu and Kager, 2014). Tested in a visual habituation paradigm, Dutch monolingual infants show a rebound in sensitivity to a tonal contrast at 17–18 months. No similar U-shaped pattern across the first 2 years after birth has been reported for the perception of non-native consonant and vowel contrasts (Liu and Kager, 2015a,b). However, since non-tone-language adult listeners are sensitive to tones (Xu et al., 2006), such rebound is not entirely unexpected. The rebounded sensitivity is attributed to infants’ acoustic (or phonetic, hereinafter) rather than linguistic (or phonological, hereinafter) sensitivity in light of non-tone-language adult listeners’ acoustic perception of tones (Hallé et al., 2004; Jongman et al., 2017). This is similar to English infants’ discrimination of non-native Zulu click sounds given the acoustic dissimilarity of these sounds to native inventory (Best et al., 1988, 1995).

Question arises whether infants growing up learning two languages follow the same trajectory as monolinguals. Following the previous study reporting U-shaped tonal perceptual trajectory (Liu and Kager, 2014), a follow-up study reports that non-tone-language learning infants from bilingual backgrounds show a rebound to the same tonal contrast at approximately 11–12 months after birth, 6 months earlier than their monolingual peers. Similar findings have been reported for the perception of consonant and vowel contrasts (Bosch and Sebastián-Gallés, 2003; Burns et al., 2007; Albareda-Castellot et al., 2011; Liu and Kager, 2016b). Similar to that of monolinguals, the rebounded sensitivity in non-tone-language learning bilingual infants matches adult data in suggesting that non-tone-language listeners perceive lexical tones acoustically (Hallé et al., 2004; Jongman et al., 2017). More generally, it matches previous literature showing that adult tone-language and non-tone-language listeners use different acoustic cues when perceiving lexical tones (Lee et al., 2008, 2010; Huang and Johnson, 2010; Cabrera et al., 2014).

Additionally, several factors such as enhanced auditory sensitivity and cognitive advantages have been proposed to account for the bilingual perceptual difference (Liu and Weidemann, 2017). The rebounded sensitivity in non-tone-language learning infants and the bilingual difference are crucial for the current study as they lead to further questions: What is the nature of non-tone-language learning infants’ tonal perception at the rebound stage: acoustic or linguistic? Does rebounded sensitivity influence infants’ learning ability of non-native words? Furthermore, does a bilingual difference in perception lead to a better outcome in learning?

These questions can be answered through label–object mapping involving non-native tonal minimal pairs. Specifically, if infants perceive tones linguistically at the rebounded stage, they are expected to be able to learn words contrasting in tone. Alternatively, if their perception is acoustically driven, non-tone-language learning infants’ tonal word learning ability should deteriorate with age. Infants initially accept a wide range of word forms (e.g., non-speech sounds; Woodward and Hoyne, 1999; Hirsh-Pasek et al., 2000; Namy, 2001), and recognize early word forms with increasing linguistic experience (Jusczyk and Hohne, 1997; Tincoff and Jusczyk, 1999; Swingley and Aslin, 2002; Fennell and Werker, 2003; Bergelson and Swingley, 2012, 2013). They are able to recognize frequently used words and map novel labels to novel objects as early as 6 months (Shukla et al., 2011; Bergelson and Swingley, 2012). At 12 months, infants have developed native phonotactics, and continue to show label–object mappings for non-native sound contrasts (Jusczyk and Luce, 1994; MacKenzie et al., 2011, 2012). Dutch infants of 18 months interpret vowel duration as lexically contrasting, but English learners of the same age do not (Dietrich et al., 2007). This is in keeping with the different vowel properties of Dutch and English. By 20 months, their ability to associate non-speech symbols or non-native sounds with objects deteriorates (Namy and Waxman, 1998; Woodward and Hoyne, 1999; May and Werker, 2014; Saffran, 2014; Hay et al., 2015). In the second year after birth, infants appear to have formed sound categories from native language which they adopt to guide the acquisition of words, suggesting the experience of a second attunement (Werker and Tees, 2005). That is, infants refine what they consider to be possible word forms, and their early linguistic learning entails not only language-relevant acoustic cues but also linguistic interpretation at appropriate levels of linguistic analysis.

Tested by a label–object mapping paradigm in which infants are required to map two novel sounds with novel objects, 14- and 20-month-olds successfully associate dissimilar-sounding words with novel objects (lif-neem; Stager and Werker, 1997; chook-dal, Bijeljac-Babic et al., 2009). They do not typically succeed in associating minimal pair acoustic features with novel objects (bih-dih) possibly limited by their low vocabulary size (Werker et al., 2002) or task difficulty (Yoshida et al., 2009). Nevertheless, they are able to do so when additional information is provided, such as (1) increased referential cues (Fennell and Waxman, 2010), (2) additional object familiarization (Fennell, 2012), (3) enhanced speaker variability (Rost and McMurray, 2009), (4) added social interaction (Mani and Plunkett, 2008), (5) reduced task difficulty (visual choice procedure, Yoshida et al., 2009), and (6) similar lexical contexts (frequently occurring phonemes, Thiessen, 2007). At 17–20 months, infants are able to associate novel objects with minimal pair words (Werker et al., 2002). Their performance is tightly correlated with current and future language comprehension and production skills (Bernhardt et al., 2007). Few studies have directly examined the tonal word learning ability among non-tone-language learning monolingual and bilingual infants. English-learning infants succeed in label–object mapping of monosyllabic words that differ in a tonal contrast in Mandarin Chinese (T2 rising vs. T4 falling) at 14 months but fail at 17 (Burnham et al., 2017) and 19 months (Hay et al., 2015). They also fail to associate objects with Mandarin T1 flat vs. T2 rising contrast at 17 months (Burnham et al., 2017). Taken together, infants’ native word learning ability increases between 14 and 20 months, while their non-native word learning ability decreases (Hay et al., 2012, 2015).

It remains unclear whether infants’ diverse linguistic experience may prolong the developmental trajectory in word development. Some studies show non-prolongation of the developmental trajectory. Monolingual and bilingual infants appear to experience linguistic milestones and developmental trajectories at similar time windows (Swain, 1972; Vihman, 1985; Pearson et al., 1993, 1995; Petitto and Kovelman, 2003; Vihman et al., 2007; Werker and Byers-Heinlein, 2008; Werker et al., 2009; Hoff et al., 2012; Werker, 2013; De Houwer et al., 2014; Singh et al., 2014; Liu et al., 2017), with matched number of lexical concepts (Pearson et al., 1993; Pearson and Fernandez, 1994; Junker and Stockman, 2002; Thordardottir et al., 2006; Hoff et al., 2012; De Houwer et al., 2014; Liu et al., 2017). When appropriate contextual carriers are given (e.g., speaker matching their language environment), monolingual and bilingual infants both learn certain minimal pairs (/bos/-/gos/, Mattock et al., 2010; /k𝜀m/-/g𝜀m/, Fennell and Byers-Heinlein, 2014) at 17 months. On the other hand, prolonged word learning trajectory in bilingual infants have also been reported (Friedrich and Friederici, 2004; Conboy and Mills, 2006; Kaushanskaya and Marian, 2009; Marchman et al., 2010; Byers-Heinlein, 2013; Singh, 2017; Singh et al., 2017). Nine-month-old Chinese–English bilingual infants recognize both Chinese and English words contrasted in tone (Singh and Foong, 2012), even though English does not use tone to differentiate meaning. While monolingual infants begin to succeed at learning two minimal paired non-words (/bI/-/dI/) around 17 months, bilinguals do not succeed until 20 months (Fennell et al., 2007; Werker, 2013). Bilingual children are behind their monolingual peers in vocabulary size when single language, especially the non-dominant language, is compared from 8 months up to 10 years (Bialystok et al., 2010; Gauthier and Genesee, 2011). Eighteen-month-old bilingual but not monolingual infants keep their flexibility for a prolonged period and continue to show the mapping of labels that minimally contrast in pitch contour to novel objects, and the ability deteriorates at 22 months (Graf Estes and Hay, 2015). Both monolingual English-learning and bilingual English–Chinese-learning infants are able to detect tonal substitutions as mispronunciations at 18 months, but monolingual infants are no longer able to do so at 24 months while the bilinguals were still able to do so. Bilingual infants’ performance appears to be language-specific: they can detect tone mispronunciations in Chinese but not English contexts (Singh et al., 2014). In sum, the results of word learning studies with monolinguals and bilinguals suggest that infants attribute linguistic relevance to tones in a language-specific fashion between 18 and 24 months. By the end of the second year, infants’ ability to use lexical tone for word learning is in accordance with their native language and exposure.

Discrepancies between monolingual and bilingual infants’ novel word learning outcomes may be attributed to a number of factors such as different use of learning strategies (Sebastián-Gallés et al., 2012) and task design (Singh et al., 2012). Infants from a multilingual environment do not use mutual exclusivity to the same degree as their monolingual peers when learning words (Byers-Heinlein and Werker, 2009). They are sensitive to environmental differences and contextual cues (Mattock et al., 2010; Fennell and Byers-Heinlein, 2014). In addition, differences in time windows at which non-tone-language learning infants’ tonal label–object mapping ability decreases may be due to different testing paradigms, which may elicit different levels of sensitivity.

Apart from these factors, acoustic properties of tonal contrast may also play a role. A number of studies use T2–T4 (rising–falling) in Mandarin Chinese as the target contrast (Singh and Foong, 2012; Hay et al., 2015), with the pitch directions close to those in the interrogative-narrative intonation patterns in many languages, such as English and Spanish. This potentially introduces an effect of perceptual assimilation, assimilating a non-native phoneme into one’s native phonemic category (Best, 1994; Soderstrom et al., 2011; Tyler et al., 2014). If perceptual assimilation plays a role in tonal processing and learning, we would expect non-native listeners’ better performance in T2–T4 (rising vs. falling) compared to the T1–T4 contrast, in which the assimilation of T1 remains unclear. In addition, an expansion of investigation to other non-native tonal contrasts and re-examination of the word learning of 18-month-old non-tone-language learning infants is necessary to further understand the impact of age, stimuli, and language background on interpretive narrowing in word learning (Stager and Werker, 1997; Hay et al., 2015), the process by which infants restrict the types of sounds that can be mapped to word meanings.

To understand the impact of linguistic diversity on novel word learning ability, we tested monolingual and bilingual infants, both of whom lacked prior experience to lexical tones. To reduce the effect of perceptual assimilation on tonal word learning, a new tonal contrast different from previous studies (T2 rising vs. T4 falling) was used. Linking the previous question concerning the nature of tone perception among infants learning non-tone-languages, the research questions are: How does non-tone-language learning Dutch infants’ label–object mapping ability for sound-to-meaning pairs involving lexical tone contrasts develop during the second year of life? Do learning patterns differ between non-tone-language learning monolingual and bilingual Dutch infants? We adopted a word learning paradigm using the same stimuli as in the previous visual habituation paradigm (Stager and Werker, 1997) and tested monolingual and bilingual infants at two age ranges (14–15 and 17–18 months). To understand the effect of contrast acoustic properties on learning and reduce the effect of potential perceptual assimilation, a contrast in Mandarin Chinese (T1 level vs. T4 falling) was used. We predicted successful learning at 14–15 months for both monolingual and bilingual Dutch infants and left the prediction open for 17- to 18-month-olds. We hypothesize that the tonal rebound is acoustic/phonetic in nature and hence would not positively affect word learning. Bilingual infants may show enhanced performance for word learning due to their flexibility in learning non-native tone-to-word pairings (Graf Estes and Hay, 2015). Alternatively, bilingual infants undergo the perceptual rebound earlier than monolinguals. These two factors may affect bilinguals’ ability to learn new words contrasting on tones at 17–18 months.

This paper investigated the ability to learn label–object associations of a non-native tonal contrast in toddlers acquiring non-tonal languages, testing the generality of the interpretive narrowing. A tonal contrast different from previous studies (Singh et al., 2014; Graf Estes and Hay, 2015) was adopted for a better understanding of the effect of acoustic properties and perceptual assimilation on learning. Results shed light on Dutch infants’ non-native interpretive narrowing process with two main findings. First, infants were able to establish associations between novel tones and objects at 14–15 months, whereas they failed to do so at 17–18 months. Second, the current results indicated similar developmental trajectories between monolingual and bilingual infants in word learning involving novel and non-native sound contrasts.

This paper addresses how early language learners determine which acoustic dimensions in their environment differentiate word meanings. Non-tone-language learning monolingual and bilingual infants are able to construct linguistic representations of Mandarin T1–T4 tones at 14–15 but not at 17–18 months. Linking the current findings with previous literature, we hypothesize that infants’ perception of non-native tones is more acoustic than linguistic in the later phase of language development, that is, mainly based on the acoustic properties of tones. In addition, provided the different outcomes across contrasts (T1–T4 vs. T2–T4) between the current and previous studies, we are inclined to suggest a role for perceptual assimilation in non-native word learning (Best, 1994). That is, non-tone-language learning infants’ tonal label–object mapping ability is affected by intonation contours from their native language, and facilitation may occur when acoustic similarities/overlaps occur between non-native tones and native intonation (e.g., T2–T4). Given that differences may also lie in the paradigms used across associative word learning studies, the suggestion should be considered with caution. Last but not least, bilingual infants appear to at least keep the same pace as their monolingual peers along the word learning of non-native tonal contrasts.

LL contributed to the experiments. LL and RK contributed to the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.