Learning words is a crucial step in learning a language, no matter whether it is one's initial, native language as an infant, or a new, non-native language as an adult. Importantly, learning new words requires the ability to process relevant phonetic information and represent it in proper phonological categories. This ability is largely based on which phonetic variations are relevant to word meaning and how phonological categories are established in one's native language. In all languages, lexical representations include segmental information related to the identity of consonants and vowels constituting the word forms. In many languages, lexical representations also include suprasegmental information, such as lexical stress, pitch accents or tones. Phonological repertoires vary across languages, and the same is true for lexically-relevant prosodic information. For instance, both Cantonese and Mandarin have tonal systems, but their systems differ in both number and identity of tones (Wang, 1963; Mandarin: Cheng, 1966; Hashimoto, 1972; Howie, 1976; Cantonese: Bauer and Benedict, 1997; Duanmu, 2000; Yip, 2002), as illustrated in Figure 1. The present study will explore the interplay between word learning and phonological processing (of consonants, vowels and tones) by comparing three groups of adults when learning minimal pairs of Cantonese words in their native (Cantonese-speaking adults) vs. a non-native (Mandarin- and French-speaking adults) language.

Decades of research have established that speech perception becomes language-specific during the first year of life, as attested by decreases in ability to discriminate many (though not all, see below) non-native phonological contrasts (that is, contrasts not used in one's native language), and increases in the ability to discriminate native contrasts. These changes have been found to happen later for consonants (by 10–12 months of age, e.g., Werker and Tees, 1984a; Best et al., 1988; Rivera-Gaxiola et al., 2005), than for vowels (by 6 months of age, Kuhl et al., 1992; Polka and Werker, 1994). The developmental timing for tones is less clear, as changes are usually reported around 10 months of age (Mattock and Burnham, 2006; Mattock et al., 2008; Liu and Kager, 2014; Cabrera et al., 2015) although evidence for changes as early as 4 months has been found in one study (Yeung et al., 2013).

These developmental changes in speech perception, which attest to the early acquisition of the phonological repertoire of the native language, have continued effects in adulthood. Speech perception difficulties have been found for the processing of non-native consonants (e.g., Werker and Tees, 1984b), non-native vowels (e.g., Polka, 1995), and non-native tones (Gandour et al., 2000; Hallé et al., 2004; So and Best, 2010). This is attested by the fact that adults will sometimes have difficulties identifying some non-native sounds, and/or discriminating between non-native sounds. For example, regarding consonant perception, the fact that the English /r/ consonant does not have an equivalent in both Japanese and German has been found to lead to differences in how Japanese- and German-speaking adults perceive this sound (in contrast to English /l/) when compared with English-speaking adults (e.g., Miyawaki et al., 1975; Iverson et al., 2003). Moreover, perception of this non-native contrast differs across the two language groups, with more difficulty observed for the Japanese-speaking adults, who appear to form only one sound category, compared to the German-speaking adults who perceive two sound categories (e.g., Iverson et al., 2003). This further shows that these processing difficulties stem from interference with the native phonological system. With respect to tones, many studies have found that speakers of languages that do not use tone contrasts at the lexical level identify and discriminate non-native tones with more difficulty than speakers of tonal languages (Gandour et al., 2000; Hallé et al., 2004; So and Best, 2010). Even though some discrimination ability is found in speakers of non-tonal languages, they appear to process tones differently. This is attested, for example, by the fact that (non-tonal) French-speaking adults, while being able to discriminate non-native Mandarin tones, perceive these tones less categorically than Mandarin-speaking adults. Some have proposed to link this to their lack of phonological categories for tones (Hallé et al., 2004).

The first goal of the present study was thus to explore the effects of such perceptual changes on adults' ability to learn new words in an unfamiliar language. Although this is a situation that adults have to cope with when learning a new language, it has received surprisingly little attention to this day (but see Chandrasekaran et al., 2010; Cooper and Wang, 2012, 2013, for training studies on English-speaking adults' processing of Cantonese or Mandarin tones in lexical contexts). Here, we evaluated monolingually-raised Mandarin- and French-speaking adults' ability to learn new words in Cantonese, and compared their performance to baseline data from Cantonese-speaking adults. This was done in a laboratory setting, during which, on each trial, adults had to learn a pair of Cantonese pseudowords that differed by either a consonant, a vowel or a tone. Given the above language specialization findings at the perceptual level, evidenced by difficulties in low-level (discrimination or identification) non-native processing, we predict that adults (and infants from 6 to 12 months onward) should have more difficulty learning new words in a non-native language than in their native language, because they are made up of some sounds that do not belong to the native phonological repertoire. Hence, overall performance should be higher for Cantonese-speakers than for Mandarin- and French-speaking adults, and it might even be that the latter two groups fail at learning. Another possibility is that linguistic distance between the native language and the language of the stimuli affects performance. Since Cantonese and Mandarin, being both Sino-Tibetan languages, share many phonological, morphological, and syntactic properties (Li, 1937; Gong, 1980; DeLancey, 2009), which is not the case with Cantonese and Indo-European French, Mandarin-speaking adults might have higher overall performance than French-speaking adults.

One important feature of our experimental design is the fact that on each trial adults had to learn a pair of new pseudowords. Therefore, for learning to take place, adults had to process the phonological contrast distinguishing the two paired words, which allowed us to explore in more detail the interplay between phonological and lexical processing in this process of acquiring new words. To begin with, the fact that the pseudowords contrasted in either consonant, vowel, or tone information allowed us to evaluate differential processing of these three phonological sound categories. This second goal of the present study was motivated by the proposal that consonants carry more information about the lexicon, whereas vowels play a more important role in syntactic and prosodic processing (Nespor et al., 2003). For example, in word reconstruction studies in which English, Dutch, and Spanish listeners hear pseudowords and have to transform them into real words, they preserve consonantal over vocalic information, changing kebra into cobra rather than zebra (van Ooijen, 1996; Cutler et al., 2000). Evidence for this bias for consonantal information in lexical processing (often referred to as the C-bias in the literature) is supported by studies with adults across several non-tonal languages (Dutch, English, French, Italian, Spanish) and a variety of different methods such as word learning (e.g., Bonatti et al., 2005; Creel et al., 2006; Toro et al., 2008; Havy et al., 2014) and lexical access (e.g., New et al., 2008; Carreiras et al., 2009; Delle Luche et al., 2014; New and Nazzi, 2014).

Importantly though, when this project was started, little was known about the C-bias in non-European languages, and in particular in tone languages. Tone languages provide a particularly interesting test of the C-bias as lexical tones are mostly carried by vowels. This might affect performance, in two opposing ways. Indeed, the need for speakers of tone languages to attend to tones to identify words might increase their attention to the vowels (which carry them), and thus increase the weight given to vowels compared to consonants in tone languages compared to non-tone languages. This might result in a lack of bias or in a reversed advantage in processing vocalic information. In contrast, the fact that vowels carry tones might make the acoustic realization of vowels more variable in tone than in non-tone languages, making them more difficult to process and identify. If so, the consonant bias in lexical processing found in non-tone languages might be even more pronounced in tone languages.

To date, only two studies have explored this issue, but have focused on levels other than word learning: lexical access to known words, and word form segmentation in an artificial language. First, in a word reconstruction study based on van Ooijen (1996) and testing lexical access, Wiener and Turnbull (2016) asked participants to transform a pseudoword into a real word by changing either a consonant, a vowel (in fact, to conform to Chinese phonology, they were asked to change the final - in Chinese phonology, and in the stimuli used in that study, the final corresponds to V, VV, or VVN), a tone, or any of the three. Results show effects of condition, corresponding to the fact that Mandarin-speaking adults appear to preferentially change tones over both consonants and vowels/finals, with vowels appearing to be the less mutable sound category, contrary to what had been found in Dutch, English, and Spanish (van Ooijen, 1996; Cutler et al., 2000). These findings suggest a different balance in the weight given to consonants and vowels, with less weight given to consonants (or more weight given to vowels), by Mandarin-speaking adults. Second, an artificial language study exploring whether Cantonese-speaking adults use consonants or vowels (and tones) to segment a fluent speech stream revealed that they could not use consonantal information alone, but could rely either on vocalic information alone (although the difference between the consonant and vowel conditions was not significant), or more likely on a combination of vocalic and tonal information (Gómez et al., 2017). This also suggests a different balance in the weight given to consonants and vowels by Cantonese-speaking adults as compared to French- or Italian-speaking adults (Bonatti et al., 2005; Toro et al., 2008). The present study will add to this literature by providing the first evaluation of this issue in a word learning task for tonal language speakers, for either the native language (Cantonese adults processing Cantonese stimuli) or a foreign language (Mandarin adults processing Cantonese stimuli). It will also provide the first evidence of whether the C-bias, found in French-speaking adults when processing native stimuli, would also extend to the processing of non-native stimuli (French adults processing Cantonese stimuli).

The results regarding the use of tonal information by French-speaking adults when learning words will also inform our understanding of the link between phonological and lexical processing. Previous studies on tone perception/identification have established that even though adult speakers of non-tonal languages have more difficulties at tone processing than speakers of tone languages (e.g., Gandour et al., 2000; Hallé et al., 2004; So and Best, 2010), they tend to perform above chance levels. Recently, Liu and Kager (2014) found that tone discrimination in non-tonal Dutch-learning infants follows a U-shaped function: while 5–6-month-olds can discriminate a Mandarin tone easily, their sensitivity declines between 8 and 15 months, but they regain sensitivity to it by 17–18 months. They suggested that this rebound in sensitivity might be related to the acquisition of the intonation of the native language. If this increased sensitivity is not limited to low levels of processing, then the French speakers in our experiment might perform at above chance levels on the tone-contrasted trials. This prediction is supported by previous findings on English-speaking adults (Chandrasekaran et al., 2010; Cooper and Wang, 2012, 2013) showing tone processing in lexical contexts, although in these studies, participants were subject to intense word training (several training sessions of about 30 min, in which some feedback was provided). It is unclear whether non-tone language speaking adults would show sensitivity to tone information in a less intensive task.

The present study used eyetracking to investigate the ability of Cantonese-, Mandarin-, and French-speaking adults to learn pairs of pseudowords in Cantonese, while processing fine phonetic information (consonant vs. vowel vs. tone information), whether used contrastively in the native language or not. On each of 24 trials, adults saw a pair of cartoons. In each cartoon, an unfamiliar object was presented visually while 6 sentences in Cantonese, each containing a pseudoword labeling that object, were heard. Between the two cartoons, the pseudowords differed by either a consonant (8 times), a vowel (8 times) or a tone (8 times). Adults were then tested on whether they had been able to learn the words following this short word learning phase, by presenting them with the two unfamiliar objects side-by-side, and observing their pattern of object looking before (prenaming phase) and after (postnaming phase) one of the objects was named. The current procedure was based on Experiment 1a of Havy et al. (2014) in which French-speaking adults were taught pairs of new pseudowords that differed either by a consonant or by a vowel. A comparison of performance in the two conditions to evaluate the consonant bias revealed that adults increased their looking times toward the target object (mean percentage of looking times at the target object in the postnaming—prenaming phase) similarly in both the consonant and vowel conditions, but that latencies in shift from the distractor to the target at the time of naming were faster in the consonant than in the vowel condition, establishing a consonant bias. To explore the relative strength of the processing of consonant, vowel, and tone information in our three linguistic groups, we similarly analyzed changes in mean percentage of looking times at the target object between the prenaming and postnaming phases (which also evaluates whether the pseudowords were learned), and latencies to shift from the distractor to the target at the time of naming. We also performed cluster-based permutation analyses on the time course of looking times in the different conditions to determine when in processing looking times to the target differ among the three conditions.

In this study, we investigated whether and how adults can quickly learn new minimal pair words in a non-native tone language, Cantonese, and whether this ability is modulated by native phonological knowledge. We tested this learning ability in Mandarin- and French-speaking adults, using Cantonese pseudowords differing minimally in either a consonant, a vowel, or a tone, and compared their performance to those of native Cantonese-speaking adults. Overall, we found that all three groups of adults performed at above chance levels in learning the pseudowords, and this held for all three types of contrasts. Also, compared to native Cantonese-speaking adults, both Mandarin- and French-speaking adults performed worse on all three types of contrasts. Furthermore, French-speaking adults performed even worse on tones when compared to Mandarin-speaking adults.

The present findings first establish that adults in all three language groups could rapidly learn new words in a computer-based situation, after solely 6 repetitions of each word. Note that the present interpretation in terms of word-learning needs to be qualified by the fact that the present study does not establish long-term establishment of lexical items, and could result from simple associations between the pseudowords and either the objects (or the side of the screen on which the objects were presented). Future studies will have to further probe our word-learning interpretation, using designs testing for word learning independent of object localization, and in long term memory, for example adapting the word-learning design used in Dittinger et al. (2016). While this word-learning finding is in part trivial for the Cantonese-speaking adults (though see more discussion on this below), it holds even when the new words were presented to Mandarin- and French-speaking adults, for whom Cantonese was a non-native language, and who had no knowledge of Cantonese prior to taking part in the experiment. Our findings reveal a significant effect of nativeness status, as overall, Cantonese-speaking adults performed better than the other two groups (in overall performance and shift latency analyses).

The effect of linguistic distance is less clearcut. Indeed, although Cantonese is closer to Mandarin than to French (at many levels including phonology, morphology and syntax, Li, 1937; Gong, 1980; DeLancey, 2009), this did not significantly impact overall performance and shift latencies, as French-speaking adults performed at the same overall level as Mandarin-speaking adults, in spite of a trend in the expected direction for overall performance (see further discussion in the Appendix for a more fine-grained approach). Our findings thus establish robust word learning abilities in a non-native language in adulthood, that contrast with the difficulties that adults have in learning some specific aspects of the phonology and syntax of non-native languages (e.g., Flege et al., 1999; Birdsong and Molis, 2001; Dupoux et al., 2008; Boll-Avetisyan et al., 2016). This difference might be due to the fact that while the acquisition of the phonology and syntax of one's native language is to a great extent completed in the first years of life, vocabulary acquisition is a lifelong, continuing process that allows for the acquisition of specialized vocabularies (as when, for example, becoming a -developmental- psychologist!) or learning the names of new objects and concepts (e.g., to “log into” a “googledoc” on one's “iphone”) in the native language.

Importantly, these word learning abilities were found in a specific learning context in which adults had to learn words presented in pairs, and in which the sound forms of the two words differed only by a consonant, vowel or tone. The fact that Mandarin- and French-speaking adults succeeded in learning the word pairs in all three conditions establishes that they could process fine segmental (consonantal and vocalic) and suprasegmental (tonal) information in doing so, and that they were establishing representations of the word forms that included specific segmental or tonal information. This finding is particularly striking for the French speakers' performance with tone contrasts, given that tones are not used in French at the lexical level. It could be due to the fact that these contrasts were introduced to the adults in minimal pairs of words, where they had to pay attention to the fine phonetic detail in order to distinguish the objects and memorize the words. Further research will be needed to explore whether our French-speaking adults would have failed to use such precise phonetic information if they had not been presented with minimal pairs, leading to lower or at chance performance. Importantly though, the ability of the French-speaking adults to use tonal information when learning words suggest that the rebound in tone discrimination found in late infancy in Dutch, another non-tonal language (Liu and Kager, 2014), interpreted in relation to the acquisition of the intonation of the native language, would not be limited to low levels of processing, but would extend to the lexical level.

Our findings also establish that adult performance is not solely based on the acoustic distance between the contrasted sounds, but is also dependent on their native phonological system. At this more fine-grained level, language distance appears to play a role, as our results clearly show that the Mandarin-speaking adults performed better than the French-speaking adults in learning words distinguished by Cantonese tonal contrasts. Since there was no difference in performance between the two language groups for consonants and vowels, this effect likely indicates that Mandarin-speaking adults, as experienced tone language users, exhibit greater ability in processing non-native tonal information at the lexical level, when compared to the non-tone user French speakers. In the Appendix, we present exploratory analyses, based on individual trials analyses, that allow some evaluation of the Perceptual Assimilation Model (PAM; for consonants: Best, 1995; for vowels: Tyler et al., 2014; for tones: Hallé et al., 2004) applied here at the level of word learning rather than speech processing.

Besides providing data on the phonological/lexical interface in processing a new, non-native language, our results also provide an evaluation of the use of tonal information in word learning, and its impact on processing consonantal and vocalic information at the lexical level in native speakers of a tone language. Regarding the use of tonal contrasts, we found that tonal contrasts are as important as consonantal and vocalic contrasts in processing word meanings for native Cantonese-speaking adults. This is revealed by the overall accuracy analyses showing that Cantonese adults perform at the same level in all three contrast conditions. Our time course analysis further shows that all three kinds of contrasts are processed at the same speed from the onset of the contrasting phonemes. For the tones, the comparison of our two analyses time-locked to consonant vs. vowel onset suggests that tonal information became available from the onset of the vowel. This might be related to the fact that 6 of the 8 pairs we presented started with unvoiced consonants, so that tonal information was mostly carried by the vowels. Whether a similar pattern would be found for syllables starting with voiced consonants would need to be evaluated in an experimental design counterbalancing the two types of consonants.

Furthermore, our work bears on the issue of the relative weight given to consonantal and vocalic information in lexical processing. Previous studies on various Indo-European languages (English, Dutch, French, Italian, Spanish) have found that adults have a consonant bias in accessing or learning words (e.g., van Ooijen, 1996; Cutler et al., 2000; Bonatti et al., 2005; Creel et al., 2006; New et al., 2008; Toro et al., 2008; Carreiras et al., 2009; Delle Luche et al., 2014; Havy et al., 2014; New and Nazzi, 2014). This supports the “division of labor” proposal by Nespor et al. (2003) that consonants are given more weight than vowels in lexical processing (while vowels are given more weight than consonants at the prosodic/syntactic levels). Accordingly, in the present study, we investigated Cantonese, a tone language, where lexical meanings are also crucially cued by tones. Our interest came from the fact that since tones are essentially associated with the voiced portions of syllables, which mostly correspond to vowels (and nasal codas) in Cantonese, and since only a few onset consonants (/j, w, m, n, η/) are voiced in that language, the relative weight given to consonants and vowels might be different from what has been found for Indo-European languages. The effect of tones could either increase the weight given to vowels (since they carry both segmental and tonal information, compared to only segmental information in non-tone languages) or decrease their weight even further (due to additional acoustic variation related to tonal differences and the fact that each vowel in Cantonese can carry 6 different tones).

Our findings did not reveal any advantage for either consonantal or vocalic information in lexical processing for Cantonese-speaking adults, as shown by their overall similar performance in the consonant and vowel contrast conditions. This finding differs from all previous findings on adult speakers of non-tonal languages, and in particular with the findings of a clear C-bias in latency analyses found when French adults learn new words (Havy et al., 2014). The present null effect (lack of difference between the C and V conditions), which thus needs to be interpreted with caution, might be taken as evidence that Cantonese-speaking adults pay more attention to vowels that carry tonal information than non-tone language users, resulting in a lack of C-bias. This interpretation needs to be considered cautiously given that a null effect was also found in the other two language groups, including in the French-speaking adults, which have been documented to have a C-bias in lexical processing when processing words in their native language (e.g., Bonatti et al., 2005; New et al., 2008; Havy et al., 2014). This lack of effect in the French- and Mandarin-speaking adults could mean that there is something in the acoustics of the stimuli used in the present study that does not support a C-bias. Alternatively, it could mean that the C-bias only operates in the native language, or in languages in which adults have sufficient experience/knowledge, hence the null effect found here for the French-speaking (and Mandarin-speaking) adults who had no or limited knowledge of Cantonese. Importantly though, our interpretation in terms of lack of a C-bias in Cantonese is corroborated by two recent studies having explored similar issues in either Mandarin- (Wiener and Turnbull, 2016) or Cantonese-speaking (Gómez et al., 2017) adults, using a word reconstruction and word form segmentation task respectively. As discussed in the introduction, their findings differ from those previously found in Indo-European languages, failing to find a clear C-bias in both languages, thus suggesting a different balance in the weight given to consonants and vowels in these two tone languages.

The above findings that begin to establish crosslinguistic differences in consonant/vowel weight between adult listeners of tonal vs. non-tonal languages are to be considered in relation to infant studies on Indo-European languages that have shown that the C-bias is modulated in infancy both developmentally and crosslinguistically (see Nazzi et al., 2016, for a complete review). Indeed, results from French and Italian show that while a C-bias is found from 8 months onward (Nazzi, 2005; Hochmann et al., 2011; Poltrock and Nazzi, 2015; Nishibayashi and Nazzi, 2016), it is not present up to 6 months of age (Benavides-Varela et al., 2012; Bouchon et al., 2015; Nishibayashi and Nazzi, 2016; Hochmann et al., 2018). Moreover, a C-bias could not be attested before 30 months in British English-learning infants (Nazzi et al., 2009; Floccia et al., 2014), and Danish-learning 20-month-olds demonstrate a V-bias (Højen and Nazzi, 2016). Taken together, these studies suggest that the C-bias is acquired and that its acquisition depends on the phonological and lexical properties of the native language.

Given that Cantonese- and Mandarin-speaking adults appear to have a reduced or reversed bias (Wiener and Turnbull, 2016; Gómez et al., 2017; present study), it is of great interest to expand research on the consonant bias to infants and toddlers learning a tone language, which was one of the original motivations for setting up the present study. At present, only one study has started to explore this issue in (Mandarin-dominant) Mandarin-English bilingual toddlers (aged 2.5–3.5 years) and preschoolers (aged 4–5 years). In a word recognition task exploring their sensitivity to mispronunciations of known words, the toddlers were found to be more sensitive to tone than consonant and vowel mispronunciations, while the reverse pattern was found in preschoolers (Singh et al., 2015). However, at both ages, no differences in sensitivity were found between consonant and vowel mispronunciations. Future studies will have to expand on this first finding, exploring such effects in younger monolingual infants learning various tone languages, and exploring various aspects of lexical processing, including both word learning and lexical comprehension.

In conclusion, the present study establishes adults' word learning abilities in an unknown language, and show that level of performance is modulated by how the phonologies of the native and non-native languages map onto each other. They also bring evidence suggesting that being a speaker of a tonal language reduces the consonant bias in lexical processing previously found in adults of several Indo-European languages, probably due to the fact that tones are carried by vowels more than by consonants. However, no clear bias could be found for either consonants or vowels, and future studies will have to further probe the link between phonological and lexical processing in tone languages. These findings nevertheless set up the foundations for equivalent developmental studies that will inform our understanding of what determines the phonological biases that are observed in lexical processing.

SP created the experimental stimuli and design, conducted data analyses and drafted the manuscript. CK assisted in stimuli creation, recruited participants and collected data in Hong Kong. HuiC interpretated the data and drafted the manuscript. HC and TN conceptualized the study and supervised all stages of the project.