Lexical competition in young children’s word learning
Introduction
To understand speech, listeners must identify the sequence of words composing each utterance. Because even words repeated by the same talker never sound exactly the same twice, this identification process requires listeners to discount perceptible but irrelevant phonetic characteristics such as speaking rate and overall pitch, while attending to phonetic characteristics that distinguish words. Furthermore, the dynamic nature of the lexicon adds a complex additional component to speech comprehension. Because new words can enter the vocabulary at any time, listeners must be alert to the possibility that a phonetic variant of a familiar word is, in fact, a novel word to be learned. Appropriately balancing interpretation of phonetic variation is particularly crucial early in language development, when many words are being learned and while phonetic perception is still undergoing language-specific tuning. In the present research we examined this process, focusing on word learning in one and 1/2-year-olds.
Interpretation at both the lexical and phonetic levels is rapid and automatic in young children—words are said to be “activated” as the signal unfolds, according to the match between the signal and representations in the lexicon (e.g., Marslen-Wilson, 1987, Swingley et al., 1999), while the acoustic signal is “assimilated” to the set of phonetic categories in the phonological inventory (e.g., Best, 1994). The need for these sources of information to be balanced carefully is seen clearly when children are confronted with novel words that sound similar to words they already know. Phonological analysis may suggest the presence of a novel word, but the resemblance of the novel sequence to a familiar lexical item may support identification of the word as familiar.
One possible resolution of this tradeoff is that the rapid pace of vocabulary expansion in young children may lead them to be especially open to considering poorly matching forms as new words, favoring expansion of the lexicon over ignoring lexical variability. An alternative possibility is that children place relatively little confidence in their phonological analyses of speech, favoring interpretation of “near misses” of familiar words as instances of those words. At present, little is known about how children weigh these different sources of information, though some evidence suggests an increasing reliance on phonological analyses from 2.5 years to 4 years and adulthood (Jarvis, Merriman, Barnett, Hanba, & Van Haitsma, 2004). Assessment of this weighting process may contribute to a better understanding of the interaction between linguistic levels in lexical acquisition.
A common assumption in models of language learning in infancy is a “bottom-up” developmental course in which phonetic categories are learned first, the early words are learned second, and syntactic categories and operations are learned last. Indeed, infants are well on their way to forming their native language’s phonetic categories by 6–12 months, and their first words are spoken at the end of this age period (e.g., Kuhl et al., 1992, Polka and Werker, 1994, Werker and Tees, 1984). Syntactic development depends upon knowing words, and generally follows infants’ acquisition of a substantial vocabulary. According to this simple “phonology first” progression, young children’s word learning should not be encumbered by phonological insensitivities.
However, it is now clear that word learning begins earlier than once thought, and also that phonological learning is a protracted process extending into middle childhood. Some evidence for precocious word learning concerns infants under the age of 6 months—children who on current evidence have not yet learned language-specific phonetic categories. For example, 4.5-month-olds prefer to listen to their own name rather than a foil (Mandel, Jusczyk, & Pisoni, 1995). Six-month-olds can use the presence of their own name or the word Mommy (though not the variant Tommy) in an utterance to help identify the boundaries of adjacent words (Bortfeld, Morgan, Golinkoff, & Rathbun, 2005), and they look appropriately at a videotape of their mother or father upon hearing Mommy or Daddy (Tincoff & Jusczyk, 1999). Thus, early in development at least some words are learned as categories that can guide interpretation of speech, although the specificity of these early lexical representations remains uncertain.
In this same 6- to 12-month age period, infants begin to refine their perception of speech to focus on phonological distinctions made in the native language. These changes in early perception are manifested empirically in infants’ developing failure to distinguish similar speech sounds that are not used contrastively in the native language. For example, both Catalan and Spanish 4-month-olds distinguish the vowels [e] and [ε], which are different vowels in Catalan but are treated as a single category in Spanish. By 8 months, Spanish learners no longer discriminate the vowels, though Catalan learners still do (Bosch & Sebastián-Gallés, 2003). Such developmental changes have been shown in vowel perception as early as 6 months (e.g., Kuhl et al., 1992, Polka and Werker, 1994) and in consonants at around 11 months (e.g., Werker & Tees, 1984; see also Cheour et al., 1998, Polka and Bohn, 1996). These early changes in speech categorization are followed by increasing refinement of phonetic categorization well into middle childhood (e.g., Hazan and Barrett, 2000, Nittrouer, 1992).
The fact that lexical and phonological categories develop contemporaneously suggests two possible directions of influence from one linguistic level to the other. First, as lexical categories become better entrenched, children might use them to help determine when two similar sounds should be counted as phonologically distinct. For example, many dialects of American English contrast the similar vowels , as in caught, and , as in cot. Both vowels are frequent in words that infants have probably encoded as familiar forms: in all, off, ball, and water, and in on, mommy, not, and got, among others. In principle, variation in the lexical contexts of these vowels might help infants recognize that the acoustic differences between them signal separate phonological categories, just as correlated cues to category identity aid in other category learning situations (e.g., Garner, 1974, Melara and Marks, 1990).
Interaction between phonological and lexical categories could also affect interpretation in the reverse direction: as phonological categories become increasingly robust, children may use them to distinguish between words, or to determine when a variant of a familiar word is or is not sufficiently different to be counted as a distinct word. Indeed, this use of phonological difference as a sufficient and, except in the case of homophones, necessary marker of lexical distinctions may be viewed as the main function of phonological categorization (e.g., Trubetskoy, 1939/1969). This interpretation process is the subject of the present experiments.
Children’s adoption of a phonological criterion for differentiating words depends upon their having stored reasonably complete phonological representations of familiar words. If, for example, children’s knowledge of the sounds of a word like ball were limited to only a vague sense that some stop consonant should be followed by a vowel and a sonorant consonant, it would be impossible for children to tell which new sound-forms differ by at least one phonological unit and which differ by less than that. Whether children do encode phonologically detailed representations of words has been a matter of debate for many years (e.g., Eilers and Oller, 1976, Shvachkin, 1948/1973, Walley, 1987, Waterson, 1971). The conclusion we draw from this debate is that for words children know well, like dog and Mommy and baby, children’s phonological representations are complete rather than vague. For words with which children have had much less experience, there is evidence for underspecification of their forms.1 Studies supporting these conclusions are reviewed below.
Some experiments examining infants’ phonological knowledge of familiar words took advantage of 7- to 11-month-olds’ preference for listening to familiar words over unfamiliar words, testing whether this preference would be maintained when the familiar words’ pronunciation was altered. Maintenance of the familiar-word preference (indifference to mispronunciation) is compatible with underspecification of phonological form, while disappearance of the preference when words are mispronounced suggests that infants retain specific phonological details in words. In a word segmentation task, Jusczyk and Aslin (1995) showed that 7.5-month-olds correctly recognized target words (cup, dog, bike, feet) after hearing those words embedded in multiword fluent speech. However, infants did not falsely recognize mispronounced versions of those target words (tup, bog, gike, zeet). Although early results showed little influence of mispronunciations on French infants’ lexical preferences (Hallé & de Boysson-Bardies, 1996), subsequent experiments with English infants, coupled with reanalysis of the French data, suggested that in fact both English and French infants are affected by mispronunciations of words, at least in stressed syllables (Vihman, Nakai, DePaolis, & Hallé, 2004). In a similar series of experiments, Swingley (2005a) found that Dutch 11-month-olds’ lexical preferences were disrupted by mispronunciation of either onset or offset consonants of monosyllabic words. Thus, infants’ phonological knowledge of consonants in at least some words appears to be fairly specific in the sense that deviations from citation forms render words poorer matches to infants’ stored representations.
Other studies have used picture-fixation or “eyetracking” tasks, together with mispronunciation manipulations, to test slightly older children. When children are presented with pairs of pictures (e.g., an apple and a ball) and a spoken utterance (e.g., Where’s the ball?) they tend to fixate the picture that is named in the utterance (Golinkoff, Hirsh-Pasek, Cauley, & Gordon, 1987). In studies of children ranging from 14 to 23 months, Swingley and Aslin, 2000, Swingley and Aslin, 2002 found that this target fixation response diminished when the target word was mispronounced. For example, upon hearing Where’s the gall?, children looked at the ball less than when hearing Where’s the ball?. In these studies, children did tend to look at the target picture even when its name was mispronounced, though not always significantly above chance. Similar effects have been demonstrated in other studies (Bailey and Plunkett, 2002, Swingley, 2003, White et al., 2004) and in related experiments using audiovisual habituation methods with 14-month-olds (Fennell & Werker, 2003). The latter studies again showed that for familiar words like ball, dog, and apple, even young 1-year-olds appear to encode words with a high level of phonological specificity.
The fact that many words in children’s early vocabularies are encoded with full phonological specification indicates that in principle children could use a phonological criterion for distinguishing novel words from familiar ones. However, other studies have revealed important limitations in young children’s interpretation of speech during word learning (Cummings and Fernald, 2003, Stager and Werker, 1997, Swingley, in press, Werker et al., 2002). For example, Werker and her collaborators have shown that under some conditions 14-month-olds apparently fail to learn two discriminable but similar-sounding novel words, while they succeed in learning two different-sounding words (e.g., Werker & Curtin, 2005). By 17 months, children succeed in the same procedure (Werker et al., 2002). In a related study, Mills et al. (2004) compared ERP responses to correct pronunciations of familiar words, mispronunciations of those words, and nonwords. At 14 months, a negative ERP component between 200 and 400 ms was found to be larger for correct pronunciations and mispronunciations than for nonwords; at 20 months, this component was larger for correct pronunciations than for mispronunciations or nonwords. These results suggest that between 14 and 20 months, children’s categorization of mispronounced words changes in some way. Though the behavioral consequences of this change are not clear, one interpretation is that the 20-month-olds treated the mispronunciations as nonwords because they used a more finely tuned phonological difference criterion to categorize the mispronunciations as nonwords.
The fact that speech processing is a limiting factor in word learning is supported by other studies revealing that individual differences in a range of speech processing measures are correlated with vocabulary development. For example, ERP measures have linked very young infants’ responses to phonetic stimuli and linguistic performance several years later (Guttorm et al., 2005, Molfese, 2000). Tsao, Liu, and Kuhl (2004) documented a significant association between 6-month-olds’ performance in discriminating a nonnative vowel contrast, and vocabulary size at 13, 16, and 24 months. By the end of the second year, children’s latency to respond to a familiar word in a picture-fixation task is strongly correlated with both lexical and grammatical measures (Fernald, Perfors, & Marchman, 2005). Even in children as young as 3 years, word and nonword repetition abilities (seen as an index of phonological working memory) are correlated with receptive vocabulary size (Gathercole & Adams, 1993).
Thus, the analytical abilities revealed by infants in learning the phonetic categories of their native language do not guarantee that young children will assume that perceptible phonological variation signals a lexical distinction. In fact, experimental evidence shows that under challenging conditions older children often fail to interpret phonological differences this way. In a series of studies, Merriman and colleagues have examined children’s responses when asked to choose between a familiar object and a novel object upon hearing a novel word (Jarvis et al., 2004, Merriman and Marazita, 1995, Merriman and Schuster, 1991). Under these conditions, when the novel word does not resemble the name of the familiar object, children generally interpret the novel word as a name for the unfamiliar object (e.g., Markman & Wachtel, 1988). But this response was not shown consistently when the novel word was phonologically similar to the familiar object. For example, children might see a flower and a palette, and hear “Point to the glower.” Although children’s responses varied with a number of factors (such as the trial structure and the nature of the phonological resemblance), in general preschool children were less likely to infer that the novel word corresponded to the novel object when that word was phonologically similar to the pictured familiar object than when it was not. Note, though, that in these studies children were placed in a particularly difficult position: to interpret a word like glower as a name for the unfamiliar object rather than the flower, they must not only detect the difference between flower and glower, but must also treat as coincidental the similarity between the novel word and the name of the familiar object. Hence, this task may be too stringent to serve as a general test of children’s ability to interpret phonological variation under normal word-learning circumstances.
The present experiments examined the interaction of the lexical and phonological levels of linguistic analysis in one and 1/2-year-olds’ word learning by presenting a novel object and labeling it with a name that was or was not similar to a word children already knew (for example, tog, a variant of dog, or shang, which is not similar to a familiar word). As described above, novel words like tog pose a problem of interpretation: the lexical level may supply the interpretation that among the words of the lexicon, dog is the best match, leading to activation of that word and its meaning. On the other hand, phonological analysis should reveal that in fact tog is a distinct word, not a match to dog. In principle, this mismatch should be sufficient to support rejection of dog as the intended referent, and consideration of tog as a potential addition to the lexicon.
The foregoing predictions contrast with those that have emerged from work with adults. In some tasks, such as speech shadowing (Marslen-Wilson & Welsh, 1978) and mispronunciation detection (e.g., Cole, Jakimik, & Cooper, 1978), phonologically novel word-forms are often interpreted as similar words even by adults. Such behavior need not be considered maladaptive; indeed, talkers may misspeak and listeners may misperceive, so loose criteria for matching heard words to lexical representations can be helpful, particularly when (as in these shadowing and detection studies) mispronunciations are slipped into meaningful sentences. However, since adults know most of the words they hear, it makes sense for them to be biased in favor of resolving utterances as sequences of familiar words. Children, by contrast, probably know fewer of the words they encounter, and know only a fraction of their ultimate vocabulary. Our initial expectation, then, was that young children would seize upon perceptible phonological distinctions (like the [t] and [d] in tog and dog), readily using the output of phonological analysis to learn new words in spite of phonological similarities. This prediction was tested in two experiments.
Section snippets
Experiment 1
In Experiment 1, children about 1.5 years old were taught two novel words for novel objects. One novel word was a phonological neighbor of a familiar word (where neighbor refers to a word differing from another word by addition, deletion, or substitution of a single segment; Luce & Pisoni, 1998). (We will refer to these taught words as “novel neighbors,” and will call the familiar words from which they were derived “familiar source words.”) The second novel word was not a neighbor of any words
Experiment 2
Children were taught either a novel phonological neighbor of a familiar source word, or a nonneighbor. For both groups of children, the first half of the testing phase consisted of both-familiar (filler) trials, and both-novel trials. As in Experiment 1, on the both-novel trials two novel objects were displayed, and children heard the word they had been taught. Learning was inferred from children’s fixation to the named picture.
In the second half of the testing phase, all children were shown a
General discussion
The present experiments provide a first step in delineating the processes by which 1.5-year-olds learn new words, with respect to the balance between phonological sensitivity and toleration of sub-lexical variability. By 18 months of age children are generally agreed to be capable of discriminating the speech sounds of their native language, though often not the speech sounds of other languages. The effect of native-language exposure is usually held to reflect children’s formation of
Acknowledgments
This work was supported by grants from the National Institutes of Child Health and Human Development (NIH HD-37082) and the National Science Foundation (SBR-9421064) to R.N.A., and from the Max-Planck Society and the Dutch National Science Foundation (NWO) via a Spinoza award to Anne Cutler. Portions of the research reported here were presented at the Boston University Conference on Language Development in 2004. The authors extend their thanks to the Rochester lab (in particular Elizabeth
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