When one cue is better than two : Lexical vs . Syntactic Cues to Verb Learning

Prior results with adults (Gillette, Gleitman, Gleitman, & Lederer, 1999; Snedeker & Gleitman, 2004) and preschoolers (Piccin & Waxman, 2007) have established that certain information sources are more powerful than others in their utility as cues for verb learners. The current paper tests the predictions of these results with infant verb learners. We show that the informativity of a cue is offset to some extent by the amount of processing required to utilize that cue. Consequently, the best condition for word learning for adults and older children is less effective for infant learners. We show that infants can learn the meaning of a novel verb better when it takes a pronominal subject than when it takes a lexical NP as subject. This effect, however, is not due to the special status of pronouns as cues to syntactic category (Childers & Tomasello, 2001; Bernal, Lidz, Millotte, & Christophe, 2007), but rather to the extra processing demands imposed by contentful lexical nouns. A well-known problem for the acquisition of language generally and of words in particular is the stimulus-free nature of language use (Chomsky 1959). Extralinguistic context is not a very good predictor of what someone is going to say. For example, when looking at a Rembrandt painting, one might say, “Dutch,” “It’s crooked,” “I prefer abstract expressionism,” “Remember our camping trip last summer?” or just about anything else. Hence, if the language learner is to learn about what is being said on the basis of what is happening in the world, the lack of reliable correlations between language and events represents a serious obstacle. Worse, even when someone is talking about the here and now, the world makes available many different possible descriptions of the visible scene. Consider a scenario in which a boy and his mother are flying a kite. A novel word used in this context might refer to the kite, the string, nylon, wind, boredom, clouds, blue, anticipation, lunch, force, flying, waiting, saying, hoping, thinking, breathing, etc. Given the multiplicity of interpretations of any given context, the learner faced with a novel word needs to determine which of the objects, events and properties made available by perception/conception that word refers to. A theory of word learning must therefore determine a procedure by which alternatives are eliminated. At minimum, such a theory must allow the learner to compare across situations to narrow down the number ofexpressionism,” “Remember our camping trip last summer?” or just about anything else. Hence, if the language learner is to learn about what is being said on the basis of what is happening in the world, the lack of reliable correlations between language and events represents a serious obstacle. Worse, even when someone is talking about the here and now, the world makes available many different possible descriptions of the visible scene. Consider a scenario in which a boy and his mother are flying a kite. A novel word used in this context might refer to the kite, the string, nylon, wind, boredom, clouds, blue, anticipation, lunch, force, flying, waiting, saying, hoping, thinking, breathing, etc. Given the multiplicity of interpretations of any given context, the learner faced with a novel word needs to determine which of the objects, events and properties made available by perception/conception that word refers to. A theory of word learning must therefore determine a procedure by which alternatives are eliminated. At minimum, such a theory must allow the learner to compare across situations to narrow down the number of alternative hypotheses. But even allowing for cross-situational comparison, a word-toworld pairing procedure still cannot overcome Quine’s (1960) cooccurrence problem: rabbits always occur with rabbit-ears, jumping always occurs with legs, giving always occurs with receiving, and sugar always occurs with sweetness. One set of solutions to the cooccurrence problem points to linguistic biases about what words encode. Such solutions are based on observations that children are likely to believe that a novel word, applied to an unfamiliar object, will refer to an object, as opposed to a property of that object or a relation between objects (Gentner, 1982; Waxman & Markow, 1995), that a novel word is likely to be interpreted as the whole object, as opposed to its parts or texture (Markman & Hutchinson, 1984; Soja, Carey, & Spelke, 1991), and that a novel word is likely to refer to a basic-level kind (Markman, 1989; Mervis, 1987; Waxman, 1992). This approach to word learning leads us to expect that nouns (more specifically, words that refer to object kinds) will be learned first because children have a bias to interpret novel words as referring to object kinds. Indeed, it has been widely observed that nouns are massively overrepresented in the earliest child vocabularies (Caselli et al., 1995; Gentner & Boroditsky, 2001; Goldin-Meadow, Seligman, & Gelman, 1976; Huttenlocher, 1974; Woodward & Markman, 1998). Of course, biases in the learner to interpret words as referring to object kinds can only take us so far, since language is filled with words referring to properties, events, and relations, and with words that quantify over objects (the, every, most) and events (always, never, sometimes). The language learner must ultimately come to understand the mapping between a wide range of linguistic devices and the meanings they convey. In recent years, an approach to word learning has developed which, first, explains the early appearance of nouns in child vocabulary and the apparent bias for early words to refer to objects and, second, presents a clear hypothesis about how learning of words that do not refer to object categories might proceed (Gleitman, 1990; see also Fisher, 1996; Fisher, Hall, Rakowitz, & Gleitman, 1994; Gillette et al., 1999; Mintz & Gleitman, 2002). On this view, the preponderance of nouns in child vocabularies derives from the different informational requirements of the different grammatical categories. Adjectives and verbs, for example, are predicates that require arguments. Thus, the meanings expressed by these kinds of words cannot be learned until after the learner has acquired some nouns to head the noun phrases that serve as arguments to these predicates. As a consequence, words referring to object categories, which have no such grammatical prerequisites, can be learned first, providing a foothold for subsequent learning of words from other grammatical categories. This syntactic bootstrapping approach highlights the different informational requirements associated with learning different kinds of words and hypothesizes that syntactic information can be used to guide lexical acquisition, especially for verbs and adjectives (Gleitman, 1990; Landau & Gleitman, 1985; Syrett & Lidz, in press; Waxman, 1999; Waxman & Booth, 2001; Waxman & Lidz, 2006). Indeed, a great deal of research has shown that infants and children can use information about syntactic distribution to identify important aspects of verb meaning (Arunachalam & Waxman, in press; Bunger & Lidz, 2004, 2006; Fisher, 1996; Fisher et al., 1994; Fisher 2002; Landau & Gleitman, 1985; Lidz, Gleitman, & Gleitman, 2003; Naigles, 1990, 1996; Waxman, Lidz, Braun, & Lavin, 2009) and that they can use syntactic information to distinguish adjectives from nouns, to identify the meaning of a novel adjective (Klibanoff & Waxman, 2000; Mintz & Gleitman, 2002; Waxman, 1999; Waxman & Booth, 2001) and to identify the subclass of a novel adjective (Syrett & Lidz, in press). Moreover, research in this line has revealed that the bias to interpret novel words as referring to objects does not reflect the initial state of the language learner. Rather, the bias to interpret words as referring to object kinds is true only of count nouns, not all words, and does not appear until 14-months of age, when the learner can distinguish (on the basis of morphosyntactic properties) count nouns from other syntactic categories. Earlier in development, both nouns and adjectives are equally likely to be interpreted as referring to either object kinds or object properties (Waxman & Booth, 2001, 2003). Consequently, it appears that at least some word learning biases emerge as a function of the acquisition of syntactic categories. The observation that word learning biases reflect knowledge of syntactic categories lends plausibility to the idea that features of syntactic distribution play a critical role in word learning. More narrowly, this observation lends initial support to the idea that asymmetries between nouns and verbs in word learning derive in part from asymmetries in the syntactic prerequisites for learning words from these categories (Snedeker & Gleitman, 2004; Waxman & Lidz, 2006). This idea is further supported by experiments with adult subjects placed in a word learning environment (Gillette et al., 1999). Whereas participants were able to learn nouns to some degree solely through observation of extralinguistic context, verb-learning required additional syntactic information. Gillette et al. (1999) presented adults with a series of video-taped scenes which all depicted the context of a parent uttering a word to their child. The participants’ task was to identify the meaning of a word presented in a range of conditions which differed by how much information was given about the linguistic context of the utterance. Crucially, the proportion of words that were correctly identified and the type of words that were identifiable varied significantly as a function of information type. When presented with no information beyond the visual scene, adult participants correctly identified nouns more often than they correctly identified verbs. For verbs, the contribution of various information sources is most evident. Participants were least successful at guessing the verb when only the visual scene was presented (7.7% correct). However, providing the