The Onset of Syntactic Bootstrapping in Word Learning: Evidence from a Computational Study

The Onset of Syntactic Bootstrapping in Word Learning: Evidence from a Computational Study Afra Alishahi Computational Linguistics and Phonetics Saarland University, Germany afra@coli.uni-saarland.de Pirita Pyykk¨onen Computational Linguistics and Phonetics Saarland University, Germany pirita@coli.uni-saarland.de Abstract their language yet. A continuous picture of the develop- mental path of word learning is lacking. In this paper, we propose a novel approach to studying this problem. We use an existing computational model of early verb learning which incrementally learns syntactic constructions of language from usage data. We adapt this model to the task of identifying target words in novel sit- uations given different sets of (perceptual and linguistic) cues. Our results show that having access to linguistic in- formation significantly facilitates identifying verbs in later stages of learning, but no such effect is observed at the earlier stages. For identifying nouns, additional linguistic information does not affect performance at all. The syntactic bootstrapping hypothesis suggests that children’s verb learning is guided by the structural cues that the linguistic context provides. However, the onset of syntactic bootstrapping in word learning is not well studied. To investigate the impact of linguistic infor- mation on word learning during early stages of language acquisition, we use a computational model of learning syntactic constructions from usage data, and adapt it to the task of identifying target words in novel situations. Our results show that having access to linguistic infor- mation significantly improves performance in identifying verbs (but not nouns) in later stages of learning, yet no such effect can be observed in earlier stages. Introduction Learning verbs is a challenging task for young children: their early vocabulary contains many more nouns than verbs, and they learn new nouns easier than new verbs of the same frequency (e.g., Imai et al., 2005; Waxman, 2006). The acquisition of nouns is mainly attributed to cross-situational evidence, or regularities across different situations in which a noun is used (Quine, 1960). In con- trast, learning verbs seems to depend on the syntactic frames that they appear in. It has been suggested that children draw on syntactic cues that the linguistic context provides in verb learning, a hypothesis known as syntactic bootstrapping (Gleitman, 1990). According to this view, verbs are learned with a delay because the linguistic in- formation that supports their acquisition is not available during the early stages of language acquisition. To investigate the impact of linguistic and extralinguis- tic cues in identifying words, Gillette et al. (1999) pro- posed the Human Simulation Paradigm (HSP): adult par- ticipants watch videos of caregivers interacting with their toddlers, and are asked to identify target words marked by a beep. Videos are displayed without sound, and sub- jects are provided with different degrees of information about the linguistic context of the target verbs. Various HSP studies have shown that having access to linguis- tic and structural cues significantly improves the perfor- mance of adults in identifying verbs. Piccin & Waxman (2007) adopted the HSP paradigm for testing school-age children, and showed that children also rely on linguistic information for identifying verbs, but their performance is inferior to adults. These findings hint at a gradual de- velopment of syntactic bootstrapping, but it is uncertain whether the same effect can be observed in much younger children who have not mastered the syntactic structure of Time Course of Syntactic Bootstrapping Several preferential-looking studies have shown that chil- dren are sensitive to the structural regularities of lan- guage from a very young age, and that they use these structural cues to find the referent of a novel word (e.g., Naigles & Hoff-Ginsberg, 1995; Gertner et al., 2006). In a typical setup, children are given more than one interpreta- tion for an utterance (e.g., different activities displayed on parallel screens), and their looking behaviour reveals their preferred interpretation. However, these studies cannot compare the impact of different cues in word learning, since the same type of input is available to subjects in different conditions. In contrast, HSP studies manipulate the number and type of cues that subjects receive for performing a task across conditions, and thus evaluate the impact of each set of cues. In their influential study, Gillette et al. (1999) provided their adult subjects with various combinations of visual cues (videos), a list of co-occurring words, the syntactic pattern of the sentence, and the full transcript of the narration. Their findings and those of later studies have consistently shown that the more linguistic infor- mation adult subjects receive, the more accurately they identify missing verbs. Piccin & Waxman (2007) used HSP for studying seven- year-olds as well as adults. Subjects in each age group were randomly assigned to either ‘no linguistic informa- tion’ (-LI) or ‘full linguistic information’ (+LI) condition. In the -LI condition, participants heard no audio other than beeps indicating the target words. In the +LI con- dition, participants heard all the surrounding speech as well as the beeps. After watching each clip, subjects were asked to guess the target word (a noun or a verb). Their