Discovering Pronoun Categories using Discourse Information

Discovering Pronoun Categories using Discourse Information Naho Orita (naho@umd.edu) Rebecca McKeown (rmckeown@umd.edu) Naomi H. Feldman (nhf@umd.edu) Jeffrey Lidz (jlidz@umd.edu) Department of Linguistics, 1401 Marie Mount Hall, University of Maryland, College Park, MD 20742 USA Jordan Boyd-Graber (jbg@umiacs.umd.edu) College of Information Studies, South Hornbake, University of Maryland, College Park, MD 20742 USA University of Maryland Institute for Advanced Computer Studies, University of Maryland, College Park, MD 20742 USA S Abstract Interpretation of a pronoun is driven by properties of syntactic distribution. Consequently, acquiring the meaning and the dis- tribution are intertwined. In order to learn that a pronoun is re- flexive, learners need to know which entity the pronoun refers to in a sentence, but in order to infer its referent they need to know that the pronoun is reflexive. This study examines whether discourse information is the information source that the learner might use to acquire grammatical categories of pro- nouns. Experimental results demonstrate that adults can use discourse information to accurately guess the referents of pro- nouns. Simulations show that a Bayesian model using guesses from the experiment as an estimate of the discourse informa- tion successfully categorizes English pronouns into categories corresponding to reflexives and non-reflexives. Together, these results suggest that knowing which entities are likely to be re- ferred to in the discourse can help learners acquire grammatical categories of pronouns. Keywords: language acquisition; Bayesian modeling VP NP Det N V NP Alice’s sister saw herself Figure 1: Syntactic tree showing a c-command relationship between the antecedent Alice’s sister and the pronoun herself. English speakers know that the sentence in (1) means that Alice saw Alice in the mirror and the sentence in (2) means that Alice saw someone else in the mirror. (1) Alice saw herself in the mirror. (2) Alice saw her in the mirror. These interpretations reflect adults’ knowledge that reflexives like herself require different syntactic relations with their an- tecedents than non-reflexives like her. Evidence shows that children acquiring various languages have knowledge of the grammatical distributions of pronouns (Jakubowicz, 1984; Crain & McKee, 1985, among many). However, it is not yet known how children acquire this knowledge. In English, the distribution of pronouns is governed by two constraints on the pronoun-antecedent relation: locality and c-command. Locality refers to the domain of the syn- tactic relation between the pronoun and its antecedent. Re- flexive pronouns must have their antecedents in the local do- main, corresponding approximately the same clause in En- glish (Chomsky, 1973). The second constraint is that re- flexive pronouns must be c-commanded by their antecedent (Reinhart, 1976). In the sentence ‘Alice’s sister saw herself’, English speakers know that the antecedent of herself is not Alice, but Alice’s sister. That is, when the hierarchical struc- ture of the sentence is represented as a tree in Figure 1, the re- flexive herself is contained in the sister node of its antecedent Alice’s sister. Non-reflexive pronouns appear in exactly those contexts where reflexives do not appear: approximately con- texts in which the antecedent is either non-local, not in a c- commanding position, or both (Chomsky, 1973). This means that the relationship between the grammatical positions of an- tecedents and pronouns, as characterized by locality and c- command, defines the distribution of grammatical categories of pronouns in English. Critically, these grammatical constraints concern the rela- tionship between a pronoun and its antecedent. This means that syntactic knowledge alone is insufficient for acquiring pronouns, because it cannot be applied without knowing the intended antecedent of a pronoun. In order to learn that her- self is reflexive, learners need to interpret the sentence in (1) as ‘Alice saw Alice’, recognizing that Alice and herself co- refer to the same entity. However, in order to interpret the meaning of the sentence, they need to use the knowledge that herself is a reflexive pronoun, whereas her is a non-reflexive pronoun. This circularity poses a potentially difficult problem for children acquiring language. In this paper we show that discourse information can help learners categorize pronouns into appropriate distributional classes. If learners use discourse information to predict that the pronoun herself in (1) is likely to refer to Alice and the pronoun her in (2) is likely to refer to someone else, this pro- vides information that can help them categorize these pro- nouns into different classes. We examine (i) to what extent discourse context is informative for determining the referent of a pronoun and (ii) whether this estimate of a pronoun’s reference is sufficient for learning to classify pronouns. The paper is organized as follows. Our next section de- scribes a behavioral experiment that measures the discourse information available to listeners. The following section