Joint acquisition of word order and word reference

Joint acquisition of word order and word reference Luke Maurits (luke.maurits@adelaide.edu.au) Amy F. Perfors (amy.perfors@adelaide.edu.au) Daniel J. Navarro (daniel.navarro@adelaide.edu.au) School of Psychology, University of Adelaide Adelaide SA 5005, Australia words have been shown to make use of word order knowledge in a comprehension task (Hirsh-Pasek & Golinkoff, 1996). This suggests that word order knowledge is acquired very early, but it is not clear how it is done. Although learn- ing word order is a more limited problem than learning the referents of words, since there are fewer possible solutions to the problem, it is still a puzzle how it can be done so quickly. Some have suggested that prosodic bootstrapping may explain a related problem, the acquisition of head direc- tion (Christophe, Nespor, Guasti, & Ooyen, 2003). Although this requires the assumption of innate knowledge of the map- ping principles between prosodic cues and head direction, and does not address the acquisition of word order itself, prosodic bootstrapping may play an important role. Abstract Inferring the mappings between words and their referents is a difficult problem that all language learners face. Similarly, learning which word orders are permitted in one’s language is one of the first grammatical learning tasks these same learn- ers must solve. We present a modeling framework which ad- dresses simple versions of both of these problems by using the joint information in each to bootstrap the other. We discover that these two distinct learning tasks may be easier to solve jointly because of the way in which the inferences in one prob- lem constrain the inferences in the other. Keywords: word learning; word order; Bayesian models; mu- tual constraint; reference; linguistics Introduction The language-learning child is faced with two simultaneous acquisition problems: acquiring the (semantic) rules that map the words she hears onto the objects and actions she per- ceives, and acquiring the (syntactic) rules that govern how those words should be combined to make grammatical sen- tences. Both are difficult learning problems in their own right, and have been the topic of considerable research. Determining the meaning of words on the basis of real-life observational evidence is quite difficult (Gillette, Gleitman, Gleitman, & Lederer, 1991), in part because of the inher- ent ambiguity of words, in part because the number of po- tential meanings is logically underconstrained (Quine, 1960). While it may be that the identification of a word’s referent is made easier by pre-existing biases (Markman, 1990), recent research has also suggested several methods by which chil- dren could explicitly learn which objects or actions a particu- lar word refers to. For instance, social cues such as pointing or gaze (Frank, Goodman, & Tenenbaum, 2007) can assist the learner, as can a sensitivity to the statistics of cross-situational word learning (Frank et al., 2007; Yu & Smith, 2008) and the ability to form theories about the abstract rules that govern the mapping of words onto categories (e.g., Kemp, Perfors, & Tenenbaum, 2007). Experiments and computational mod- eling suggest that the difficulties and ambiguities inherent in cross-situational word learning can be at least partially alle- viated by these techniques. Acquiring the rules of syntax is also a famously difficult problem. Even if we restrict ourselves to more tractable sub- problems – for instance, the acquisition of word order – the empirical data present some difficult issues. Children make few mistakes in word order when they start combining words (e.g., Brown, 1973), and even children who do not combine In this paper we propose that both of these acquisition problems can be made more tractable by addressing them jointly. On the one hand, if the learner believes that word or- derings tend to be consistent, constraints are imposed on the manner in which words may be mapped onto entities in the world. On the other hand, even knowing a few word mean- ings is enough to provide a great deal of evidence about word order. These intuitions suggest that viewing the problem as a joint acquisition problem can make both individual prob- lems easier. While in one sense the proposition may appear counterintuitive – after all, there is in some sense ‘more’ to learn in the joint problem – to the extent that each problem mutually constrains the other, the acquisition problem should be made less difficult, rather than more. This basic idea is not a new one: for instance, earlier work noted its poten- tial (Siskind, 1990, 1991). However, performing inferences about both syntactic and semantic information was beyond the computational capabilities of the time, and in practice, that work simply demonstrated that hardwired syntactic in- formation could make the learning of semantics easier. Our research goes beyond this work in two ways: first, because we demonstrate that truly joint inference, in which both as- pects of the problem mutually bootstrap each other, can make the learning problem easier; and second, because the syntac- tic information is simpler and sparser (word order rather than X-bar theory or richer grammatical knowledge). Our study presents two models that seek to establish word-referent map- pings on the basis of cross-situational learning statistics: one model also seeks to acquire word order, and uses this to as- sist word-referent mapping learning, and one does not. We demonstrate that solving the joint acquisition problem results in more rapid learning of word reference.